Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 142341
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
641 PM CDT Mon Aug 14 2017

.UPDATE...
Issued at 641 PM CDT Mon Aug 14 2017

Interesting setup right now across far northeast IA into southeast
MN and far southwest WI with the development of showers taking on
mini-supercellular characteristics. Per RAP trends, 0-1km shear is
maximized in that area, just ahead of an approaching cold front.
Low level lapse rates aren`t terribly great, but have already have
one photo of a likely funnel with possible very brief touchdown
earlier in Houston County. Per guidance trends, this setup will
only stick around another hour or so before we lose heating and
shear weakens, but interesting nonetheless. SPS issued earlier for
possible funnels, but already seeing some signs per radar trends
that current "storms" (with a notable lack of lightning) are
starting to trend downward.

&&

.SHORT TERM...(This evening through Wednesday night)
Issued at 213 PM CDT Mon Aug 14 2017

Local and regional radars still showing a few showers over
portions of central Wisconsin in the cyclonic flow of the
departing short wave trough over the Upper Great Lakes. These
showers could possibly hold onto into the early evening over the
far eastern portions of the area, otherwise the rest of tonight
and into Tuesday look to be dry as high pressure over Minnesota
and western Ontario briefly builds in over the area and as upper
level ridging moves across the region.

The ridging aloft and at the surface will get pushed east of the
area starting Tuesday night. Water vapor satellite imagery
currently shows the next system was over Oregon and northern
California and will quickly move east toward the region. There
looks to be several short wave troughs embedded in the mean trough
with this system. The first of these short wave troughs will move
across the region Wednesday with another for Wednesday night. As
these waves move across the area they look to produce periods of
weak to moderate pv advection in the 500-300 mb layer. The low
level moisture transport looks respectable with these waves, but
never looks to be overly strong. The leading and convergent edge
of this moisture transport looks to start spreading in very late
Tuesday night or early into Wednesday morning and then starts to
diminish Wednesday evening as the first short wave troughs moves
past the region. The warm air advection ahead of the short wave
troughs looks to produce a period of 1 to 3 ubar/s (at times a
little higher) of up glide on the 305K isentropic surface.

The surface front currently across northern Iowa and southern
Wisconsin will continued to get pushed south as the surface high
builds in tonight. This front will start to return back to the
area Wednesday and how far north is gets will determine how much
instability and the coverage of storms that will occur. It
currently appears that the general consensus from the 14.12Z
models is that the front may get back into the southern portions
of the forecast area Wednesday afternoon. Recent runs of the GFS
brought in the most amount of CAPE, but the 14.12Z run now only
shows 500-1000 J/Kg of ML CAPE south of about Interstate 90. The
deep shear still looks to be lacking with maybe around 30 knots of
shear in the 0-3 km layer. This could be enough for a strong storm
or two but would not anticipate much severe weather. A larger
concern is with the possiblity of some heavy rain. The models
remain consistent in showing the warm cloud depths in the 3.5 to 4
km range with precipitable waters approaching 2 inches. This
should allow for some locally heavy rain but with the recent dry
stretch, the area should be able to take some rain and not
concerned about any flooding issues at this time.

.LONG TERM...(Thursday through Monday)
Issued at 213 PM CDT Mon Aug 14 2017

The last of the short wave trough with this system will move
across the area Thursday. By this time, the best low level support
will be well past the area, being tied to the lead short wave
troughs, so only expecting some lingering showers and storms to be
around. Northwest flow aloft looks to set up behind this system
which should allow another short wave trough to move across the
Upper Midwest for late in the week into the start of the weekend.
Differences between the GFS and 14.12Z ECMWF on how strong this
wave may be and how much rainfall that could occur. Either way,
enough to include a small rain chance for Friday and Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 641 PM CDT Mon Aug 14 2017

Scattered showers continue along a cold front slowly drifting east
through the region, but just about to clear LSE by 00Z. Behind
that front, confidence is very low regarding redevelopment of low
clouds, as earlier expectations for widespread stratus have
dwindled somewhat as clouds upstream have really mixed out. Gut
feeling based on the setup is we may be dealing more with a fog
setup for RST as skies clear out quite a bit overnight, with a
period of VLIFR in dense fog now expected, centered either side
of sunrise.

LSE is much more tricky, as current low clouds over central
Wisconsin could be shoved back southwest as the low level flow
turns northeasterly overnight. Should that occur, a period of MVFR
or even briefly IFR ceilings is likely. However, should clouds
fail to materialize, as suggested by recent runs of the RAP model,
a fog threat could develop in LSE. For now, have split the
difference with some light fog and lower stratus centered either
side of sunrise, but will really need to watch trends through the
night.

Fog and/or stratus should gradually lift through the morning
hours, with scattered to perhaps broken ceilings into the
afternoon with cumulus redevelopment as some thicker cirrus also
arrives. Winds will remain under 10 knots through the period,
shifting from the north to the east with time.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...Lawrence



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