Tuesday, July 28, 2015

Heat Warning tomorrow, then watch for falling dewpoints!

As of early this afternoon, temperatures climbing into the mid 90s and high dewpoints (a measure of humidity) combined to meet the NWS definition of excessive heat (heat indices of 110°+), and the Heat Advisory that was in effect was upgraded to an Excessive Heat Warning. That warning continues through 8pm Wednesday, as we have one more very hot and humid day to deal with tomorrow. In fact, there's little relief overnight as the projected low in the city is 81°, which would tie the record warmest low temperature for July 29.

You've no doubt heard about the cold front that will move through the region Wednesday night. Many times in the summer, moisture and heat "pool" ahead of these fronts, resulting in hot and humid days. This is exactly what we expect tomorrow as temperatures once again reach the upper 90s with dewpoints in the mid 70s to near 80 in some spots in the Delta, making it feel much like it did today, if not a bit hotter!

So what about rain chances? Unfortunately, it appears the best chance of thunderstorms will remain to the east of the metro with only a slight (20%) chance locally on Wednesday and Wednesday night. Part of the reason for the low rain chance is that upper level high pressure will continue to be strong, limiting the support for storms, despite the presence of a front. Even the front itself will have little in the way of converging air along it, which would typically aid in making the air rise and increasing chances of storms.

The GFS computer model forecast at the jet stream level Wednesday evening shows the strongest wind well north of the area and a broad area of high pressure over the south-central U.S. The strong high will prevent any organized storms from affecting the region as the cold front passes.

The good news arrives Thursday as the front clears the area and drier air begins to push into the region. Temperatures will also peak about 5-7° lower than ahead of the front, so although it will still be hot (lower 90s), dewpoints will be a good 10° cooler, in the mid 60s. So rather than feeling like 110°, low 90s will FEEL like low 90s! I think we can live with that!

Looking towards the weekend, although temperatures will remain near or slightly above seasonal norms (lower to mid 90s), dewpoints & afternoon humidity values will remain below normal, so it should be a little easier to get the yard work done or get in a run! The long-range outlook continues the hot pattern into next week but with continued mid 60s dewpoints, thus heat indices should remain below the danger levels seen the past couple of weeks.

Be conscious of the heat for one more day, then we can look forward to hot without the humidity!

Erik Proseus
MWN Meteorologist

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Saturday, July 25, 2015

A hot mess - and why dewpoint is more important than humidity

An Excessive Heat Warning has been issued for the majority of the metro (Fayette County is technically under a Heat Advisory, but I'm not quibbling when the heat index has reached the danger level). The Excessive Heat Warning is in effect until Sunday at 8pm, but I have high confidence that we'll see it extended, possibly through Wednesday, before a front finally starts to move in our direction by week's end.

Excessive Heat Warning through Sunday in pink, Heat Advisory in orange. Graphic courtesy NWS-Memphis.

After a short break from the mid and upper 90s (but not the high humidity) earlier this week, plus a few rounds of strong thunderstorms, upper level ridging builds anew and can be thanked (or cursed) for pushing temps back into the mid 90s to near 100 for the next several days.  Rain chances are minimal to nil through Wednesday as well. These are the dog days... woof.

If it's any consolation, we are nearing the end of the hottest part of the year, climatologically. When eliminating rounding of temperatures to the nearest degree, the period from July 11-27 has the highest average daily temperature at 82.8°. Starting next week we slip a tenth of a degree or so! OK, that didn't help you any.  Maybe this will...

Perhaps this will make you feel a little better Memphis!
Posted by MemphisWeather.net on Saturday, July 25, 2015

So, besides hot temperatures, why are we seeing heat index values near or above 110°? Maybe you've heard this before: "It's not the heat, it's the humidity." (Well, in this case, it's the heat too. Let's not kid ourselves!) But there is truth to that statement. Let's look a little closer at humidity.

