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Area forecast discussion...updated 
National Weather Service Aberdeen South Dakota 
1058 am CST Wednesday Jan 7 2009 


Update...rest of today 
an abundance of cloud cover remains across much of the 
region...except for the far northeastern County Warning Area where clearing is 
slowly working in from the north/northeast. Did increase cloud 
cover a bit from the James Valley and points west as it appears 
clouds will be around most of the day. Radars around the region 
showing flurries and light snow under these clouds. Did extend 
flurries into the early afternoon hours as observation upstream still 
showing flurries. Visibilities being reduced to around 2sm at times. 
Better chance for accumulating snow today remains across western 
South Dakota and the extreme western abr County Warning Area. Did increase probability of precipitation to likely 
across Murdo area as web cams confirming light snow at this time. 
Accumulations will remain under an inch today. Also lowered highs 
today a couple degrees across the eastern half of the County Warning Area where 
temperatures are having a tough time climbing this morning. 




&& 
Previous discussion... 


Short term...today through Friday 
forecast challenges include snow chances out west today and 
tonight...followed by freezing rain and/or sleet chances Thursday 
afternoon and night...followed by snow and blowing snow potential 
on Friday. 


Currently...weak high pressure resides across the region along 
with scattered-broken MVFR and VFR ceilings. The regional mosaic 88d display 
also shows light snow/flurry-type activity has increased a bit in 
areal coverage over the past few hours. Have stretched out the 
flurries mention through 18z of the today period to cover these 
clouds and radar returns. 


With a strong polar jet diving down The Spine of the northern High 
Plains today...will be watching banded snow signature begin to 
pick up west of this County Warning Area by late morning and persist through the 
day as weak shortwave energy embedded within the fast nnwrly flow 
aloft spills southward and works over a fairly notable low to middle 
level baroclinic zone that has set up over the western Dakotas. 
Expecting that by late this evening some of this light banded 
snowfall activity will have shifted slightly eastward dropping 
rather light snowfall amounts /generally around an inch or less/ 
across the far western forecast zones. 


Then...the forecast takes on a rather low confidence look and feel 
for Thursday through Friday. The 00z NAM produced an outlier 
solution. So...the 12z/00z ec and 00z GFS were leaned on the most 
for the details surrounding Thursday and Friday. Basically...a 
warm layer advects across the forecast area Thursday and Thursday 
night. While doing so...there may be adequate forcing and lift 
working across the region to produce a deep enough cloud layer 
/basically between 850hpa and 600hpa/ to produce some light 
precipitation that would melt in the warm layer and the either 
refreeze into sleet or just freeze on impact with the ground 
Thursday afternoon and Thursday night. The very front edge of 
precipitation may fall as some light snow briefly before this 
transition through sleet and/or freezing rain takes place. So...in 
addition to the freezing rain and sleet mention...went ahead and 
tossed in snow mention as well...although...make no mistake about 
it. The most critical aviation and public component being 
forecasted here is freezing rain and sleet. These would be the 
forecast weather types that prompt a Freezing Rain Advisory over the 
next 24 to 36 hours...if need be. Again...this would...from all 
appearances...be a moving band of light precipitation heading 
north and east toward eastern North Dakota and Minnesota. Kept quantitative precipitation forecast 
guidance numbers on the low end of things for pretty much the 
entire event. Finally...once the best forcing/lift combo departs 
this County Warning Area late tonight...low level winds remain from the south and 
thermal advection continues to be either weak warm air advection 
or neutral advection...so tossed in mention of some very light 
freezing drizzle across the eastern half of forecast zones where 
super-cooled low stratus is expected to be hanging out through the 
first half of the day on Friday. 


Then...assuming there is a light crusting of freezing rain over 
the County Warning Area-wide Snow Field by Friday morning...when the really strong 
northwest winds pick up...will the Snow Field lay dormant beneath 
the layer of ice. Or...will any freezing precipitation be 
negligible causing widespread blowing and drifting snow problems 
over the County Warning Area during the day on Friday. Somewhat concerned about 
the strength of winds on Friday. These strong northwest winds are 
likely going to force some form of hazardous weather headline for 
Friday. The 00z GFS produced 18mb and 20mb 6-hourly pressure 
tendencies on Friday. Also...during the strongest surge of 
cold air advection...850hpa winds are prognosticated in the GFS model to be 50 to 55 
knots...with 925hpa winds of 35 to 40 knots beneath it. Sustained 
winds of 40 miles per hour or higher may not be achieved on Friday...but 
concern is growing for some 55 to 60 miles per hour gusts early on during 
the cold surge on Friday. 


No changes of substance were made to inherited maximum/min T grids 
with this forecast package. 




Long term...Friday night through Tuesday 
all of the models and GFS ensembles continue to be consistent and 
agree well on the large scale long wave pattern across the US 
through the long term. The upper ridge axis in the western US with 
an eastern US trough remain through the long term with 
amplification towards the end of the period. For Friday 
night...expect winds to drop off and skies to clear as surface 
high pressure builds across the County Warning Area. May see temperatures fall 
even farther below zero than the current forecast for the 
northeast County Warning Area. Otherwise...for Saturday through Sunday night...the 
County Warning Area remains in warmer southwest to west northwest flow at the 
surface with the main frontal zone and short waves remaining east 
of the County Warning Area. Increased temperatures for both Saturday and Sunday 
and will likely have to increase them even more as latest mex 
guidance came in warmer. Also...kept it dry through Sunday night. 
The GFS...ec and ensembles all agree that a strong Arctic front 
will drop across the region Monday and Monday night. Could see a 
lot of wind with this along with some light snow and blowing snow 
Monday into Tuesday morning. Current forecast for Monday and 
tuesday's temperatures may have to be lowered in the next forecast. 




&& 


Aviation... 
IFR/MVFR clouds will continue to slide south under an area of 
high pressure...affecting mainly kmbg/kabr and Katy terminal 
sites. Some of these low clouds may also continue brief periods of 
light snow...which could result in visibilities in the 3-5sm range. Kpir 
is expected to stay low VFR ceilings...although they may also experienced 
periods of MVFR visibilities from 4-5sm with haze and flurries. 


As the high pressure system continues to slide south...look for 
improving conds by 22z...with VFR conds expected from 22z to 12z 
Thursday for all terminal sites. 




&& 


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


$$ 
Update...tmt 
short term...dorn 
long term...mohr 
aviation...Hintz 


Weather.Gov/Aberdeen 

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