Sunday, December 27, 2015

One more round of strong storms early Monday, then winter arrives

If Mid-South weather could set it's "status" for this weekend, it would be "It's Complicated."

Wavering cold front

A cold front that originally came through Christmas Eve night, retreated to the north Christmas night to allow record-setting warm air to return the Mid-South on Saturday, has dropped south again earlier this morning. This has allowed temperatures in the 50s back into west TN, separating the northern Mid-South from the 70s just south of the metro. However, it's not done vacillating yet. It will pull back north of the metro once more later this evening and placing us in the warm sector one more time as - mercifully - a much stronger cold front arrives from the west early Monday morning, putting an end to the whimsical nonsense,

Metro temps as of 2:30pm Sunday, in the wake of a cold front that sits along the MS/TN state line.

Overnight weather details

As the Mid-South returns to the warmer airmass overnight, it will set the stage for one more chance of severe storms as the potent spring-ending cold front arrives from the west. Showers and a few thunderstorms are expected to be scattered as the front retreats back to the north this evening, then as we near dawn Monday, the front pushes a squall line into the region from AR. The squall line will be moving into an area that is favorable for strong thunderstorms, a few of which could be rotating.

Damaging wind along the line will be the most likely severe weather threat, along with very heavy rainfall and a chance of flash flooding, but a couple of tornadoes will be possible, mainly south and west of the metro in the wee hours Monday. The most likely timing for the squall line to pass through the metro is between 4am-9am Monday with a few strong storms also possible ahead of the line, after about 3am. A Flash Flood Watch is in effect for widespread 2-5" rainfall amounts from Sunday afternoon through Monday at noon.

Low pressure over the southern Plains will drag a cold front into the Mid-South early Monday, as a second cold front retreats just to our north overnight, placing the Mid-South in the warm sector one last time overnight. Map above valid at midnight.

Simulated radar from the HRRR model shows a squall line of storms advancing on the Mid-South overnight from AR and LA as snow and ice wrap around low pressure over the Southern Plains. Loop ends at 4am, just before the line moves through the metro. Graphic courtesy WxBell. Model data - timing may not be exact.

Monday - post-front weather

Behind the line, low pressure to our northwest and strong high pressure over the southeast U.S. will set up a scenario for very gusty wind throughout the day Monday. Southwest wind to 30-40 mph will be possible, resulting in the issuance of a Wind Advisory for the metro through 6pm Monday. All metro residents are encouraged to tie down or bring in any outdoor items that could blow away. Wind will diminish Monday evening to 10-15 mph.

Bottom line

To recap, scattered showers and storms are expected most anytime through the wee hours Monday morning. The severe weather threat begins after about 3am Monday and is most likely with a squall line of storms that should be over the metro between 4am-9am Monday. Damaging wind, small hail, and an isolated tornado (mainly southwest of the metro) is possible. Flash flooding is also possible due to very heavy rainfall and strong gusty southwest wind will continue throughout Monday.

The rest of the week

Once the system exits Monday, temperatures drop to much more seasonal levels with highs in the low 50s and lows in the 40s Tuesday and Wednesday, before dropping even further as we head towards New Year's Eve and the start of 2016. A chance of showers exists ahead of the reinforcing push of cold air on Tuesday night and Wednesday. Reference the MWN Forecast in our mobile app or at the links below for those details. We'll have any additional updates on our social media channels the next 24 hours.

Erik Proseus
MWN Meteorologist

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Thursday, December 24, 2015

After yesterday, severe weather is now in the rear-view mirror... right?

Wow, what a day. An outbreak of severe thunderstorms swept through areas from the Midwest and Ohio Valley into the Mid-South on Wednesday afternoon and evening, bringing high wind, large hail, and at least one long track tornado. The north Mississippi supercell that dropped a tornado near Clarksdale, MS continued east-northeast at near highway speeds as it tore through places like Como, Holly Springs, and Ashland. Preliminarily, the NWS in Memphis has assigned an EF-3 rating to the twister that at times was up to 3/4 mile wide, though a full assessment of the nearly 150 mile path will take days.

The forecast from the Storm Prediction Center and local NWS offices, and relayed through media partners and private enterprises like MWN, was overall very good. See the mid-day SPC risk area below with preliminary storm reports below. The Enhanced Risk area in particular matched up well with where severe weather actually occurred, while the Moderate Risk and "Particularly Dangerous Situation" Tornado Watch correctly captured the north Mississippi beast.

