Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200505

Area Forecast Discussion
National Weather Service La Crosse WI
1205 AM CDT SAT AUG 20 2016

.SHORT TERM...(Tonight thru Saturday night)
Issued at 230 PM CDT Fri Aug 19 2016

Main forecast concerns this period are SHRA/TSRA chances tonight/
Sat and heavy rain threat with them, cooler temps.

18z data analysis had a broad trough of low pressure over the region
from lk Superior to southwest IA. Very moist/deeply saturated
airmass was in place across IA/WI/southeast 2/3 of MN. Higher of the
CAPE/warmer airmass was over the south end of the region where some
sunshine allowed for diurnal warming and temps in the 80s vs. 70s
further north under the more persistent bkn/ovc cloud decks. SHRA/
TSRA early this afternoon mainly across northern IA where some weak
SW low level flow was pushing the deeply saturated/unstable airmass
northeast over the cooler airmass further north.

Model runs of 19.12z initialized quite well. Solutions offering a
tighter consensus as the MT shortwave energy swings E-SE and deepens
mid level troughing over the Upper Midwest tonight thru Sat night.
Trend favors a compromise of the earlier runs on both timing/
strength as the main shortwave energy approaches tonight and passes
Sat. Still some smaller meso-scale/convective detail difference but
models more similar than they have been Wed/Thu. Short-term fcst
confidence is on the good side this cycle.

For the short term, after relative lull in convection this afternoon,
deep layered forcing/lift increase across the area tonight into Sat
morning. Rather impressive 850-500mb FN convergence signals over the
area later tonight/Sat morning. This with increasing PV advection
aloft ahead of the rather strong approaching shortwave and under the
deepening trough/broader divergence aloft. Looking more and more
like a stronger deformation band of precip is going to spread across
the fcst area later tonight thru Sat morning. Initial backing/
strengthening of the low level flow ahead of the trough sends the
moist/unstable airmass just south of the fcst area northward over
the cooler dome of air over much of WI/MN/northern IA. SHRA/TSRA
chances increase from south to north across the area late this
afternoon and thru the evening. This also works to deepen the
saturation of the column ahead of the approaching shortwave and
strengthening deep layered QG and FN convergence with/ahead of it.
PW values in the inflow airmass in the 1.5 inch range, along with
500-1000 J/KG MUCAPE late tonight/Sat morning. SHRA/TSRA tonight
into Sat morning will continue to be efficient rain makers and
capable of producing more heavy rains. Max rain rates likely to be
more in the 1/4 to 3/4 inch/hr range, but duration of the rain at
any one location looking to be longer as the deformation band would
rotate/lift across the area. Will issue a flash flood watch for
portions of the area that have been hit by heavy rains with in the
past week. See hydro section below for more details.

Shortwave and mid level trough axis passes Sat afternoon, with SHRA/
isolated TSRA chances and heavy rain threat decreasing from SW to NE
across the fcst area thru Sat afternoon/night. Deeper layered
subsidence behind the trough/shortwave spreads across the area Sat
night helping clear out the clouds, especially across the west/south
sides of the fcst area. Mdt/strong low level cold advection spreads
across Sat afternoon/night. Coolest of 925-850mb airmass is over the
area by 12z Sun, but W/NW gradient winds of 5-10 mph to keep
boundary layer stirred Sat night (along with residual low level
moisture) and lows from bottoming too far. Blend of guidance
lows/highs for tonight thru Sat night generally looks good but highs
Sat may be a few degrees too warm if SHRA/ovc skies dominate thru
the day.

.LONG TERM...(Sunday through Friday)
Issued at 230 PM CDT Fri Aug 19 2016

For Sunday thru Monday night: main fcst concern this period is

19.12z model runs in good agreement for the Sat trough to quickly
progress east Sun/Sun night, with shortwave ridging building over
the region by Mon night. Confidence Sun thru Mon night in what is
shaping up to be a quieter weather period is good this cycle.

