Sunday, January 14, 2018

Turning our eyes to Monday night - more snow??

Our first winter weather event (and "snow day") is in the rearview mirror, but impacts linger this weekend as bitterly cold air settles over the region. Most areas within the metro ended up with anywhere from 1-1.5" of sleet and snow and very light amounts of ice glaze on Friday. The sleet being the predominant precipitation type resulted in icy roadways, and pretty much everything else.

While sun on Saturday aided many areas greatly, despite temperatures remaining below freezing, areas that were wet from melting re-froze last night, with icy patches still around, especially on secondary roadways. Temperatures today will just slide in under the freezing (or melting?) point and with overcast conditions, there will be less progress made on the icy spots. In addition, an upper level wave is bringing light snow to Arkansas and some flurries could make it into the metro this afternoon. Current radar is shown below.

Current loop from StormView Radar
Monday will see additional warming that should help a great deal with the remaining ice as high temperatures head for 40° or better with partial sun and southerly wind. However, don't get used to the relative "warmth." By evening, the next significant push of Arctic air arrives thanks to an annoying Clipper.


No, not that annoying clipper... one from Alberta, Canada! But feel free to "T" this one up too if you want, as it'll bring another round of #StupidCold to the Mid-South by mid-week. (Off-topic: man, I still miss Z-Bo... go ahead, watch that loop a few more times. I digress...) The clipper will also be partially responsible for our next snow event right on its heels Monday night into Tuesday morning. Given that we are still recovering from one snow day and continued impacts, perhaps this one comes #TooSoon?

What to Expect

One the plus side: no ice and relatively minor amounts. On the negative side: very cold air results in high snow ratios as we head towards Tuesday morning rush hour.

So what do we expect with this system? Temperatures will fall below freezing Monday evening, but the roads will likely retain a little of the daytime heat as light snow begins to fall before midnight, resulting in initial melting and wet streets. As the night goes on, snow chances increase and temperatures continue to drop as Arctic air pours in. If the European and NAM models are correct, snow appears likely after midnight through about 8-9am Tuesday morning. By the time light snow ends, temperatures will likely be near or just below 20°.

High-resolution NAM model forecast temperatures at 6am Tuesday. It shows 21° in Memphis. (WxBell)
Snow totals will be strongly affected by the cold air pouring in, causing snow ratios to rise. A snow ratio is the amount of liquid for each inch of snow. Typically we see roughly 10:1 ratios, which means that for every 1" of liquid, we would receive 10" of snow (or more likely, for every 0.10" of liquid we see 1" of snow). Higher ratios occur in colder air when the snow is drier (think less "pack-able" for snowballs). As our temperatures drop down towards 20°, our snow ratio could exceed 15:1. That means that for the same 0.10" of liquid, we would get 1.5" of snow instead of 1.0".


So how much snow are we talking? Between 9pm Monday and about 9am Tuesday, I think an inch is a fairly decent bet, but with higher snow ratios, 1-2" feels like a good possibility, especially south of I-40 where I believe the slightly higher amounts could occur vs. north of I-40. (Oh, you don't like I-40 always being the dividing line? OK, I'll use Highway 70, which no one in MS knows anything about. See what I mean - you have to use an interstate. But again, I digress...)

Below, see the average from the European ensemble members (at a 10:1 snow ratio), as well as the NWS forecast:

The European model ensemble (51 Euro models run with slightly different parameters) average snowfall is shown above given a 10:1 snow ratio. Above, we talked about how it could be higher, thus snow totals could be underestimated, but only if this model is even correct. (weathermodels.com)



Road Conditions

As far as expected road conditions are concerned, once again very cold air could come into play. I can't help but look back to January 28, 2014, when Birmingham and Atlanta experienced "Snowmageddon" with only 1-2" of snow. Do you know why it was such a major event? Not because "it's the south," but because it was about  20°, the snow melted a bit on initial contact from warmth the day before and warm tires driving on it, then it essentially "flash froze," creating a massive skating rink. Our forecast conditions are nearly identical. The exception may be that their snow began around the morning rush hour when warm tires were prevalent, whereas most of our event will occur overnight with light traffic. Perhaps that limits the possibility of a devastating event like Snowmaggedon, as the light snow could stick to the ground overnight and the tires driving on it won't melt it. I simply don't know how roads will be impacted, but I raise the possibility of a treacherous commute because I have seen it before in these conditions. We won't know until it happens. Here's how an NWS forecaster in Amarillo describes this setup, borrowed from the AlabamaWx blog:

