Tuesday, August 26, 2014

Summer continues for now, but tropical moisture will affect holiday weekend plans

A massive upper-level high pressure ridge has receded a bit, but hot and dry weather continues until tropical moisture arrives, just in time for the holiday weekend!

Abnormally dry weather over the past month has resulted in very dry vegetation and soil. A strong upper-level ridge that was in place over the weekend combined with the dry conditions to create record heat, with the mercury reaching the century mark for the first time in over two years on Sunday and tying the record high for the date. (The last time Memphis hit 100 - actually 101 - was July 30, 2012). Another record was tied Monday morning as the low only dropped to 79. Thankfully the ridge receded a bit and temperatures have responded, only reaching the mid 90s Monday and lower 90s today. With wind out of the east to southeast during the middle of this week, dewpoints will also fall just a bit. Though highs will remain in the mid 90s through the end of the week, heat indices won't top 100 thanks to slightly lower afternoon humidity.

A large ridge of high pressure at the mid levels of the atmosphere (18,000' feet) dominates the eastern 2/3 of the nation Monday night. This is keeping hot and dry weather in place across the Mid-South. Hurricanes Cristobal and Marie are also noted. Neither pose a threat to land areas. Click for larger image.

However, to our south, tropical moisture is sitting over the northwestern Gulf of Mexico, off the Louisiana and Texas coastline. The National Hurricane Center is monitoring this area and a short-lived tropical depression is possible. However, even if it doesn't, as high pressure moves east, southerly flow will push that tropical moisture north, putting the lower Mississippi Valley and Mid-South in its path.

Precipitable water (PW) values as of mid-day Wednesday. PW shows areas with high moisture content in the atmosphere. In this case, tropical moisture is sitting over the northwestern Gulf off the coast of TX. This moisture will be pulled north this weekend on southerly flow. There is also obviously a great deal of moisture in Hurricane Cristobal.

We'll notice this pattern shift by Friday and especially into the holiday weekend as dewpoints (humidity) rise, cloud cover increases, and showers and thunderstorms enter the forecast. In fact, this weekend may bring the best chances of widespread rain to the area in over a month. For now, severe weather is not a big concern, though heavy rainfall will be possible in areas that receive multiple storms.

By Friday evening, the ridge over the Pacfic strengthens, the ridge in the middle of the country moves east to the southeastern U.S., and well-established southerly flow exists over the Mid-South. This setup pulls Gulf moisture north into the region, resulting in decent rain chances. Click for larger image.
Forecast rainfall amounts from Friday evening through Sunday by the National Weather Service. A welcome 0.5"-1" of rain is possible and more could be expected in areas that receive multiple storms. Click for larger image.
So, for the rest of the week, enjoy hot, but not overly humid, days and prepare for the possibility of occasional rain on outdoor plans this Labor Day weekend, including the University of Memphis football team's home opener against Austin Peay on Saturday evening and the opening weekend of the Delta Fair. Find the complete MWN Forecast on our mobile apps (links below) or click here for our mobile website forecast.

Erik Proseus
MWN Meteorologist

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