Saturday, August 21, 2021

One more storm chance, then hot and humid + a note on Henri

Some of us have gotten some beneficial rainfall this past week! Others (I'm looking at you eastern Shelby County) have had more than your fair share, while others could definitely use some more. Such is the nature of summertime convection, even when it is more organized than a typical summer day.

Radar-estimated rainfall over the past 72 hours indicates northwest Shelby and Tipton Counties with less than an inch, while the eastern metro and points east received 2-4"! (MRMS data via NOAA)

Today: last rain chance for a while

The "wet" pattern is transitioning to the "hot" pattern via the "humid" route this weekend. Today is the last decent chance of thunderstorms in the Mid-South until later next week as a large ridge of high pressure aloft begins to squash rain chances as it builds east from the southern plains. Today's rain chances are driven primarily by a couple of outflows moving through unstable and very moist air. 

The first outflow, which is now to our south, has sparked scattered showers south of Memphis. The next outflow from storms in Missouri earlier this morning will arrive early this evening and encounter more unstable air, as temperatures reach the 90 degree mark. Precipitable water  (PW) values, which measure total moisture in the air (not just the humidity we feel at the ground) are very high for August - above 2". This combination will likely spark scattered storms around the early-dinner hour and into the evening. High-res models expect those to form somewhere close by and then drop south into north MS. So the best chances will be south of I-40, basically increasing from very low just north of the metro, to likely in north MS. 

The 9am run of the HRRR model predicts initial showers to drop to our south this morning, a lull in activity, then additional storms to form, mainly in north MS, early this evening (after 5-6pm). The loop runs through midnight. Remember, model ≠ gospel. (WeatheBell)

Threats with Saturday storms

Whoever gets some of these storms will be visited by very heavy rain (owing to the high PW values) and some gusty wind, as downbursts are likely in these summer storms and outflows will be prevalent.  Our area is under a Marginal Risk (level 1) of severe weather today, so a few gusts could reach 40-50 mph in spots. In addition, we are in a level 2 Slight Risk of excessive rainfall, so storms could quickly drop a couple inches of rain and cause some flooding issues.

A Marginal Risk (level 1) of a few scattered severe storms exists today, mainly bringing a gusty wind and heavy rain threat.

A Slight Risk of excessive rainfall (level 2) is forecast for most of the metro today. (NWS)

Sunday-Wednesday: hot and humid

Once we are on the backside of this rain chance, he building ridge aloft and high pressure at the surface will result in dry but very humid, weather Sunday into the first half of next week. Highs will climb to the mid 90s by Monday. Combined with dewpoints in the mid 70s (which borders on insufferable), heat indices will easily reach the danger level of 105 degrees and could go above 110. Heat Advisories are a sure bet, and so is heat illness if you aren't avoiding long stretches out in it without hydrating properly and taking frequent breaks. This pattern continues through about Wednesday.

Sprawling high pressure aloft (a "ridge") encompasses a large portion of the U.S. as shown by the European model valid Tuesday night. The ridge results in hot weather and diminished rain chances. (WeatherBell)

Later in the week: minor relief

As we reach mid-week, the ridge will start to weaken a bit and afternoon rain chances start to tick up a bit. This will help to lower high temperatures slightly, back to the lower 90s or maybe near 90 by next weekend. Overnight lows will remain rather uncomfortable. In all, summer looks to continue through the end of August!

A note on Hurricane Henri

First, this isn't Derrick Henry of the Titans, it's pronounced "ahn-REE" (pretend you are a French chef when you say it). Hurricane Henri is beating a path towards southern New England with category 1 wind, a large storm surge, and very heavy rain. Landfall is forecast on Long Island east of New York City Sunday afternoon, then it will slow down and take it's time exiting New England. 

Forecast track for Hurricane Henri as of Saturday morning.

Henri will likely be the most significant storm to hit this area since at least Sandy in 2012 and perhaps Hurricane Bob in 1981. Expect to see footage of trees and power poles down, excessive flooding, coastal erosion, and the like. Officials are bracing for power outages that could last for many days. Not really what that area wants to be dealing with right now.

Erik Proseus
MWN Meteorologist

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