The warnings issued by the local NWS office also were spot on and correctly highlighted the threats that were imminent or ongoing, including the use of the rare "Tornado Emergency" Warning for Holly Springs, well before the tornado devastated southern portions of the town. The cumulative warning map below shows just how widespread the threat was. Fortunately, most of the metro outside of Holly Springs, was spared nature's wrath, despite some reports of up to quarter size hail in Bartlett and strong wind gusts as the storms moved through. We were fortunate indeed!

NWS Severe Thunderstorm (yellow) and Tornado (red) warnings issued on December 23, 2015.
So now we get a break for a while right? Well... from high threat severe weather, yes, but from lower-end strong storms and rainfall, no.  As you have noticed, it doesn't feel like Christmas Eve outside, despite a cold front moving through yesterday. The overall upper air pattern has changed very little and near-record warmth continues.

Despite a very pleasant spring (err, winter) day today, clouds and rain move back in tonight as yesterday's front turns back north as a warm front and puts us right back in the warm sector for the weekend. As it slowly meanders through the area tonight and tomorrow, rain chances return and thunderstorm chances also appear high, starting after midnight tonight and continuing off and on through Christmas Day. Unless Santa arrives before midnight tonight, he's going to need rain gear and a lightning rod on his sleigh! Rudolph's nose might come in handy tonight.

The chance of severe weather with tonight's storms is low, but not zero. SPC has a Marginal Risk (category 1 of 5) over the metro, mainly for a few strong wind gusts or some hail. However, along the front, there is a fair amount of "atmospheric spin," which COULD result in a weak tornado. However, this threat is nowhere close to yesterday's. We think you should go to sleep tonight dreaming of a White Christmas and just have the weather radio or StormWatch+ in your app on, just in case. We don't think you'll need it.

The SPC severe weather outlook for tonight shows a low-end risk of severe weather, mainly after midnight.
On Christmas Day, storms will remain possible along the corridor marked by the frontal boundary. Once again, a Marginal Risk of severe weather is indicated by SPC, as shown below. The main threats remain a stray damaging wind or hail report, with a very small risk of a weak tornado during the morning or afternoon. Again, remain aware but don't change any plans, other than making sure your plans are all indoors, despite the warm weather! Rainfall on Christmas Day could reach an inch, especially in north MS.

The SPC severe weather outlook for Christmas Day also shows a low-end risk of severe weather, mainly morning and afternoon
As we become firmly entrenched in tropical Gulf of Mexico air Saturday, record highs in the mid 70s will be threatened and the rain threat shifts north of the metro, outside of a stray shower or three. It'll feel like April. On Sunday, the front moves back into the area as low pressure forms over the Southern Plains. This will mean increased rain and thunderstorm chances. Then as the low, or its attendant cold front, move through the region sometime between Sunday night and Monday night, we continue with high rain chances, but with temperatures falling a bit (but still above normal). Severe weather during this period is still a question mark. The more definite scenario is a LOT of rain. The NWS says as much as 4-5" is possible in some areas between now and Tuesday morning with much of it falling Sunday and Monday. Flash Flood Watches appear likely for that period.

Total precipitation forecast by the NWS through Tuesday morning. The Mid-South is in an area that could receive up to 5" of rain while even higher totals are expected to our north and west, where flooding is nearly certain.
 Behind that system, it appears we finally get to experience more seasonal temperatures as the pattern flips heading into the New Year. For trend purposes only, check out the GFS forecast temperatures for the next two weeks. It looks more like January as January arrives!

GFS model forecast temps for the next two weeks. Use for trend purposes only (numbers are not exactly right). Winter is coming!
To all of our friends and followers, we wish you a blessed Christmas! We'll be watching the weather so feel free to check in and we'll be sure to keep you abreast on Facebook and Twitter. (And if you're still looking for a last minute Christmas gift, get yourself the MWN mobile app and go ahead and pay the $8 for StormWatch+. Or give someone else $8 and tell them to get it! After yesterday, I'm not sure why you wouldn't.)

Merry Christmas!

Erik Proseus
MWN Meteorologist

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Tuesday, December 22, 2015

Tuesday evening update on severe weather expected Wednesday

Here's the latest on tomorrow's severe weather potential as of 7:30pm Tuesday night. Take 11 minutes to watch and get all the information you need to prepare, then another 10-15 minutes to take those preparation steps tonight. Severe weather is LIKELY tomorrow. 

As mentioned in the video, be sure to check out StormWatch+ in the mobile apps for iOS and Android. For $7.99 (plus $0.99 for the base app), you get a portable, customizable, PRECISION severe weather alert notification system. Think of it like a 2015 version of a weather radio that only alerts you if you need to be alerted! Link for more information and to download is below.

We'll have more updates starting early Wednesday on Twitter and Facebook.

