Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 162311
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
511 PM CST Fri Nov 16 2018

.SHORT TERM...(This evening through Saturday)
Issued at 245 PM CST Fri Nov 16 2018

Active stretch of weather into the overnight hours, with the main
forecast concern obviously being placement of the heaviest snowfall
potential tonight. Early afternoon analysis shows the main player in
action, with an elongated shortwave skirting east through Montana
and a notable low level frontogenetic response(and mid level
frontogenesis/frontolysis coupled response) responsible for a band
of snow sneaking east through the Dakotas. That upper wave is on
track to sag south in our direction tonight, taking its time while
awaiting another piece of energy digging south from central Canada
to kick it farther south into Saturday.

That setup may have some ramifications in terms of potential
pivoting of better banded snow somewhere across southeast Minnesota
or northeast Iowa later tonight, but in general the expectation for
tonight has not changed all that much. Biggest change is a southward
shift in the best frontogenetic response and associated heavier
snowfall potential, such that locations near and north of Highway 29
may not see much at all from this event as they`re located on the
stronger downward branch of the FGEN circulation. Farther to the
southwest, we should pick up a gradient of 2-4 inches of
accumulation, and given the shift, have expanded the Winter Weather
Advisory to cover all of northeast Iowa in addition to the
previously advised counties.

So...what could go wrong? Do have concerns that a narrow band of
heavier snow may pivot across southeast MN/northeast IA as the lead
wave slows down and the frontogenetic response re-orients from a
northwest-southeast orientation to one of a more east-west
direction. Also concerned that where that FGEN is focused we may
over-achieve on ratios with cross sections showing deeper lift
"bulls-eyed" through the -12C to -18C dendritic growth layer (though
it is a little shallow for solid dendrite formation). Interesting to
note the NBM (national blend of models) depicting a stripe of 5+
inches over southwest parts of the area, so something to watch
closely through the evening. Regardless, there will be travel
headaches given accumulation on the road, and the current Advisory
should suffice where it is in place, even with slightly lower
amounts on the northern edge. Snow tapers quickly toward or just
after sunrise, with gradual clearing and cooler air working into the
area for the afternoon. Thankfully not much in the way of wind to
deal with this go around. Winter weather fans - enjoy!

.LONG TERM...(Saturday night through Friday)
Issued at 245 PM CST Fri Nov 16 2018

The large scale should break down from a northwest flow regime
/colder than normal/ into a more zonal-to-southwest flow by the
middle of next week /warmer than normal/. Northwest flow regimes are
always on the lower confidence side for system strength and track
but over overall it seems like a fairly quiet period with no impact
weather. Thus, have kept a model consensus blend as the backbone of
the forecast.

Have made some changes to the southern areas, south of I-90,
dropping the Sunday morning lows into the single digits to near 10F
with snow cover on the ground.

There is consistency on a trough moving through on Monday but
differences exist on the track. The 16.12Z ECMWF is far northeast
with a dry Monday. So, continued the blend in this time frame and a
chance of snow.

On Wednesday, ridge building and warm advection begin with a warm up
back to normal or just above normal temperatures. No impacts seen to
Thanksgiving travel seen at this time. There does continue to be
some signal for precipitation in the ECMWF for Black Friday but at
this time it looks to be warm enough for rain in southwest flow.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 511 PM CST Fri Nov 16 2018

An area of snow stretching from the Dakotas into southern
Minnesota late this afternoon will reach KRST and KLSE by early
to mid evening, continuing through the early morning hours. Expect
IFR to LIFR conditions in heavier snow bands. Improving
conditions are expected on Saturday as high pressure begins to
build across the area, but confidence in timing cloud trends is
somewhat low at this point. Light winds this evening will become
northerly around 10 kts overnight into Saturday.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 6 AM CST Saturday for WIZ032>034-
     041>044-053>055-061.

MN...Winter Weather Advisory until 6 AM CST Saturday for MNZ079-
     086>088-094>096.

IA...Winter Weather Advisory until 6 AM CST Saturday for IAZ008>011-
     018-019-029-030.

&&

$$

SHORT TERM...Lawrence
LONG TERM...Baumgardt
AVIATION...JM


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