Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191136
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
535 AM CST Wed Dec 19 2018

.SHORT TERM...(Today through Thursday Night)
Issued at 322 AM CST Wed Dec 19 2018

GOES-16 water vapor imagery was showing a long wave trough moving
through the Plains. Embedded shortwave trough currently rotating
northeast through eastern IA. This was spreading mid-cloud
altocumulus into the area. At the surface, the forecast area was
sitting in pressure gradient between high pressure over the Ohio
River Valley and Plains trough. This was producing relatively mild
southerly flow over the region with temperatures as of 2 am in the
30s.

Forecast a bit complicated today through Thursday, having to deal
with saturation through the column and forcing with incoming trough.
For today, will continue to see mild/moistening southerly flow into
the area ahead of the Plains trough. RAP sounding shows little if
any stratus development with all moisture remaining in the mid-layer
and maybe a few sprinkles from time to time. The NAM on the
otherhand insists on stratus development and the possibility of some
drizzle. Confidence is low on stratus development since GOES
IR/surface obs not indicating any low cloud upstream over the central
Plains. And, since there is now a lack of snow cover across
northeast IA/far southwest WI, feel mild flow will not cause low
level boundary layer saturation. So, bottom line, will see ample mid-
level cloud with maybe a few sprinkles today. May have to watch
areas north of I-94 as temperatures there early on this morning will
be hovering right around 30-32 degrees. If sprinkles do occur could
see some patchy ice. Will keep a close eye on this. Otherwise,
temperatures warm today, topping off in the upper 30s to middle 40s.

Main long wave trough and deeper saturation approach the area
tonight into Thursday morning. Model soundings do not show much ice
in the column tonight into Thursday but do show lower levels
saturating leading to stratus development and some occasional
drizzle. Could be enough ice from time to time to mix in some light
snow, but looks mostly like drizzle. May have to watch again for
some patchy light freezing drizzle late tonight into early Thursday
morning as temperatures lower to near 32 in cold air advection.

Trough exits east of the area later Thursday afternoon with a ridge
of high pressure building in for Thursday night. This will provide
drying with highs Thursday in the 32 to 40 range and lows Thursday
night in the low/mid 20s.


.LONG TERM...(Friday through Tuesday)
Issued at 322 AM CST Wed Dec 19 2018

Friday will be mostly sunny with the ridge drifting east across the
area. A little chillier though with highs in the upper 20s to the
lower 30s.

Ridge of high pressure holds Friday night with an increase in
mid/high clouds ahead of another trough of low pressure moving
through the Dakotas/MN.

Will see a slight chance of light snow brushing portions of
southeast MN into northern WI as that Dakotas/MN trough rotates
northeast toward Ontario. Otherwise, a mostly cloudy day with highs
again in the upper 20s to middle 30s.

High pressure builds in/moves across the region Sunday through
Christmas Eve Day for dry weather and near seasonal normal
temperatures.

Perhaps some light snow for Christmas Eve/Christmas Day? Latest
GFS/ECMWF show a weak mid-level trough pushing through the Northern
Plains into the Upper Mississippi River Valley region. Appears it
will come down to enough forcing to overcome drier lower levels.
Either way, any resulting snowfall looks very light at this point.
Plan on lows Christmas Eve in the teens to lower 20s with highs
Christmas Day in the middle 20s to middle 30s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 535 AM CST Wed Dec 19 2018

Forecast challenges focus on when (if?) the low cigs finally
develop/move in and any potential pcpn/fog chances and related vsby
impacts.

Cigs: low level saturation never materialized the way the NAM
suggested it would, with only mid-level VFR cigs across the entire
region. Models still suggesting this will eventually happen as the
southerly fetch continues ahead of an approaching sfc front/upper
level shortwave off to the west. However, consensus is favoring the
drop into IFR/MVFR after later this afternoon. That said, see some
potential for this to hold off until after 06z tonight. For the
moment, will trend toward a drop into MVFR before 00z, lowering
further overnight. Once in, the low clouds should hold across the
region into Friday.

WX/vsby: low level warming will help fuel a smattering of pcpn
across the area today and tonight. High cloud bases suggest only
sprinkles today, with perhaps drizzle (freezing?) or light snow
tonight-Thu morning. Minimal if any accumulations, and not enough
confidence to include in the forecast at this time. Some vsby
restrictions west of KRST from BR this morning, and could see a
short period at KRST. Better chances for MVFR BR overnight at both
TAF sites.

Winds: holding south around 10kts or so into tonight, swinging to
the west/northwest overnight as a cold front sweeps through.


&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...DAS
LONG TERM...DAS
AVIATION...Wetenkamp


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