331
FXUS63 KARX 032015
AFDARX
Area Forecast Discussion
National Weather Service La Crosse WI
215 PM CST Tue Dec 3 2024
.KEY MESSAGES...
- VERY WINDY Wed afternoon-evening. Wind Advisory for most locations
west of the Mississippi river (could be expanded east). Wind gusts
35 to 45 mph for entire area with sustained from 20 to 30 mph. Winds
WILL impact travel and blow lightweight objects around (holiday
decorations).
- BITTER COLD start to Thu with combination of wind and cold air
producing 5 to 15 below wind chills. Bundle up for your commute to
work/school.
- SCATTERED SNOW SHOWERS Wed (20-50%). Accumulations look minimal at
this time, but could provide further travel impacts with sudden
reductions in visibility, some slick spots on roads.
- Cold is short-lived with above normal air moving in for the
weekend - back in the 40s for some!
&&
.DISCUSSION...
Issued at 215 PM CST Tue Dec 3 2024
- WED/THU MORNING: windy, colder and low end snow shower chances (20-
50%)
OVERVIEW: upper level shortwave trough still on track to drop
southeast out of Canada tonight, digging across the great lakes Wed
night, developing a negative tilt as it drives over the new england
states by thu evening. Although the associated sfc low will hold
across southern Canada, a couple related sfc fronts will move across
the region. Very strong cold air advection, some low level
instability, and tightening pressure gradient accompany the system,
promising strong/gusty winds, much colder air, and some snow shower
chances. The bulk of its impacts will be shifting east with the
storm Thu morning.
WINDS: a lot of elements progged to come together and foster a very
windy period for the region with Wed afternoon-evening likely
harboring the strongest gusts for the local area.
1) Tightening sfc pressure gradient
2) Unidirectional winds (northwest) through nearly the entire column
Wed aft/eve.
3) Mixing upwards of 825 mb with RAP/HRRR/NAM/GFS bufkit soundings
suggesting upper 30s to around 40 kts will be brought to the sfc for
the afternoon/early evening Wed.
4) Trop fold could bring even stronger winds surfaceward, with the
1.5 PV sfc reaching 750-800 mb in the NAM/GFS, but as low as 850 mb
in the EC. The EC has been the most robust with this.
All this has been a very consistent signal in the models the past
several days. The EC (as already alluded to) has been the most
bullish on strong winds. Essentially 100% of its ensemble members
paint 40 mph gusts for the entire forecast area for Wed aft/eve. In
its 03.12z run, 20 of the EPS members suggest 50+ mph could be
attained Wed afternoon at KRST. In addition, 03.00z EFIs are
0.8+...an increase over already high values from previous runs
(0.7+). The EC is "all in" for a high wind event, perhaps too much
as it is running at the top end of the guidance. That said, ample
support from the other short/medium range guidance that 45 mph gusts
could be reached, along with periods of sustained 30 mph.
Per coordination with surrounding offices, will issue a Wind
Advisory for most of SE MN/NE IA for 18z Wed through 03z Fri. May
need to expand this eastward - and will let further model updates,
realtime observations, steer that need.
Eitherway, it will be a very windy day Wed/Wed night. In addition,
with how cold it is expected to get, any potential loss of power due
to strong winds will have added impacts on heating. Prepare
accordingly.
TEMPS: don`t expect much drop in temps from the early evening
through the overnight as the southerly fetch/low level warming
persists. Much colder air starts to return after 12z Wed, in earnest
post the second front. Highs for most will be by noon, with steady
or falling through the afternoon. 850 mb temp anomalies via the
NAEFS and ECMWF are around -2.5 for Wed night while determinist runs
of the EC/GFS 850 mb temps fall from near 0 at 12z Wed to -15 C by
12z Thu. The Grand Ensemble places a 80-100% for single digit lows
west of the Mississippi River Thu morning with less chances north of
I-94 where clouds could be more prevalent. Add in the strong
northwest winds and wind chills from -5 to -15 should be planned
for. Bundle up early morning commuters to work/school Thu.
PCPN CHANCES: a variety of lifting mechanisms will provide multiple
means to force snow showers Wed into Wed evening. A loft, the upper
level trough drops southeast across WI with mostly mid/upper level
QG convergence with it. At the sfc, while the system`s associated
low will be tracking west-east just north of Great Lakes, 2 cold
fronts will stretch south/southwest of the low. The first is progged
to slide southeast across the region in the early morning hours (12-
15). The second takes a similar track during the afternoon. Strong
cold air advection in the low levels will promote 1000:850 mb lapse
rates of +9 C/km (mostly behind the second front). Meanwhile
RAP/HRRR Bufkit soundings depict weak instability through the day.
Same soundings point to enough saturation to form clouds/showers.
While there is initially weak, disjointed low level Fgen along both
sfc fronts, CAMS modeled reflectivity depicts mostly cloud
streets/convection parallel to the mean wind (not in relation to the
frontal lift). RAP does become more organized and increases the low
level Fgen as the second front shifts southeast of the local area,
so perhaps frontal orientated convection would increase then.
Short term guidance is low with its snow chances - perhaps more
reflective of their low QPF output. The forcing, however, says
chances should be higher. Will increase above the model blend with
this in mind. Where snow falls, amounts do look relatively minor.
That said, if parallel bands do form, very localized snow could drop
for a longer period of time - and thus, higher amounts could be
realized. A lot of "ifs" here, but its a scenario to monitor.
- WEEKEND: warming up!
Change in the upper level pattern moving into the weekend with the
zonal/broad ridging taking over. Latest deterministic runs have 850
mb temps climbing from the teens below zero at 12z Thu to the single
digits above for Sat-Mon. High temps in EPS suite of ensemble
members have placed 75% of its members in the low/mid 40s for highs
Sun, just a few degrees cooler either side of that day. A very
consistent signal in the EPS. The GEFS has been cooler than the EPS,
but has shown some increase over its past few runs. Either way, all
trends point to at/above normal temps moving in for the weekend and
the start of the new work week.
The change in the pattern could allow for bits of shortwave energy
to spin near/across the region. Current outlay of the ensembles
suggest the higher pcpn probabilities would lie north/south of the
forecast area.
&&
.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1137 AM CST Tue Dec 3 2024
An IFR-MVFR stratus deck has been primarily impacting smaller
airports in central Wisconsin earlier this morning. Expect these to
lift farther northeast later this afternoon. KLSE may scatter out at
times given satellite imagery up to KMSP.
A trough from the Central to Northern Plains will continue east over
the Upper Mississippi River Valley tonight into early Wednesday
morning. Increased south winds, seen on surface observations to the
west, will therefore move over the forecast area. Initial winds
gusts 20+ kts late tonight, increasing through Wednesday morning,
peaking near 40kts late Wednesday.
Flight restrictions move from northwest to southeast through
Wednesday morning.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Wind Advisory from noon to 9 PM CST Wednesday for MNZ079-086-
087-094-095.
IA...Wind Advisory from noon to 9 PM CST Wednesday for IAZ008>010-
018-019-029.
&&
$$
DISCUSSION...Rieck
AVIATION.....JAR
NWS ARX Office Area Forecast Discussion