Friday, March 16, 2012

Possible BIG Rain Event Next Week

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12:45 PM Saturday Update... I have been in the process of reviewing the latest model data and I still have some uncertainties next week.  First of all, I axis of heaviest rainfall isn't set in stone, but it appears eastern Oklahoma into western Arkansas will see the most.  The HPC says 7'' will be possible in this location.  Do I buy that? While it's completely within the realm of possibilities, I'm a bit skeptical.  Again, this all depends on the speed of the system.  If it takes a slow approach, yes, that amount will be possible.   If it speeds up, those amounts will not be realized.  This is a not a matter of "if it will rain?", but "how much will it rain?"  At this point, I think it's safe to say western Arkansas could see 3-6''.  As we get closer, we'll nail down the details.

One other aspect to the system will be the severe weather potential.  It's there, but I think the potential for heavy rain and flooding will be higher especially since the rain will fall across hilly terrain.

As this area of low pressure aloft cuts off and meanders through the mid south Wednesday and Thursday, there will be a pocket of very cold air aloft.  I'm also going to watch for the potential for widely scattered thunderstorms if any surface heating can take place.  In this situation, these storms would produce hail. Right now, the models show very little in the way of instability, but it's something worth watching.  There's so much on the table to watch with a system like this and I'll do that for you right here on the Arkansas Weather Blog.
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As you know, I have been keeping an eye on an incredibly strong area of low pressure aloft which will affect the plains into the mid south next week.  While severe weather will be possible, the bigger threat may end up being very heavy rainfall.  The specifics can't be answered yet as this is still in the long range, but I think it would be wise to start talking about it now especially in light of what was released from the National Weather Service recently.  While, it's too early to predict the severity of the rainfall, keep in mind Arkansas ranked #1 in the nation for flash flood fatalities in 2011.  The National Weather Service has run a campaign over the years called, "Turn Around, Don't Drown."  Many of those fatalities were due to drivers trying to cross flooded roads.

Many uncertainties still exist as this event unfolds next week.  We still don't know where exactly the axis of heaviest rain will set up and how fast the system will move.  Obviously, the slower the system, the heavier the rainfall amounts will be.  As intense as this storm will be, there's an equally intense area of high pressure in front of it which should make it a slow mover.  We really need this to speed up and keep the heavy rainfall to a minimum.  Only time will tell as the models get a better handle on the situation.

The next four maps are from consecutive runs of the GFS showing how difficult it is at this point to predict where the heaviest rain will occur.

The 00Z run of the GFS  from Friday shows the axis of heaviest rain across eastern Oklahoma with amounts more than 5 inches.  The amounts decrease the further east you go.
The 06Z Friday run continues to show the axis of heaviest rainfall across eastern Oklahoma, but increase amounts over Arkansas.  IF this verifies, we're talking about 3-5 inches.
The next run of the GFS (12Z) shows the axis of heaviest rainfall now over extreme eastern Oklahoma into western Arkansas.  If this verifies, we're talking about amounts in excess of 5'' in a hilly area of the state.
The 18Z Friday run of the GFS now shows more of eastern Oklahoma and much of Arkansas with 5'' or more of rainfall next week.  Now you see the forecasting problems and this is just 1 days worth of model information from the Global Forecasting System.  Like I said above, knowing where this axis sets up is uncertain.  Also, if the speed is faster, the amounts will be less.  If the low moves slower, the amounts could be more.
The official forecast from HPC shows 5'' or more from northeast Texas into western Arkansas.
You're looking at the 500 mb pattern from the NAM late Sunday.  Notice the area of low pressure moving through the western United States.  The high dominating our weather will ease to the east.
This is the GFS early Wednesday morning.  Notice the area of low pressure aloft is cut off over Oklahoma.  When this happens, they can become very difficult to predict.  As along as it stays west of the state, we will have the threat for heavy rainfall and thunderstorms.  Again, we hope it speeds up.

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