Erik Proseus
MWN Meteorologist

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Monday, December 21, 2015

MWN Video Discussion - Severe weather threat Wed and Christmas weather meteorologist Erik Proseus takes a look at your Christmas week forecast, including a travel outlook for those leaving the area this week to visit family and friends. An in-depth examination of the meteorology behind our severe weather threat on Wednesday is also discussed for you weather nerds. If you just want to know what to expect this week from a forecast perspective, fast forward to the 21:00 mark!

We advise you to have multiple ways of getting severe weather information on Wednesday, including NOAA Weather Radio, local TV/radio, and a smartphone warning app. We recommend our own app, of course! for iPhone and Android includes an optional push notification warning service called StormWatch+. For $7.99 one-time, you get a fully-customizable mobile weather radio that alerts you only if you are in the path of a storm, not if your county is. Get more information and download the app at the link provided below.

Stay with MWN on Facebook and Twitter for updates as warranted throughout the week. Have a safe and happy Christmas week!

Erik Proseus
MWN Meteorologist

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Saturday, December 19, 2015

More detail on a warm and wet holiday week

A brief Saturday afternoon blog post as eyes start turning towards Christmas week and, as most of you know by now, weather that looks nothing like the first days of winter (which begins Monday at 10:48pm). We described it like this on social media this morning:

And I know what you're thinking...

What's up is a large subtropical high pressure ridge over the southeastern U.S., an Arctic jet stream that is well north of its usual winter solstice position, and the peak of El Nino conditions in the Pacific Ocean. Putting all that together results in temperature anomalies (departure from average) for the period December 21-26 that look like what is shown below, according to the American model ensemble. Yes, that is at least 10°F above average for much of the eastern U.S. and Canada! (Normal highs this week are near 50 with lows in the mid 30s. I expect we'll see several lows that are well above our average high temperatures this week.)

Temperature anomalies from Dec 21-26 from the GEFS model ensemble indicating well above normal temperatures for the eastern U.S./Canada during Christmas week. Graphic courtesy WxBell.
 That's why we won't be seeing anything close to a white Christmas. But it doesn't mean we'll be dry. In fact, the American GFS model predicts precipitation for the next 10 days looks like the next figure - 4-5" of rain through the morning of December 29th. Honestly, for some areas of the Mid-South that could be a little low looking at other long-range model solutions.

Total precipitation forecast by the GFS model through Tuesday morning, December 29. Graphic courtesy WxBell.
With excessive warm air and lots of moisture in place, as well as passing low pressure systems, we will also be in for some thunderstorms. The chance for severe weather is still questionable, but we'll be keeping an eye on it. Possibly the best chance is Wednesday as low pressure and a cold front affect the region and temperatures reach near 70° but it is too soon to tell. It does appear that we may get a relative "break" from the rain behind that Wednesday system which could mean a drier Christmas eve.

However, by Christmas Day, it looks like we're back into the southerly flow, which would mean a return of rain chances. Both the American GFS and European models project a potent low pressure system and cold front move around the 27th-28th. That could be our best chance of severe storms, but it's way too soon to know how that will pan out.  Here's our best shot at the next 6 days:

Instead of hot chocolate, we could be saying "pass the sweet tea" on Christmas morning. Stay tuned for additional detail the next few days.

Erik Proseus
MWN Meteorologist

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Wednesday, December 16, 2015

December-like weather (for a minute) and our first look at Christmas week

Warm December weather continued into early this week, despite a cold front over the weekend. High pressure behind that front was Pacific in origin, so it didn't bring a great deal of cold air. Hence we stayed in the 60s for highs (actually 70° officially yesterday).

Today, another cold front moves through and this one will bring some more December-like air with it. Ahead of the front, showers and even a few strokes of lightning occurred and moved over the metro this morning. Expect to see a chance of showers continue this afternoon until the front passes by rush hour. The strongest storms with this system will be closer to the Gulf of Mexico today.

Behind today's front, our 60°+ highs will be gone for several days. In fact, the high pressure system that builds into the region will have some pretty decent cold air with it. Thursday will be a transition day with seasonal temperatures (highs in the lower 50s and lows in the upper 30s), then we drop below normal Friday with highs in the mid 40s and much of the area dropping below freezing (see surface map below showing the large dome of cool high pressure over the southern U.S.).

The surface forecast map for Friday morning shows cool high pressure moving across the southern plains, resulting in weather we are more accustomed to in December. But it will be short-lived as the high moves east and the Mid-South gets back into southerly flow by late in the weekend.
As we head into the weekend though, that high moves to our east and starts funneling warm air into the region again on southerly wind from the Gulf of Mexico. An "average" day on Saturday transitions us to above normal once again as we head into Christmas week. Clouds increase Sunday with rain chances looking decent for Monday as high temperatures rebound back into the 60s, a good 10° above normal. The MWN Forecast below outlines the next 6 days for you.

