Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
309 PM CST Thursday Dec 18 2014
Short term...(this evening through friday)
issued at 307 PM CST Thursday Dec 18 2014
A narrow band of drier air around 92h has led to some clearing or
partly sunny skies across portions of central and southern Minnesota early
this afternoon. In far ec Minnesota/wc WI...earlier cloud layer and weak
convergence in this layer led to a period of -sn...which briefly
caused some minor accumulations across the Twin Cities. This area of
-sn has moved off to the north/NE and dissipated...or became very
light for radar not to detect it.
Although there will be a brief period of mostly clear skies moving
across the area this evening...it will quickly become cloudy again
as a weak shortwave moves across the region tonight from the Dakotas.
In addition to the increasing cloud cover...a weak area of lift
across central Minnesota may produce some light freezing drizzle/flurries.
Deeper moisture will be north of mpx County Warning Area...but enough to include
some light precipitation across the far northern County Warning Area by Friday
morning. No major problems with the freezing precipitation is
expected due to limited coverage and duration.
The strong inversion that developed last week and led to a long
period of cloudy skies...low ceilings and fog...is likely to develop
once again by Friday afternoon as temperatures above the boundary layer rise from
the warmer SW flow underneath the cloud deck.
Long term...(friday night through thursday)
issued at 307 PM CST Thursday Dec 18 2014
Main concerns during the long term remain fzdz potential
Saturday/Saturday night...followed by precipitation chances Sunday through
Wednesday night. Included in the precipitation chance next week is the
issue of p-type Sunday night into Monday.
For this forecast...model preference was a blend of the GFS/European model (ecmwf) as
the Gem quickly becomes an outlier on Monday next week. Differences
between the GFS/European model (ecmwf) are fairly minimal until Thursday...but after
that...we are beyond this forecast period.
The first problem with the forecast shows up Saturday as we get into
increasing srly flow. One thing that looks like a certainty is more
time spent beneath a deck of stratus. Still questionable Sat/Sat
night is if moisture is deep enough to generate dz. For now...kept
Saturday dry...but kept the fzdz mention going for Sat night. The
reasoning here is that during the day Saturday...we will have
shortwave ridging and subsidence working across the area...while
Saturday night...the next weak shortwave moving across the area will
provide a little burst of lift and deeper moisture. Confidence is
still low on if we will see any dz or precipitation at all...but on
Saturday night...most models show some light quantitative precipitation forecast spreading across
the area...which is indicative of dz.
For Sunday...srly winds will be increasing in strength...but we will
see the added benefit of seeing moisture return to the dgz...along
chances for precipitation other than dz. Moisture transport increases as
well on Sunday...but it is oriented toward eastern areas...so continued
to have chance probability of precipitation for western WI.
Sunday night...the upper wave that will be with US more or less to
Christmas makes its appearance. This will send a surface low across southern
Minnesota/northern Iowa Monday. The upper wave will still be taking on a negative
tilt and closing off...so the surface low will make it only as far as
Michigan by Wednesday. Also on Wednesday...a southern wave will round the
trough...with a coastal low taking over by Thursday. The European model (ecmwf) still
shows this low deepening to under 970 mb...but it is a bit farther
east than earlier runs. Other notable change with the system is in
the temperature department. Because the upper low gets so deep...the
cold air gets stopped up in Canada. With the 17.12 runs...the
European model (ecmwf)/GFS were showing 925-850 temperatures cooler than -12c by
Thursday...but with the 18.12 runs temperatures in that same layer are
forecast to be warmer than -10c...so we only look to cool back down
to about normal by the end of the week...with the most noticeable
change in the forecast for next week being temperatures about 5 degrees warmer
than what we had.
As for potential sensible weather next week...we still look to spend
a prolonged period of time within cyclonic flow...which looks to
result in a prolonged period of light to moderate snow Monday night
through Wednesday night. In addition...as the low is moving across
the area on Monday...it looks to bring a fairly warm tongue of air
up in the area...with the European model (ecmwf)/GFS showing surface wet bulb temperatures
increasing into the middle 30s Sunday night into Monday. Forecast
soundings from the GFS continue to show p-type being rain Sunday
night through Monday morning for much of the area outside of central
Minnesota and northwest WI. Once this system moves off to the east...profiles
still look to cool enough to switch things over to snow...though the
European model (ecmwf) is slow enough in bringing the cold air in to push the threat
for a rain/snow mix out into Tuesday in western WI. Both the GFS and European model (ecmwf)
continue to show several inches of accumulation next week...but we
get those several inches because we look to pick up an inch or two
of snow a day Monday through Wednesday. Winds also look to pick up
on Tuesday...though with the sub 970mb low staying farther east in
the latest models runs...winds do not look like they will be as
strong as they were looking yesterday...but still would be strong
enough to lead to blowing snow issues depending on how much snow we
get next week. Still lots of uncertainty surrounding this
system...but given its timing during a busy travel week...still has
the potential to adversely impact much of the area in one way or
another next week.
Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1100 am CST Thursday Dec 18 2014
Earlier this morning a cloud layer around 1.5 to 2.5k above ground level moved
from wc WI...into ec Minnesota as boundary layer winds slowly became
east-southeast/southeast this morning. As temperatures in the cloud layer held near -10c to
-12c...ice Crystal formation continued...plus the added affect of
weak convergence in this layer...led to -sn. Temperatures will slowly
warm this afternoon and winds will veer to the S/SW...so the MVFR cloud
deck should move off to the north/NE and only affect krnh/Keau through
the next 6 hours. Elsewhere...kstc will see VFR conds by 21-23z. After
00z...another area of MVFR ceilings will move from NE/Iowa back into SW
Minnesota and cause krwf/kaxn to lower to MVFR ceilings once again later
tonight. Confidence remains high on the MVFR ceilings for ec Minnesota/wc
WI...with lower confidence on the return of MVFR ceilings across SW/wc
Minnesota after 00z. This is mainly a timing issue...not so much of will
it occur or not. Light winds from the east-southeast/southeast will slowly increase
by the end of the taf period.
Last batch of -sn should move through the Airport terminal area by
18-19z. MVFR ceilings will slowly erode from the west/SW and advect into
the metropolitan area by 20-22z. Timing remains the may problem...but
confidence is high enough to introduce VFR conds by 21z. MVFR ceilings
will likely come back into ec Minnesota Friday...but timing remains the
main dilemma. Will continue with VFR conds until we have a better
handle on timing. Winds will hold from the east-southeast/southeast through the
period...and slowly increase from 3-5 kts this afternoon...to 6-8 kts
by the end of the taf period.
/Outlook for kmsp/
Friday...VFR early...possibly MVFR by aftn/evening. Winds south-southeast 6-8 kts.
Sat...MVFR ceilings. Chance -fzdz late. Winds south-southeast 5 kts.
Sun...MVFR/IFR. Chance -fzdz/-ra/-sn. Winds south-southeast 7-10kts.