Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 11/13/19
Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
625 PM CST Tue Nov 12 2019
Issued at 620 PM CST Tue Nov 12 2019
Updated to include 00z 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 QPF amounts. Some of the models also hint on
elevated instability which could also increase the QPF 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 QPF 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 QPF 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 00Z TAFS through 00Z Wednesday evening)
Issued at 620 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 tomorrow,
producing MVFR-to-IFR snow for much of central MN into western WI.
Have made only minor tweaks from the 12/18z TAF set but the idea
is the same 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 MN and during the evening in WI. The main issue
remains how quickly the atmosphere can saturate to realize
snowfall on the ground instead of evaporating aloft. Breezy south
winds will persist overnight in advance of the low pressure system
then swap to W and NW with its passage.
KMSP...Will still look for IFR conditions during the middle of the
day tomorrow, 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.
Ceilings will likely be in the 1000-2000ft range throughout the
duration of the snowfall.
/OUTLOOK FOR KMSP/
Thu...VFR. Wind WNW 5-10 kts.
Fri...VFR. Wind NE 5-10 kts.
Sat...MVFR psbl. Wind S 10-20 kts.