Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 242341

Area Forecast Discussion
National Weather Service La Crosse WI
641 PM CDT Sat Sep 24 2016

.SHORT TERM...(Tonight thru Sunday night)
Issued at 230 PM CDT Sat Sep 24 2016

Main forecast concerns this period are the heavy rainfall potential
later tonight/Sun and renewed flash flooding/flooding threats.

18z data analysis had a cold front from a low in western ND to SE
SD, thru eastern Neb and central KS. Another deep/rich moisture
plume with PWs in the 1.5 to 1.8 inch range advecting north ahead of
this front and the strong mid level low/trough over MT/WY/CO.
Increasing lower level forcing with the front and divergence aloft
ahead of the trough, combined with 2K-3K J/KG SB/MUCAPE producing
increasing bands of SHRA/TSRA from SE SD thru E Neb/W IA into
eastern KS. The SHRA/TSRA lifting toward SW MN early this afternoon.
Closer to home, strato-cu stuck under an inversion near 900mb
remained over much of eastern MN/northeast IA/western WI. Temps
under the clouds remained mostly in the 60s, with 70s/low 80s
further west with some sunshine and ahead of the front.

No issues noted with 24.12z model initializations. Models
offering a rather tight consensus of solutions as the NE WY mid
level trough/ low opens up and lifts into NE ND by 12z Sun.
Improving agreement Sun/Sun night as secondary shortwave energy
dropping down the back side of the trough deepens it over the
Upper Midwest, with another mid level in the vicinity of western
lk Superior by 12z Mon. Trend favors stronger of early runs with
the trough over the region by Sun night. Short-term fcst
confidence is good this cycle.

For the short term, mid level low/trough and shortwave lifts into
NE ND tonight, takes the sfc low to near lk Winnipeg and pushes
the trailing cold front to just east of I-35 by 12z Sun. Sfc low
continues lifting into NE Manitoba Sun with the cold front
sweeping east across the area to near lake MI by 00z Mon. Message
here is for the system to impact the area tonight/Sun to be
progressive, and move rather quickly into/across the area.
However, any TSRA with the front looking to remain capable of high
rainfall rates/brief heavy rains. PW values progged to increase to
the 1.5 to 2 inch range in the moisture plume ahead of the front,
along with warm cloud depths in the 3500-4000m range, MUCAPE in
the 250-500 J/KG range and the deep layered forcing/lift ahead of
the front thru mid level trough. TSRA tonight/Sun morning will be
efficient rain makers once again, with potential rain rates of 1
to 2 inches/hr. Difference this time is this system to be a north-
south line of SHRA/TSRA progressing west to east across the area.
Unlike this past Tue night thru Wed night, any heavy rain
potential this time not expected to last very long at any one
location. Another however though, most if not all of the local
landscape is saturated with high water/flooding continues on many
rivers and streams, and cannot take an inch of rain in a short
period of time without some runoff occurring. Any runoff right
now has the capability to aggravate/renew flooding problems in
those areas hard hit by heavy rains a few days ago. Not expecting
widespread renewed problems, but will err on the side of caution
and issue a flash flood watch for much of the fcst area that
received the heavy rains this past Tue night-Wed night. Main
threat time still setting up to be late tonight/Sun morning.

Once the front sweeps thru, PW values decrease and the main forcing/
lift exits east of the area with decreasing SHRA/TSRA chances thru
Sun afternoon. By Sun night any lingering SHRA chances confined to
the NE end of the fcst area under the deeper/colder cyclonic flow
aloft. Used a blend of the guidance lows/highs for tonight thru Sun
night, with low level NW flow/cold advection behind the front
returning lows Sun night to near the late Sept normals.

.LONG TERM...(Monday through Saturday)
Issued at 230 PM CDT Sat Sep 24 2016

For Monday thru Tuesday night: main fcst concerns this period
include brisk NW winds Mon, lingering SHRA chances NE of I-94 Mon,
temperatures thru the period.

24.12z models in decent agreement on the mid level low drifting east
across lk Superior Mon/Mon night, with deep/cool NW flow aloft over
MN/IA/WI. Looser consensus by Tue/Tue night but trend is for the mid
level low to wobble over the eastern great lakes keeping N/NW flow
aloft over the region into the middle of next week, favoring
slower/stronger of the earlier runs. Fcst confidence is good for
this to to be a cooler and needed drier period.