Christmas week will bear watching closely.  Not only will we be back into a very un-Christmas-like pattern with warm, moist air on strong south wind, but disturbances in the force (obligatory Star Wars reference) will result in rounds of showers and thunderstorms through at least Wednesday (into Christmas Day if you believe some fairly reliable models). In fact, there is some support for potential severe weather and temperatures back near 70° Tuesday and Wednesday of next week. It's too early to define specific or timing, but the signals are definitely strong for a very warm and wet few days leading up to Christmas.

Below are and temperature and precipitation probability forecasts from the NWS Climate Prediction Center for the period December 21-25, indicating a 60% chance of above normal precipitation and a 90% chance (as definite as it gets) of above normal temperatures for a large section of the eastern U.S. The best chance for a White Christmas is unfortunately going to be a plane ride away in the higher elevations of the western U.S. it appears. Not only is grit and grind dead, but so is any lingering hope of a White Christmas in Memphis.

NWS Climate Prediction Center (CPC) temperature outlook for Dec. 21-25 showing a 90% chance of above normal temperatures for the eastern half of the U.S. As close to a sure bet as it comes.

The NWS CPC also indicates a high likelihood of above average precipitation. Combined with the warm temperatures, we'll be watching for the chance of thunderstorms during Christmas week.
Erik Proseus
MWN Meteorologist

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Saturday, December 12, 2015

November 2015 Climate Data and Forecast Accuracy

November Recap

Above average temperatures continued for the third straight month in November. In fact, this was the 4th warmest November on record. The month was also much wetter than average, becoming the 8th wettest November in recorded history, thanks to multiple heavy rain events. For the year, the average temperature at Memphis International Airport is 65.0°, which is 0.8 degrees above normal. After running a deficit for much of the year, precipitation for 2015 is up to 47.89", which is right at normal (0.05" below average).

There were fairly minor severe weather reports in November during a few different events. Wind damage, mainly in the formed of downed trees, occurred on the evening of the 17th in multiple locations across Shelby County as a line of storms moved through. Flooding was reported in western DeSoto County on the evening of the 17th. Also, isolated wind damage occurred in Tipton County on the morning of the 6th and the evening of the 11th.

Meteorological fall, defined by the months of September, October, and November, concluded with above average temps and precipitation. In fact, it was the 5th warmest fall on record with an average temperature of 67.2 degrees (3.0 degrees above normal). Precipitation totaled 13.92", which was 1.36" above normal.

Memphis International Airport, Memphis, TN

Average temperature: 57.8 degrees (4.6 degrees above average)
Average high temperature: 66.7 degrees (4.1 degrees above average)
Average low temperature: 48.9 degrees (5.2 degrees above average)
Warmest temperature: 81 degrees (5th)
Coolest temperature: 28 degrees (23rd)
Records set or tied: Maximum low temperature records were set on the 4th (67°) and tied on the 5th (66°) and 27th (63°).
Comments: November 2015 was the 4th warmest November on record. Only 3 daily average temperatures were cooler than 1 degree below normal. Eight days were at least 10 degrees above average.

Monthly total: 10.16" (4.67" above average)
Days with measurable precipitation: 11 (1.5 day above normal)
Wettest 24-hour period: 3.31" (17th to 18th)
Total Snowfall: None
Records set or tied: None
Comments: November 2015 was the 8th wettest November on record

Peak wind: Southeast/47 mph (17th)
Average wind: 7.1 mph
Average relative humidity: 67%
Average sky cover: 60%

Click here for a daily statistical recap for Memphis International Airport.

Cirrus Weather Solutions /, Bartlett, TN

Average temperature: 53.1 degrees
Average high temperature: 65.9 degrees
Average low temperature: 46.1 degrees
Warmest temperature: 79.7 degrees (5th)
Coolest temperature: 25.1 degrees (23rd)
Comments: None.

Monthly total: 10.39" (automated rain gauge), 11.41" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 11
Wettest date: 3.59" (17th) (via automated gauge)
Total Snowfall: None
Comments: None

Peak wind: East/33 mph (17th)
Average relative humidity: 75%

Click here for a daily statistical recap for Bartlett, TN.

MWN Forecast Accuracy

MWN average temperature error: 2.59 degrees
MWN forecast temperatures within 2 degrees of actual: 56%
MWN average dewpoint error: 2.51 degrees
MWN forecast dewpoints within 2 degrees of actual: 60%

MWN's forecasts extend out five periods (2.5 days, or roughly 60 hours). Historical accuracy statistics can be found here.