Hgts already rising Sun morning and continue thru Mon night under
the progressive flow. Sfc ridge axis approaches Sun then slides
across the area Sun night. Deeper layered anti-cyclonic flow Sun
should make for a mostly sunny day over all but the northeast end of
the fcst area where a small -SHRA chance may linger into Sun
morning. Some 925mb warming already occurs by late Sun, but mixed
925mb temps at 00z Mon would only support highs in the upper
60s/lower 70s. sfc ridge axis moves across the area Sun night, for
what should be the coolest night under clear/mostly clear skies and
light/decoupled winds. Did not add it to the grids at this time, but
will have to watch late Sun night/early Mon morning for valley fog
formation. Progressive pattern/ridge building keeps the sfc high
moving east Mon/Mon night. Pressure gradient increase over the area
Mon as the high moves off and the next trough takes shape in the
northern plains. 925mb temps of 19C-23C by Mon afternoon along with
south gradient winds of 10-20 mph for mixing to return highs Mon to
near normal. Modest gradient continues over the area Mon night. This
along with the low level warming, Mon night lows looking to be some
10F to 15F warmer than those of Sun night. Models returning
moisture/instability already Mon night. However what lower level
thermo-dynamic forcing/lift there is looks to be just north of the
fcst area. Small SHRA/TSRA chance mainly north of I-94 Mon night OK
for now. Used a blend of the guidance highs/lows for Sun/Mon/Mon
night. Leaned toward cooler of the guidance lows for Sun night.

For Tuesday thru Friday (days 4 to 7): main fcst concerns this period
include SHRA/TSRA chances centered on Wed, temperatures.

Medium range model runs of 19.00z/19.12z in rather good agreement
for shortwave ridging to be moving across the Upper Midwest Tue,
with trough/shortwaves to then approach/move across the region Tue
night into Thu. Some shortwave detail differences by Fri (to be
expected) but consensus for broader troughing to remain over the
north-central CONUS Fri. With at least reasonable model agreement
thru Fri, fcst confidence in the day 4-7 period is average to good
this cycle.

Shortwave ridging aloft moves across the region Tue, but at the sfc
troughing progged to be approaching from the plains. Tighter
gradient over the area Tue, for brisk south/southwest winds but
stronger of the lower level warm advection will have lifted north of
the area. With little low level thermo-dynamic forcing and warm mid
level temps, Tue should be a dry/warm day with highs back to
near/above normal. Hgts fall, moisture/instability increase and
shortwaves/the sfc-mid level trough approach/pass Tue night into
Thu. Some timing differences but consensus would have the stronger
of the forcing/deeper of the moisture move across Wed/Wed night. 30-
50% SHRA/TSRA chances centered on Wed/Wed night good for now. Thu
night/Fri would see cooler/drier high pressure building in behind
the departing trough/front. Model/ensemble consensus of highs/lows
for Tue-Fri looking well trended at this time.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1205 AM CDT Sat Aug 20 2016

The large area of rain has been dissipating on the southwest side
as the moisture transport shifts off to the east of area. The
20.03Z HRRR has handled this dry slot effect very well and shows
only some occasional light rain for both sites for the rest of
tonight into Saturday morning. The concern then becomes if there
has been enough moisture introduced into the lower sections of the
atmosphere to allow low clouds and fog to form. Looking downstream
to where is was raining most of the evening, the conditions have
remained almost exclusively VFR. However, the office sky cam does
show some lower clouds floating by. With KLSE having received more
rain than KRST, confidence is higher that some low MVFR ceilings
will form at KLSE along with some fog to reduce the visibility to
IFR. With less rain at KRST so far, will only take the forecast
down to MVFR. The rain still looks like it will end Saturday
morning with conditions returning to VFR during the afternoon.


.HYDROLOGY...Tonight and Saturday.
Issued at 230 PM CDT Fri Aug 19 2016

The potential for heavy rains, flooding/flash flooding continue
tonight and Sat as the system winds up/deepens/lifts across the
region. Tightening signal for a deformation band of precip to rotate
across the fcst area mainly late tonight/Sat morning. This plus
SHRA/TSRA late this afternoon/evening, potential exits for another 1
to locally 3 inches of rain across the fcst area. Given recent
rounds of heavy rain/flooding/flash flooding, will issue a flash
flood watch for tonight/Sat in areas where another 1 to 3 inches
of rain poses the greatest threat of more water problems (Wabasha
east thru Jackson Co and Monroe east thru Adams Co).


.ARX Watches/Warnings/Advisories...
WI...Flash Flood Watch through Saturday evening for WIZ032>034-

MN...Flash Flood Watch through Saturday evening for MNZ079.



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