560 
fxus63 kmpx 082146 
afdmpx 


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


Discussion... 


Forecast period continues to be active with several bouts of snow 
possible as well as a return to bitter cold Arctic air by the middle 
of next week. First round of snow is on the way tonight and Friday. 
Overall model continuity not all that great so confidence in snow 
chances this weekend and early next week is not too high. Afternoon 
water vapor imagery shows broad upper trough continuing to spin over 
the eastern U.S. With trough axis shifting east along the eastern 
Seaboard. Another weaker upper trough is dropping southward along 
the Ontario/Manitoba border down the backside of the eastern 
longwave trough. Shortwave ridging is progressing eastward across 
The Rockies while a sharp trough has moved onshore over the West 
Coast and continues inland approaching The Rockies. Several little 
waves of energy are embedded along this feature and it is the 
handling of these waves that is creating some difficulty with near 
term snowfall. The trough will pass later Friday with the whole 
pattern amplifying through the weekend with a Sharp Ridge building 
off the West Coast and a broad longwave trough becoming established 
over the Great Lakes. This pattern will hold through the rest of the 
forecast period resulting in cold weather and periodic 
shortwaves/surface troughs transiting the region. 


The details...tough forecast today with guidance continuing to show 
a range of solutions and little run to run continuity with regard to 
details for snow tonight and tomorrow. Above mentioned trough 
working into The Rockies continues to push weak upper ridging 
eastward. Warm advection is developing across the Dakotas with radar 
returns appearing as well. Weak surface high pressure is squarely over 
the forecast area this afternoon with dew points in the lower single 
digits. Middle level warm advection will March northeastward into 
southwest Minnesota early this evening with 280-295 k layer showing fairly rapid 
decrease in condensation pressure deficits indicating fairly rapid 
saturation. Isentropic adiabatic Omega on the 295 surface increases 
markedly in the southwest this evening. Expect that snow will break 
out across west central and western portions of south central Minnesota by 06z 
before expanding further northeast towards the I-94 corridor from 
St. Cloud to New Richmond and points south by 09z or so. This band 
of warm advection will continue to March northward but lose its 
punch as saturation becomes more difficult across west central WI by 12z. Middle 
level trough will drop southeastward across southwest Minnesota 
Friday afternoon. May see a break in good snow accumulation from 
late Friday morning into early afternoon across the southern County Warning Area as 
the warm air advection will be weaker and to the northeast and the middle level trough 
will be entering the northern County Warning Area. Probability of precipitation may be a bit overdone south 
from 15 to 21z Friday. Low confidence in lowering them due to some 
faster model solutions keeping precipitation more steady but ending 
earlier. Precipitation type is a concern for a small corner of the area 
near Fairmont to St. James to Springfield as middle level warm air may 
warm enough to allow partial melting after 09z Friday. Have 
mentioned a mix of sleet in these areas after 09z until 15z at which 
point forcing weakens and middle levels dry a bit keeping crystals from 
falling into the saturated layers lower in the atmosphere. Therefore 
freezing drizzle is a possibility in these areas from 15 to 18 or 21z 
until crystals return with the approach of the middle level wave. Snow 
is expected at all other areas through the period. 


Have made some adjustments to the snow amounts with another slight 
push south and west. Expect a sharp gradient in snow amounts along 
and north of Interstate 94 from about St. Cloud to the Twin Cities 
to Eau Claire. This cutoff is due to expected weakening middle level 
warm advection then fairly persistent subsidence to the north and 
east of the open wave evidenced by upper level deformation in the 
500-300 mb layer Friday. Model continuity has been poor in the 
precipitation department but trend has been downward. Duration of forcing 
between the warm advection and the passing upper wave argue for 
greatest amounts near the Minnesota River Valley from Montevideo southeast 
to Mankato then east through Owatonna. Best upward motion is 
displaced above the favored Crystal growth layer during the warm 
advection period favors smaller snow flakes and ultimately less 
snow. Absence of strong frontogenesis through period...minus the 
leading edge of the warm air advection which is only marginally saturated...also 
argues for slightly lower snow ratios closer to 14 to 15 to 1. 
Areas along and north of Interstate 94 may have a tough time meeting 
advisory criteria but there has been enough fluctuation in forecast 
guidance to keep the advisory as well as the fact the decent 
snowfall should occur during the morning commute. Overall amounts of 
1 to 6 inches across the forecast area with the lowest amounts from 
around Little Falls to Chippewa Falls Wisconsin...areas which may 
ultimately see only around an inch. Just to the southwest...from St. 
Cloud to the northern twin cites metropolitan to Eau Claire should see 1 to 3 
inches while areas from Alexandria to the central and southern Twin 
Cities metropolitan to Durand Wisconsin should see 2 to 4 inches.../a very sharp 
gradient is looking more likely across the metropolitan area with least 
northeast and most southwest/. Amounts near 6 inches may accumulate 
in the Minnesota River Valley from Montevideo to Mankato and then east to 
Owatonna...with slightly lower amounts southwest where sleet may mix 
in and snow briefly stops midday Friday. 


Saturday through Thursday...surface high pressure will be building 
in behind the departing trough Saturday. The main question will be 
how fast it can move in. Colder air will be moving to the region but 
is not significantly colder than Friday. Highs in the teens are 
expected. The high quickly departs Saturday night with a series of 
clippers approaching from the northwest. The location and magnitude 
of these clippers varies considerably among the NAM/GFS/ECMWF...but 
each does have them..just in different places at different times. 
With a trough crossing the region will maintain consistency with the 
previous forecast which includes slight and low chances probability of precipitation Sunday 
and Sunday night. Snow amounts still look light. Am also concerned 
that probability of precipitation may need to be shifted back into Saturday night. A 
stronger clipper is forecast by the GFS to cross just to the 
southwest of the County Warning Area Monday... with the European model (ecmwf) further south. The GFS 
track would favor several inches of accumulation. Have increased 
snow chances across the southwest half of the area Monday. The 
passage of this low will bring the first blast of Arctic air for the 
week as 850 mb temperatures tumble to around -20 c with high pressure quickly 
crossing the area Monday night. 


With surface high pressure overhead currently...min temperatures will occur 
this evening then warming is expected as the high shifts east and 
southeasterly winds take over with warm advection. Highs tomorrow 
will probably occur early in afternoon then become Stead or slowly 
fall in the northwest as winds turn northerly and cold air advection 
commences. 


Warmer air returns on the back side of the high Tuesday with the GFS 
very excited about a strong low coming out of South Dakota and 
across central Iowa. The GFS ensemble and European model (ecmwf) point to a low but 
of much lower intensity. Have included slight probability of precipitation of the southwest 
on Tuesday. Initial surge of cold air behind the low is impressive 
as 850 mb temperatures drop to -23 c or so with another wave dropping down the 
back of the mean upper trough into Thursday as 1050+ mb high settles 
into the Dakotas. Coldest 850 mb air of -26c tries to fill into the 
eastern County Warning Area for late Wednesday/early Thursday. This would result in 
highs struggling to reach zero by middle week across many areas. 
Current forecast may be a bit too warm should the magnitude of cold 
air mentioned above occur. There are several systems to move through 
the area first and likely changes in timing and magnitude will have 
an impact on the magnitude of the arriving cold air. Will lower 
temperatures slightly from previous forecast but be a bit conservative at 
5+ days out. ..mdb.. 


&& 


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. 


&& 


$$ 


Mdb/trh/mpg 

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