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Friday, December 11, 2015

Is it still December? The weather is acting more like April!

Very warm air is in place across much of the eastern half of the nation in a season known more for snowflakes up north and cool breezes down south than shorts and t-shirts!  Today's high (as of 3pm) of 77° ties the record for the date set in 2007, and we aren't done with record warmth!

Check out all of the potential records that could be tied or broken below. First, tonight's overnight lows, which will be in the mid 60s in the metro. Circled temperatures are potential "warmest minimum temperature" records:

Saturday's highs will be break records all across the Lower and Mid Mississippi Valley into the Ohio Valley and Midwest:

And again Sunday morning, more record warm low temperatures are likely to be broken, including here at Memphis (the caveat to those cities on the western side of the map being that Sunday's frontal system will likely mean daily lows will be set in the evening rather than morning):

With another couple of days of warmth ahead, that must mean the frontal system will be a doozy huh? Well, maybe not... at least as far as severe weather. The Storm Prediction Center's outlook for Sunday (below) shows the most likely chance of severe weather well to our south, along the western and central Gulf Coastal region into eastern TX and Louisiana. The light green area (which includes the metro) basically means that thunder is possible but severe weather is not anticipated.

SPC severe weather outlook for Sunday. The best chance of severe storms will be well to our south, though a rumble of thunder will be possible with the PM rain.
We think that this situation will need to be monitored fairly closely, so we'll do that. With temperatures already running above what computer models thought we'd see, and a pre-frontal trough (basically a weak front ahead of the main cold front) moving through near or just after peak heating on Sunday (late afternoon to early evening), we could end up with a little more unstable environment than the models currently depict. Until this morning, the SPC risk zone also stretched a little further north, up to the MS/TN state line. So it bears watching. Like our triplicate systems in November, this one will again have plenty of wind energy but is lacking in instability. Stay tuned.

As far as precipitation, a pretty healthy amount of water will be falling from the sky Sunday late afternoon through evening. We agree with this NWS map below (which agrees with computer models) that a good inch is likely and some places could see up to 2" is multiple downpours move overhead. Precipitable water (which measures the amount of moisture in a column of air over a point) will be nearly 1.5", which is near record highs as well (similar to a couple of those wet November events).

Forecast rainfall amounts through Monday, according to the NWS.
The other factor on Sunday in particular will be wind ahead of the weather system. We encourage you to tie down anything outside that you don't want blowing a few blocks to your north. Deflate the blow up Christmas decorations unless you want to show the kids how reindeer really do fly.

Behind Sunday's system, it'll be a bit cooler, but still well above normal temperature-wise. A mid-week system brings another chance of rain (much lower chances and less precip) but finally pushes temperatures back down to near normal (lower 50s for high, mid 30s for lows) to end the week.

Be sure to follow our Facebook and Twitter accounts listed below for the latest details on this weekend's weather and use the opportunity provided by warm weather to (finally) get those Christmas decorations up. Just be wary of the wind!

Erik Proseus
MWN Meteorologist

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Saturday, December 5, 2015

December turns warm, and will stay that way for a while

Spectacular weather for the first weekend in December, which is always a busy one! The Memphis Marathon couldn't really have asked for finer conditions (remember the ice storm a couple years ago?) with chilly temps early that warmed quickly and light wind. Christmas Parades abound today as highs reach the lower 60s with sunny skies. Normal highs are in the mid 50s with lows in the mid 30s this time of year.

Current metro temperatures from
Above average temperatures will continue for the foreseeable future as upper level ridging dominates the region (with a few quick passes of transient upper level disturbances) and high pressure at the surface resides over the eastern U.S. We'll likely get used to afternoon temperatures in the 60°+ range over the next week! The first upper level "imperfection," and its weak surface cold front pass, by tomorrow, bringing cloud cover and maybe a few sprinkles, but a very small chance of measurable rainfall. Despite the cloud cover, temperatures should still reach 60°.

Upper level (500mb, or 18,000') loop showing an approaching trough and disturbance (red to purple colors) moving across the south-central U.S. through noon tomorrow. Graphic courtesy Rapid Refresh model.
By Monday, the passing trough is replaced by another ridge of high pressure as the sun returns and temperatures climb into the upper 50s. Tuesday and Wednesday features more of the same with slightly warmer weather. Highs return to 60° or a bit higher each day.

