Wednesday, September 11, 2013

Precip chances this afternoon? Let's see what the high-res models says...

[EP: Today, MWN intern William looks at the potential for precip around the metro this afternoon using the HRRR model and a well-conceptualized and written discussion.]

Gonna give short term forecasting a shot this morning with the possibility of storms this afternoon using the 11Z HRRR as my primary analysis tool. Lets start off with current data.

Visible satellite imagery at 1345 UTC. This image shows you what clouds would look like from space.
Visible satellite imagery shows a mid-level deck of stratus clouds across the area around 9 AM. Lets take a look at the 3 hour forecast of the HRRR to see how it did on predicting this deck of clouds.

Total cloud cover in percentage at 1400 UTC.
The HRRR did not do a very good job considering it was only forecasting out 3 hours. It picked up on some possible cloud cover in the area but really didn't nail it. This is the first thing I noticed when considering t'storms this afternoon because if clouds stick around long enough there may not be enough instability to get some t'storms going like the HRRR is predicting. However, temps are already in the 80s and I'm not expecting these clouds to stick around long. Now lets see what the HRRR is showing in terms of upward motion.

Vertical Velocities at 1900 UTC.
This map is of vertical velocities at a level where we expect cloud formation. This map indicates positive vertical velocities in the area by 2 PM, a good indication of cloud and possible shower development.

Most Unstable CAPE at 1800 UTC.
This map shows CAPE values which are a measure of available potential energy for t'storm development. At about 1 PM this indicates 2000+ J/kg of potential energy, these are pretty good values if you can get storms initiated.

Previous hour total precipitation at 1900 UTC.
This map shows precipitation from 1-2 PM just northeast of Memphis. Outflow boundaries from these storms moving into Shelby County around 2 PM should initiate more storms in the Memphis area.

To sum everything up, I've used a short term model to try to predict timing of t'storms this afternoon. The HRRR indicates a good chance of t'storms but it was also not very accurate in predicting a deck of clouds just a few hours after initializing. However, I think there's still a good chance of at least some isolated t'storms this afternoon between 12-4 PM and will have to watch to see how things unfold.

--William Churchill, MWN Social Media Intern

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