Scientific Forecaster Discussion
fxus63 karx 172312
Area forecast discussion
National Weather Service La Crosse WI
515 PM CST sun Dec 17 2017
Short term...(this evening through monday)
issued at 301 PM CST sun Dec 17 2017
Main forecast concerns for tonight are on the potential for areas
of dense fog. If the fog forms visibility would be reduced to 1/2
mile or less and a layer of frost/ice may develop on untreated
roads and bridges.
A band of light snow/flurries has developed over southeast
Minnesota into southwest Wisconsin late this afternoon. Not
expecting much accumulation out of this but forecast models
suggest this band may pick up a bit before weakening later this
afternoon and evening. Frontogenesis in the 700 mb layer is the
main driver of band of light snow. This is expected to edge east
closer to central Wisconsin late this afternoon into this evening.
A few spots may measure 1/10 to 1/4 inch before ending.
Focus then turns to dense fog potential this evening into the
overnight hours. Light winds along with higher relative humidity
values near the surface will set the stage for fog development this
evening. Visibility restrictions continue over much of the area so
as temperatures cool slightly this evening we should see
visibilities drop in fog. With temperatures below freezing, we
could see the fog freeze upon untreated roads and sidewalks
leading to slippery travel. The fog could linger into the early
Monday morning commute. Another item for this evening is a weak
shortwave moving across southern Wisconsin that may produce some
light rain, mainly over Grant County. Thinking temperatures will
remain warm enough to not see much if any icing on untreated Road
surfaces. Confidence is low on the rain making it into the area.
Long term...(monday night through sunday)
issued at 301 PM CST sun Dec 17 2017
Main forecast concerns in the long term are on potential for
accumulating snow and possibly mixed precipitation for some areas
Thursday into Friday.
A trough is expected to dig into the southwest conus Wednesday into
Wednesday night as surface low pressure develops over eastern
Colorado. A band of snow then looks to setup across portions of the
upper Mississippi River valley Wednesday night into Thursday but
exactly where this band sets up is uncertain. Also, warmer air
being pulled into the region ahead of this developing system
introduces precipitation type concerns. Keep in mind there remains
large spreads in 850 temperatures and overall precipitation
amounts in the gefs plumes. So, this means there is considerable
uncertainty on the exact details of this system. Looking at the
operational European model (ecmwf) and GFS, the low does intensify over the upper
Mississippi River valley Thursday afternoon into Thursday night
with the potential for snow and mixed precipitation in the
forecast area. The Canadian model has a different solution, with
weaker system overall. So, it's a little early to determine
precipitation amounts. What is known is that precipitation is
looking more likely for the area and it will probably be in the
form of snow for many areas with the potential for a mix in spots.
Travel impacts are also looking more likely for Thursday through
Friday morning. Continue to monitor later forecasts closely as we
hone in on details.
Much colder temperatures look to arrive next weekend, impacting the
Christmas Holiday. There are strong signals for below normal
temperatures. Winds chills may also become a concern Friday night
through Sunday, with values potentially dropping below 0 and
possibly to around -20 over the open areas of southeast Minnesota.
Plan for colder weather if you are traveling next weekend.
Aviation...(for the 00z tafs through 00z Monday evening)
issued at 515 PM CST sun Dec 17 2017
Cigs: snow melt, lingering moisture and a light wind field will work
to keep low cigs locked in at the taf sites into Mon morning -
mostly LIFR/IFR. Once winds start to pick up from the southwest Mon
morning as responsible shortwave trough exits east, cigs should
rise/scatter out. NAM more pessimistic than the GFS/rap on holding
onto the low clouds longer - opting with the quicker solutions for
Weather/vsby: a lot of MVFR br out there right now (per sfc obs) and meso
models suggest this will continue into Monday morning. How low it
GOES it uncertain. Without much wind and a decent amount of
saturation, see the potential for IFR, possibly lower. Won't get too
aggressive at the moment - monitor and adjust if necessary. Fog
should clear as winds pick up Monday morning.
Winds: light and variable tonight, increasing and becoming
southwest/south for Monday. Some gustiness possible.