Area Forecast Discussion
Issued by NWS La Crosse, WI

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949
FXUS63 KARX 140230
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
830 PM CST Fri Jan 13 2017

.UPDATE...
Issued at 830 PM CST Fri Jan 13 2017

Fairly quiet evening at the ol` WFO here, but clouds are being a
bit of a pain. Per satellite trends, stratus in the 4-7kft layer
extends well to the west still, well into eastern South Dakota,
ever-so-slowly inching east but taking its sweet time. Given the
setup of weakish flow with a more southerly component through the
early morning hours, just don`t think clouds will be in all that
much of a hurry to depart, with us locally having to wait for the
passage of an 850-750mb trough axis roughly 10-14Z to fully scour
the lower clouds (some cirrus aloft will remain over southern
areas). Not a big deal in the grand scheme of things, but worth an
update, with temps overnight likely to move very little under
those clouds despite ridging at the surface. Will need to nudge
overnight lows up several degrees across the north.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 315 PM CST Fri Jan 13 2017

A weak upper shortwave trough embedded in westerly flow aloft has
been dampening as it has progressed eastward into the region
within confluent flow this afternoon. Some light snow has occurred
farther west into north central Iowa/south central Minnesota in
association with 700 mb frontogenesis. However, with the wave
weakening and dry air below 800 mb that must be overcome,
confidence in snow is relatively low for our area, but have kept
some low chances south of I-90 through early evening given the
potential. Behind this system, a strong 1040 mb surface high will
impact the region through Saturday night with dry weather and
slowly moderating temps.

.LONG TERM...(Sunday through Friday)
Issued at 315 PM CST Fri Jan 13 2017

The main concern through this period continues to be the wintry
mix of precip across the region Sunday night into Tuesday and
associated impacts.

The upshot at this point is that another messy system is likely to
impact the region with a variety of precip types still expected
Sunday night through Tuesday and potential impacts to travel.
Confidence is high in wintry precip occurring, but details
regarding types and duration of these ptypes (which would affect
impacts) remain the main forecast challenge.

Getting into the details...dry surface high pressure remains over
the region through Sunday. This will keep a dry low-level air mass
in place into Sunday evening. An upper low currently evident off the
southern CA coast will be ejecting into the southern plains Sun
evening. Low-level moisture will surge northward Sun night and
Monday with increasing isentropic ascent/warm advection leading
to a gradual increase in precip from the south later Sun Night
into Monday. The GFS indicates pwats rising up to near 0.70
inches over southern parts of the region on Monday, well above
climo, indicative of the strong moisture transport into the area.

However, looking at the big picture, 13.12Z global models continue
to differ with the interaction between the southern stream wave
ejecting northeast and a northern stream shortwave diving southeast.
These differences are evident in still considerable spread among
GEFS ensemble members both with precip amounts and thermal profiles.
The operational 13.12Z GFS is colder and farther west compared
with what the ECMWF has shown, which would impact ptypes.
Regarding precip types, it will be a question of how strong and
how far north an elevated warm nose reaches into the area and then
how fast surface temps respond. In these situations a few degrees
at the surface and/or aloft can make a big difference in what ends
up reaching the ground.

Given the remaining differences, continued to stick with a model
blend for thermal profiles, which yields a wintry mix of freezing
rain/sleet/snow, with a better chance of precip changing to plain
rain on Monday/Monday evening over portions of the region,
especially farther south. With surface temps likely to be slow to
respond and cold ground temps, even in areas where air temps climb
near or above freezing, there could still be some icy roads well
into Monday/Monday night. The precip will likely continue into
Monday night and Tuesday as the deformation zone swings through before
the system pulls away, but we will have to watch for any dry
slotting, depending on the track of the surface low. There`s a
good likelihood winter headlines will be needed eventually, but
with much of the event not expected until Monday, have opted not
to issue at this point.

Unlike the system earlier this week, there will not be much cold air
behind this system.  In fact, a more zonal to eventually southwest
upper flow will keep the Arctic air bottled up well to the north.
Highs in the 40s will be possible into late next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 528 PM CST Fri Jan 13 2017

High pressure remains draped across the region, with a
continuation of VFR conditions expected through Saturday evening.
A lingering batch of 4-7kft clouds will gradually erode through
midnight, leaving some cirrus at times into Saturday, though with
even those diminishing with time. Expect winds to be rather
variable and under 8 knots the next 24 hours.

&&

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

$$

UPDATE...Lawrence
SHORT TERM...JM
LONG TERM...JM
AVIATION...Lawrence



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