Area Forecast Discussion
Issued by NWS La Crosse, WI

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988
FXUS63 KARX 242349
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
549 PM CST Tue Jan 24 2017

...00Z Aviation Update...

.SHORT TERM...(This Evening through Thursday - AKA "The Storm")
Issued at 135 PM CST Tue Jan 24 2017

High Impact Weather Potential: Significant winter storm will impact
the region mainly tonight into Wednesday. Still some chance amounts
will shift a bit, with the potential for a narrow band of quite
heavy snow pending the exact storm track. No changes to going
headlines at this time.

Well...let`s just jump right into it. Significant accumulating
snowfall pretty much a given at this point, as quite the strong
closing shortwave over the Central High Plains continues to develop
while meandering eastward toward our region. Not much in the way of
any big changes from the going forecast, though the biggest concern
is how much mixing of precip will occur into parts of northeast Iowa
and southwest Wisconsin as well as just where the heaviest banding
will set up and how much snow could potentially fall beneath it
(hint: it could be quite a bit).

Snowfall overall has trended just a tad slower the past 12 hours,
just arriving over far southwest counties late this afternoon and
likely not until after midnight north of I-94. However, strong mid
level difluence coupled with a deep layer warm advection and
frontogenetic signal and moisture transport of 0.5+ inch PWAT values
will work to bring a band of heavier precip through the area this
evening and overnight. Signals the past 24 hours have consistently
pointed to areas across northeast Iowa and far southern Minnesota
(centered near and west of I-35) seeing the heaviest snowfall totals
overnight, with rates of 1-2 inches per hour possible. That lead
snow band looks to wane with northward/eastward extent as the better
forcing gradually wanes, though do have some concerns that NAM/GFS
may be weakening the initial band a little too fast (HRRR much more
robust).

Strongly sloped frontogenetic signal overtopped by negative EPV
suggests banding elements are likely, and my biggest concern is we
end up with higher amounts closer to the I-35 corridor. In fact,
experimental HRRR/RAP numbers are a little disconcerting with QPF
amounts pushing 1" just this evening/overnight alone. Unfortunately,
confidence in where the best banding will set up is still somewhat
low (as it always is in these cases), but suffice it to say amounts
may need to be "upped" for some areas once the snow bands start to
show their hand. Overall totals of 3-6" tonight look in line mainly
along/south of I-90, with some thunder also possible, northern Iowa
and far southwest Wisconsin where negative  theta-e lapse rates
makes an appearance above a stable boundary layer tonight. Farther
north, amounts overnight look to be much lower north of I-94 (around
1-2"), owing to weaker forcing and late precip arrival. Meanwhile,
farther south closer to Highway 20 and into far southwest Wisconsin,
mixing concerns are quite large with a warm nose edging in aloft but
also the potential for upright convective elements at times likely
messing with precip types as stronger omega briefly overcomes any
warming. Looks like more of a snow/sleet/rain mix down that way,
with amounts of maybe 1-3 inches (locally more possible).

Forcing then transitions to more of a TROWAL setup into Wednesday as
the surface-700mb circulation centers pass just south of the area.
Don`t envision extremely heavy snowfall rates in this setup, but
several more inches are likely through the day, pending
temperatures. Similar to tonight, do have some concerns that valley
locations could hold near or just above freezing, mitigating
accumulation potential save for during periods of heavier rates. One
thing of much higher confidence is that precipitation will hang on
longer through the day, likely into the evening over portions of
western/central Wisconsin. Temperatures should also begin to fall
more below freezing during this time as well, such that any slushy
snow on roads will have a greater risk to freeze. Additionally,
chances for some snow showers/flurries will continue into Thursday
as another weaker upper wave drops toward the region and broad
cyclonic flow prevails aloft.

All in all, looking like a very messy storm for all areas,
especially given the wet nature and overall lower ratios of expected
snow. There is some risk we may struggle to technically reach
warning criteria over our eastern counties toward central Wisconsin,
though impacts alone justify such a headline.

.LONG TERM...(Thursday Night through Tuesday)
Issued at 135 PM CST Tue Jan 24 2017

A longwave trough will become established across eastern North
America late this week with ridging to the west. A series of upper
level short wave troughs will translate southeastward embedded in
northwesterly flow aloft over the Midwest. Model guidance is in good
agreement showing this evolution, although there are some timing
differences with individual upper shortwaves through the weekend.
Given this pattern, overall it looks like skies will remain on the
cloudier side through the weekend with model soundings suggesting
plenty of low stratus around. Light snow showers/flurries are
possible at times over the weekend with any of these systems as weak
forcing from the upper shortwave troughs combine with deeper
moisture/steep low level lapse rates. Temps will turn cooler through
the weekend back to more seasonable late January levels.

Both the 24.12Z GFS/ECMWF bring brief shortwave ridging/low-level
warm advection back across the area Monday with temps rebounding
slightly above average ahead of a stronger surface low tracking
across the northern Great Lakes.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 549 PM CST Tue Jan 24 2017

Snow will overspread the TAF sites this evening with conditions
quickly deteriorating to IFR by late evening. Periods of LIFR
conditions are also expected in the heaviest snowfall when
visibilities could fall to around 1/4 mile at times. Look for the
snow to continue through at least midday on Wednesday before
gradually tapering off. IFR conditions will likely persist through
the afternoon hours on Wednesday.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning until 6 PM CST Wednesday for WIZ032>034-
     041>044-053>055.

     Winter Weather Advisory until 6 PM CST Wednesday for WIZ061.

     Winter Weather Advisory from 9 PM this evening to 6 PM CST
     Wednesday for WIZ017-029.

MN...Winter Storm Warning until 6 PM CST Wednesday for MNZ094.

     Winter Storm Warning until 6 PM CST Wednesday for MNZ079-086>088-
     095-096.

IA...Winter Storm Warning until 6 PM CST Wednesday for IAZ008>011-018-
     019.

     Winter Weather Advisory until 6 PM CST Wednesday for IAZ029-030.

&&

$$

SHORT TERM...Lawrence
LONG TERM...Lawrence
AVIATION...Wetenkamp



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