Wednesday, May 21, 2014

The Upper Low... Where Will It Go?

You really need to watch these upper lows this time of the year.  A closed off low doesn't have much influence from the main jet to push it along and therefore they are slow moving.  This time of the year, these features are well known for producing flooding and some of it can be flash flooding.  The atmosphere is juiced up with moisture and temperatures are quite warm.  These slow moving systems have a ton of moisture to work with and can produce tremendous rainfall amounts.  Due to the cut off nature to the low, projecting the exact path and timing can be difficult in the long range.  Remember, low pressure has a counterclockwise spin to it in the northern hemisphere.  This allows it to grab Gulf moisture and force it out in front of the storm system.  The "lift" associated with these lows can produce heavy rain and storms just east of the center while the western side is drier, but not completely immune to rain and storms.
  The time period we really need to watch is the beginning to middle of next week.  Memorial Day? Yes, but most of the data points to the highest chance for rain starting next Tuesday.

This is the Wednesday morning run of the GFS and it's valid next Wednesday morning at 7AM.  Needless to say, there are going to be changes.  Like I said, in the long range, the speed and exact track of these are tough to forecast.  You're looking at 500 mb which is almost 20 thousand feet up into the atmosphere.  See the closed black line on the border with Oklahoma and Kansas?  That's the upper low location according to the GFS at this time period.  See how the black lines ahead of it make a ridge?  That's the current ridge of high pressure bringing this nice, warm weather this week.  It moves away and the storm system moves in during the Tuesday/Wednesday time period.  Ahead of the low, Gulf moisture is in place and lift associated with the system produces rain and storms.
The European model valid next Wednesday morning has the low over Missouri so you can see the differences between the two long range models.
The Weather Prediction Center shows a target area for the heaviest rainfall over western Arkansas over into eastern Oklahoma and northeast Texas.  These amount are mainly for late Monday into Wednesday morning and they could exceed 3 inches.  It goes without saying some storms can produce more than that.  This will change as the models get a better handle on the situation.

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