“We typically get our worst travel problems and most accidents with 1-2 inches of snow. It actually seems like we have more problems with the dry, powdery snows than the wet, slushy snows. Just from my short experience out here, air temperatures of 28 F and lower seem to be when snow and ice accretion really start developing quickly on the roads.” - NWS-Amarillo Meteorologist


Other impacts

I believe the airport will fare better than Friday's event, mainly because most of the snow falls at a low traffic time and dry snow is much easier to deice than sleet. As always, you'll want to check with your airline if you fly through Memphis on Tuesday and of course allow plenty of time to get to the airport - double your normal travel time, or more.

I still won't predict school closings, though this one feels more like a "get up in the morning and see how it looks" kind of event. Friday's was easier with ice predicted during the morning commute. This event is not the same. Road conditions Tuesday morning will likely drive the decision.


Once again, with a #StupidCold airmass in place, low temperatures could get down into the upper single digits on Wednesday morning (especially if there is snow cover), and only reach the mid 20s both Tuesday and Wednesday. We won't be back above freezing until mid-day Thursday. Impacts could be prolonged once again. Click here to check the MWN Forecast for this week's temperatures.

Forecast Bust Potential?

There always is. In this case, not all models are sold on anything more than a light dusting. In fact, the American GFS model is one of those that is not convinced. I currently favor the European model, which has been very consistent for several days, and in fact, drops more snow than we are currently forecasting (as much as double). The American-born NAM Nest model, which is much higher resolution and only available out to 60 hours, agrees with the global Euro model, but with a bit less snow. None predict ice, and all are sold on very cold air by Tuesday morning.

We'll have the latest on Monday with another blog or perhaps video on Monday night so you can plan accordingly for Tuesday morning. Perhaps a 4-day weekend turns into 5 days?? Time will tell!


In the meantime, download the MWN mobile app for radar, current conditions, human-generated forecast, and the latest from our social media feeds, which are all also linked below!

Erik Proseus
MWN Meteorologist

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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Wednesday, January 10, 2018

Friday's winter weather event - our first call at the details

The best chance of wintry precipitation so far this year arrives this Friday. While (most) everyone looks forward to "snow days," if you end up with a change of plans from a normal day this time, it will most likely be due to ice, not snow. Snowball fights are currently not in the cards!

First a reminder that, as of this writing Friday evening, we're still about 36 hours away from the onset of winter precipitation, and while the computer models are generally coming into some alignment, we also see some "wild hairs" out there that lead to reduced confidence in some areas (which is not atypical for a southern winter event). Exact details are yet to be resolved and the situation remains fluid.

I'll start with the bullet points, based on the latest data and sound meteorological reasoning, unaffected by the wishes of any particular lobbying group. :-)

A Winter Storm Watch is in effect Friday for the blue counties, which are areas that have the greatest potential for impact to the general population. The counties under a watch, and later a warning or advisory, could change with new information.

What will fall: Besides temperatures, rain, then freezing rain, then a mix with sleet and snow, ending as light snow.
Start time: Rain transitions to freezing rain (ice) early Friday morning, probably between 3-5am
End time: Perhaps the lowest confidence portion of the forecast, but most likely by mid-day Friday
How much: Freezing rain: up to 1/4"; sleet and snow: 1" or less

Now for some details...

Despite very warm temperatures today and Thursday, a major cold front will escort our next Arctic air mass into the Mid-South Thursday late evening. This front will cause temperatures to drop rapidly towards freezing by the early morning hours on Friday on a strong and gusty north wind. Rain could occasionally be heavy along and just behind the front and a rumble of thunder can't be ruled out. Precipitation intensity should let up a bit as the temperature nears freezing. Once it drops to 32°, rain continues to fall (because the near-surface layer is cooling more rapidly than temperatures aloft) and starts freezing to objects that also drop below freezing - power lines, trees, and other exposed objects.