Relative humidity vs. Dewpoint

When humidity values are reported, it's actually "relative humidity" that we're talking about. It's relative to the temperature. For instance, if you track humidity during a normal day, you'll see that it peaks around sunrise and bottoms out in the late afternoon. However, even though the air is closer to saturation when relative humidity is highest, relative humidity doesn't actually tell you how much water content is in the air. Besides temperature, water content is the most important factor in determining how "uncomfortable" it feels.

Consider a calm, clear spring morning with temperatures in the mid 50s. The relative humidity is 90%. Now consider early this morning, with temperatures near 80 and relative humidity of 80%. In our example, the humidity was 10% lower this morning, so it wasn't as sticky as our spring example morning right? Wrong! Behold, I give you the more appropriate measure of water content (and uncomfortability - yes I made that up): DEWPOINT.  Dewpoint is the temperature to which air must be cooled to be saturated (or achieve 100% relative humidity).

Let's look at yesterday's observations from Memphis International Airport (below). Notice the humidity (yellow highlight column) falls from 88% in the morning when the temperature was 76° to 49% in the afternoon when the temperature was 95° (orange highlighted rows). That humidity value is "relative" to the temperature. Did the amount of water content in the air change from one time to another? Barely. We know this by looking at dewpoint (green highlight column). It was 72° when the temperature was 76° and 73° when the temperature was 95°. It inched up 1°, thus the amount of water content of the air changed very little. You'll also notice that the dewpoint is generally is more consistent (in the 72-76° range all day) than relative humidity, which varied from about 50-90%.

So, you say "50% humidity doesn't sound all that high!" Did you go outside yesterday afternoon? The air felt very humid! 50% in the morning is low humidity, but 50% in the afternoon is high! Dewpoint helps straighten all this out, since it measures the amount of water vapor in the air no matter the temperature!

Dewpoints above 65° usually cause people to start to notice the "humidity" in the air. Dewpoints above about 72° are very uncomfortable and when a dewpoint reaches the upper 70s to near 80° it's best to just avoid the outdoors altogether, especially if you have health issues! The amount of water vapor in the air at a dewpoint of 80° is so high that it can cause heat illness fairly quickly for those who aren't taking every possible precaution.

It's rare to see dewpoints this high at properly sited professional equipment that is well-maintained and in open areas, such as at large airports. However, in the MWN backyard, the grass is green and thick, moisture is constantly evaporating due to the heat, and there is less mixing of the air due to being sited in a neighborhood with obstructions. Even with well-maintained semi-professional equipment, the dewpoint has been reading 80° recently. I can assure you, having to mow that green thick grass that I can feel every degree of that ridiculous dewpoint! The sweat does little evaporating with that much moisture already in the air.

MWN's Bartlett reporting station, surrounded by grass and slightly sheltered by fences. These factors contribute to a higher dewpoint in this locale than at the airport.
So when someone comments on the humidity from now on, you can take a mental note that it's actually the dewpoint that determines how uncomfortable it feels!

Erik Proseus
MWN Meteorologist

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Sunday, July 19, 2015

MWN Video Forecast Discussion - 7/19/15

In this edition of the MWN Video Forecast Discussion, we details how long this Mid-South heat wave will last (hint: there a brief respite in sight!) and when to expect our next rain chances. (Video runs 11 minutes.)

Total projected rainfall through Friday, as predicted by NOAA's Weather Prediction Center. The Mid-South will hopefully see widespread amounts of 1/2-1".
The main hazards the next few days will be heat Monday and scattered thunderstorms towards midweek.

Erik Proseus
MWN Meteorologist

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Thursday, July 16, 2015

The dog days are here... hot, hot and more hot!

Temperatures have been heating up the past week as broad high pressure at the upper levels of the atmosphere has built over the region. Though not atypical of summer in the south, humidity levels have also been high, which has prompted daily Heat Advisories as heat indices have topped out in the 105-115° range.

High temperatures during the current heat wave.

