Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 280902

Area Forecast Discussion
National Weather Service La Crosse WI
302 AM CST Tue Feb 28 2017

.SHORT TERM...(Today through Wednesday)
Issued at 302 AM CST Tue Feb 28 2017

Water vapor imagery early this morning showed a broad longwave
trough across the western US with several upper shortwaves embedded
within southwest flow aloft downstream into the central US. A weak
broad surface low was located across the central plains. Strong 700-
850 mb warm advection was occurring ahead of the low with increasing
low-level moisture transport into the region that will eventually be
shunted to the east as the surface low ejects northeast today. Radar
echoes associated with the 850-700 mb warm advection/frontogenesis
had been slow to translate into precip at the surface with an
initially dry sub-cloud layer, but deep enough saturation has
occurred for rain as the band has continued northeastward across
southwest WI.

Expect the current band of showers to continue lifting northeast
early this morning with high res models generally indicating another
round of showers later this morning, especially across the southeast
half of the area with a push of warm advection ahead of the warm
front. The warm front will try to lift northward today, but should
remain near or just south of the area as the surface low tracks
into eastern Iowa by late today. Despite this, mild temps mainly
in the 40s and 50s are likely, warmest across far southwest WI
into northeast IA.

The surface low will deepen as it passes east of the area tonight
into Wednesday as the primary upper trough moves into the Great
Lakes region. A period of stronger upper-level forcing with the
upper wave along with mid-level frontogenesis will keep precip
chances going tonight into Wednesday. As the boundary layer cools, a
transition to snow is expected. The 00Z GFS/ECMWF indicate more
organized precip in the def zone/trowal region tonight into Wednesday
and greater saturation in the snow growth region. That should
help limit freezing drizzle/rain potential and instead give mainly
a rain to snow scenario, with only a brief window of mixed precip
potential. However, if forcing ends up weaker, could not rule out
some potential for freezing drizzle/rain. There is variability
regarding QPF over the area tonight/Wed with the GEM/GFS on the
lower side compared with the ECMWF/NAM. A few inches of snow would
be possible, right now favoring eastern areas. However, will have
to monitor to see if later model runs show higher precip amounts
like the ECMWF, which could lead to higher amounts. Of course,
placement of the band will also impact amounts across the area.

.LONG TERM...(Wednesday night through Monday)
Issued at 302 AM CST Tue Feb 28 2017

A few upper level shortwave troughs will dive southeast in northwest
flow aloft across the region late this week, while temps remain near
seasonal averages. The GFS remains a bit more robust with the
Thursday wave with stronger 700-300 mb Q-vector convergence and low-
level frontogenesis compared with the ECMWF, but any accumulations
would be light. Given the remaining uncertainty in track and
intensity, have kept mid range precip chances.

Another wave on Friday will track southeast with strong mid-level
warm advection developing as the upper flow transitions to more
zonal. Some light precip could develop within the warm advection
regime Friday or Friday night, but very low amounts.

The weekend is shaping up to be mild and dry with low amplitude
upper ridging developing over the area and 850 mb temps climbing
back into the 5-10C range. Many locations will be back into the 50s
again for the weekend, maybe even near 60 by Sunday.

A Pacific trough is poised to eject out of the Rockies early next
week bringing potentially higher precip chances to the region.
However, confidence on the evolution of this system is on the low
side at this time range.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1140 PM CST Mon Feb 27 2017

Despite an active looking radar as of 28.0530Z, rain is spotty at
best with not much precipitation reaching the ground given deeper
dry sub-cloud layer. Cloud ceilings are also still well above 3000
ft agl, but this should gradually change through the night with
saturation of the near surface layer. Forecast guidance shows
rapid development of low MVFR/IFR ceilings with possible
advection fog near daybreak on Tuesday. Delayed lower conditions a
couple hours compared to previous forecast, but otherwise
introduced IFR conditions between 28.12 to 28.14Z. If fog becomes
dense, visibility reductions below 1SM are possible, especially at
KRST, but confidence in timing/coverage is still low at this
time. Once IFR conditions develop, they will be tough to dissipate
through the rest of the period. Light south/southeast wind
overnight will gradually shift to the northwest by late in the




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