Scientific Forecaster Discussion
fxus63 karx 082342
Area forecast discussion
National Weather Service La Crosse WI
542 PM CST sun Dec 8 2019
Short term...(tonight through monday)
issued at 300 PM CST sun Dec 8 2019
Challenging forecast period through Monday. Todays trend is a more
vigorous return southerly flow of moist air into Wisconsin later
tonight. This increases precipitation chances there, and freezing
drizzle chances near and north of I-90 in central WI.
Latest surface analysis shows a cold front slowly making its way
across the area from kmcw-klse-kmfi at 20z. Post-frontal low-cloud
is present, but visibilities have remained above 3 miles nearest to
southeast Minnesota. Further west, a dense fog advisory and very low
visibility and fzfg is found in swrn Minnesota. This stratus will continue
to shift slowly southeast a few more counties before stalling in
southwest/central WI/nern Iowa.
Shortwave trough energy over both Montana and Colorado per GOES water vapor
imagery at 20z will phase to provide cyclogenesis over the Central
Plains tonight and Monday. Falling pressures at low-levels will
bring a surge in moisture northward into swrn/central WI, deepening
the surface based layer to about 2km with weak vertical motion in
the layer /5ub/S upward/. Cold front should provide some cooler air
into the WI area to provide a higher threat of icing from drizzle
later tonight. Therefore, with snow transition and 1-3 inches by
Monday afternoon, confidence on a travel hazard prompted a Winter
Weather Advisory for central WI.
Further west, the moist layer in place will deepen later tonight
from about 1 km to 1.5 km, and deep forcing will not have yet
arrived, setting the stage for possible fzdz and icing for the
morning commute. Via collaboration, and with confidence not being as
high as WI, it was decided on to wait on an advisory further west
along I-90 and north roughly.
Dense fog is also possible with the surge tonight and frontal cloud
layer. 08.12z href visibility probabilities suggest northeast Iowa
and southern WI may see fog of <1mi. This too will need to be
Key points: confidence is highest in central WI for freezing drizzle
and light icing for the Monday morning commute. There may be fzdz
further west along I-90. Development would be after 3 am and into
the morning. A transition to snow will occur Monday from west to
east, accumulations greatest /3"/ north of Highway 29.
Long term...(monday night through sunday)
issued at 300 PM CST sun Dec 8 2019
As large upper trough takes hold, expect precipitation to clear the
area by Monday evening. Focus after that will certainly be
plummeting temperatures, a signal that remains consistent in
guidance. Coldest air expected Tuesday through Wednesday before some
moderation takes place. At coldest, readings of 2.0/2.5 Standard
anomalies below normal predicted. While not records, could see highs
finish in the top 10 of coldest highs for Dec.10 and 11th. Wind
chills could approach advisory levels as well.
Initially, as cold air advection peaks and trough axis works
through, a few flurries are possible. Some medium range guidance
also suggesting a weak impulse swings through in northwest flow
going into Wednesday. After some coordination with Iowa offices,
have added some light snow to forecast but speed of feature and
relatively dry air should keep any accumulation to bare minimums.
After that, flow becomes a bit more zonal supporting the idea of
moderating temperatures. Another short wave trough in guidance could
bring light snow in north /mainly along and north of I-94/ Thursday
but will have to work out those details later in the week.
Temperatures closer to normals expected.
Some amplification in flow is also suggested going into next weekend
so could be another light precipitation event in offing, along with
Aviation...(for the 00z tafs through 00z Monday evening)
issued at 542 PM CST sun Dec 8 2019
A complex aviation forecast going forward as a winter system is set
to impact the area late tonight into Monday. MVFR to IFR conditions
located to the west have been slowly spreading east-southeast into
the area this evening. Should reach krst here shortly, with lower
confidence in timing for klse. Have MVFR conditions moving in at
klse around 04z, but this will need to be monitored closely and
adjusted if necessary. The threat for drizzle/freezing drizzle and
fog (mainly west) are introduced late tonight/overnight, bringing
IFR to LIFR conditions. A transition to snow should occur by mid to
late morning on Monday and then end by Monday afternoon, allowing
improvements in both ceilings and visibility. However, winds will
pick up by Monday afternoon as the surface low exits, with west-
northwest winds 15-20 kts and gusts up to 30 kts likely. Looking
ahead, VFR conditions should make a return by late Monday night with
winds gradually lessening.
WI...Winter Weather Advisory from midnight tonight to 6 PM CST Monday