By 6am Tuesday, high pressure covers most of the central and eastern U.S. leading to tranquil weather and above average temperatures for the Mid-South. With the high to our east, wind becomes southerly on Tuesday. Graphic courtesy NOAA.
As we head into the latter half of the week and next weekend, strong southerly flow (marked by moderate south wind and moisture feeding into the region from the Gulf of Mexico) establishes itself over the southern U.S. This pattern means a couple things: more clouds and periodic rain chances, but also even warmer temperatures.

Towards the end of the week, strong southerly wind pushes temperatures up but also means periodic rain chances. The GFS model depicts this pattern well for Friday night. Graphic courtesy Pivotal Weather.
Medium range models are having a tough time so far with timing of low pressure systems and cold fronts, thus also rain chances. We're carrying a low end rain chance (20%) Thursday with 30-40% chances on Friday and Saturday. By Saturday night or Sunday, a large-scale surface and upper-level system will push through, bringing a round of showers and maybe a few thunderstorms and dropping temperatures from the mid (to perhaps upper) 60s back down into the 50s to start the third week of December. We'll have to re-address the timing of said systems as we get closer to the mid-week timeframe. Until then, enjoy the relative warmth! (For those keeping score, our MWN Winter Outlook predicted above normal temperatures for much of December. So far we look to be right on track.)

The Climate Prediction Center temperature outlook for the third week in December shows a 60-70% chance of continued above normal temperatures for the metro. In fact, most of the nation east of the Rockies could experience above normal temperatures, on average, from December 13-19.
Erik Proseus
MWN Meteorologist

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Friday, November 27, 2015

A warm Turkey Day becomes a wet holiday weekend

Well, at least we can say it was a nice holiday! In fact, the high temperature in Memphis on Thanksgiving was 75°. That is not a record for the date, but I did go back 20 years into the archives and it was the warmest Thanksgiving over the past two decades.  In reviewing those years I found lots of highs in the 40s and 60s. I also found that rain has fallen on only 2 Thanksgivings in the past 20 years (2000 and 2003).
Despite a few sprinkles, Black Friday has also turned out pretty decent too, warm and cloudy but mainly dry so far. Temperatures have been in the mid 60s most of the day. If we don't drop below this morning's low of 63°, we'll tie the daily record for warmest low temperature. We can be thankful we're not dealing with the ice storm that some in the Plains are experiencing! That speaks to the strength of the cold front to our west, if not to its (lack of) forward motion.

Heading into the rest of the weekend, a persistent upper level pattern has caused the surface front to nearly stall out as it attempts to move east through the Mid-South. This upper pattern features a fairly strong ridge of high pressure over the lower Mississippi Valley and Gulf of Mexico and a deep upper level low over the western states. In between, a broad southwesterly flow extends from the Desert Southwest to the Great Lakes, or parallel to the surface front. When this occurs, there is very little to "push" the front through and our surface pattern also stagnates.
Jet stream level (39,000') setup as of Friday evening, showing a strong southwest to northeast flow across the U.S. with a ridge of high pressure over the Gulf of Mexico and a deep and large low pressure trough over the western U.S. A surface front lined up with the jet stream aloft typically doesn't move very fast.
Eventually, high pressure over the Plains will push the front through the Memphis area, likely around noon tomorrow. Though rain is expected off and on tonight into early tomorrow (with the heaviest to our northwest), the steadier rain will move into the metro behind the front later Saturday as moisture-laden air also resides behind the front, which stalls again over north MS. Upper level disturbances moving parallel to the front will keep rain chances high right through Sunday and into Monday. This is when low pressure forms along the front and helps to finally push it east as the upper level flow flattens out, becoming zonal (meaning the air moves nearly west to east). It could be early Tuesday before we finally shake the rain chances entirely however.
By Monday evening, the southern U.S. is dominated by a strong west-to-east jet stream (what we refer to as "zonal flow") which helps to keep frontal system more progressive at the surface. Also visible is the weakened trough that was in the western U.S. moving over the jet stream across the northern Plains.
In the meantime, rainfall totals across the metro will likely end up in the 3"+ range by Monday afternoon, most of that falling between Saturday evening and Monday morning, though light amounts are to be expected tonight and Saturday, as well as during the day Monday. Flash Flood Watches are posted just northwest of the metro in Arkansas where rain totals (which include that which has already fallen) could be upwards of 6".

NWS rainfall forecast from 6pm Friday through 6pm Monday. Though the rain has held off so far, a lot is on the way, particularly from Saturday PM through Monday AM. Graphic courtesy NWS.
So keep your galoshes, ponchos, umbrellas, and rain boots handy for the next few days! For those attending Senior Day at the Liberty Bowl Saturday morning as the Memphis Tigers football team plays their final regular season game of the year against SMU, periods of light rain are possible from early day tailgating right through the game, but we're not expecting major washouts. Temperatures will be mild in the morning (lower 60s) before starting to fall into the 50s by early afternoon as the front moves through around noon, so overall not bad temperature-wise.

