Friday, July 8, 2016

Heat & drought, storms, then more heat

June was hot - the seventh hottest on record in Memphis. July is continuing that trend. In fact, as of 8am this morning, the official reporting station at Memphis International Airport had not dropped below 80° in 72 hours and that streak will continue today and likely until early Saturday morning when the forecast low is 79°.

Overall, it's also been dry, though some places have received the benefit of scattered thunderstorms dropping heavy rain in localized areas over the past month. Areas just to our south and across the interior southeast are dealing with very dry conditions - drought in many cases. North Mississippi, including the southern metro, is on the edge of that drought area.

Precipitation anomaly (departure from normal) over the past 90 days shows large areas of the southeast U.S. with a deficit of multiple inches of rain through the spring and early summer. Graphic courtesy WxBell.

Short-term drought (appearing in the last 6 months) is present throughout the interior southeast with greatest effects in northern GA/AL and the southern Appalachians. A close-up view of MS is shown below, indicating dryness into the southern metro (Tate, Tunica, and Marshall Co's). Graphics courtesy US Drought Monitor.

As we head into the weekend, some much-needed rain is expected, though we are hopeful that severe weather does not accompany it. A weak cold front will be located just north of the metro with the possibility of storm systems riding along the front through the area. They will feed on the very unstable air that is in place south of the front, driven by the heat and humidity. The good news, I suppose, is that increased cloud cover and rain in the area should hold temperatures down just a bit this weekend.

A weak cold front will stall over the Mid-South this weekend, becoming a focus for possible convective complexes with a chance of strong storms and heavy rain.
If a storm complex or two (called MCC's, or mesoscale convective complexes) forms, they tend to continue through the night, not ending when the sun goes down. This scenario is very unpredictable beyond about 12-24 hours, thus nailing down timing of potential storms this weekend is difficult.

The atmospheric pattern that favors these complexes will be partially in place however, with a general northwest to southeast flow at the mid-levels (what we call "northwest flow"), a weakness in the mid-level pressure pattern developing over the area, shortwave troughs (or upper air disturbances) moving near the region, and plenty of heat and moisture to feed the storms.

The GFS model of pressure at the 500mb (18,000') level at 7pm Saturday shows wind flow "ridging" over high pressure over NM with wind flow from northwest to southeast across the Mid-South. This is what is referred to as "northwest flow" and is a favorable pattern in the summertime for convective complexes to follow that wind flow. Graphic courtesy Pivotal Weather.
The first of these storm clusters or complexes appears to skirt the northern metro early this evening according to this morning's high-resolution short term models. Thus, for what I think is the third time this week, areas just to our north are under a risk of severe weather, as forecast by the Storm Prediction Center. As of 2pm, the risk is in category 2 of 5, or Slight Risk, with the main period to monitor being late afternoon to early evening. The primary severe weather threat will be damaging wind. Northeast AR and northwest TN have the best chance of storms with the tail end of the storms possibly reaching as far south as I-40 in the metro.

Heading into the weekend, we'll simply advise that you keep a close eye on the forecast and our social media feeds as severe weather cannot be ruled out Saturday, Sunday or Monday while the front hangs over the area. Again, timing of storm complexes, or just pop-up storms, is nearly impossible to pinpoint right now. A few storms could get strong enough this weekend to produce areas of damaging wind or large hail, as well as very heavy rain.

The one time period that has a little higher confidence level on is overnight Saturday night into early Sunday. Multiple models have depicted storms developing during that period. We're hopeful that they hold off until after Saturday evening outdoor activities have wound down, including the Levitt Shell ticketed event featuring Sharon Jones and the Memphis Redbirds Christmas in July evening with baseball, fireworks, and Santa (I wouldn't want to be the one in the Santa suit outside in July)!

By early next week, upper level ridging (high pressure) begins building back into the region, pushing the thunderstorm threat back to our north and allowing mid-summer heat to rebuild with highs back in the mid 90s and heat indices above 100. The long-range outlook for the third week of July is predicted to have a high likelihood of more "above normal" temperatures. Hang in there and stay cool!

The week 2 outlook from NOAA's Climate Prediction Center indicates a 70% chance of temperatures remaining above normal. In other words, a respite from the summer heat doesn't appear to be in the offing during this period!

Erik Proseus
MWN Meteorologist

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