Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
322 PM CST Wednesday Dec 4 2013
Short term...(this evening through thursday)
issued at 320 PM CST Wednesday Dec 4 2013
Deepening low pressure was located in northeast Iowa at middle
afternoon. The low will continue to strengthen as it moves northeast
across Wisconsin...Lake Superior...and into Ontario overnight. The
cirrocumulus product from the kmpx dual pol radar nicely depicted the snow/mix
precipitation line moving slowly east across the St Croix River
Valley into Wisconsin early this afternoon. 850mb temperatures over
Eau Claire remain very warm...but will collapse during the late
afternoon with rain changing to sleet and snow and then to all snow.
So...portions of west central Wisconsin will see limited snowfall
totals due to the expected low level warmth. Still...sleet and
freezing rain will certainly have an impact on travel conditions
there. Widespread snowfall will continue lifting north through
Minnesota for the remainder of the afternoon and into the evening.
The forecast carries likely probability of precipitation through 03z across much of the
area. The 15z run of the hopwrf supported this idea as it developed
a secondary area of snow developing around 00z near the South Dakota
border and moving east-northeast through 06z across central Minnesota. This secondary
area will be firmly in the cold air that is coming...meaning it will
be much lighter /with higher snow to liquid ratios/. The concern
with this round of light snow is the blowing snow and the poor
visibilities that it may cause from west central into south central
Minnesota. Therefore...with the potential for white-out conditions
west and south...accumulating snow slowly diminishing north and
mixed precipitation changing to all snow east...will keep current
headlines as they are.
The strong northwest winds will usher in much colder air into the
region overnight...as temperatures in the 20s and 30s this afternoon
plummet overnight to near zero across western Minnesota and around 10 above
in western WI. Wind chill values will drop into the minus teens
overnight across the area. In addition... expect to see flurries
Thursday morning. Temperatures Thursday really will not moderate as
cold air advection continues on the back side of the exiting low
Long term...(thursday night through wednesday)
issued at 320 PM CST Wednesday Dec 4 2013
The two weather highlights in the extended are the cold
temperatures/wind chills...and the chance for light snow Sunday and
Monday. At the start of the period...bitterly cold air will be in
place across the region...but the cold advection will have ended for
the most part. 3hr surface pressure rises off the NAM are on the
order of 2mb/3hr...so winds will decrease overnight to around 10mph.
Low temperatures will drop to near -15f across western Minnesota...and -5f
in eastern Minnesota and western WI. Wind chill values will be -20 to -30f
on Friday morning...so will likely address these with wind chill
advisories once the Winter Storm Warning expires Thursday morning.
The cold air will remain in place throughout the period...with
temperatures expected to be 20 to 30 degrees below the seasonal
average for early December.
The next feature of interest is the potential for a low density
snowfall centered around Sunday night. The GFS/Gem/ECMWF 04.12 all
have a shortwave moving southeast of the region. Despite the
4-corners origination...the thermal profile of this wave will be
much more clipper like in nature. Forecast soundings off the GFS
show light winds veering with height and a deep dendritic zone. This
fits the conceptual model of high snow ratios...so have increased
the values in excess of 30:1. Although quantitative precipitation forecast amounts will be
light...as of now could see around 3 inches in southeastern Minnesota and
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1150 am CST Wednesday Dec 4 2013
Low level warm layer with temperatures a little above 0 degree c is
progressing slowly east of the Twin Cities per the kmpx dual
polarization products. This would suggest more of a snow scenario
for kmsp with a mix of freezing rain and sleet for krnh into middle
afternoon. The profile is warm enough for all rain at Keau through
early afternoon as well with a mix late afternoon with some snow
this evening. Kaxn...kstc and krwf all cold enough for all snow.
Ceilings for the most part this afternoon below 1k with ceilings
increasing to 1k-2k from SW to NE as the storm lifts out this
evening. One big concern in the wake of the synoptic snow will be
the increase in west-northwest winds this evening that will persist into
Thursday morning. This should produce blowing and drifting snow
for kaxn...krwf and kstc which may result temporary white-out
conditions. Northwest winds from 20 to 25 knots forecast with gusts near
30 knots. Ie...vlifr conditions possible. This is something future
taf issuance/S can focus on.
Kmsp...the airfield is going to be on the edge of mixed precipitation for
a few hours this afternoon before the thermal profile turns cold
enough for all snow. This will leave a few hours in the middle to
late afternoon for accumulating snow before winds switch to the
west and the better forcing for snow moves northeast. Increasing
west-northwest winds this evening from 15 to 20 knots with gusts around 25
/Outlook for kmsp/
Thursday night...VFR. West wind 5-15 knots.
Friday...MVFR ceilings possible. West wind 5-15 knots.
Friday night...VFR. West wind less than 10 knots.
Sat...VFR. Variable wind 5 knots or less.
Minnesota...Winter Weather Advisory until 6 am CST Thursday for mnz069-070-
Winter Storm Warning until 6 am CST Thursday for mnz041>045-
WI...Winter Weather Advisory until 6 am CST Thursday for wiz024>028.
Winter Storm Warning until 6 am CST Thursday for wiz014>016-023.