Surface map valid 6pm Saturday evening showing the cold front just southeast of Memphis and widespread rainfall expected along and behind it as low pressure systems ride along the front from southwest to northeast. Graphic courtesy NWS.
Much of next week features cooler weather and mainly dry, though high clouds could be abundant. Lows will drop into the 30s with highs in the lower 50s at best by mid-week. Click here for the complete "human-powered" forecast from MWN and be sure to follow us throughout the weekend on social media as we bring you the latest information!

Also, for a few more hours (ending tonight at midnight), our mobile app for iPhone/iPad is FREE for Black Friday! Click the link below to grab it before it goes back up to $0.99! (Android users will still have to pay $0.99 since Google doesn't give us the flexibility pricing that Apple does. Sorry!) We also encourage you to upgrade within the apps to add StormWatch+, or precision severe weather alert technology. You'll find it to be a very advanced version of a NOAA Weather Radio that is mobile, works nationwide, and is about 1/4 the cost of a standard weather radio. Check it out and thanks for your support in helping to make the rest of what we offer free!

Erik Proseus
MWN Meteorologist

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Sunday, November 15, 2015

Heavy rain and strong wind on track for the Mid-South

A lot of rain to be sure, but Biblical in nature? Don't hunt for gopher wood or pair up the animals just yet...

Another strong transition-season cold front will move across the country this week. Similar to the past two events of last week and the week before, there will be plenty of wind at all levels of the atmosphere with this system, but little instability. The lack of unstable air will greatly reduce the threat of severe weather, but it won't make much difference in precipitation rates as the heart of the system moves through. Let's take the next few days a piece at a time.

Tonight / early Monday

Clouds thicken overnight tonight as upper level moisture increases on west-southwest wind aloft. Atmospheric energy originating from an upper-level low over the Desert Southwest will move over the area late tonight and early tomorrow. These "upper level disturbances" as they are sometimes called are areas where the air is lifted in the vertical. Where moisture is also present, the lifted air cools and condenses, resulting in precipitation. Thus scattered light showers that result will help to moisten the lower levels of the atmosphere and begin priming the pump for heavier rain to come. Rainfall will generally be 0.10" or less and should mainly affect areas north of I-40 with measurable precipitation.

North American Model (NAM) forecast depicting a few upper-level disturbances, including a couple strong ones in red/purple, moving through southwest flow aloft early Monday morning. They will trigger scattered showers, mainly for northern areas of the Mid-South. Graphic courtesy Pivotal Weather.

Monday afternoon / night

By Monday late afternoon and evening, steadier light rain is expected to begin, lasting through the overnight hours. This additional shower activity will be in response to increasing moisture content in the atmosphere, additional upper level disturbances moving through southwest wind aloft and providing lift necessary for precipitation, and a rapidly-strengthening low level jet stream Monday night. As low pressure over the Front Range strengthens and moves slowly east into the Central Plains, wind increases ahead (or downstream) of it. At the low levels (a couple thousand feet up), south wind will increase to nearly 50 knots (almost 60 mph) by early Tuesday morning! This strong wind above the surface but in the low levels of the atmosphere is termed a low-level jet (LLJ).

North American Model (NAM) forecast showing low-level wind increasing to over 50 mph straight off the Gulf of Mexico by Tuesday morning, bringing abundant moisture into the region. Graphic courtesy Pivotal Weather
With the low-level wind straight out of the south, a flood of atmospheric moisture from the Gulf of Mexico will inundate the lower Mississippi Valley and Mid-South. Despite this, overnight rainfall amounts Monday night should still be less than 0.50" as the best upper level dynamics remain well to the west of the region. Surface wind will also increase in response to the low level jet with steady south wind likely reaching or exceeding 20 mph.

The NWS Weather Prediction Center forecast of total rainfall from Sunday night through Tuesday at 6pm indicates only about 1/2" of rain is expected in the metro. The band of heaviest rainfall totals shows up to our west in OK, AR, and MO.

Tuesday daytime

A relative lull in precipitation will occur Tuesday, though the metro will continue to see the pump primed for heavy precipitation later as very strong southerly flow begins to increase moisture content in the mid levels as well. With no significant upper-level disturbances traversing the area, significant precipitation is not expected before dusk, though scattered showers certainly are possible given the amount of moisture in the air above us. Plan for plenty of leaves to fall as wind will remain strong and gusty at the surface Tuesday with south wind frequently gusting to 25-35 mph. A Wind Advisory may be needed on Tuesday. With a break from steady rainfall and south wind pushing warm air into the area, expect highs to eclipse 70°, even with overcast skies.

