Saturday, July 10, 2021

Pattern shift brings highest rain chances in a month

Prior to yesterday's deluge (for some of you), the previous month has been abnormally dry. We started June with very wet weather, particularly south of I-40 and into north MS, but once it dried out, it did so for all of us. 

Below, images show the rainfall departures from normal for June 10-July 9 and Thursday's Drought Monitor for Tennessee. For the past 30 days, the metro had accumulated a deficit of 2-3" of rain, or roughly only 30-40% than normal. Parts of Shelby and Tipton Counties were officially classified in D0 status, or "abnormally dry." My lawn has needed water weekly for the past few weeks now to keep it from browning up.

Rainfall departure from normal for June 10-July 9. Precipitation deficits of 2-3" (or 60-70%) were observed in the metro. (HRAP Precip Analysis via WeatherBell)

The Drought Monitor on July 8 shows "abnormally dry" conditions in portions of Tipton and Shelby Counties.

That atmospheric pattern started to change yesterday though, and the new pattern will continue into the middle of next week, as an upper level low pressure system forms to our north.

Friday retroactively rated "outperform"

Before we look into the weekend ahead, Friday's event was definitely interesting, and admittedly not well-forecast. The setup was basically a remnant outflow, or surface, boundary that moved into the metro Thursday night. You may recall a narrow line of storms making it to northern Tipton County Thursday evening. Many of us north of I-40 had a pretty decent light show after dark from distant lightning (not heat lightning!). 

As those storms fizzled, the boundary slipped south to around the TN/MS state line. Our best computer models really didn't pick up on much activity along that boundary early Friday, other than maybe a few isolated showers. I honestly didn't expect much. (In fact, I left my passenger side car window cracked a couple of inches when I went to work by the airport Friday morning!) Well, "outperform" would have been the proper rating if this were the stock market. 

With moist air flowing up from the southwest over the boundary and very muggy and unstable air in place by mid-morning, storms fired up and moved east-southeast along the boundary draped along the state line, while continuously regenerating back to the west along I-40 in east AR. Over and over, storms trained along the boundary from southern Crittenden Co along the Shelby/DeSoto border, into northern Marshall County, a "state line special." By noon, 4-6" of rain had fallen in that corridor, a couple of inches fell at the airport (some into my front passenger seat!) - setting a daily rainfall record, and most of the rest of the metro was wondering what all the fuss was about! My home in Bartlett received exactly zero drops. 

Below is the radar estimation of rainfall totals for the day, including scattered storms that formed in the afternoon in northeast AR and made it into northwest portions of the metro and lingering light rain for Shelby County in the evening. It was quite a day!

Radar estimation of rainfall for Friday's localized deluge (MRMS data via NSSL)

This weekend - northwest flow

For those who still haven't gotten their "fair share" of rain, all hope is not lost. Yesterday was a prelude to a transition in which the upper air pattern becomes northwesterly, allowing storm systems from the north to move into the Mid-South - commonly called northwest flow. It is not unusual to see this pattern a couple of times each summer and it is also a difficult one to forecast details more than about 24 hours out. The northwest flow will be driven by the development of low pressure in the mid levels of the atmosphere over the Upper Midwest that drops into the Missouri over the next few days, before weakening and lifting northeast by about Tuesday night.

The mid-level (500mb or 18,000') flow pattern from Saturday morning through Tuesday evening features a developing low pressure center over the Missouri River valley, resulting in "northwest flow" for our area tonight into Sunday, then a trough over Arkansas to start the week. (Euro model via WeatherBell)

Today starts mainly dry as we haven't quite transitioned from a high pressure/summertime setup just yet. A few afternoon showers or thunderstorms are possible in the heat of the day as temperatures reach 90 degrees, but short-term high-res models disagree on the extent and intensity of any pop-ups. Don't be surprised by some thunder and localized downpours - remember to evacuate the pool if thunder and lightning are nearby!

As we head into tonight, the first of the "northwest flow" system moves in. This one seems to be fairly well predicted by the morning model data. A line of storms is likely to drop through Missouri this afternoon and into northern AR and northwest TN this evening. That line appears likely to reach the metro around midnight, or perhaps a bit before, midnight. 

The 7am model runs of the HRRR (left) and NAM3 (right) show what they think radar will look like at midnight tonight. Subsequent iterations of the HRRR, which runs hourly, are a touch earlier with storms. Prepare for strong wind and heavy rain late this evening. (WeatherBell)

We are outlooked in a Marginal to Slight Risk of severe weather (level 1-2), with the primary threat being strong wind gusts with the line. The threat appears to be highest to our north, and the line may be weakening a bit as it gets here. Sometimes these things don't go according to plan though. The tornado and hail risk is very low, but the heavy rain threat is high as showers continue overnight after the main line. Stay tuned throughout the day and evening for updates, and prepare for the possibility of scattered power outages and minor wind damage overnight, especially in northeast AR and west TN. You'll want to have multiple methods of receiving warning information, just in case!

By Sunday morning, lingering showers will likely be falling apart, but a muggy airmass is likely to recover (or become more unstable) by afternoon and scattered showers and thunderstorms are again expected. These could last well into the evening. Though it will be cooler (mid 80s for highs), it'll still be muggy with southwest wind becoming gusty at times and a bit of sun peeking through during the afternoon as well. I expect all of us will have received some decent rain by Sunday night. In fact, the NWS has outlooked parts of the area, mainly south of I-40, in a Slight Risk (level 2) of excessive rainfall (not severe storms) for Sunday.

Starting the work week with continued rain chances

As the upper level low spins over Missouri, more showers and thunderstorms are likely on Monday. These may be more diurnally driven, meaning they are most likely during the warmest hours of the day. Keep that in mind as you make your plans for Monday. Many of us could get more rain and a few storms, most likely below severe limits, on Monday as highs remain in the mid 80s.

Tuesday will see rain chances drop off some as the low starts to weaken to our north, however scattered showers and thunderstorms appear to be a good bet in the afternoon and evening hours. Look for temperatures to start to recover and humidity to remain high. High temps will be in the upper 80s.

The latter half of next week appears to be more typical of summer (and much like this past week) as high pressure builds back in. High temperatures in the lower 90s and lows in the mid 70s will be common with isolated to widely scattered storms Wednesday-Friday.

Erik Proseus
MWN Meteorologist

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