Forecast temperatures at midnight Friday just after the cold front passes through Memphis. Northeast MS is in the lower 60s, while north-central AR is in the upper 20s. (WxBell)

The European model from Wednesday morning shows the primary precipitation type between 12-6am Friday to be freezing rain, with some trailing snow to our west. Note that the precipitation does not extend very far to the west, indicating that it likely won't last long after 6am. This solution is one that favors an early departure to the precipitation and little snow. THis is just one of many models that are available to the meteorologist however. (WxBell)

Initially, roads should still be fine as they will be warm, but they will also cool quickly, particularly those with cold air blowing under them (think about an overpass). It won't take long for those to start feeling the effects of the freezing rain. As the air just aloft also cools, the rain (that freezes on contact) should start to change over to some sleet (frozen raindrops), and then probably light snow as precipitation rates continue to fall. This entire process will probably take 3-6 hours. So, from when the surface temperature first reaches freezing (right now, I'm saying 3-5 am) to when it's cold enough to snow will take 3-6 hours, or sometime during the morning hours. During that time, depending on how much sleet mixes in, we could see upwards of 1/4" of freezing rain.

During the course of Friday's event, we expect to see all of these precipitation types, moving from the left to right as warm air is overtaken by cold air, starting at the surface and moving up.
Models all agree on the details to this point fairly well. It's after this time that they start to diverge. There are multiple indications that once it gets cold enough to snow, the moisture moves out. Others keep a little around for a short time. Either way, the probability of more than a dusting to an inch of snow is not high. Then there is the occasional "outlier model" that keeps snow falling well into the afternoon. You can hope for that, but my hard-earned money is not on it!



Confidence Levels

Given we are still a little ways out, the models aren't in as good of agreement as I'd like, and we're in the south, confidence levels are worth a short discussion. Here is my confidence in the following events:

Onset time: Medium-High. If I'm wrong, it could be a bit later, but not by much.
End time: Medium-low. This could go either of two ways. A couple of solutions point to the precip ending not long after sunrise. An outlier from this afternoon (I'm looking at you 18Z NAM) says it'll snow all day. I'd pick the under if I were you.
Precip amounts: Medium. If I'm wrong, I am aiming high on the freezing rain total. I wouldn't be surprised to see it stop at 0.10" or so. That's still a real problem, so based on impact, it's not a big difference. I'm fairly convinced we won't see much snow. Less than an inch seems a solid (educated) guess right now.
Schools Cancelled: You thought I was going to bite on that didn't you...

What is the bust potential? There are valid model solutions that show precipitation ending not long after it gets cold enough to start causing problems. This is GOOD because nobody wants an ice storm, but it could result in minimal accumulations and no snow. If the precipitation ends early, snow will be the first sacrifice.

Impacts to Business and Commerce

Roads - Contrary to popular opinion, Memphis and surrounding jurisdictions do have the ability to salt/sand roadways, as does TDOT, which handles the interstates. However, given that these events are not frequent and the number of lane-miles in the metro, every road (even every major road) cannot be treated with current assets. Priority is given to the most critical and accident-prone areas, especially including elevated roads, such as Bill Morris Parkway and various interstate flyovers and ramps. In this event, rain will be falling prior to the arrival of the cold air, so pre-treating with salt brine would be a waste of money. It'll wash right off. Expect roads to be treated once freezing begins, and avoid known trouble spots altogether, even if you leave early for work Friday morning. If even light amounts of freezing rain occur, the elevated roadways will be slick and could be closed for treatment at times.



Airport - Believe it or not, the Airport Authority and FedEx are probably better equipped to handle winter weather events than the road crews are around here. They have plenty of equipment, and there's just not a ton of traffic. Cancellations into relatively minor events like this are lower than you think. The runways will be cleared, even if one at a time (that's all you really need), airplanes will be deiced, etc. The main concerns are: A) if freezing rain is more than forecast, more cancellations would occur, and it would be on flights heading into Memphis (which then means there is no plane to take you somewhere else), and B) weather elsewhere. This storm system will affect a lot of places on Friday. Cancellations elsewhere will ripple through the system like a boulder thrown into a pond. As always, check with your airline or your airline's website or app for the latest before your cross multiple iced up bridges and overpasses to get to the airport and find out your flight is cancelled.



