Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1133 am CST Monday Dec 22 2014
Short term...(today and tonight)
issued at 439 am CST Monday Dec 22 2014
Confidence is finally high for today and tonight. What you see is
pretty much what you get today. Low clouds and occasional light
rain or drizzle will persist into the late afternoon as low
pressure swings southeast from eastern South Dakota into Iowa.
Backing winds north of the system will introduce colder air into
the system late this afternoon and this evening...with the light
rain or drizzle turning to light snow from northeast to southwest.
Lift is not all that impressive...but the moisture depth is so
think there will be occasional bouts of light snow but nothing
that will accumulate all that aggressively...particularly with
temperatures hovering near freezing tonight. Quantitative precipitation forecast for the next 24
hours is at most fifteen hundredths of an inch...and taking into
account rain today...with wet and relatively mild surface
conditions tonight...snow amounts will be less than an inch
It should be noted the pop forecast was the most difficult thing to
handle for the short term. There will be many dry hours or
non-measurable drizzle...but could not pinpoint those hours easily.
Thus...very high probability of precipitation prevail through the period with the occasional
qualifying term attached to the weather elements.
Long term...(tuesday through sunday)
issued at 439 am CST Monday Dec 22 2014
Long term concerns remain the transition of mixed rain/snow to all
snow on Tuesday. Then the development of trough over the over the
western Continental U.S. With possible storm development over northern
plains/upper Mississippi Valley region Thursday and Friday.
Models continue to indicate mainly snow across the County Warning Area for
Tuesday morning as the upper low and associated surface low lifts
northeast over Wisconsin. Strongest forcing lifts over the eastern
portion of the County Warning Area during the morning as well. This should result
in the best chance for snow accumulation over the area. Currently
have 1 to 3 inches of accumulation for Tuesday...with the 2-3 inch
amounts into a portion of west central Wisconsin...mainly north of
I-94. Could see a rain snow mix develop again over the eastern
areas into Tuesday afternoon as the stronger forcing lifts
northeast. This along with surface temperatures still in the lower
and middle 30s should keep significant accumulations at Bay. Will
issue an advisory for a couple counties in west central Wisconsin
where confidence is higher of seeing 3 inches of snow on Tuesday.
Should see near seasonal temperatures in the wake of this system.
The next threat of snow develops later Christmas day into Friday.
Both the GFS and the European model (ecmwf) trend another trough over the western
Continental U.S. With cyclogenesis possible. GFS ensembles showing large
spread in movement of any surface low with possible phasing issues
with the trough energy as well. The trough that is responsible for
this threat is currently over the the eastern Pacific. Will likely
continue to see varied solutions for this system until it moves
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1132 am CST Monday Dec 22 2014
Less than MVFR ceilings along with IFR/MVFR visibilities will
prevail at the taf sites during the forecast period as an area of
low pressure over South Dakota moves southeast into Iowa. This will mean a
gradual backing in the surface winds from southeast to NE. As the winds
become more easterly this evening...the low level thermal profile
begins to cool from the east with kstc...krnh and Keau susceptible
to a changeover to snow quicker than the taf indicates.
Hence...confidence is low on exactly when the changeover occurs.
Confidence does increase by Tuesday morning that a band of
light to moderate snow will develop across south central Minnesota
through the southern Twin Cities and on across west central WI.
Confidence is also increasing that snow accumulation in this band
will be in the 2 to 4 inch range. The taf site in the bulls eye
right now for 4 inches in Keau. Hence...moderate snow indicated at
Keau after 12z. Krnh and kmsp will be on the western edge...but a
quick 2 inches of accumulation in the morning is looking likely.
Kmsp...short term model solutions point to all rain at the
airfield through 03z. This was incorporated into the taf. A mix of
rain and snow developing thereafter with an all snow scenario by
daybreak Tuesday. Several inches of accumulation likely during the day.
Southeast winds this afternoon backing to the northeast this evening then
becoming north by Tuesday afternoon. Speeds 10-15 knots.
/Outlook for kmsp/
Wednesday...MVFR. Winds northwest 10-15 kts.
Thursday...VFR. Winds west 5-10 kts.
Friday...MVFR chance -sn. Winds northwest 10 kts.
WI...Winter Weather Advisory from midnight tonight to 6 PM CST
Tuesday for wiz016-027.