Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
540 am CST sun Nov 29 2015
Short term...(today and tonight)
issued at 335 am CST sun Nov 29 2015
Early this morning...surface high pressure is slowly working through
eastern Minnesota and western WI. Middle level cloud cover continues to
advance northeast through the southwestern Minnesota...where overnight
temperatures have been held in the 20s thanks to the flow turning
southerly and the increased cloud cover. For today...increasing middle
level cloudiness will continue thanks to middle level southwesterly
flow ahead of the deep upper low moving east over The Rockies. No
precipitation is expected through as very dry air near the ground
will prevent anything from reaching the ground. For high
temperatures today...very similar to the previous forecast with
highs in the low to middle 30s looking likely.
The weather will turn more active tonight...mainly during the
overnight period. By late tonight...we'll begin to see a defined
surface cyclone emerge to the east of the Rocky Mountain
region...while warm air and moisture advection increases through
Iowa into southern Minnesota overnight. Hi-res cams are in good agreement
of seeing scattered precipitation break out after midnight in Iowa and
advance north of the Minnesota border after 2am. The problem with
the initial onset of this precipitation is the lack of deep moisture. Model
soundings indicate lift associated with the precipitation down around 850-
700mb with dry air above that layer. Temperatures in that layer will hover
near freezing...hence there will be difficulty in having ice
crystals present in the clouds until the deeper moisture arrives a
few hours later. Surface temperatures will be in the 20s...so
expecting a mix that includes freezing drizzle overnight in southern
Minnesota. But...as we get closer to daybreak tomorrow...deeper moisture
and lift will arrive and turn the precipitation over to snow. This
concern is mainly for areas from Mankato and to the south...as the
majority of the forecast area wont see precipitation until after 12z Monday.
Long term...(monday through saturday)
issued at 335 am CST sun Nov 29 2015
Primary concern... and really the only precipitation event through the
forecast period... is the storm system impacting the area Monday
and Tuesday. Model guidance continues to show fairly good
agreement on timing of the system... and the track of the surface
and upper level lows. The European model (ecmwf) remains a bit west of the GFS...
and is more similar to the NAM in that respect. However...
forecast precipitation from the various guidance remains quite
consistent... with totals from around 0.75 to 1.00 inches. The
main difficulty with the system... and an area where the small
differences in the guidance leads to lingering uncertainty... is
with respect to liquid/snowfall ratios. Initially... things are
fairly warm across the area.... particularly the southeast and
east... where an elevated warm nose could lead to some sleet on
Monday. In addition... near surface temperatures Monday and Monday
night across the east look to be in the lower to middle 30s...
which could cause rain to mix in at times... particularly when the
precipitation is lighter.
Used a combination of model consensus and Air Force ratio methods
to try and get a handle on things. That approach leads to values
at or below 10 to 1 into Monday night from near I-35 eastward... and at or above
10 to 1 over the west. These increase from southwest to northeast
later Monday night into Tuesday as the upper low and associated
colder thermodynamic profile works across the area... with things
getting closer the climatological normal of around 13 to 1.
Decent isentropic lift works in Monday and persists into Monday
night... with stronger upper divergence and DPVA prognosticated to arrive
later Monday evening into the overnight hours. The fairly
impressive synoptic scale lift leads to precipitation output from the
thaler-qg output which is actually nearly the same as the explicit
model output... which suggest decent confidence in widespread precipitation
of a quarter inch or more. Combining those expectations with the
aforementioned snow ratios continues to support snowfall totals
at or above 6 inches across much of the area... with the west and
southwest portion of the area still looking to see the most at
this point in time. Forecast soundings from the NAM and GFS both
suggest the presence of convective instability above the frontal
inversion... which could help to locally boost vertical motion in
the vicinity of the dendritic growth zone... which is otherwise
not very impressive given its lack of vertical depth. Cam output
from the namnest and hopwrf-ts is also suggestive of some
convective elements based on their reflectivity and maximum upward vertical velocity
output. So... could certainly see localized areas with greater
Given uncertainties in snow ratios and some precipitation type concerns
initially... chose to continue the watch as is without any
expansion in area. Eventual 12 and 24 hour totals should be near
or above warning criteria over portions of the area... but
confidence still isn/T sufficient to transition from a watch
anywhere. Some snow looks to linger into Tuesday night... but the
main time frame for precipitation continues to be from late Monday morning
through the overnight hours Monday night... with things generally
arriving from southwest to northeast. Pacific high pressure will
then build across the Continental U.S. For the balance of the week... with
near to above normal temperatures and dry weather.
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 534 am CST sun Nov 29 2015
One more relatively quiet aviation period before the incoming
winter storm impacts travel. For today...middle clouds will be
widespread...but VFR ceilings everywhere. Tonight we will see ceilings
lower but at this point expect them around 5k feet...so still VFR.
The precipitation associated with the approaching storm will just be
entering southern Minnesota at the end of the forecast period.
Kmsp...because the msp taf is 30 hours...added some initial
thinking to the onset of precipitation tomorrow morning. Timing of the
onset of the snow can easily change by a few hours in either
direction at this point.
/Outlook for kmsp/
Monday...VFR becoming IFR or lower in sn/+sn. Winds east 5-10 kts.
Tuesday...IFR in snow early. Poss VFR late. Winds NE becoming northwest 5-15 kts.
Wednesday...VFR. Winds northwest 5-10 kts.
Minnesota...Winter Storm Watch from Monday morning through Tuesday afternoon