Schools - Fortunately, I don't have to make this decision! However, the fact that (currently) the event is supposed to start very early in the morning COULD make their decision a bit easier. By 5am when many school decisions have to be made, we should know what we're dealing with. From experience, districts don't tend to mess around with ice. Kids - do your homework, then stick the spoon under your pillow. Hopefully you get to stay home and enjoy a four-day weekend (Monday is MLK Day)!


Bottom line it for me, Erik

Things are lining up for a potentially impactful winter weather event, particularly from the perspective of ice. At this time, I don't expect to see massive and widespread power outages or trees downed, but travel could definitely be hazardous to treacherous, especially Friday morning, and there could definitely be an impact to business and commerce. Don't be surprised at little to no snow though. Plan ahead now and exercise caution later. If not necessary to get out Friday morning and ice is on the ground, don't. I'm planning another formal update Thursday evening, hopefully via video. Stay tuned to our social channels listed below for updates prior to then, and of course during the event.

If you appreciate the service we provide, tell a friend or a few, and go get our app, also linked below. You'll find regular updates there, including radar, forecast, and social, as well as the opportunity to get personalized severe and winter weather notifications via the StormWatch+ in-app upgrade. I appreciate you checking it out!

Erik Proseus
MWN Meteorologist

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Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Tuesday, January 9, 2018

December 2017 Climate Data for Memphis, TN

December Climate Recap

The month of December was marked by a series of frontal systems that brought large temperature swings and periods of wet weather. After starting very warm, multiple days of cool weather followed a front that brought about an inch of rain. Despite a couple of warmer days, temperatures were a bit below average from the 5th through 15th. Another warm and wet spell resulted in several inches of rain in the third week of the month. As Christmas approached, temperatures dropped precipitously with well below average conditions experienced through the final week of the month. That week put the nail in the coffin of a colder than average month with above normal rainfall that nearly put the year right on par for precipitation. Despite the very cold temperatures at the end of the month (the period from Christmas to New Year's Eve was the 12th coldest on record), the only record set was a daily rainfall mark, eclipsed on December 22 when over 3" of rain fell. Flash Flood Warnings were issued for portions of the metro on that day, otherwise no severe weather occurred during the month.

Memphis International Airport, Memphis, TN


Temperature
Average temperature: 42.4 degrees (1.2 degrees below average)
Average high temperature: 51.2 degrees (0.9 degrees below average)
Average low temperature: 33.6 degrees (1.5 degrees below average)
Warmest temperature: 71 degrees (4th)
Coolest temperature: 14 degrees (31st)
Heating Degrees Days: 693 (28 above average)
Cooling Degree Days: 0 (2 below average)
Records set or tied: None
Comments: None

Precipitation
Monthly total: 7.26" (1.52" above average)
Days with measurable precipitation: 8
Wettest 24-hour period: 3.33" (22nd)
Snowfall: None
Records set or tied: Record daily precipitation of 3.33" was recorded on the 22nd.
Comments:  None

Miscellaneous
Peak wind: Northwest/37 mph (22nd)
Average wind: 7.7 mph
Average relative humidity: 66%
Average sky cover: 60%

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

Cirrus Weather Solutions / MemphisWeather.net, Bartlett, TN


Temperature
Average temperature: 41.3 degrees
Average high temperature: 51.4 degrees
Average low temperature: 31.4 degrees
Warmest temperature: 72.0 degrees (4th)
Coolest temperature: 13.2 degrees (31st)
Comments: None

Precipitation
Monthly total: 7.18" (automated rain gauge), 7.96" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 9
Wettest date: 3.26" (22nd) (via automated gauge)
Snowfall: None
Comments: None

Miscellaneous
Peak wind: South/21 mph (13th)
Average relative humidity: 73%
Average barometric pressure: 30.21 in. Hg
Comments: None

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

MWN Forecast Accuracy

MWN average temperature error: 2.70 degrees
MWN forecast temperatures within 2 degrees of actual: 50%
MWN average dewpoint error: 2.38 degrees
MWN forecast dewpoints within 2 degrees of actual: 59%

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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Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!
MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder