Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181948

Area Forecast Discussion
National Weather Service La Crosse WI
248 PM CDT Sun Sep 18 2016

.SHORT TERM...(This evening through Monday)
Issued at 248 PM CDT Sun Sep 18 2016

Main forecast concerns in the short term are on Thunderstorm chances
tonight and again late Monday afternoon into Monday evening.

Isolated to scattered thunderstorms are possible late tonight into
Tuesday as a cold front marches southeast across the region. The
nose of the best 850 mb moisture transport and upper level forcing
are across northern Wisconsin tonight. Therefore, will keep the
highest chances for showers and storms confined to mainly Taylor and
far northern Clark counties. The RAP forecast sounding at MDZ
indicates around 1300 J/KG CAPE by 08Z tonight with 1-6 KM Bulk
Shear values of 25 kts. Cannot rule out a strong storm or two withthe
potential for large hail. Better shear noted across northwest
Wisconsin so thinking these areas stand a higher chance of seeing
some large hail if storms develop.

The cold front marches through the areas during the day on Monday
and thunderstorms most likely won`t develop along it until its in
far southern Wisconsin. The delay in thunderstorm development is
from a strong cap in place. Thunderstorms could initiate along the
front between 20 and 23Z.  The main area we will really have to
watch is Grant County in far southwest Wisconsin and possibly
Clayton County in northeast Iowa, where the storms could initiate.
It appears the storms will develop just south and east of the
forecast area. The storms will probably be supercells initially with
SBCAPE values of 2500 j/kg and 0-6 km bulk shear of 45 to 50 kts.
They may form into line segments too with 0-3 km bulk shear of 30
to 35 kts.

.LONG TERM...(Monday night through Sunday)
Issued at 248 PM CDT Sun Sep 18 2016

Main forecast concerns in the long term are on flooding potential
starting late Tuesday night through Thursday with periods of heavy
rainfall possible.

High pressure builds in Monday night into Tuesday with light wind
and mostly clear skies expected. This will set the stage for areas
of fog, especially along river valleys and central Wisconsin. Will
be adding fog to the forecast form late Monday night into Tuesday
morning. The high quickly shifts off to the east Tuesday afternoon
into Tuesday evening as low pressure develops across the plains. A
warm front is expected to lift into the Upper Mississippi River
Valley Tuesday night with Thunderstorms developing along it.
Moisture will be quick to return on southerly flow with precipitable
water values climbing into the 1.5 to 1.9 inch range. Warm cloud
depth increases to 3.5 to 4.0 km too. 850 mb moisture transport
increases into the region as the low level jet ramps up with
Moisture Transport Vector magnitude of 250 to 400 and is focused
into the warm front. This will set the stage for a possible flash
flood situation. Confidence was not high enough on where the feature
will setup to issue a watch but one may be needed soon if model
trends continue. The heavy rain environment remains in place
through the day on Wednesday the ramps up again Wednesday night
into Thursday morning. This second overnight round of heavy rain
Wednesday night may focus just a little further north that the
first round. If both rounds of heavy rain fall over the same area,
a serious flooding situation may evolve. Considering the local
area is over 200 percent of mean precipitation for the past 30
days, the soils are nearly saturated. A lot of uncertainty at
this time on where the heaviest rain will be so continue to
monitor later forecasts closely.

Thursday through Sunday

Good model agreement that precipitation will be ongoing at the
beginning of this period, as a warm front and shortwave trough work
through the area. Widespread precipitable water values over 1.5
inches and warm cloud depths near 4 km point to yet another heavy
rain event. Following the shortwave, the boundary stalls over the
region while a closed low over the Dakotas continues to bring
moisture into the area allowing precipitation to last into the
weekend, which is when the model solutions diverge. The 18.12Z GFS
sends the low northeastward into Canada, quickly moving the cold
front and additional precipitation through the area, while the
18.00Z ECMWF has the low farther south, keeping the area wet for a
bit longer. The 18.12Z ECMWF comes into better agreement with the
GFS regarding the placement of the low, but is still slower in
moving the cold front through. With the region receiving well above
normal precipitation over the last few weeks, a consistent heavy
rain signal like this from the models certainly makes flooding a
major concern.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1231 PM CDT Sun Sep 18 2016

VFR conditions expected through the period. A push of warm and
moist air into the area will kick off some altocumulus overnight
along with a few shra/tsra. The bulk of this shra/ts activity is
expected to stay north/east of the KRST/KLSE taf sites. For this
reason...left shra/ts mention out for now but will continue to
monitor. Otherwise...will likely see some LLWS this evening into
the early morning hours as stronger southwest winds around 40kt
set up above the nocturnal inversion.


.HYDROLOGY...Tuesday night into the weekend
Issued at 248 PM CDT Sun Sep 18 2016

Flooding possible this week. Periods of heavy rainfall are possible
starting Tuesday night persisting into Thursday.  Thunderstorms
could develop and repeat or train over locations starting late
Tuesday night and persisting into Thursday. With soils nearly
saturated at many locations, flash flooding is possible. In
addition flooding may develop on area rivers and this flooding
could persist through late week into the weekend. Flood watches
may be issued once confidence increases on where the heaviest rain
will be. Also, more heavy rain is possible later in the week going
into the weekend, making any flooding issues worse. Monitor
forecasts closely over the next few days.




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