Scientific Forecaster Discussion
fxus63 kmpx 130451
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1051 PM CST Tue Nov 12 2019
issued at 1045 PM CST Tue Nov 12 2019
Updated to include 06z aviation discussion below.
Short term...(this evening through Wednesday night)
issued at 230 PM CST Tue Nov 12 2019
Breezy conditions have developed across western Minnesota today
as the departing Arctic air mass slid off to the southeast, and a
surface trough intensifies in the northern plains. Although
temperatures are still quite cold for mid November, the worst of
the cold temperatures has past. Clouds have already increased
ahead of a developing storm system moving out of the Canadian
rockies this afternoon. Strong warm air advection across the
northern plains has led to mid/high level cloudiness spread across
the upper Midwest. Although regional radar hasn't shown any echoes
which implies precipitation, the air mass overhead is extremely
dry and will likely take some time to saturate before any
precipitation reaches the ground.
The main forecast concern is when will the atmosphere saturate in
the wake of this dry air mass in place.
First, any deep moisture will need to be developed by means of
isentropic lift or fgen which leads to where will this develop,
the strength of the forcing, and timing. As with most type of
clippers that originate over the plains of Canada, moisture is
limited, and most observations across Montana and North Dakota are
only showing mid-level clouds. Current model trends have stronger
Theta-E advection (abv 700mb) later this afternoon/evening across
Minnesota which leads to more mid-level moisture developing. Most
models, and the latest rap support little low level moisture even
with strong warm air advection developing overnight. Most of the
saturation occurs where the best fgen (500-600mb) coincides with
the stronger warm air advection at low levels across central
Minnesota between 6- 12z. This is the area that have the higher
percentage and higher quantitative precipitation forecast amounts. Some of the models also hint on
elevated instability which could also increase the quantitative precipitation forecast amounts,
and associated saturation in the boundary layer which the models
support. Not until 12-15z, does southern Minnesota, south of I-94,
become saturated enough to warrant increasing percentages. Even
at this point as the fgen focuses north of I-94, large scale lift
that is associated with the short wave, could lead to more
widespread precipitation across southern Minnesota by mid/late
morning. However, based on the amount of quantitative precipitation forecast expected, I wouldn't
be surprised to see snowfall amounts less than a half inch south
of I-94 by Wednesday afternoon. The best chance of 1-2 inches will
occur in central Minnesota where again the better forcing and
faster saturation in the boundary layer develops. Forecasters need
to monitor where the fgen develops, the layer in which is
develops, strength, and how fast upstream observations develop
cloud bases less than 5k.
Long term...(thursday through tuesday)
issued at 235 PM CST Tue Nov 12 2019
By Wednesday evening, and as the short wave exits the upper
Midwest, a dry period is expected until this weekend.
Two frontal boundaries are expected to pass across the upper
Midwest over the next week. Both of these frontal boundaries do
support some light quantitative precipitation forecast amounts, but timing and location of the
best forcing remains questionable this far out. Confidence is low
on any chance of precipitation past tonight/Wednesday with a
gradual warming trend.
This warming trend is based on how several weather systems moving
across the eastern Pacific and slowly break down the upper ridge
that has been plaguing this area for the last two weeks. There
will be bouts cooler weather as cold fronts will move across the
area as a semi-quasi zonal flow develops , but the type of air
masses that are expected to move across the upper Midwest will
have a combination of Pacific, and Canadian air mass
characteristics. Thus, it will not be as cold as the current
Arctic air mass moves farther north into Canada.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1045 PM CST Tue Nov 12 2019
VFR conditions expected for much of the night in advance of a weak
clipper-type system which will drive through the region Wednesday,
producing MVFR-to-IFR snow for much of central Minnesota into western
WI. Have slowed down the system a bit as there are more
indications that the atmosphere will saturate from top down a bit
slower than previous model runs. Nevertheless, indications are
that a roughly 4-6 hour period of -sn can be expected mid-to-
late morning into the afternoon hours, ending by late afternoon in
Minnesota and during the evening in WI. Breezy south winds will persist
overnight in advance of the low pressure system then swap to west and
northwest with its passage.
Kmsp...will look for mainly MVFR conditions during the middle of
the day Wednesday, after the morning push but before the
afternoon push. That said, some flurries and/or light snow showers
are very possible during the latter portion of the morning push,
along with some lingering light snow into the early part of the
evening push. In addition, some periods of IFR visibilities cannot
be ruled out but confidence is a bit lower on such a scenario so
have held off advertising IFR visibilities in the 06z set. Ceilings
will likely be in the 2000-3000ft range throughout the duration
of the snowfall, but a few periods of sub-1700ft ceilings also
cannot be completely ruled out.
/Outlook for kmsp/
Thu...VFR. Wind west-northwest 5-10 kts.
Fri...VFR. Wind NE 5-10 kts.
Sat...MVFR psbl. Wind S 10-20 kts.