A bit of history

However, for those who have been around the Mid-South their whole life (or at least the past 35 years), you'll recall the wicked hot summer of 1980. During July of that year, there were 19 days with highs at or above 100°, including a record stretch of 15 consecutive days. That was also the year that the all-time record hottest temperature occurred in Memphis - 108° on July 13th.

High temperatures at or above 100° during the summer of 1980.

A brief interruption in the heat

Earlier this week, a series of convective complexes made their way through the Midwest in the "ring of fire" - the zone around the periphery of the high that convective systems follow. While most of these stayed to the east of the Mid-South, a small cluster of storms formed in eastern KS and tracked through southern MO into northeast AR and across the metro on Tuesday night.

As the storms reached the Mid-South during peak heating, they encountered a highly unstable airmass. The result was a summer storm that brought widespread 50-60 mph wind and many smaller pockets of 60-70 mph wind to the metro. In fact, an 87 mph gust was recorded in Atoka in Tipton County!  Nearly 30,000 MLGW customers were without power after the storm and tree debris (in some cases whole trees) littered the landscape. A video of the radar data from that storm system is shown below. Notice also that storms fired up quickly in the late afternoon heat to our north, in an east-west fashion ahead of a cold front that remained to our north.

Into the weekend

The high pressure system has re-established its hold on our weather now though and it appears to remain entrenched right through the weekend. Thus we will see more mid to upper 90s highs, lows barely below 80, heat indices in the danger category, and nearly a zero percent chance of rain right into early next week.

Next week

By Tuesday/Wednesday, the ridge weakens a bit again (similar to earlier this week), which could allow for some airmass "popcorn" thunderstorms ahead of a weak front on those days. And just like earlier this week, the front itself will not move south of us. Thus cooler temperatures and lower humidity will again have to wait. Early signs are that the ridge of high pressure once again establishes itself over the region to end next week and temperatures remain well above normal as we head towards the end of July. Unfortunately there is little in the way of good news for those who like a break from the heat and humidity of the "dog days" of summer!

Heat Safety

We remind you that the effects of heat are cumulative - the longer it stays hot, the less capable people are of tolerating it. This is especially true for the youngest children, our older adult community, and those with medical conditions. Also, don't forget about your pets! It may be the dog days, but that doesn't mean your canine friends enjoy it more than you! Our complete guide to staying safe in the Memphis heat can be found here or check out the tips below to stay safe, if not completely comfortable, during these hot days!

Erik Proseus
MWN Meteorologist

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Saturday, July 11, 2015

A new meaning for the term "crap app"... or a 59% chance of bird droppings

I've ranted before on the uselessness of "crap apps" and that ALL weather is local, therefore your weather information sources should also be as local as possible. A classic example was placed in my lap last night.

As many of you know, MemphisWeather.net is the official weather partner of the Memphis Redbirds. So it goes without saying that I communicate with them (the head groundskeeper in particular) regularly on details of the forecast and, even more often, short-term trends during inclement weather. Last night, as the Birds were chewing up and spitting out the Nashville Sounds on a warm night with fair skies and a 0% chance of rain, I was sent the following screenshot of the hourly forecast on a national weather app by the groundskeeper:

Being a prudent and thorough groundskeeper, he is apt to check multiple sources of weather information, even on calm nights. When he came across the forecast above, with its elevated rain chances, he checked radar and noticed an echo over downtown. With a forecast of rain from that app (I don't mind calling it out - The Weather Channel) and the radar image with an "echo" on it, plus the knowledge that it wasn't supposed to rain, he reached out to me. I then grabbed the screenshot below from RadarScope.

Radar reflectivity showing a region of enhanced returns over downtown, which were persistent for perhaps an hour Friday evening prior to sunset.

I was obviously cynical, but also curious, so I did some additional radar interrogation, including looking at one of the new dual-polarization products (correlation coefficient, or CC) shown in the right panel below. It shows blue colors co-located with the reflectivity returns on the left indicating SOMETHING present in the air. The bright blue colors are low CC values, meaning it was NOT precipitation being detected. The objects being detected were non-uniform in shape/size.

