Tuesday, May 29, 2012

Arrival of a strong cold front will end heat (briefly)

Memorial Day weekend was just as advertised, and as we have come to expect in the Mid-South - dominated by heat!  With temperatures in the lower to mid 90s for the holiday weekend, the only minor relief was that the humidity was not off the charts, so heat index readings didn't quite reach 100.  The heat continued today as we got within 1 degree of the record high for the date, topping out at 95.

The strong persistent high pressure responsible for the heat and lack of storms is weakening and moving east though, as a trough of low pressure replaces it.  The trough will push a strong cold front through the region on Thursday.  Ahead of the front, the Storm Prediction Center has initially placed the entire region under a Slight Risk for severe weather on Thursday (see graphic below).

For Wednesday, a few more storms may pop up than we have seen the past day or two as a weak cold front straddles the region. Dry conditions will persist one more day for most areas, even though increased cloud cover should hold high temperatures down into the lower 90s.  On Wednesday night, strong to severe thunderstorms across the central and southern Plains will move east-southeast towards the Ozarks and eventually the Mid-South.  These storms will be the fly in the ointment for Thursday as far as our severe weather chances are concerned.

Though the storms could be severe as they move across Arkansas Wednesday night, it is not unusual at all in this pattern for storms to be in a weakening state as they arrive in the metro, especially if it is near dawn, when the atmosphere is generally least conducive for severe weather.  So, while they may bring some heavy rain and a few claps of thunder, many times this scenario produces showers and extensive cloud cover.  The cloud cover then sticks around a good part of the day, outflow boundaries from the morning storms move by to the east of the area and, when afternoon and evening storms form along the cold front, they do so east of the immediate area and rob the region of not only a threat of severe weather, but some much-needed rainfall (the region stands more than a foot below normal rainfall amounts by this time of year).

Above is a graphic showing the GFS model's interpretation of the weather scenario at mid-day on Thursday. The positioning of the frontal systems and low pressure are good for possible severe weather, but if the precipitation the GFS shows well out ahead of the cold front materializes, our slight risk may be overstated.  We'll see how later model runs handle the scenario.  The best scenario for severe weather would be to have limited cloud cover in the morning with no rain and a primed atmosphere when the front comes through late in the day.

Behind the front, much cooler air will settle in for a couple of days. Friday's high will be only in the 70s with widespread 50s on Saturday morning. The cool-down will be brief, however, as strong summertime high pressure re-builds late in the weekend, bringing the mercury back up to the 90s by the start of next week!

However the severe weather scenario works out, you can count on MWN to be monitoring the situation and bringing you the latest via social media and the MWN Forecast.  If you haven't yet downloaded the MWN app for iPhone or Android devices, now is a good time to do so.  Be sure to activate StormWatch+ in the app so that you can be alerted when and if severe weather threatens your location!

For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

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