Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
240 PM CST Wednesday Dec 17 2014
Short term...(tonight through Thursday night)
issued at 240 PM CST Wednesday Dec 17 2014
Main forecast concerns this period...clouds and temperatures.
18z data analysis had high pressure ridging from western ont to
eastern ND to MO/AR. Skies had generally cleared under/near the
ridge axis across much of Minnesota/IA/eastern Dakotas. Further east...a
trough/front was slowly dropping south across Lake Superior. Clouds
lingered in the low level convergence ahead of this trough across
the southwest 1/2 to 2/3 of WI...with the north-northeast flow
behind the front/trough spreading more clouds southwestward across
Lake Superior...the Michigan u.P. And northeast WI. Even with the sunshine
across MN/IA...temperatures slow to warm...even where the ground is snow-
17.12z models generally initialized quite well but all were 1 to 2mb
too weak/far northwest with the MO to ND to western ont ridge axis.
Solutions very similar as weak shortwave ridging slides across the
region tonight/Thu. Tighter signal for a weakening shortwave to move
into the north half of the region Thursday night...being squeezed by
ridging trying to develop between East Coast/West Coast troughing.
Trend favors faster/stronger of the earlier model runs. Short-term
forecast confidence is good this cycle.
For the short term...sfc-850mb troughing/front continues to drop
south across the area tonight...east of the weak middle level ridge
axis. Plenty of 925mb moisture trapped under an inversion prognosticated to
persist across the northeast half of the forecast area tonight in/near
this trough axis. Would think with the surface-850mb ridge axis drifting
across the area Thursday that the low clouds would break up/clear but
925mb relative humidity forecasts show lower level moisture firmly stuck over the area
under the strong inversion near 925mb. Then for late tonight through
Thursday night...mid/high level moisture ahead of the shortwave coming
across the northern plains spreads across the area...so even if the
lower clouds break/clear...more middle/high clouds looking to spread
across the area later tonight into Thursday night. Trended toward more
cloud cover through the short-term...especially over the northeast 2/3
of the forecast area tonight/Thu. With more clouds...trended toward
warmer of guidance lows tonight/Thursday night and cooler of guidance
Long term...(friday through wednesday)
issued at 240 PM CST Wednesday Dec 17 2014
Main forecast concerns for Friday through Sat night...clouds/temperatures.
17.12z models remain in good agreement Friday as one shortwave tracks
across the north end of the region and more energy digs toward
western Iowa. Good agreement continues Friday night Sat as these features
pass north/south of the forecast area and yet another stronger wave
tracks across the northern rockies Friday night and into the northern
plains Sat. Models then diverge with the evolution of this shortwave
Sat night...European model (ecmwf) takes most of the shortwave energy north of the
US/can border...can-Gem quite amplified digging much of the energy
into neb/Dakotas...GFS in the middle. Overall trend is slower/
stronger with central/northern plains troughing Sat night. Forecast
confidence is good Friday through Sat then average Sat night.
Bulk of the forcing/lift and 850-500mb saturation with the wave
approaching Thursday night passes north of the forecast area Friday. That with
the wave moving into western Iowa Sat morning passes south of the forecast
area Sat. Forecast area looks to remain dry Friday through Sat...but plenty of
moisture above 500mb spreads across the area and some moisture below
850mb lingers across the area...for at least partly to mostly cloudy
skies Friday through Sat. Southerly low level flow prognosticated to prevail
across the forecast area for Friday through Sat night. A slow moderation of
925mb temperatures Friday through Sat night as weak to MDT low level warm
advection persists across the region. With a general lack of snow
cover...temperatures look to warm a few degrees each day...even with plenty
of clouds expected. The southerly flow eventually pulls increasing
surface-850mb moisture northward into the area Sat night...with
increasing/thickening low clouds and warmer lows. Used a blend of
the guidance highs/lows for Friday through Sat night.
For Sunday through Wednesday...day 4 to 7...
Main forecast concerns this period...precipitation chances and temperatures
through the period.
Medium range model runs of 17.00z/17.12z in reasonable agreement for
longwave troughing to be developing over central noam sun/Mon.
However plenty of detail differences...and impacts on the sensible
weather for the region these periods...depending on the interaction
of wave into the northern plains at 12z sun and the next feature
digging into the central/northern plains behind it Sun night/Mon.
Good consensus for a central noam longwave trough continues
Tue/Wed...but plenty of detail differences continue with how the
shortwave pieces interact and impact the upper Midwest weather
heading into the Christmas Holiday. Overall trend is stronger...
more amplified with the trough/energy for the first half of next
week. Day 4-7 forecast confidence is average this cycle.
Increasing low level warm advection spreads in Sunday ahead of the
deepening troughing into the central/northern plains. Question is
how quickly deeper moisture returns with this flow. Small mainly -sn
chance sun per the model/ensemble consensus reasonable for now.
Moisture continues to slowly increases Sun night through Tuesday in
persistent southerly flow ahead of troughing/low pressure slowly
approaching and moving into the upper Midwest. Models differ in the
lower level/meso-scale details by Sun night through Tuesday...as far as
925mb temperatures and potential precipitation types. Differences with the
timing/strength/position of the surface-850mb low into/across the area
Tuesday as well. Until the details sort themselves out...20-50 percent
-ra/-sn chances for much of the Sun night through Tuesday night period
reasonable at this point. Precipitation chances should be decreasing and
temperatures cooling Wednesday as the surface through middle level low/trough would be
exiting east of the region. Given the stronger trend and potential
for system to slow even more...lingering 20-40 percent -sn chances
on Wednesday okay for now. Model/ensemble consensus of the highs/lows for
sun-Wednesday appeared good.
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1215 PM CST Wednesday Dec 17 2014
Challenging forecast period for the taf sites of klse and krst.
Currently the region is under a sharp and deep temperature inversion
which is acting to limit mixing and contain moisture in the low-
levels /hz or br/. While some clearing has worked through krst and
is in vicinity of klse...clearing is leading to more cumulus development with
enough vertical cloud depth to produce MVFR -shsn. So...periods of
scattered-broken ceilings should be around through the afternoon. Krst may be
right on the IFR/MVFR boundary near 1kft /bkn008-bkn012/.
Once the sun begins to set...the cumulus should dissipate and a
period of clear skies seems probable at krst...possible at klse.
However...overnight winds above the surface are expected to be more
from the northeast and surge more cloud into the taf sites. Until
that time...there is a chance krst could get into IFR fog should
skies remain clear long enough. Have not addressed that in the taf
as of yet but did keep MVFR fog.
Overall...have kept klse in the clouds although there may be some
clearing periods until later tonight. Have a bit more optimism for
the krst taf.