Radar reflectivity (left) and correlation coefficient (CC - right). CC values indicate that the returns over downtown were not precipitation, but objects of irregular size and shape.

I returned a message to the groundskeeper, assuring him there was still a 0% chance of rain and that his national app was wrong (surprise, surprise). To me, it appeared that the radar was picking up on waterfowl returning to roost for the night on the banks of the river near downtown. I have seen this before (on a repeating basis in fact) just before sunset in roughly the same place. I believe Mud Island is a favorite "home" overnight for birds and have had some corroboration of that fact from downtown residents in the past.

What is the chance that these birds are detected by Doppler radar? About 59% apparently.

So what to make of the crap app with the 59% chance of rain? Most national apps use a single computer model for their forecast, which results in not only a POOR forecast when the model isn't performing well, but one that is also very inconsistent, often changing several times a day.

In the case of The Weather Channel app, I understand that it is a little  more sophisticated and actually performs slightly better than the typical national app. So what do I THINK happened? Though I can't prove it, I believe that TWC's app picked up on the radar echoes around downtown and "smartly" adjusted the rain chances for the first couple of hours of the forecast! If this was indeed birds that tripped the app up, it gives a whole new meaning to the term "crap app."

By the way, as opposed to the crap app, I never changed my forecast from the original 0% chance of rain. That's the difference between using a national app and shopping local. MemphisWeather.net's app and the forecast contained therein are HUMAN-POWERED.  I write the forecast, using all available model data, plus intuition, local knowledge and experience. Most of the time, that will result in a much more accurate product and one that is definitely more consistent. I think that's worth far more than the $0.99 I charge for it. Check it out for yourself.

Screenshot of the forecast from the human-powered MWN app
(And special thanks to the Redbirds groundskeeper for providing me an opportunity to give him, and all of you, yet another reason to stick with your trusted local weather source!)

Erik Proseus
MWN Meteorologist

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Monday, July 6, 2015

June 2015 Climate Data and Forecast Accuracy

June Recap

Temperatures and precipitation were above normal in Memphis in May and that trend continued in June. In fact, daily average temperatures were at or above normal for 22 consecutive days during the month. Precipitation was below normal until a deluge of more than 2" to end the month. However, most of the metro was below normal for the month. Through the first half of the year, Memphis International Airport still has a year-to-date rainfall deficit of 3.22", or about 88% of normal. As far as temperatures go, the year-to-date average temperature is 1.1 degrees below normal. Both of these numbers are trending closer to normal though over the past few months.

Despite above normal precipitation at the airport, most of the metro was below normal for the month of June, some areas by a couple of inches.

There were a few rounds of thunderstorms in the metro with scattered severe weather reports. Wind damage reports were received on the 9th and 26th in the metro and on the 24th in Tunica County. There was also both hail and wind reports in the metro on the 30th.

Memphis International Airport, Memphis, TN

Average temperature: 80.7 degrees (1.1 degrees above average)
Average high temperature: 89.9 degrees (1.0 degrees above average)
Average low temperature: 71.5 degrees (1.2 degrees above average)
Warmest temperature: 98 degrees (24th)
Coolest temperature: 60 degrees (2nd, 4th)
Records set or tied: None.
Comments: Every day from the 5th through the 26th was above normal, bookended by below normal temperatures to start and end the month.

Monthly total: 4.47" (0.84" above average)
Days with measurable precipitation: 8 (1.0 day below normal)
Wettest 24-hour period: 2.07" (29th)
Total Snowfall: None
Records set or tied: None
Comments: Over 2" of rain on the 29th resulted in a drier than normal month ending wetter than normal. It was the only day with more than 1" of rainfall for the month. It was also the second wettest June 29th on record behind the 5.87" recorded just one year earlier in 2014.