North American Model (NAM) forecast showing surface wind increasing to 20-25 mph with higher gusts Tuesday afternoon. Parallel lines from south to north indicate a flow of air from the Gulf of Mexico. Graphic courtesy Pivotal Weather.

Tuesday night

Tuesday night is the time period of most concern, with the main threat being flooding and flash flooding and, secondarily, wind. As the north-south aligned cold front moves through the area, it will provide the necessary lift promote heavy precipitation and will open the faucet wide. For 24 hours prior to the front arriving, strong southerly wind carrying Gulf moisture-laden air will provide abundant moisture for the front to tap into. Upper level dynamics increase as wind increases to around 30 mph at the surface and 60-75 knots (70-85 mph) from 2,000 feet to almost 15,000 feet. In addition, jetstream wind about 35,000 feet up will increase to over 100 knots (115 mph). Wind Advisories will likely continue into the night.

Atmospheric profile from the NAM model at 9pm Tuesday showing very strong southerly to southwesterly wind in the low levels of the atmosphere and a strong southwesterly upper level jetstream above. In addition, the red line (temperature) and green line (dewpoint) are very close together from the surface to about 25,000', indicating high relative humidity through most of the column of air above Memphis.
With all of this wind coming from the south, it is originating from the Mid-South's biggest moisture source - the Gulf of Mexico. Precipitable water (PW) values, which are a measure of total atmospheric water content, will approach 2.00", which would be record territory for this time of year according to climatology provided by the Storm Prediction Center. With a slow-moving cold front and record atmospheric moisture levels, the stage is set for possible flooding and flash flooding Tuesday night. Various computer model solutions and Weather Prediction Center forecasts paint a picture of rainfall totals between 2-3" in a roughly 6-hour window overnight.

North American Model (NAM) forecast showing precipitable water values reaching near-record levels by Tuesday evening with abundant moisture in all levels of the atmosphere. Notice also behind the cold front how quickly moisture falls off. Graphic courtesy Pivotal Weather.

There are 3 big concerns related to the flooding threat on Tuesday night:

1) Precipitation rates. Widespread 2-3" amounts in a roughly 6-hour window, on top of precipitation that has already fallen from Monday morning through Tuesday, will lead to high water in the usual places. Drainage ditches, streams, and creeks will fill rapidly.
2) Leaves. It's fall, y'all! I drove around some today and saw piles of leaves sitting on curbs and in gutters, not to mention all over yards. We have a lot of trees in the Mid-South and the leaves are falling (and if they aren't yet, wait until the wind has been blowing 20-30 mph for 24 hours!). The leaves will block natural areas that water flows, namely along the sides of streets in the gutters. With drainage grates potentially blocked by leaves, street flooding becomes a bigger hazard as the water can't run off as quickly.
3) Darkness. All of the heavy rain will occur when it is dark out. Pair darkness with torrential rain and leaves blocking gutters and we have a recipe for big problems for motorists. Streets that haven't flooded in a long time will. We have one suggestion: if you don't HAVE to get out Tuesday night, DON'T! Get your errands done on the way home from work then stay put overnight.

Moving on, the threat of severe weather will be low Tuesday night as the lack of instability (as with the previous systems the past couple of weeks) and overall saturation of the atmospheric column will limit the formation of strong storms. Convective precipitation (that which occurs in an area of rising air) and thunder is certainly possible and would serve to increase rainfall rates, but the bulk of the severe weather should remain to our southwest. If there is a severe weather threat locally, it will be strong(er) wind from a possible squall line of storms. Again ,we expect that threat to remain to our south, but it's worth mentioning.

The severe weather threat on Tuesday will be southwest of the Mid-South where instability is greater and a squall line could develop, according to the Storm Prediction Center convective outlook.

As the cold front passes sometime in the wee hours Wednesday, the faucet will shut off as wind shifts westerly, cutting off the feed from the Gulf and ushering in drier air from the west (as seen in the precipitable water graphic above). By dawn, the rain will probably be over (and your rain gauge could be overflowing). Your school's car line should be dry.

Wednesday - Friday

The second half of the week will be a time to dry out as high pressure of Pacific origin moves overhead. Temperatures will remain seasonal with highs in the mid 60s and lows in the 40s. Clouds will depart early Wednesday, leaving the rest of the week mostly sunny.

Follow us on Facebook and Twitter and on our mobile apps (all links below) for the latest information on this week's wind and heavy rain threats, as well as any changes to the forecast.

Stay safe and (relatively) dry!

Erik Proseus
MWN Meteorologist

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