Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 171636

Area Forecast Discussion
National Weather Service La Crosse WI
1136 AM CDT Mon Jul 17 2017

Issued at 1133 AM CDT Mon Jul 17 2017

Although there are a few clouds around today, decided to bump
high temperatures about 2-3F this afternoon. This was mainly based
on a more southerly component to the wind and 925 MB warming of
about 2C from Sunday and the warming seen this morning. Warmest
air is moving in west of the Miss River in the lowest levels.

Tuesday forecast looks to be on track for increasing thunderstorm
chances in the afternoon as the front approaches from the west.
Consensus of short term models that forecast into later Tuesday
have 70-90% precipitation chances around 7 pm areawide.

.SHORT TERM...(Today through Wednesday)
Issued at 303 AM CDT Mon Jul 17 2017

Early this morning, water vapor satellite indicated the center of
the upper level ridge was over the Missouri River Valley with the
ridge axis northeast to Hudsons Bay. The models continue to
suggest this ridge will become more west to east oriented with
time allowing for a nearly zonal flow to set up across the Upper
Midwest. This zonal flow looks to be rather dirty with numerous
weak short wave trough embedded in it that either come in off the
Pacific Ocean or rotate around the large upper level low over
central and northern Canada. With this pattern, the models
continue to show differences in strength, timing and placement of
the short wave troughs leading to a low confidence forecast. As
the upper level flow becomes zonal, a cold front will get pushed
rapidly east across southern Canada while the trailing end of the
front will drop slowly southeast toward the area Monday night and
then into the area Tuesday. This front then could either stall out
over the area per the 17.00Z GFS or get pushed to the south like
the 17.00Z NAM is showing. Again, these differences do not lend
much confidence to the expected outcomes.

As the front moves into the area Tuesday, the NAM continues to be
the most robust with the short wave trough coming through the
flow. It has the wave timed with the front and brings it into the
region during the max heating of the day. The GFS also continues
its trend of being weaker with the short wave trough as well as
having a faster timing. It then looks to develop a MCV Tuesday
afternoon and drops that south of the area Tuesday evening along
with an associated convective complex. The 17.00Z ECMWF looks to
be somewhat of a compromise between the two models showing some
activity along the front similar in placement to the NAM, but not
nearly as high with the QPF, and then has some activity Tuesday
evening farther south like the GFS, but again much lighter on the
QPF. Overall, they all show some activity along the front as it
comes in Tuesday afternoon, especially across the northwest and
then taking that across the northern half of the area Tuesday
evening. So based on this, having 50 to 70 percent chances seems
reasonable for now. Once this activity moves out of the area late
Tuesday night, there is general agreement that much, if not all,
of Wednesday should be dry with a weak area of high pressure that
briefly drops down over the Great Lakes.

Any threat for severe weather will be tied to the front Tuesday.
CAPE ahead of the front looks to be rather modest with up to 2000
J/Kg of ML CAPE in place. The NAM continues to suggest that right
along the front there will be 40 to 50 knots of shear in the 0-6
km layer while the GFS keeps the stronger shear well behind the
front and has maybe 20 knots of shear along the front. If the NAM
scenario is correct, there would be enough shear to support a
damaging wind and large hail threat, but confidence in this is
rather low.

.LONG TERM...(Wednesday night through Sunday)
Issued at 303 AM CDT Mon Jul 17 2017

The upper level ridge should gradually move east and be centered
over the southeast U.S. by the weekend. This will essentially keep
the zonal flow in place over the region with weak short wave
troughs embedded in it. The position of the upper level ridge will
not allow the front to advance to the south and the expectation is
that it will waffle north and south but remain close to the area.
Additional convective complexes should form along the front,
maybe more so at night as the low level jet impinges on it, and
timed with any short wave troughs that move across the area. With
nothing to really push the front out of the area, will show the
rain chances in every period through the weekend.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1133 AM CDT Mon Jul 17 2017

Expect VFR conditions through the period. Overnight the LLWS will
be borderline for inclusion into the TAFS with about 20kts of
shear in a thin layer near 1500 ft AGL. Have chosen not to include
it at this time.

Thunderstorm chances begin to increase Tuesday afternoon at the
TAF sites. Future TAFS will probably need a period of TEMPO TSRA
later Tuesday afternoon and evening.




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