Thursday, September 19, 2013

Wet Weather to Start the Weekend, Beautiful Weather to End It!

[EP: Though this discussion is a little more technical than our typical blog, it does highlight the different atmospheric processes that will promote rain and thunderstorms Friday and Friday night. William is obviously getting a good grasp of the atmosphere in his meteorology classes! For the official MWN Forecast, click here.]


A unique set up with tropical moisture to our south and a digging trough to our north will provide interesting weather heading into the weekend. However, this weather will be short-lived, bringing beautiful fall conditions to wrap up the weekend.


Below we will review what one of the weather models (NAM) is showing to see why rain looks so likely tomorrow into Saturday morning.

NAM 200 mb (38,000') winds at 03Z Saturday (10pm Friday)
In the highest parts of the troposphere we see a jet streak positioned over the region. In this particular image, valid for 10 PM Friday night, we see Memphis in the right-rear quadrant of the jet-streak. This is an area we look for to provide rising motion through the atmosphere indicative of showers and thunderstorms.

NAM 500 mb (18,000') vorticity at 00Z Saturday (7pm Friday)
Traversing down in the atmosphere we find areas of positive vorticity advection in the Mid-South around 7 PM Friday. This is another great dynamic ingredient of rising motion, further supporting rainfall during this time frame.

NAM 850 mb (5,000') temperatures and winds at 00Z Saturday (7pm Friday)
Warm air advection in the lower levels of the atmosphere is one of the last things we look for to identify areas of rising air, and it just so happens on this particular weather model the best chance at a bit of that is right around the same time frame.

NAM Composite Reflectivity (dBZ) at 00Z Saturday (7pm Friday) 
Then it is not much of a surprise that the simulated radar image of that same time, 7 PM Friday evening, shows the heaviest precipitation over the Memphis area.

This analysis of one of the models is not an exact forecast of what is going to happen, but it does give you an idea of the processes that are occurring to suggest the rainfall that we're expected to receive. Other weather models are similar in their presentation and the hardest part is going to be nailing amounts and exact timing of precipitation.


Widespread precipitation looks quite possible as early as tomorrow morning, but the best dynamics look to be moving through late tomorrow afternoon and well into the evening. Mid-level moisture will spread over the region with upper-level energy providing most of the uplift for showers and thunderstorms. Clearing looks possible mid-day Saturday with things shaping up for a beautiful fall weekend.

William's Memphis Forecast

Friday: Overcast with a high of 82, low of 65. Near 100% chance of rain. Precipitation amounts of about an inch.

Saturday: Decreasing clouds with a high of 79, low of 58.

Sunday: Sunny skies with a high of 83, low of 60.

--William Churchill (MWN Social Media Intern)

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