NWS La Crosse, WI wrote its last area forecast discussion before the first event at Dec 14, 10:25 PM CST, 2 hours 5 minutes earlier.
Area forecast discussion, Dec 14, 10:25 PM CST, 2 hours 5 minutes before
Area Forecast Discussion
National Weather Service La Crosse WI
1025 PM CST Tue Dec 14 2021
.SHORT TERM...(This evening through Wednesday night)
Issued at 226 PM CST Tue Dec 14 2021
As has been well-advertised over the past several days, a period
of extremely anomalous mid-December weather is shaping up for
tonight through Wednesday night with potential fog, record high
temps, thunderstorms, and very strong, potentially damaging wind
gusts.
Broad W/SW flow aloft and increasing high level moisture was
occurring across the region ahead of a strong shortwave trough
just off the CA coast. Warm advection is already ongoing across
the area with stronger moisture transport/isentropic likely to
result in low stratus/fog development tonight along with gradually
climbing temps overnight. With the cold snowpack in place,
especially over northern areas, wouldn't be surprised to see some
dense fog development as moisture quickly overspreads the region,
but will continue to assess this potential through the evening.
Model soundings do saturate up through 1 km in areas, particularly
east, so some drizzle could develop as well.
The aforementioned compact shortwave trough will quickly eject
from the Fours Corners region early Wednesday to near Lake
Superior lake Wednesday night taking on a negative tilt. The
attendant surface low will quickly deepen to below 980 mb by late
evening Wednesday over the MN arrowhead. As has been mentioned in
prior discussions, the temperature, moisture, and wind fields with
this system are all extremely anomalous for this time of year.
Strong moisture transport/warm advection will continue through the
day on Wednesday propelling temps in the 50s and 60s, near or
above daily records in many locales. The strongest moisture surge
will occur just ahead of the shortwave trough later in the day
with dew points climbing into the 50s and 60s, with 70 not out of
the question in the southern areas.
Low-level wind fields rapidly increase through the day with 50+
kt winds around 1 km AGL. However, mixing will likely be
inefficient through the day with the stout warm nose aloft and low
clouds in the warm sector. That said, wind gusts could reach 30
to 40+ mph, particularly in the afternoon.
As the surface low approaches by early evening, CAMs are in good
agreement that a strongly forced line of convection will very
quickly sweep east across the area. This will be a high shear-low
CAPE type of event with very high 0-1 km bulk shear around 40-50
kt and SBCAPE up to around 500 J/kg. Given the very fast storm
motions and reservoir of strong winds aloft, any convection could
produce severe wind gusts. HREF max convective wind speeds do show
this potential well. The weak buoyancy certainly limits
confidence, but given the degree of low-level shear, rotating
updrafts and a conditional risk for a tornado. The storms will be
moving very quickly, with the window of concern mainly 6 to 10 pm,
and perhaps a diminishing trend with eastward extent through
evening.
Behind the front, strong cold advection/drying with an impressive
pressure rise/fall couplet progressing northeast during the
evening/overnight. Consistent with the strong EPS signal and short
term guidance, non-convective wind gusts look to be maximized
behind the front through the early morning hours before gradually
diminishing by sunrise. With 60+ kt winds at 1 km, steepening
lapse rates in the cold advective regime should help tap some of
these stronger winds, with potential for 55 to 60+ mph wind gusts
capable of wind damage. Given the consistent signal for the
anomalous winds, will upgrade to a high wind warning.
.LONG TERM...(Thursday through Tuesday)
Issued at 226 PM CST Tue Dec 14 2021
In the wake of the strong mid-week system, an area of high
pressure is expected to build in over the region for Thursday. A
short wave trough looks to swing across the area Thursday but the
presence of the area of high pressure along with a lack of deep
moisture looks to keep the area free of precipitation. This high
looks to retreat back into Canada Thursday ahead of the next
system. A northern stream short wave trough should dig southeast
over the Upper Midwest late Friday night into Saturday, but there
is not much model consensus right now on how strong this wave will
be or the track it will take. This system looks to have the
potential to produce a little bit of light snow for parts of the
area Friday and Friday night.
&&
.AVIATION...(For the 00Z TAFS through 00Z Wednesday night)
Issued at 1025 PM CST Tue Dec 14 2021
Strong storm system to bring a variety of aviation impacts to the
region Wed-Wed night...wind, low cigs, potential br/fg, and a short
window for a storm risk.
Cigs: lower cigs have moved in, and should hold through much of Wed.
There are a couple windows where cigs could pop back up to VFR for a
few hours - mid/late afternoon prior to cold front and then mid
evening post the front, in the storm system's dry slot. Have tried
to add some detail where confidence was higher. Cigs should drop
back into mvfr/ifr overnight Wed.
WX/vsby: also meso models favor fog developing overnight, mostly in
areas where there is (or recently was) a snowpack. Not much evidence
for fog yet, but will continue some mention - mostly MVFR - for now.
Toward 12z Wed could have enough low level warming through the cloud
layer to produce patchy/areas of drizzle. Won't add to forecast yet,
but if it manifests, expect some vsby restrictions. Then, a line of
storms is looking likely, roughly impacting KRST by 01z Thu and KLSE
by 02z. VERY fast moving and will enhance the already strong winds
(storm gusts of 70mph possible). The storms will move in and out
very quickly, prob not lasting 1/2 hour.
Winds: winds on the increase through the day Wed, becoming very,
very strong Wed night. Southeast becoming southerly Thu, then
swinging west overnight Thu with passage of a cold front. Strongest
winds will be Thu night sustained 25 to 35 kts with gusts 45 to
55. Expect rapid improvement by/after 12z Thu as the storm exits.
In addition, LLWS will be a concern from time to time and will try
to highlight time periods/locations when the threat is greatest.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...High Wind Warning from 6 PM Wednesday to 6 AM CST Thursday for
WIZ017-029-032>034-041>044-053>055-061.
MN...High Wind Warning from 3 PM Wednesday to 3 AM CST Thursday for
MNZ079-086>088-094>096.
IA...High Wind Warning from 3 PM Wednesday to 3 AM CST Thursday for
IAZ008>011-018-019-029-030.
&&
SHORT TERM...JM
LONG TERM...04
AVIATION...Rieck