Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
609 PM CST Sat Dec 20 2014
Short term...(this evening through sunday)
issued at 354 PM CST Sat Dec 20 2014
Most of the fog and light precipitation has ended today...but could
redevelop late tonight. Temperatures will hang near freezing for
the next 18 hours. Therefore...slick spots on local roads are the
biggest concern in the short term.
This morning brought a little light freezing drizzle/rain to
portions of central Minnesota and areas of freezing fog to south
central Minnesota. The result...a few slick roads and some accidents
reported by mndot...especially on I-35 and I-90 near Albert Lea.
Since then...temperatures have warmed by a few degrees and the fog
and light wintry precipitation has come to an end. The positive news is
that there is little risk of much freezing precipitation through the
rest of the afternoon and the evening. Tonight...however...there
should be an area of 925-850mb moisture advection that comes out of
the Dakotas and into west central and northern Minnesota. This
advection isn't overly impressive...but you can already see the
light radar returns in ND and South Dakota this afternoon. A second area of
warm/moisture advection becomes enhanced tonight across Iowa and early
tomorrow morning in southern Minnesota. This is due to an increased SW flow
that is in response to the developing system in the northern rockies
tonight. The depth of the moisture isn't great...but we should
continue to be saturated in the lower atmosphere. This should bring
about light precipitation in the form of drizzle /with fog/ across much of
eastern Minnesota and western WI - a few snow flakes are also possible. The
precipitation will be light and might not even be enough to wet the ground
in most locations. That being said...temperatures will likely hang
between 30-33 degrees through tomorrow morning...so any drizzle that
occurs with a surface temperature at or below freezing could lead to
minor ice accumulation. Again...this will be mostly true across
eastern Minnesota and western WI tomorrow morning.
Long term...(sunday night through saturday)
issued at 354 PM CST Sat Dec 20 2014
Quite the weather pattern unfolding for next week across the
central and eastern Continental U.S. As northern and southern streams phase
around midweek...driving a deepening low pressure system northward
across the Ohio Valley and into the central Great Lakes. A
conference call with wpc today led to their assessment that
todays 12z GFS was an outlier when compared to the gefs and the
European model (ecmwf) ensemble. Even the parallel run of the GFS has reversed
course with the midweek storm shown near the Middle Atlantic States.
The deterministic European model (ecmwf) remains the most consistent solution over
the past several days with this pattern...both on placement and
timing. Fortunately...the middle week system is a little too far east
to affect US directly...but its the Monday through Tuesday period
when these upper air features are beginning to come together that
causes US concern. The 12z/18z NAM and 09z/15z sref are in very
good agreement with the European model (ecmwf) with the surface and upper air
pattern from Monday into Tuesday. This raised confidence in the
forecast that the low level thermal profile will support rain
developing across west central and southwest Minnesota Sunday night and
spreading into the Twin Cities and west central WI Monday morning.
Therefore...continued the trend of raising probability of precipitation with categorical
for portions of the eastern and southern County Warning Area on Monday. The
NAM/sref and European model (ecmwf) hold the northern stream upper low across
central Minnesota for Monday night and much of Tuesday. This allows for a
dry slot to spread northward across far eastern Minnesota and western WI
Monday night through Tuesday...diminishing the precipitation...
while temperatures hover around or slightly above the freezing
mark. Hence...some light snow is forecast for the Twin Cities and
points east and south Monday night and Tuesday but little
accumulation is expected with temperatures remaining warm. The
deformation zone precipitation remains across the eastern Dakotas
and western Minnesota Monday night and Tuesday. The thermal profile there
does support several inches of snow accumulation from Canby up to
Some light snow may occur across the forecast area on Wednesday on the
backside of the upper low. Again...amounts look light but it may be
enough to whiten the ground. Another chance for snow remains in
the forecast from Thursday into Friday as a fast moving short wave
moves through from the Pacific northwest. The European model (ecmwf) has pushed
this feature a little farther south today...something that it has
done now for the past three runs...with snow chances better across
far southern Minnesota and northern Iowa. This is a feature that will need
to be watched as it is highly dependent on how the middle week system
comes together and eventually moves away. It does appear that next
weekend will be much colder with high/low temperatures trending
below seasonal normals.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 609 PM CST Sat Dec 20 2014
Forecast begins with widespread IFR ceilings from SW Minnesota into
central and northern Minnesota... and low end MVFR ceilings from south
central Minnesota into west central wisc. Ceilings should gradually lower
this evening over south central Minnesota into wisc... with Keau likely
the last to drop to IFR late this evening. Visibilities should
mostly stay MVFR but some spots that never improved too much today
should drop back to IFR during the evening.
Other concern is for drizzle and possibly freezing drizzle after
10z Sunday from Albert Lea Minnesota area to Keau and maybe krpd by 14z in
Wisconsin. Moisture appears to be shallow but some lift is
evident. Key question will be surface temperatures by the time drizzle
reaches this area. Temperatures will likely be 30-32 degrees 12z-
17z... and this could mean spotty ice accumulation. Have not yet
added fzdz to Keau but may need to amend this evening.
MVFR ceilings should remain MVFR for a few hours this evening...
but ceilings should slowly lower...reaching IFR during the 04z-
06z time...very common in this type of setup. Daytime heating by
just a few degrees should allow for a return to MVFR late Sunday
morning. Other concern is drizzle but that will likely barely miss
kmsp to the south and east. If kmsp does see any precipitation around 12z
Sunday... it may be fzdz with surface temperatures around 30-32 degrees.
Will be watching this closely.
/Outlook for kmsp/
Sun night...MVFR/IFR. Chance -ra/-ip. Winds south-southeast 10-15 kts.
Monday...IFR. -Ra with -ra/-sn mix late. Winds southeast 8-15 kts.
Tuesday...MVFR/IFR. -Sn or -ra/-sn mix. Winds northwest 5-10 kts.
Wednesday...MVFR. Slight chance -sn. Winds northwest 10-15 kts.