Saturday, April 30, 2016

"Wake low" causes wind damage across the metro Saturday morning

For the second day in a row, a line of thunderstorms dissipated over the metro region. Both times a 'wake low' formed over the area. Saturday morning's was much stronger and produced the most impact. It made its appearance known in the form of high wind that occurred between about 10am and noon, starting in far eastern AR and making its way slowly east across the city and into the eastern suburbs.

So what is a wake low? According to the American Meteorological Society, a wake low is defined as:
"A surface low pressure area or mesolow (or the envelope of several low pressure areas) to the rear of a squall line; most commonly found in squall lines with trailing stratiform precipitation regions, in which case the axis of the low is positioned near the back edge of the stratiform rain area."
In other words, sometimes trailing a squall line a mesoscale (small scale) low pressure system will form. Typically it will trail a mesoscale high pressure system than forms immediately behind the squall line. In many instances, the meso, or wake, low will be aligned along the back edge of stratiform (steady rain) precipitation trailing the line of storms as air sinks, thus warming and also drying the back edge of precipitation. The pressure gradient between the meso high and meso low creates a scenario in which a localized area of high wind can develop. (Weather geeks will recall that wind increases when the gradient between two pressure systems increases. A strong high pressure system in close proximity to a strong low pressure system produces very strong wind.)

A wake low can form on the back edge of an area of precipitation due to sinking air that warms as it sinks. The pressure gradient between the meso high (shown as the 'cold pool' above) and the trailing low can contribute to increased wind speeds between the high and low.
In today's case, there was about a 3 millibar pressure difference between Memphis International Airport and West Memphis airport, a fairly large difference over such a short distance. The result was peak wind in the 50-55 mph range in many areas around the metro, including Tom Lee Park, where Beale Street Music Festival infrastructure was setup (though patrons were not in the park). Damage at Tom Lee Park included tents, trash cans, and many other loose objects overturned and blown apart and debris scattered throughout the park, which caused a 2 1/2 hour delay in opening the festival and 4 cancelled musical acts. There were also many reports of trees and power lines down throughout the Bluff City and into DeSoto County with over 22,000 customers without power on the MLG&W grid at one point.

Below are some pictures submitted to us on Twitter, as well as a 6 hour radar loop from 5-11am showing the decay of the squall line. The wake low formed in the last hour of the loop on the back side of the light precipitation.







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

Thursday, April 28, 2016

Memphis in May starts this weekend - so of course it's going to rain!

Last year, the first weekend in May, and by association the Beale Street Music Festival, was blessed with amazing weather. But as you might expect from "Memphis in Mud," consecutive years would be taking it a bit too far! So, get those galoshes, rain boots, and ponchos ready (because umbrellas aren't allowed), and let's dig into the details of what to expect this weekend, whether you're headed downtown, to Arlington in April, or just having a spring garage sale!


Though the general pattern from Friday through Sunday is a wet one, there will definitely be some "lulls" as it won't rain the entire weekend. Hopefully this will provide some idea of when those might occur. A couple of "up-front" notes: severe weather chances are very low, so we don't expect any large hail, high wind, or tornadoes to strike a crowded outdoor event. However, rain will be heavy at times and lightning is also expected. We recommend you have a plan for what to do if you start hearing thunder - and standing under a lone tall tree ain't it.


Friday

The pleasant spring weather we've had today quickly gets inundated by Friday's lunch hour as low pressure forms over west TX, pulling a warm front north from the Deep South into the Mid-South. While the morning hours will be mainly dry, the afternoon will be mainly wet with periods of heavy rain possible, but minimal lightning.

The surface map for mid-day Friday shows a stationary front to our south as it gets ready to move north. Dark green is where highest rain chances are and the area to our west will expand east during the afternoon hours.
As the warm front approaches in the evening, some thunderstorms will also be likely and rain could continue deep into the overnight hours. Rainfall amounts Friday afternoon through early Saturday will likely be pretty close to an inch with much heavier amounts to our west. Again, severe weather is not expected. Temperatures Friday will top out in the upper 70s and remain mild (near 70) in the evening hours.

The "most likely" precipitation total map through midnight Friday night indicates 3/4" or more of rain is expected with the initial wave of precipitation Friday.  2-3" may fall across  parts of Arkansas.

Saturday

The daytime hours Saturday will probably see a break from the steady rain with just scattered showers or thunderstorms possible. Honestly, I wouldn't be surprised to see some fairly lengthy dry periods from morning through mid-afternoon. With the warm front to our north and the next cold front well to the west, there won't be a lot to trigger storm initiation outside of a weak passing upper level disturbance. With gusty southerly wind pushing temperatures to near 80 and very humid air in place, it won't take much to pop off a few thunderstorm cells though, so I'm not willing to call it a dry day.

By Saturday night, the low pressure center will be moving across the Missouri River Valley, dragging the cold front closer and rain and thunderstorm chances increase. If it stays fairly dry during the day, perhaps even with a peek of sunshine, instability will be pretty high, so lightning and thunder appears likely by evening. Severe weather chances should remain minimal with the main threats being a strong gust of wind or small hail, besides heavy rainfall. Evening temperatures will again be in the 70 degree range. Total rainfall Saturday and Saturday night could be another inch or so.

The surface map for Saturday early evening shows precipitation chances increasing again as a cold front approaches from the west. By Sunday morning, that front will be along the Mississippi River, marking an end to steady precipitation. Thunderstorms will also be possible as the front approaches Saturday night.

The "most likely" precipitation totals map for the period Saturday morning through the overnight hours Saturday night shows another 3/4" or so of rain will fall. 

Sunday

The cold front moves through Sunday morning. It's passage should end just about all rain chances, though a post-frontal trough could wrap around the low to our north and bring a shower or two. Overall, we believe chances are low and by noon we should start seeing some sunshine. The rest of the day looks downright pleasant with highs in the upper 70s to near 80 and a breeze with a northerly component whisking the humidity to the south.

Total precipitation forecast by the NWS through Sunday afternoon indicates at least 2" of rain is expected in the Memphis metro this weekend.
Overall, we advise keeping rain gear handy and remaining weather aware throughout the weekend. We'll be nowcasting for the entire metro throughout the weekend on our social media channels below with an emphasis on Music Fest. You'll also be able to find radar, live tweets, and more through our mobile apps for Android and iOS (links below). Stay safe and stay dry (if possible!).

Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook, Twitter, and Google+
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