Peak wind: Northwest/41 mph (5th)
Average wind: 6.8 mph
Average relative humidity: 69%
Average sky cover: 50%

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

Cirrus Weather Solutions / MemphisWeather.net, Bartlett, TN

Average temperature: 79.8 degrees
Average high temperature: 90.1 degrees
Average low temperature: 70.2 degrees
Warmest temperature: 96.7 degrees (24th)
Coolest temperature: 55.6 degrees (3rd)
Comments: None

Monthly total: 3.28" (automated rain gauge), 3.39" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 9
Wettest date: 0.77" (29th) (via automated gauge)
Total Snowfall: None
Comments: None

Peak wind: Northwest/28 mph (8th)
Average relative humidity: 77%

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

MWN Forecast Accuracy

MWN average temperature error: 1.62 degrees
MWN forecast temperatures within 2 degrees of actual: 78%
MWN average dewpoint error: 1.62 degrees
MWN forecast dewpoints within 2 degrees of actual: 82%

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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Holiday weekend rain gives way to building high pressure and summer heat

Before we dive into weather, I have one thing to get off my chest...

OK, so now that that is over with we can move onto less important things!

Holiday weekend rain

First, here's a good picture to recap the holiday weekend we just experienced, courtesy of the National Weather Service in Memphis:

Add caption

This doesn't count Saturday's rain, and, because it's interpolated, the 10" reading from New Albany, MS is smoothed over. Honestly, with all the rain (including doubling the previous record precipitation on Friday at Memphis International) and some areas of flash flooding that occurred, the timing wasn't terrible. Major evening holiday events (with the exception of a brief shower at fireworks downtown Saturday evening) stayed dry for the most part. Temperatures also stayed unseasonably cool with highs in the 70s both Friday and Saturday.

Looking ahead

Of course, being July we knew that wouldn't last. Sure enough, we're back in the 80s now and will see 90° readings very soon. One factor that will has kept temperatures at bay a bit Sunday and Monday is the very wet ground. With lower humidity and dry ground, the sun's rays heat the lower atmosphere, resulting in hot temperatures. With all the humidity and wetness, more of the sun's energy has been used by the evaporation process, resulting in less to heat the air. In addition, the abundant low level moisture and evaporation resulted in more cloud cover as the rising air cooled and condensed (remember your water cycle from elementary school?) which turned water molecules from a gas into a liquid and produced clouds, which also reduced the amount of heating that occurred. The result was below normal temperatures the past couple of days.

Now that the ground is drying and fewer clouds will be around, especially by mid-week, our temperatures will respond to the sun's heating power with highs back to near normal levels for early July.

Next 24 hours

There is one more anomaly to deal with in the next 24 hours before the hot July weather can take over though, and that is an approaching front. While it won't make it all the way to the metro (typical of July), it will get close enough, and combine with a mid-level shortwave of energy, to produce a low chance of thunderstorms, mainly in AR, tonight and tomorrow. Rain chances are 20-30%, so don't be surprised if you hear some thunder or get a downpour from one of these widely scattered storms.

HRRR (high-res model output) "future radar" for the overnight hours, showing the possibility of a few t'storms, mainly in AR tonight. The early line of storms and steadier rain is closer to a front that will not move into the metro. Graphic courtesy WeatherBell.

Rest of the week and weekend

Beginning Wednesday, high pressure at the surface and aloft moves overhead, capping off the atmosphere under a heat dome, limiting cloud cover, and warming things up! If you want sunbathing, pool-diving, "Memphis summer" weather, you'll get your wish the latter half of the week into the weekend. Temperatures will reach the lower to mid 90s each day with virtually no rain chances that we see at this point and partly cloudy skies. Heat indices will also likely rise over the 100° mark this weekend.

Upper level (18,000' up) high pressure is noted in red area building from east to west over the next several days. This high will be responsible for capping off any rain chances and causing temps to rise heading into the weekend. Loop is from Tuesday through next Monday , when the high moves to our west, allowing for the possibility of cooler and more unsettled weather early next week. Graphics courtesy WeatherBell.

Here's the "at-a-glance" forecast then for this week...

Erik Proseus
MWN Meteorologist

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