Wednesday, December 18, 2013

Santa Brings Pre-Christmas Severe Threat


Wow, have things changed from a week ago.  What first appeared as a threat for wintry weather, has changed to a heavy rain and storm threat.  There will be wintry weather in Oklahoma, Kansas, Missouri, and points northeast.  This storm system will affect millions in the central and eastern United States this weekend which is not good news for last minute shoppers and travelers.

Is severe weather this time of year unusual?  NO!!!  I can already hear the anchors on the "NATIONAL NEWS"  calling this "rare and unusual", "a surprise storm".  It's like they copy and paste the same script from storm to storm.  So let's just disprove both of those points.  No surprise... we have been warning you for days now.  Rare and unusual?  Again, no!  Severe weather in winter and spring is typical in the south.  I remember tornadoes on Christmas Eve in 1982 and taking cover at church.  Remember the big January 1999 outbreak?  Winter severe weather happens here.  It is rare in New York which is the center of their world so that may be why they think that.  By the way,  I'm not using the term "outbreak" for this event at this time.

I want to address a point brought up by a meteorologist I have tremendous respect for.   I said the models did a 1-80.  They really didn't.  They have had this system all along, but differed in the specifics.  I guess the truth should be, I did a 1-80 from the threat for wintry weather to rain/storms.  The data is there for us to look at and there's really too much.  It's information overload and we can make a simple situation too complex.  It's our job to figure out which model has a better handle on a particular situation.  No model will be perfect.  The term "guidance" is better to use.  It gives us direction with our forecast.

Now onto the weekend.  As I said above, a very large and potent storm system will have a huge impact across several states.  That impact will be snow/ice north to severe weather south.  Also, heavy rain will be a problem and I think that's a big threat too.  In terms of impact on Arkansas in order...

1. Heavy rain
2. Severe storms (wind)
3. Severe storms (tornadoes)
4. Winter weather

I think the heavy rainfall threat is problematic due to dormant vegetation.  This will help increase runoff with rises expected along rivers and streams.  Street flooding will be possible and low lying areas which are typically flood prone will need to be watched.  Total rainfall amounts could exceed 3 inches especially where thunderstorms occur.  The severe weather threat is secondary, but still possibly significant.  While it is unclear at this time exactly how this will unfold, I think the highest threat will be the southeast half of the state with the worst over LA and MS.  High winds will be a problem, but isolated tornadoes can't be ruled out either.

The winter weather threat is last.  There could be a change over for far northwest Arkansas before ending, but this should not be a big deal.  Here are a few maps...

This is valid Saturday at 6PM...from weatherbell.com.  The 00Z GFS shows the surface low across SW Arkansas.  It's along and southeast of this track which has the potential for severe weather. The map above shows CAPE or instability.  I question just how much of this will be realized with the heavy amounts of rain and cloud cover around the state.  This is why the SE half of the state needs to be watched. 

By midnight, the severe threat is shifting east.

The WPC indicates more than 3'' of rain will be possible near and north of the surface low track.
The Storm Prediction Center has decided to blanket the entire state.  This indicates the area they are watching carefully Saturday.

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