Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
354 PM CST Sat Jan 24 2015
Short term...(this evening through sunday)
issued at 354 PM CST Sat Jan 24 2015
Main concern in the short term period is the clipper system expected
to impact the region tonight.
Current water vapor imagery shows this wave dropping southeast
through western North Dakota...with a precipitation shield of mostly
rain moving southeast through the eastern Dakotas. This feature is
west of what earlier model runs had indicated...and the latest
guidance has converged on a solution of the heaviest precipitation
falling south and west of a line from Appleton through Mankato
toward Rochester. Hi-res guidance such as the
rap/hrrr/nmm/arw/hopwrf all indicate this southwestern solution as
well. Overall...snow amounts were slightly reduced and the heaviest
axis of about 2" shifted slightly southwest.
It will take some time to saturate the low levels once the precipitation
shield is overhead...and it should start out as rain or a rain/snow
mix in southwestern Minnesota. Model soundings do indicate that there
should be rapid cooling to below freezing in the low levels above
the surface thanks to evaporative cooling...so only expect rain to
last one to two hours initially before becoming mostly snow. The
overall onset of the precipitation has also been delayed...so areas further
north and east should see snow as the dominant precipitation type.
Some of the latest hi-res guidance we've received this afternoon
suggests lesser total quantitative precipitation forecast in the heaviest axis...so also trended
the forecast in that direction. The pv anomaly with this wave is
impressive...but moves through quite quickly once the precipitation
arrives. Parts of southwest Minnesota will lose some snow accumulation to
warm temperatures as well. Temperatures late this afternoon were commonly in the
upper 30s including a few sites in the 40s.
The snow should be pretty much over with by daybreak Sunday. Temperatures
tomorrow will be slightly cooler...but still above normal. Cloud
cover will decrease through the morning. The temperature forecast for
tomorrow may have to be adjusted upward just like today if abundant
sunshine can return.
Long term...(sunday night through saturday)
issued at 354 PM CST Sat Jan 24 2015
A weak clipper will be headed across northeast Minnesota and WI Sunday
night and Monday. This track will keep the significant
precipitation well to the north and east of the Twin Cities.
Hence...only some light snow is forecast during the early morning
hours on Monday across eastern Minnesota and western Wisconsin with
accumulations at this time a half inch or less. The loss of ice
aloft Monday morning results in some light freezing drizzle being
carried into the afternoon across our west central Wisconsin
counties. Even with the clipper passing by...Monday will be
another day with highs well above normal with readings from the
middle 30s in the western WI to the middle 40s in west central and
south central Minnesota.
In the wake of this feature...a massive ridge aloft will spread
into the central portion of the Continental U.S. By Wednesday...continuing
our above normal streak on temperatures. With less snow forecast
for tonight across our County Warning Area...went more aggressive on high
temperatures for Tuesday and Wednesday. Pretty much used the
ecemos for Wednesday which drove highs into the upper 30s at
Keau...the lower 40s in the Twin Cities with middle to upper 40s
across west central through south central Minnesota.
A two piece Pacific system will flatten the ridge Wednesday night
and Thursday. One piece of energy is a low off Baja California right now
while the second is a short wave riding in through the Pacific northwest.
The two are prognosticated to come together over the northern High Plains
on Wednesday. It still doesn't seem to be a big system as it moves
through our area with chance probability of precipitation at this point. Due to the
low level warmth...some of the initial precipitation Wednesday
evening is light rain.
A more vigorous clipper is indicated to move through early next
weekend by the European model (ecmwf). The GFS is a little different but eventually
unleashes Arctic air by late in the weekend. Certainly a more
northerly flow pattern with temperatures dropping below normal to
begin the month of February.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1201 PM CST Sat Jan 24 2015
Guidance continues to be overzealous on low level
moisture...especially the NAM. 850mb moisture reveals a decent
wedge of dry air lingering through much of the afternoon right
through our forecast area. Hence...VFR ceilings expected to last through about
23z near axn...to 02-04z from eastern Minnesota through western WI.
Overall trend in the guidance was to slow down the incoming
clipper system and to shunt it further southwest. So...have
delayed the start of -sn at all taf sites. Heaviest precipitation axis
now lines up toward rwf in a northwest to southeast oriented band. Very little
snow expected for the Twin Cities metropolitan and areas east thanks to
the southwestern shift. Generally west winds will become light and
variable this evening and turn NE thereafter. Expecting ceilings to clear
through the morning from northwest to southeast.
Kmsp...continued SW trend in shifting the precipitation with the incoming
clipper. I feel confident that msp will receive less than an inch
of snow from this system. Otherwise...no change from main
discussion...trended toward a more optimistic forecast with a
slower onset of ceilings and -sn.
/Outlook for kmsp/
Sunday night...VFR. Becoming MVFR ceiling/chance -sn late. Wind east-southeast 5
kts in the evening becoming S-SW 10kts late.
Monday...chance MVFR ceiling/-sn early then VFR. Wind west-SW 10-15 kts.
Tuesday...VFR. Wind northwest 5 to 8kts.