Area Forecast Discussion
Issued by NWS La Crosse, WI

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

011
FXUS63 KARX 132115
AFDARX

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

.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 18Z TAFS through 18Z Saturday afternoon)
Issued at 1146 AM CST Fri Jan 13 2017

The band of light snow across northwest Iowa into southwest
Minnesota is expected to have a hard time surviving as it move
east this afternoon. High pressure was still centered over western
Wisconsin late this morning and was keeping dry low level air in
place. In addition, the short wave trough was moving into
confluent flow aloft, which will cause it to weaken and diminish
the forcing to keep the snow going. If any snow does survive, it
will likely go south of both airports and will continue with a dry
forecast. The VFR ceilings will remain into this evening and the
early overnight before moving east allowing skies to scatter out.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...04



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.