Thursday, August 30, 2012

Issac Hits Arkansas. Rain and A "Slight" tornado Risk Detailed

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IT'S OFFICIAL... WINNER OF CONTEST #1 IS AMANDA PAIGE WITH HER GUESS OF 91, 100 DEGREE DAYS.  

THE WINNER OF CONTEST #2 IS DONNY COOK WITH THE AVERAGE SEASONAL TEMPERATURE OF 83.50 DEGREES.

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12:30 PM Saturday Update... based upon preliminary calculations, Donny Cook wins contest #2 with his guess of 83.50 degrees.  HE ABSOLUTELY NAILED IT! The average seasonal temperature according to my calculation was 83.50.  I'm awaiting conformation from the NWS.

What is confirmed is the winner of contest #1.  Congratulations to Amanda Paige who won contest #1 with the correct guess of 91, 100 degree plus days. 

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5:30 AM Friday Update... Isaac has pretty much gone as planned.  The system is moving northwest into northern Arkansas and we're still in the front right quadrant. Early this morning bands of rain and storms have been developing across southern and eastern Arkansas as well as Louisiana.  If we get some breaks in the clouds and some daytime heating, there's enough shear in the atmosphere to cause some brief rotation within some of the storms.  Once again, these are not your typical tornadoes.  They are weak and brief.  The risk today remains low.  Check out the map below and the post from last night below as well.

The area in yellow is the risk area issued by the Storm Prediction Center in Norman, OK
Total rainfall through 5 AM Friday
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He's here.  The much talked about remnants of Isaac are affecting the state with several thousands already in the dark.  It's really amazing to have that many outages with winds up to 40 mph and some rain.  Weakened trees from the drought are probably playing a role in that high number.  Sorry to tell you, but there will be several thousand more as we continue to see bands of heavy rainfall across the state and some wind.

That rain should continue into Friday, but I wanted to blog about the tornado potential since SPC has placed much of the state in a "slight risk" Friday.  As I have said all along, any tornadoes that do form should be brief and isolated.  With tropical systems like this, they usually rate on the lower end of the EF scale.  Once again, it's rain that will be the biggest story with Isaac, but here are a few thoughts on that SLIGHT risk Friday.

The area of low pressure (Isaac) will be pulling away and weakening.  The NAM (North American Model) shows the coverage of rain decreasing Friday afternoon and that COULD allow for breaks in the clouds.  Heating of course is not a good thing when it comes to severe weather.  The amount of shear in the atmosphere may be elevated just enough to cause any storms that do form to rotate.  Again, this threat is not high, but it's there.

Check out the maps below.  BTW, thanks so much for coming here and reading the Arkansas Weather Blog.  It's something I'm very proud of and love doing for you.  The numbers this week are incredible.  Thanks for coming here to get your information and if you're a weather geek, I share your passion for the weather as well.

This is the hi res NAM simulated radar Friday at 6 PM.  Notice the solid area of rain is gone and their could be some breaks.  The "swirl" in SE Kansas is the remnant low.  This model is developing scattered thunderstorms across portions of AR.
By 1 AM, the model shows much of it decreasing and moving to the north with the lose of daytime heating.
The NAM shows some instability.  The green areas are CAPE values around 1000 J/KG or higher.  This is some noticeable instability.
The EHI ( Energy Helicity Index) combines 2 severe weather parameters. Instability and Shear.  Look at the areas in green, these are SLIGHTLY elevated areas.  This is why we'll watch that carefully Friday, especially during the PM.
The Storm Prediction Center has placed much of Arkansas under a slight risk for severe weather.
The bigger story of course is the rain.  This was updated by HPC late Thursday afternoon.  Rainfall amounts could reach 5'' or more across portions of central and northern AR.  I think final totals will likely average around 2-5''.  Not drought busting, but drought denting for sure!

2 comments:

Linda T said...

Great Blog! Glad I found it - it's very informative. Keep up the good work!

Anonymous said...

Next blog post, a cool front coming in next week?

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