Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 290832

Area Forecast Discussion
National Weather Service La Crosse WI
332 AM CDT Wed Mar 29 2017

.SHORT TERM...(Today through Thursday night)
Issued at 332 AM CDT Wed Mar 29 2017

Water vapor imagery early this morning shows a pronounced upper low
over New Mexico with with a plume of mid/upper moisture extending
to the north and east with increasing large scale ascent ahead of
the wave. This system will be our main weather maker over the
next 36-48 hours as it gradually lifts northeastward. 00Z MPX/GRB
soundings indicated quite a bit of dry air in place, but continued
mid/upper forcing will help to saturate the lower levels through
the day. By late morning showers should be increasing across
northeast Iowa, possibly encountering some resistance with
stubborn low-level dry air remaining. Rain will continue to
increase from the southwest through the afternoon, tied to
stronger 850- 700 mb warm advection. This warm advection along
with mid-level frontogenesis will help focus the precip through
the overnight hours into Thursday morning. The heaviest precip
totals are expected to be across far southwest WI and northeast IA
where an inch or a bit higher of rain is possible, with amounts
tapering to around a tenth of an inch over north central WI. The
29.00Z ECMWF remains more progressive with most of the precipitation
exiting to the east Thursday afternoon. However, the NAM/GFS hold
onto deformation zone precip longer into Thursday/Thursday night
with more of a pronounced TROWAL feature evident on 700 mb theta-e
fields. The NAM would even linger some precip into Friday in
western Wisconsin. Thus, will keep high precip chances going
through Thursday, especially over the southeast half of the area
decreasing to a mainly dry forecast on Friday.

There are still a few potential issues with precip type late this
evening through early Thursday, especially across central/north-
central WI. Surface temps should fall into the low to mid 30s
overnight tonight. NAM soundings in central WI suggest a period of
wet snow is possible especially north and east of I-94 overnight,
highly dependent on lower tropospheric temps and possibly precip
rate as well. However, NAM soundings in this area indicate some
potential for loss of ice overnight, which would lead to a liquid
scenario with surface temps critical. GFS soundings, on the other
hand, keep cloud ice through the night and show a rain/snow
scenario. With a rather deep nearly isothermal layer near 0C,
definitely a tricky forecast. All in all, it is possible there
could be some minor snow accumulations tonight mainly north/east
of I-94, but with warm ground temps and marginal air temps, it
will likely take high snow rates to create any travel issues.

.LONG TERM...(Friday through Tuesday)
Issued at 332 AM CDT Wed Mar 29 2017

Weak surface high pressure should bring a return to dry weather
Friday into Saturday, although Friday looks to be on the cloudy
side. Another southern stream upper low is expected to move from
the southern Rockies on Saturday eastward through early next week.
It is possible some light precipitation could spread north into
the area Saturday night into Sunday as a weak upper shortwave
ejects ahead of the main upper low, but at this time the higher
rain chances with this system look to be to the south. The
GFS/ECMWF then show a digging northern stream wave early next week
into the western CONUS, but there is poor model agreement and run
to run consistency in this pattern with the timing and
amplification of waves, so there is very low forecast confidence
for early next week. At this point, average to slightly above
average temps are favored.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1127 PM CDT Tue Mar 28 2017

The 29.00z NAM (like many of the meso models) is a bit faster at
bringing the rain into the TAF sites. It has the rain arriving my
early afternoon. Meanwhile, the GFS is just a couple hours later.
Due to this, lowered the ceilings and visibilities a bit faster
into the MVFR range. This should occur at KRST around 29.19z and
at KLSE around 29.21z.

One thing interesting in the latest HRRR run is that it suggests
that the rain may end by 30.03z as the best 850 mb moisture
transport shifts east of the forecast area.  If this occurs, the
visibilities may improve to VFR by the end of the TAF period.




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