The deep N/NW flow this period spreads cooler and much drier air
across the region thru the period. Given the deep, tight, cold
cyclonic flow, some shortwave energy rotating around the mid level
low and perhaps some weak CAPE during the afternoon, a small -SHRA
chance across the NE end of the fcst area on Mon seems reasonable.
This blends with the neighboring grids too. Tighter pressure
gradient over the area on Mon behind the front. Model soundings
showing mixing to about 850mb Mon afternoon, with 30-35kts of wind
in the 925-850mb portion of the mixed layer. Mon looking to be the
first brisk/windy day in a while, with NW winds 15-25mph gusting 25-
35mph in the afternoon. Did raise winds a bit Mon afternoon with the
30-35kts above 925mb. Gradient relaxes a bit Mon night but W-NW
winds 5 to 15 mph looking to continue thru the night. Will have to
watch Tue for some breezy 15-25 mph NW winds in the afternoon.

By 12z Tue PW values progged to be 0.5 to 1 std deviation below
normal. Rounds of lower level cold advection send 925mb temps into
the 7C to 10C range by Tue morning and again Wed morning, with
highs/lows this period trending a category or 2 below normal. This
with what should be sunny to partly sunny skies. With some winds for
boundary layer mixing thru the period, used a blend of the guidance
highs/lows thru the period.

For Wednesday thru Saturday (days 4 to 7): main fcst concerns this
period is temperatures thru the period.

Medium range model runs of 24.00z/24.12z in good agreement on the
trough/upper low to exit east of the region Wed and ridging aloft to
build over the north-central CONUS for Thu. Reasonably good
agreement continues on Fri for hgts to lower/ridge axis to move east
of the region as some energy ejects out of renewed western NOAM
troughing into the plains. Some differences by Sat but large scale
flow is western NOAM troughing and eastern NOAM ridging with some
form of SW flow aloft over the central CONUS. Fcst confidence this
period is average to good this cycle.

Day 4 thru 7 is shaping up as a dry period with slowly warming temps
under the rising hgts/ridging Wed/Thu then SW flow aloft Fri/Sat.
Rising hgts, some low level warm advection and plenty of sunshine
Wed/Thu looking to allow temps these days to warm to near/above the
late Sept normals. Perhaps more clouds Fri/Sat with some increase of
moisture in the southwest flow, but the deeper low/mid level
southwest flow aloft should continue to bring in warmer air with
temps later next week into next weekend near/above the normals.
Model/ensemble consensus of highs/lows for Wed-Sat looks good at
this time.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 641 PM CDT Sat Sep 24 2016

First stronger low pressure system of the fall season remains on
track to swing a cold front through the region very late tonight
through Sunday afternoon. In advance of that front, persistent
lower MVFR stratus remains intact for LSE/RST but should gradually
erode through the early morning hours as we continue to warm
significant into that cloud deck. Thereafter, still looking for a
band of showers and perhaps a few embedded storms to work across
the region late tonight through Sunday, likely diminishing in
coverage through the night given a notable lack of instability
before perhaps redeveloping east of LSE into early Sunday
afternoon. At this time, confidence is nowhere near high enough to
maintain any thunder mention, and in fact it`s not out of the
question that only a very brief period of showers actually
survives this far east ahead of the cold front.

Regardless, should see conditions improve into the afternoon with
any residual lower ceilings quickly lifting in a notable dry slot,
but with winds shifting west and gusting 20-25 knots for most
locations. That clearer weather won`t last too long, with incoming
cold advection and associated stratus expected to return from the
west after 21Z. At the moment, it appears ceilings will initially
remain VFR but may dip to MVFR levels just after 00Z


.HYDROLOGY...Through Sunday
Issued at 230 PM CDT Sat Sep 24 2016

Another round of showers/storms expected late tonight through Sunday
as a north-south orientated cold front slides eastward across the
region. QPF in the various models isn/t all that much, and would
normally not illicit a lot of concern if not for the very heavy
rain/significant amounts much of the local area received from Tue
night-Fri morning. In addition, a summery airmass holds...with 4 kft
warm cloud depths and nearly 2 inches PW. Same airmass produced over
an inch of rain in an hour over central WI early this morning - with
a somewhat meager looking shower/storm. Storms late tonight/Sun will
certainly be capable of that.

All said, issuing a Flash Flood Watch for almost the entire area
from late tonight through much of Sunday. Again, coverage and
amounts of rainfall should not be close to what we experienced
earlier this week, but the ground can`t hold any more water. Any
heavy rain will run off into already high rivers/streams and could
result in further flash flooding.


WI...Flash Flood Watch from midnight CDT tonight through Sunday
     evening for WIZ029-032>034-041>044-053>055.

MN...Flash Flood Watch from midnight CDT tonight through Sunday
     afternoon for MNZ079-086-087-094-095.

     Flash Flood Watch from midnight CDT tonight through Sunday
     evening for MNZ088-096.

IA...Flash Flood Watch from midnight CDT tonight through Sunday
     afternoon for IAZ008-009-018-019.

     Flash Flood Watch from midnight CDT tonight through Sunday
     evening for IAZ010-011-029-030.



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