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Area forecast discussion 
National Weather Service Aberdeen South Dakota 
242 PM CST Sat Nov 21 2009 


Short term...tonight through Monday night 
forecast challenges include temperatures throughout the period and 
precipitation chances mainly late Sunday night through Monday night. 


Currently...a cold front is working slowly into the far western 
forecast zones. Out ahead of the front...breezy to windy south 
winds have been combining with the November sunshine to pump up 
temperatures into the 50s and 60s throughout the region. 


This cold front makes its way across the forecast area 
tonight...relaxing surface winds...while turning them around to a 
northwest direction. However...the main upper low of interest in 
the short range of the forecast is approaching the region from the 
Pacific northwest on Sunday. So...those northwest winds will quickly Switch Back 
around to the southeast...where they will reside until late Sunday 
night and Monday when the low pressure system begins to lift out 
of The Rockies and head southeast across the western portions of 
South Dakota and Nebraska. During this transition from The Rockies 
to the plains states...the low pressure system will begin to 
deepen. While doing so...the atmosphere tries to saturate enough 
to get some precipitation reaching the ground out across the 
western forecast zones. Maintained the low end probability of precipitation across the 
western forecast zones during the first half of the Sunday night 
period...but then increased them to 30 to 40 percent chances of 
mainly rain across the eastern forecast zones late Sunday night 
where better moisture/lift and forcing will make it alot easier to 
produce full saturation and ground-reaching precipitation. That 
emphasis on precipitation falling over the eastern third of 
forecast zones appears to be maintained now by just about all the 
models. Again...the thermal profile seems to support a mainly rain 
event...perhaps starting out as a little bit of freezing rain or 
snow out west Sunday night as it begins to lift out of The Rockies 
and before it can effectively work to saturate the atmosphere with 
clouds/precipitation. By Monday...the focus for precipitation 
chances is definitely switching over to the eastern third of the 
forecast area...with rain the primary p-type. Monday night...as 
deformation zone precipitation begins to work south and east out 
of this area...the back edge of the precipitation band may switch 
over to light snow...and possibly produce a light coating of snow 
in spots. Currently maintaining up to an inch or so in the snow 
grids across the central portion of the forecast area /appx the 
James River Valley region/. Temperatures will switch over to 
cooler readings for Sunday and Monday with cooler air in place and 
clouds/precipitation possible. 




Long term...Tuesday through Saturday 
the extended period starts off with an upper level trough over the 
plains. The system shifts east during the day...then a secondary 
low drops out of Canada and over the northern plains Tuesday 
night. The GFS holds energy more over the eastern plains and moves 
the wave out about 6 to 12 hours faster than the European model (ecmwf). Strong 
ridging that will be over The Rockies slides over the Western 
Plains Thursday night...and over the plains Friday. Models diverge 
quite a bit at this point as the GFS drops an upper low across 
southern Canada toward the Great Lakes...while the European model (ecmwf) indicates 
a fairly deep trough tracking across The Rockies to the plains by 
the end of the period. 


At the surface...a low pressure system will be over Minnesota/Iowa 
Tuesday morning...with ongoing precipitation across mainly the 
eastern half of the County Warning Area. The low tracks northeast toward the Great 
Lakes Tuesday and Tuesday night...with broad high pressure over 
the western half of the country. Model time sections differ on 
temperatures in the lower levels...with the NAM keeping 
temperatures above zero as high as 800 mb...and the GFS showing 
temperatures at the surface falling below freezing on Tuesday. For 
that reason...will keep the precipitation on Tuesday as a mention 
of rain or snow. Will keep a schc pop for snow Tuesday night 
across the eastern County Warning Area as the secondary upper wave moves overhead. 
The surface high to the west begins to broaden eastward 
Wednesday...with the high remaining dominant through the day 
Friday. At that time...a trough begins to dig down the Lee of The 
Rockies...and tracks across the plains as a cold front Friday 
night. Little in the way of moisture around...so will not mention 
precipitation with the frontal passage. High pressure starts to 
nose back over the region behind the exiting front on Saturday. 


850 mb temperatures remain below zero through much of the period...with the 
exception of Friday when warm air advection develops ahead of the approaching 
cold front. Generally looking at near to slightly above normal 
temperatures through the period...with Friday being the warmest 
day. 




&& 


Aviation... 
VFR conditions will prevail tonight with scattered-broken high clouds at or above 15k 
feet. May see some high level IFR to MVFR clouds stream over the 
far eastern County Warning Area late tonight and early Sunday morning...mainly 
affecting Katy. 




&& 


Abr watches/warnings/advisories... 
South Dakota...none. 
Minnesota...none. 
&& 


$$ 
Short term...dorn 
long term...Parkin 
aviation...Parkin 


Weather.Gov/Aberdeen