Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201728
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1228 PM CDT Thu Jul 20 2017

.UPDATE...
Issued at 1150 AM CDT Thu Jul 20 2017

After assessing rainfall potential for Friday into early Saturday
morning, decided to pull the trigger on a Flash Flood Watch valid
Noon Friday through 9 am Saturday. Expecting scattered storms to
upscale during the day Friday with increasing moisture transport
This complex then looks to drop southeast into moisture-
rich/CAPE pool. Lots of flooding/damage from last night`s heavy
rain, coupled with possible additional 1 to 3 inches/locally
higher amounts, prompted this headline decision.

&&

.SHORT TERM...(Today and tonight)
Issued at 245 AM CDT Thu Jul 20 2017

Showers/storms on the nose of the low-level jet/stronger moisture
transport producing locally heavy rainfall will continue to shift
eastward as the low-level jet veers early this morning. Otherwise,
it should be a pretty quiet day as the low-level surface boundary
gets shifted southward again. Could not rule out a stray
shower/storm, especially farther south closer to the boundary, but
anything would be isolated. It will still be warm and quite
humid, especially across southern areas closer to the boundary
with highs in the 80s.

Another potential severe event/heavy rain event still appears on
track for Friday. Storms should fire across the northern plains
Thursday night in advance of an upper shortwave trough north of the
surface boundary. Sustained low-level warm advection and increasing
moisture transport north of the surface boundary will help focus
redevelopment of storms as they progress eastward out of the plains
through the day. Once again, the storms likely will focus on the
edge of the stronger instability which will cut across the area.
Deep layer 0-6 km shear around 40 kts would support a severe threat,
including hail and damaging winds. Showers/storms should continue
into Friday night ahead of a weak surface low with continued
moisture transport on the nose of the low-level jet before it veers
westerly and helps end to the precip.

The environment will remain very favorable for heavy rainfall during
this time with seasonably high precipitable water values and very
deep warm cloud depths exceeding 4 km. With a few inches of rain
having fallen in some areas already this week, will have to
carefully watch for flash flood potential Friday/Friday night. See
the hydrology section for more details below.

Confidence is lower in the placement of surface features on Saturday
in the wake of the convection Friday night. Some showers/storms
could linger at least into Saturday morning, though. A strong
upper shortwave trough approaching from the northwest could help
to trigger additional scattered storms in the afternoon and
evening over parts of the area with some potential for strong
storms.

.LONG TERM...(Friday through Wednesday)
Issued at 245 AM CDT Thu Jul 20 2017

A cooler and drier airmass will begin to advect into the area on
Sunday as northwest flow aloft sets up with surface high building
in from the northwest. With upper troughing digging into the
northern Great Lakes, a few showers or storms could develop on
Sunday.

Otherwise, Monday and Tuesday right now look dry under the influence
of high pressure, with highs in the 70s to low 80s.

As the ridge slides eastward by the middle of the week, deeper
moisture will surge northward again as the upper flow pattern
becomes more zonal, with some potential for showers/storms pending
timing of weak embedded upper waves.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1228 PM CDT Thu Jul 20 2017

Other than some sct diurnal cumulus in the 2000-3000 ft range,
generally good VFR expected this afternoon into this evening with
the area between weather systems. The recent rains coupled with
light winds and mostly clear skies, introduced some late night early
morning MVFR BR at both KRST/KLSE. Will have to watch KLSE for
potential of IFR cigs/vsbys in some thicker BR/FG in the 10-13z time
frame. There may be more of a bkn mid cloud deck spreading across
the area after about 09z, so did not include denser BR/FG mention at
KLSE at this time. Mixed signal among models on convection potential
Fri morning, with some much more robust than others. The variability
lowers confidence for TSRA in the last 3 to 6 hrs of the taf period.
Included a -SHRA mention, but given the lesser confidence, left the
VCTS/CB mentions out for now. Great chances of TSRA impacting the
taf sites trending to be later Fri afternoon/night.

&&

.HYDROLOGY...
Issued at 555 AM CDT Thu Jul 20 2017

Locally heavy rainfall overnight, locally in excess of 6 inches,
has occurred over parts of western Wisconsin and far southeast
Minnesota with flooding continuing early this morning. Gradually
improving conditions are expected with much of the rain done for
the day.

The rainfall has caused rises on some area rivers, including the
Trempealeau and the Kickapoo, with some locations along these rivers
already at or expected to reach flood stage. The Kickapoo River
at Ontario is peaking in major flood stage with some additional
locations expected to crest near moderate flood stage.

Additional locally heavy rainfall is expected Friday into Friday
night. With several inches of rain having already fallen this week
in some spots, these areas will be particularly susceptible to flash
flooding with more heavy rain. Although there is some uncertainty
where the heaviest rain will occur, the highest chances look to be
south of I-94 at this point. Additional rises and flooding on area
rivers/streams are expected.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch from Friday afternoon through Saturday morning
     for WIZ032>034-041>044-053>055-061.

MN...Flash Flood Watch from Friday afternoon through Saturday morning
     for MNZ079-086>088-094>096.

IA...Flash Flood Watch from Friday afternoon through Saturday morning
     for IAZ009>011-030.

&&

$$

UPDATE...DAS
SHORT TERM...JM
LONG TERM...JM
AVIATION...04
HYDROLOGY...JM


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