Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 161709
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1209 PM CDT Mon Apr 16 2018

.SHORT TERM...(Today and tonight)
Issued at 322 AM CDT Mon Apr 16 2018

Main short-term fcst concerns are cloud and temperature trends
today/tonight.

06z data analysis had a broad area of low pressure over the eastern
CONUS with a ridge of high pressure in the plains. The Upper Midwest
remained under the circulation of this low and the mid level low
centered near Chicago. Moisture wrapping around these circulations
continued to produce some -SN across the east half of the region.
Closer to the plains ridging, clouds were breaking/clearing over
western MN/IA. Under the cold, cyclonic flow, early morning temps
across the region were mostly in the 20s, even under clouds and NW
winds of 10-20 mph.

Model runs of 16/00z initialized well. Solutions similar for as the
mid level low gradually exits east of the region. Hgts rise,
especially tonight, but region is left under cyclonic NW flow aloft
thru 12z Tue. Short-term fcst confidence is generally good this
cycle.

For the short-term, deep, cold cyclonic flow remains over the area
today, with at least the east side of the fcst area east of the mid
level jet axis. Some small -SN/flurry mentions mainly near/NE of I-
94 are reasonable as the circulation continues to slowly pull away.
Plenty of 925-800mb moisture to remain over the area today, stuck
in/under an inversion near 850mb. This along with some weak
instability in the 900-850mb layer. May see a few holes in the
clouds today, but these would quickly fill in with diurnal strato-
cu. Diurnal warming/mixing today to result in some brisk/gusty NW
winds later this morning/afternoon. Mostly cloudy to cloudy skies,
NW winds, snowcover, the blend of guidance highs mostly 30-35 looks
good.

Deeper layered subsidence and neutral to anti-cyclonic low level
flow moves east over the area tonight as hgts rise. For now, will
remain optimistic for clouds to decrease/clear across the fcst area
tonight, but with moisture stuck under an inversion, clouds could
hang on longer than anticipated. Some NW gradient winds to
persist across the area tonight to keep the BL at least partly
stirred and low from dropping too far. Will have to watch this
with the fresh snow cover though. If winds decouple and clouds
clear, lows in the normally colder low laying areas along/NE of
I-94 could dip into the single digits above zero. Given a bit of
uncertainty with clouds clearing, especially over the NE side of
the fcst area, stayed near the blend of the guidance lows tonight.

.LONG TERM...(Tuesday through Sunday)
Issued at 322 AM CDT Mon Apr 16 2018

For Tuesday thru Wednesday night: main fcst concern this period are
the next impending wintry system centered on Wed and potential
snowfall.

Main focus of this cycle is on this period. 16.00z models in rather
good agreement Tue as shortwave ridging builds across the region and
the next strong shortwave of interest crosses the central/southern
rockies. Solutions then diverge a bit on the timing/track of this
shortwave/trough across the central plains Tue night, across the
mid/upper MS valley Wed and quickly into the OH valley Wed night.
Fcst confidence good Tue, then average to good Tue night-Wed night.

Sfc-mid level ridging builds over the area Tue, for what should be a
mostly sunny day. However 925-850mb flow remains NE, with weak cold
advection and the area stuck under the inversion. Will have to watch
for the potential of some lower level moisture/clouds to advect SW
under the inversion Tue, which could limit our mostly sunny day.
Based on 925mb temps, Tue highs in the upper 30s-mid 40s look quite
reasonable.

Needless to say, Model detail differences Tue night-Wed night
raising havoc with fcst of a very dynamic system to impact the
area centered on Wed. GFS faster, Nam slower, EC/Gem in the middle
on timing. Then there is some north-south placement differences
of the heaviest precip axis. Trough progged to come in negative
tilt later Tue night/Wed, with impressive lift/divergence aloft
signal ahead of the shortwave and trough developing a closed 500mb
low. Rather strong lower level thermo-dynamic forcing signal as
well ahead of the sfc-850mb low to lift toward the WI/IL border as
the mid level circulation comes across. Model soundings showing
an 850-700mb warm layer aloft over roughly the south 2/3 of the
fcst area at 12z Wed, with saturation issues in the column.
Initial onset of precip late Tue night into Wed morning looking
to be a wintry mix. Per model sounding/X-sections, column quickly
saturates with arrival of the deeper lift, with quick erosion of
the warm aloft with dynamic lift/falling hgts aloft with the mid
level trough. Precip Wed morning should quickly transition to
snow. BL temps may initially be warm as well, but frozen precip
load load from aloft should quickly cool BL temps into the 32-34F
range, allowing precip to fall as snow during the heaviest
periods. Snow rates at times could be 1-2"/hr with the strongest/
deepest lift. Models struggling with snow amounts given warmer BL
temps. Models producing upwards of 1/2 to 3/4 inch of qpf in a
band with this system, but vary on where in the fcst area.
Consensus at this time seems to be the west half. SHould this qpf
fall mainly as snow, there is potential for 6+ inches somewhere in
the fcst area. Best consensus for this now is the west end of the
fcst area. Will issue a winter storm watch for 06z Wed to 06z Thu
for areas along/west of a KRST/KFKA- New Hampton IA line with the
greatest potential of 6+ inches. Did trend Wed highs downward
across much of the area with thick clouds and snowfall expected to
limit warming or drop them back to that 32-34F range for at least
part of the day.

For Thursday thru Sunday (days 4 to 7): a quieter period with main
fcst concern being slowly moderating temperatures.

Medium range model runs of 16.00z in good agreement for ridging
aloft to build across the region for Thu/Fri as the next mid level
low moves into the central/southern Rockies. reasonable agreement
for a NW flow wave to drop across the region Sat with more ridging
aloft for Sun. Day 4-7 fcst confidence is average to good this cycle.

Model consensus has this period dominated by high pressure at the
sfc, even as the NW flow wave passes Sat. Trend of dry for Thu-Sun
looks good at this time. With what should be more sunshine than
we`ve seen of late (under the sfc high and general ridging aloft),
temps should come up out of the April deep freeze. There will be the
snow cover to temper the warming and the sfc-925mb ridge axis over
the region will limit warm advection. Lower level airmass will have
to modify in place Thu thru Sun. Consensus highs in the 40s Thu
slowly warming to mainly the 50s by Sun look good at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1208 PM CDT Mon Apr 16 2018

MVFR stratocumulus clouds expected to erode/scatter out this
evening, becoming VFR through the remainder of the period as high
pressure builds in. Look for northwest winds to diminish this
evening as well.

&&

.HYDROLOGY...
Issued at 322 AM CDT Mon Apr 16 2018

Runoff into the rivers has really slowed down with the colder
temperatures. However, still dealing with some minor flooding on the
Kickapoo River; see the latest FLSARX (WGUS83) for details.
Potential exits for some rises along the lower Trempealeau River to
near or above flood stage during the upcoming weekend and early next
week.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Winter Storm Watch from late Tuesday night through late
     Wednesday night for MNZ086-087-094-095.

IA...Winter Storm Watch from late Tuesday night through late
     Wednesday night for IAZ008-009-018-019.

&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...DAS
HYDROLOGY....RRS



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