Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
537 am CDT sun Mar 9 2014
issued at 530 am CDT sun Mar 9 2014
Updated to include 12z aviation discussion below.
Short term...(today and tonight)
issued at 336 am CDT sun Mar 9 2014
Primary concerns...high temperatures and winds for today.
Overview/synopsis...current water vapor imagery shows the upper
ridge broadening and extending into the region at the current
time... with middle-high clouds descending down the ridge axis. No
discernible shortwave energy is apparent upstream which will have an
impact on US today or tonight... with the only such feature staying
to our North/East late tonight into Monday morning. Current surface
analysis shows pressure gradient tightening over the western
forecast area with warm advection from western Minnesota into the
Dakotas and Central Plains. Surface observations bear this out with
wind already gusting above 20kt over southwest Minnesota... and
temperatures rising from there back into the Dakotas and Nebraska.
Model preferences...the models are in decent agreement in terms of
the overall mass fields today and tonight... with the main
differences being in their low level temperature fields. The land
surface component of the models still appear to be struggling with
warming temperatures sufficiently over the snowpack. The higher
albedo will certainly mitigate warming to some degree... but not to
the extend that the raw model output indicates. On the other hand...
the statistical guidance may be a bit too warm for some locations.
For tonight... the NAM wrings some light precipitation out across the area as
a weak shortwave moves by to our north. Feel this is overdone... and
likely due in part to it being too moist in the lower levels from
not warming temperatures sufficiently in the boundary layer today.
Generally stuck with a blend of the guidance... but continued to be
warmer than most of the gridded guidance would suggest for highs
Analytical discussion...strong warm advection will be present across
the area today... with good isentropic ascent through the day.
However... condensation pressure deficits are quite high... so the
main result of the warm advection will be middle/high clouds. This
cloud cover could keep temperature in check to some extent... but
mechanical mixing from the strong winds will likely make up for
some of what insolation is unable to do today. A shortwave moving
around the periphery of the upper ridge will move by to our north
late tonight... but its forcing should stay to our north along with
any potential precipitation. The thaler qg diagnostics for the GFS keep any
vertical motion to our North/East tonight... with subsidence working
back into our area as the strong warm advection from today is
replaced by some cold advection as west/northwest winds develop.
Sensible weather conclusions...as mentioned... continued to go with
the higher end of guidance in terms of temperatures today... and
above it for most locations. Mixing from 925mb would actually
support highs above 60 out west and well into the 50s toward I-35.
However... the albedo effects of the snowpack should help prevent
that from happening as we hang onto a fairly low inversion. For
highs... used a blend of the mix-down potential along with a
consensus of the short range guidance. Even without deep mixing...
we/ll still see fairly strong southwest winds today into this
evening... with gusts above 30kt a good bet for locations that area
able to warm above 50. There will be quite a bit of middle/high clouds
around today into tonight... but filtered sunshine should still be
Long term...(monday through saturday)
issued at 336 am CDT sun Mar 9 2014
Fairly quiet long term with really just the one chance for precipitation
coming Monday night into Tuesday. Otherwise...we're still looking
at a warm start to the week...followed by a cooling trend.
Much like what was discussed in the short term...high confidence
in well above normal temperatures carrying over into Monday as
well. The difference on Monday being we expect a cold frontal
passage during the afternoon that could cool at least the northern
forecast area during the late afternoon. The Post frontal winds
only slightly to the northwest and predominately stay west...so
we're not expecting strong cold advection until Tuesday. The
system responsible for the precipitation chances Monday night and Tuesday
somewhat resembles a Colorado low...but is actually more of a
hybrid of a Pacific northwest system and Colorado low. The latest
model trends (last few model cycles) are as such:
GFS - most consistent little chance in track/timing/intensity
NAM - slowing/weakening and tracking farther south
European model (ecmwf) - maintaining intensity/speeding up and moving north
At this point...the European model (ecmwf) provides the most bullish solution for
accumulating precipitation for most of the forecast area. The NAM
virtually has the precipitation missing the entire forecast area and the
GFS kind of falls in the middle. The Gem is also more GFS like in
timing and placement /with just a slight northward displacement/.
The current forecast represents a Gem/GFS blend. We like the
forecast consistency of the GFS and it would argue for high probability of precipitation
in southern Minnesota and just chance probability of precipitation elsewhere - except leaving the
far northern County Warning Area dry. We kept the p-type limited to just two
weather types - rain or snow. We think the surface temperatures should be
above freezing and the chance of refreezing looks low based on the
progression of the forecast soundings...therefore freezing rain or
sleet looks unlikely at this time. We wouldn't be surprised to see
1-2" of snow in south central Minnesota. The cold advection that takes
place on Tuesday should give US a very cool Wednesday...in
fact...we might be a little too warm with our temperatures.
Otherwise...high confidence in near to above normal temperatures over the
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 530 am CDT sun Mar 9 2014
VFR conditions will prevail through the forecast period... with
some middle/high clouds generally at or above 10k feet above ground level through around 03z.
Winds will be brisk today... with some potential for gusts above
20kt where temperatures are able to warm. In fact... wouldn/T rule
out some gusts near 30kt if mixing proves to be better than what
forecast soundings would suggest. Winds should decrease some
tonight... but still remain above 5kt as they veer around to the
Kmsp...main uncertainty is with gust potential of winds. Didn/T
get too carried away... but if temperatures warm to near 50 as
forecast then mixing may become sufficiently deep for some gusts
near 30kt given the 30+ knots winds which are not too far off the
/Outlook for kmsp/
Monday afternoon...VFR. Northwest wind 5 to 15 knots.
Monday night...VFR. Northwest wind less than 10 knots becoming north.
Tuesday and Tuesday night...MVFR ceilings and visibilities
possible with a chance of rain and snow. Northwest wind 5 to 15 knots.
Wednesday...VFR. Northwest wind less than 10 knots backing west.
Wednesday night and Thursday...VFR. West wind 5 to 10 knots backing