Sunday, March 18, 2012

Spring Break Soaker Update


There are still some uncertainties when it comes to the forecast as this intense area of low pressure slowly inches towards Arkansas.  As you know, this will be "cut-off" from the main jet stream flow.   How much rain falls if a function of where this low tracks and at what speed.  The data over the past 24 hours has trended towards slowing down the storm system and cutting it off further to the south and west.  This trend continues to show a high chance for very heavy rainfall, especially for western Arkansas.  The slow movement to the storm system means it will take awhile for central Arkansas to see the rainfall.  The heavy rains should reach our portion of the state either Tuesday afternoon or even as late as Tuesday evening.  As this low moves to the east, the rainfall amounts will lessen as you head towards the Mississippi River.  While amounts could be significant there, it won't be as much as western areas of the Natural State.  There will be a very noticeable rainfall gradient across the state.  This means the amounts will change greatly in a short distance as one travels from west to east.  Where this gradient sets up is dependent upon the track and speed of the storm system.

Because the rainfall will occur over the hilly terrain of western Arkansas, rapid runoff into streams and rivers could be significant.  Since it's spring break, please get the word out to campers and hikers they need to be weather aware or rearrange their plans.  One other item that needs to be discussed is the downstream effect of heavy rainfall along the Arkansas River.  Much of this will fall in eastern Oklahoma into western Arkansas causing the river to rise eventually.

Check out the maps below.

HPC is forecasting amounts near 8'' along the Oklahoma/Arkansas border.  Notice, there is a very strong rainfall gradient.   This means there will be  a large rainfall difference from west to east.  Where this gradient sets up is dependent upon the track and speed of the storm system.
This is just showing you the 5 day rainfall map above with contours.  Notice the sharp rainfall gradient from west to east.
The NAM simulated radar valid late Tuesday at 7 PM.  According to this one piece of data, the heavy and steady rain is just entering the metro area.  Again, this is only one piece of data and the timing can change.  Eastern areas of the state may have to wait until later Tuesday night and Wednesday for the rain.
Through 7 PM Tuesday, the accumulated rainfall will be highest according to the NAM across OK, TX, and far western Arkansas.
By Wednesday at 7 PM, the GFS shows a dry slot pushing in from the southwest.  That's what you see in the purple.  The blue areas contain the highest moisture.  With the counterclockwise rotation around the system, which is located over Texas, it's entraining a punch of dry air.
By Thursday 1 PM, the dry punch is affecting most of western Arkansas with the rainfall across eastern portions of the state where you see the higher moisture values in blue.  The storm is centered over TX and OK, but should lift to the north and east.  Rainfall will be decreasing at this point!

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