551 
fxus63 kmpx 081809 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
1209 PM CST Thursday Jan 8 2009 


Update... 


Added 18z aviation discussion below. 


&& 


Discussion... 
the initial forecast concern is the clipper system 
on track to impact the area tonight and Friday. The trend that 
Wednesday model runs started toward a more southerly and slower 
track were held onto with the 08.00z and 08.06z runs. 
Therefore...confidence is increasing with regard to a plowable 
snowfall across much of the forecast area. Isentropic lift on the 
290k surface and lowering condensational pressure deficits Blossom 
across west/south central Minnesota around 06z Friday...expand toward 
east central Minnesota toward 12z Friday...and eventually reach west 
central WI after 12z Friday. Still somewhat concerned about the 
potential for light freezing rain south of the Minnesota River Valley on 
Thursday night...although the likelihood of that is shifting more 
South/West of the County Warning Area with each model run. Although there are 
still differences between the NAM and GFS...both forecast a decent 
area of 700-600mb frontogenesis and -15c Omega to pass across 
central/south central Minnesota tonight and Friday. Specific humidity 
values of 3-4 g/kg illustrate the potential for near-warning 
criteria snowfall amounts /6 inches/ across west/south central 
Minnesota...although any mixed precipitation there would reduce that potential. 
Opted for an advisory at this point...especially since widespread 
6+ snowfall amounts from this type of system are climatologically 
unfavorable. The NAM/GFS Cobb BUFKIT amounts are also supportive 
of a 3-6 inch variety snowfall. Models are increasingly indicative 
of a sharp cutoff on snowfall amounts on the northeastern fringes 
of the County Warning Area...and envision Milaca to Ladysmith locales may only see 
around an inch out of this system. Winds are not looking quite as 
robust in the wake of trough...but still think 20-30 knot speeds 
are possible Friday eve/night...mainly across west/south central 
Minnesota. 


Colder than normal conditions prevail over the weekend as surface 
ridging sets in...with highs generally in the 10 degrees to middle 
teens range. 


The secondary concern is the blast of Arctic air that is forecast 
to begin invading the region at the beginning of next week. By 
Tuesday...most of the County Warning Area will likely be experiencing highs around 
zero. Multiple weak impulses/clippers will dive across the upper 
Midwest as the large scale trough digs over the central/eastern 
Continental U.S.. have retained attendant 20-30 probability of precipitation on Sunday and Monday. 
Winds also appear intense late Sunday night and Monday...with 
sustained winds between 20 and 30 miles per hour making for frigid conditions. 


&& 






Aviation.../18z taf issuance/ 
benign weather currently seen will rapidly deteriorate overnight 
as a clipper develops over Montana and dives southeast across the 
Dakotas. Considerable disagreements still exist with models... 
mainly on timing and duration of snow. One area of agreement comes 
with warm advection snow in the morning...as GFS/NAM/ECMWF all 
indicate a heavier northwest to southeast oriented band of snow 
moving across the area between 08z and 15z. For now...it appears 
rwf and msp have the best chance of getting within the strongest 
part of the band...while the other terminals will likely be on the 
northern edge of the heavier snow. GFS basically ends snowfall 
with the warm air advection...but the NAM and European model (ecmwf) continue it through at least 
00z Saturday as the 500 mb short wave moves southeast through the 
region. NAM and European model (ecmwf) indicate a secondary heavy burst of snow 
would be possible across Minnesota around 18z as forcing from the 
shortwave moves overhead. At this time...will keep visibilities 1sm or higher 
with ceilings 1k feet or higher. However...heavier snow bursts will 
likely see conditions worse than this...but will let later 
forecasts Handel those times with tempo groups when there is a 
better idea for when the heavier snow will occur. On a final 
note...winds will remain under 10kts during the taf period as they 
turn from their current 270-300 direction to more of an 080-050 
direction by the end...the low speeds should help keep visibilities 
up during the event as we will not have to contend with blowing 
snow. 


&& 


Mpx watches/warnings/advisories... 
Minnesota...Winter Weather Advisory from midnight tonight to 9 PM CST Friday 
for Anoka-Benton-Carver-Dakota-Freeborn-Goodhue-Hennepin-Le 
Sueur-Morrison-Ramsey-Rice-Scott-Sherburne-Steele-Waseca- 
Washington-Wright. 


Winter Weather Advisory from 9 PM this evening to 9 PM CST 
Friday for Blue Earth-Brown-Chippewa-Douglas-Faribault- 
Kandiyohi-Lac Qui Parle-Martin-McLeod-Meeker-Nicollet-Pope- 
Redwood-Renville-Sibley-Stearns-Stevens-swift-Todd-Watonwan- 
Yellow Medicine. 


WI...Winter Weather Advisory from 6 am to 9 PM CST Friday for Dunn- 
Eau Claire-Pepin-Pierce-St. Croix. 


&& 


$$ 


Mtf/mpg