Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1104 am CST Tuesday Dec 1 2015
Short term...(today and tonight)
issued at 321 am CST Tuesday Dec 1 2015
Main forecast concerns this period...lingering -sn/-dz/-fzdz chances
Data analysis at 06z had low pressure centered over southwest
Minnesota...just northeast of the 500mb low near the Iowa/SD/neb border. WV
imagery showed a broad middle level dry slot east of the 500mb low
lifting north across eastern Iowa/western Illinois into southern Minnesota/western
WI. Areas radars show bulk of the heavier precipitation has wrapped well
east/north/west of the surface low as this dry slot spreads north across
the area. Lift continued ahead of the low in the surface-700mb layer
where moisture remained. With lack of ice in the clouds...this
producing a secondary band/arc of -dz/-ra lifting north across the
forecast area. Per surface observation...south edge of this band was approaching
northeast early this morning...with precipitation ending across the
southwest 2/3 of Iowa.
01.00z model look to have initialized well. Solutions very similar
for today/tonight as the South Dakota/IA/neb middle level low moves slowly east
to a position over south-central WI at 12z Wednesday. Trend is slower
with this low moving east today/tonight...even slower than slowest
of the earlier model runs. However...with the tight model
consensus...short- term forecast confidence remains on the good side
For the short term...models quickly take the bulk of the remaining
forcing/lift east/north of the area early this morning as the
stronger shortwave energy rotates up and around the east side of the
middle level low. With the middle level dry slot having moved over the
area...the heavier precipitation with this system is done for the area.
Once the secondary band of -dz/-ra rotates north through the area early
this morning...precipitation chances for the area drop into the 20-50
percent range for the late morning hours through tonight. Appears much
of the area will be dry later this morning into early this
afternoon...having to wait until some deeper moisture and lift with
the surface-500mb circulation moves east across the area later this
afternoon/tonight. This deeper moisture...to at least 700mb...and
700mb temperatures cooling to below -10c re-introduces ice into the
clouds as the surface-500mb circulation and moisture move across the
area. After mainly -ra/-dz/perhaps some -fzdz chances through the
morning...main precipitation type this afternoon and tonight to be -sn.
However...any amounts/accumulations looking to be quite light.
With mainly liquid precipitation the rest of this morning...then 20-50
percent chances of -sn this afternoon/tonight...will cancel the
Winter Weather Advisory with the 4 am package. This lines up well
with the neighbors to the west. With clouds and weak/MDT low level
cold advection ahead of the 925-500mb low...temperatures looking to be
nearly steady today. Low level cooling continues tonight but with
the cloud blanket and generally snow free and unfrozen ground
across the area...lows in the forecast grids for tonight may be too
cool. Stayed with a blend of the guidance highs/lows for today/
tonight at this time.
Long term...(wednesday through monday)
issued at 321 am CST Tuesday Dec 1 2015
For Wednesday through Thursday night...
Main forecast concerns this period...small -sn/flurry chances Wednesday...
temperatures through the period.
Good consensus among 01.0z models for the middle level low to move east
Wednesday...but more slowly than the earlier runs. Heights do start to rise
later Wednesday...with shortwave ridging to build across the upper Midwest
Wednesday night/Thursday morning. Tighter signal for a northern stream wave to
drop across the Lake Superior area late Thursday with even stronger ridging
aloft to build into the upper Midwest behind it Thursday night. Given the
rather good model agreement...Wednesday through Thursday night forecast confidence is
good this cycle.
With the slower eastward progress of the surface-middle level low Wednesday...
area stays under deeper cyclonic flow...the middle level cold pool and
more surface-700mb moisture much of the day. Small -sn chance/flurries
looks good Wednesday morning...with scattered flurries lingering into the
afternoon. Deep layered subsidence and drying above 850mb spreads
in late Wednesday/Wednesday night as the surface-middle level low finally moves east
of the region. Skies to generally remain cloudy through Wednesday. Will
remain optimistic for decreasing/clouds clearing Wednesday night as the
surface-850mb ridge axis drops southeast across the area.
However...with a strengthening subsidence inversion near 900mb
over the area some of the lower level moisture/clouds may end up
trapped over much of the area at least Wednesday evening. More westerly
low level flow spreads across the area Thursday behind the low level
ridge axis and this should advect drier low level air into the
area...clearing out any potentially lingering clouds from Wednesday
night. Either way...appears Thursday to be a mostly sunny day with low
level warm advection spreading across the area. Shortwave dropping
across Lake Superior late Thursday/early Thursday evening quickly drags a weak
cold front southeast across the area. Little in the way of low
level cold advection prognosticated behind this front...being quickly
overtaken by the next stronger low level ridging to build into the
upper Midwest. Blend of guidance highs in the 30s and lows mostly
in the 20s for Wednesday through Thursday night...around 3-5f above normal
appear well trended. Will have to watch that highs in the forecast
grids are too cool...especially with a mostly sunny day and a
generally snow-free landscape across the forecast area.
For Friday through Monday...days 4 to 7...
Main forecast concerns this period...above to much above normal
temperatures through the period.
Medium range model runs of 01.00z in rather good agreement through
the Friday-Monday period. This as ridging aloft to build across the
upper Midwest Fri/Sat...as troughing moves across the area sun
then as rising heights/ridging quickly builds in behind it for Monday.
With the good model consensus through the period...forecast confidence in
the day 4 to 7 period is above average this cycle.
Surface high pressure...ridging aloft...south/southwest low level flow
and warm advection looking to dominate the upper Midwest Fri/Sat.
925mb temperatures are prognosticated in the +3c to +8c range Friday/Sat
afternoons...easily supporting highs in the 40s to even lower 50s.
However question is how deeply will the boundary layer mix this time
of year. Model soundings only showing to about 950mb. With plenty of
sunshine expected over a snow-free landscape in early Dec...have to
believe some highs in the middle-upper 40s are possible in at least the
MS/WI river valleys both Fri/Sat. Even behind the weak front Sat
night...which comes through moisture starved...925mb temperatures in the +3c
to +6c range Sunday as the next low level ridge axis drifts across
the region. Stronger south/southwest low level flow looking to
return on the back side of this high/ahead of next plains troughing
Monday. Friday-Monday shaping up to be a mostly sunny/mostly clear...dry
period with highs at least 5f to 10f above normal...perhaps as much
as 10f to 15f above normal Fri/Sat. For now stayed with the
model/ensemble consensus of highs/lows through the period as plenty of
time to trend the highs upward if needed.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1104 am CST Tuesday Dec 1 2015
MVFR ceilings presently exist across the taf sites...and with low
pressure over southwest Minnesota moving east bringing more low
clouds with it of MVFR/IFR variety...believe the ceilings will
stay at or below MVFR. Nighttime cooling should help lower
ceilings tonight...and have kept the IFR forecast at rst being
higher elevation. Ceilings should rise with diurnal warming on
Despite the area of low pressure moving in...only anticipating a
brief period of light snow...mainly between 22-05z. Visibility
restrictions expected in the MVFR range with only a couple tenths
of accumulation. Otherwise...plan on VFR visibilities.
Winds look to remain generally at or below 10 knots...with the
strongest winds out of the southwest around 10 knots this afternoon.
Winds switch to the northwest late tonight.