Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 202333
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
633 PM CDT Tue Sep 20 2016

.SHORT TERM...(This evening through Wednesday)
Issued at 325 PM CDT Tue Sep 20 2016

Main concerns are on heavy rainfall tonight through Thursday and the
possibility for flash flooding. This could potentially be a
serious flooding situation!

Ongoing showers and isolated thunderstorms will weaken this
afternoon then severe thunderstorms are expected to develop late
this evening into the overnight hours with the main threat of
heavy rainfall. Some of the storms may also produce damaging winds
and perhaps large hail if updrafts are rotating.

A warm front lifts north into the area tonight brining a tropical
airmass into the area with precipitable water values climbing to 1.5
to 1.8. These values combined with warm cloud depth of 3.5 to 4 km
will set the stage for a potential flash flood event. The nose of
the low level jet focuses into the front tonight with 850 mb
moisture transport of 300 to 500 units slamming into the front.
All of this combined with 30 day mean precipitation values of 200
to 300 percent over the mean, primes the area for flash flooding.
Storms look to initiate over portions of southeast Minnesota into
west central  Wisconsin along the front and then edge southward
with time. So, the storms/heavy rainfall may build south into
northeast Iowa and southwest Wisconsin. The storms will capable of
producing very heavy rainfall, 2 inch per hour rates are possible.
Damaging winds are also possible. Storms will be ongoing by
sunrise on Wednesday then are expected to gradually taper off
through the morning hours.

Round two of heavy rainfall takes aim on the area late in the day on
Wednesday into the overnight hours. This is when some of the most
serious flooding could develop, especially if this rain falls over
the same areas as Tuesday night into Wednesday morning. A few
forecast models attempt to push this second round of heavy rain
to the north. Not really buying into that given that outflow from
the storms will help to drive the warm front south...potentially
closer to the interstate 90 corridor. The heavy rain threat
finally tapers off by Thursday evening.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 325 PM CDT Tue Sep 20 2016

For Thursday night into Friday...shower and thunderstorm chances
continue, its just a questions of forcing for the storms. The
warm front meanders in the neighborhood so there is the chance of
seeing more rain...potentially heavy. Will continue to monitor
closely since this would only worsen any ongoing flooding issues.

Friday night through Tuesday....

A deep upper-level trough will be in place over the Rockies for the
start of the weekend. Ahead of the trough, lower-level
WAA/isentropic lift will provide forcing for a broad band of
showers, possibly with isolated thunder, stretching from the central
Dakotas into southeast Minnesota/southern Wisconsin Friday night.
20.12z GFS/20.00z ECMWF are in agreement with lifting the thermal
gradient northward into Saturday, temporarily clearing our CWA of
rain chances. Models become increasingly divergent thereafter. As an
upper-level cut-off low detaches itself from the main flow over the
Four Corners region, the GFS becomes much more progressive, racing
the upper-level trough the Great Lakes region by Monday night.
Meanwhile, the ECMWF holds the trough over the Northern Plains until
Tuesday. This will be the determining factor for the timing and
duration of another round of rain Sunday into Monday. After this
system departs the region, high pressure is progged to build in.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 633 PM CDT Tue Sep 20 2016

The latest runs of the hi-res meso models continue to indicate the
warm front over central Iowa will lift north toward the area this
evening. Convection is expected to form north of this front as the
low level jet starts to impinge on it late this evening. The
20.22Z run of the HRRR is the first to bring this activity in and
have gone with a VCTS on its faster timing and then gone with
categorical showers and storms for much of the overnight. Initial
VFR conditions should drop down to MVFR in the convection with
some IFR possible in the heavier rains. This activity should
continue into Wednesday morning until the low level jet weakens
allowing for a break in the rain. Guidance is mixed on whether
ceilings will drop to IFR behind the rain or go up to VFR and
opted to stay with persistence from the previous forecast with
VFR. The front will still be in the area for Wednesday afternoon
with another round of convection expected to develop in the
afternoon. For now, will show a VCTS at both sites all afternoon
but this could hold off until mid afternoon once the low level jet
starts to increase and interact with the front again.

&&

.HYDROLOGY...Tonight through Thursday
Issued at 325 PM CDT Tue Sep 20 2016

A potentially serious flooding situation is taking shape for tonight
through Thursday. Two main rounds of heavy rainfall.  The first,
tonight into Wednesday morning. The second round is expected
Wednesday evening into the overnight hours. Rainfall amounts of 2
to 3 inches with locally higher amounts in excess of 7 inches
possible. Given the very wet conditions across the region flooding
could develop quickly. If these two rounds of heavy rainfall occur
over the same area the flooding could be some of the worst
experienced by many. Stay weather aware and be ready to move to
higher ground! Several rivers could go into flood. Minor to major
river flooding is possible.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch from 1 AM CDT Wednesday through Thursday
     evening for WIZ017-029-032>034-041>044-053>055-061.

MN...Flash Flood Watch from 1 AM CDT Wednesday through Thursday
     evening for MNZ079-086>088-094>096.

IA...Flash Flood Watch from 1 AM CDT Wednesday through Thursday
     evening for IAZ008>011-018-019-029-030.

&&

$$

SHORT TERM...WETENKAMP
LONG TERM...WETENKAMP
AVIATION...04
HYDROLOGY...WETENKAMP



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