|Frequency of Mid-South tornadoes by month since 1950. Graphic courtesy NWS-Memphis. Note the secondary peak of tornadoes in November as summer transitions to winter. Notice also that tornadoes can occur any time of year in the Mid-South.|
Storm Prediction Center severe weather outlookThe first of these types of systems to move through the Mid-South this fall arrives Thursday night as a strong cold front moves into the area. The Storm Prediction Center has placed parts of the Mid-South, specifically northeast into central AR, under a Slight Risk of severe weather (see graphic below). This risk level, which is the 2nd of 5 severe weather outlook categories, indicates scattered severe storms are possible. The Memphis metro is in the lowest of the 5 severe weather categories, a Marginal Risk, which can produce isolated severe thunderstorms. The main severe weather threat Thursday night in the Memphis metro will be straight line wind damage.
|Severe weather outlook categories, as defined by the Storm Prediction Center, define the threat of severe weather in a given area. Click for larger image.|
|Severe weather outlook for Thursday into Thursday night for the Mid-South. A Marginal Risk (category 1 of 5) encompasses the Memphis metro. Graphic courtesy Storm Prediction Center.|
Atmospheric setup conducive to a few severe thunderstormsYou've probably already noticed that, despite cloud cover the past few days, temperatures have been warm, especially in the mornings. In fact, this morning's low of 67° was nearly 20° above normal and set a record for the warmest low temperature on this date. Low temperatures Thursday and Friday mornings, also projected in the mid to upper 60s, could also set daily warm minimum records, while high temperatures reach well into the 70s to near 80° (or about 5-10° above average) despite abundant daytime cloud cover.
|A warm airmass covers much of the U.S. east of the Rocky Mountains, including temperatures still above 70° at 7pm Wednesday evening. Click for larger image. Graphic courtesy Wright-Weather.com.|
Dewpoint is an absolute measure of humidity, describing the amount of moisture in the air irrespective of the temperature. With dewpoint temperatures running above 60° ahead of the approaching front, there is plenty of moisture in the air to provide one ingredient for thunderstorms. In fact, as shown in the graphic below, dewpoints will likely rise into the mid to upper 60s as the front moves through on Thursday night, as a southerly flow of air from the Gulf pushes even more moisture into the region.
|The GFS/American model forecasts strong southwest wind in the Mid-South, on the order of 55-60 mph, as the front approaches early Friday. Graphic courtesy TwisterData.com.|
Thursday night, instability is the one negative factor that is present. Because the storms are arriving during the coolest part of the day - the early morning hours - daytime heating is at a minimum. Certainly the combination of temperatures in the upper 60s and dewpoints in the mid 60s will be enough to provide a modest amount of instability as described above, but it is not strong enough to result in a high threat of severe storms. The graphic below shows that the most unstable air will reside over the Gulf of Mexico early Friday morning with small amounts of instability over the Mid-South, as measured by an index called CAPE, or Convective Available Potential Energy.
|Once again, using the GFS model forecast early Friday, instability is limited in the Mid-South with the most unstable air well to our south over the Gulf of Mexico. Graphic courtesy TwisterData.com.|
The combination of an approaching front to provide lift, strong wind aloft, but weak instability, results in a likelihood of thunderstorms but a muted chance of severe weather with the most likely threat being the strong wind at 5,000 feet being pushed to the surface by downdrafts in thunderstorms. Thus, the Storm Prediction Center's Marginal risk of severe weather for the Memphis metro is a good call.
What to expect in the metro
So here is the bottom line: rain and thunderstorms are expected to move into the Mid-South Thursday overnight, following a day that will be mainly dry, but warm (upper 70s), humid (dewpoints in the 60s), and breezy (south wind at 10-15 mph). The most likely time for storms will be between 1am-5am Friday. A few storms will be capable of producing strong wind gusts to 60 mph, but widespread severe weather is NOT anticipated. Rainfall totals will likely be on the order of 1/2" or so due to the fairly progressive, or fast-moving, nature of this system. Prolonged heavy rainfall is not anticipated, though downpours are likely as storms move overhead.
|The GFS/American computer model forecasts total overnight precipitation Thursday night to be near or above 1/2" in the metro with heavier amounts further west where storms are expected to be stronger. Graphic courtesy TwisterData.com.|
As always, we encourage you to have multiple ways of receiving severe weather information prior to a potential event. At nighttime, it is particularly important that you have methods that will wake you up, including a NOAA Weather Radio programmed for your county. We also highly recommend that you download the MemphisWeather.net mobile app if you have a smartphone. In the app, you can activate StormWatch+, our precision severe weather notification service that only alerts you for the specific location(s) you input and for the types of watches and warnings you wish to receive. Learn more and/or download the app by clicking here.
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