Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 292336
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
636 PM CDT MON AUG 29 2016

.SHORT TERM...(Tonight through Tuesday night)
Issued at 232 PM CDT Mon Aug 29 2016

Main fcst concerns this period: SHRA/TSRA chances with the weak
front to approach/pass later tonight thru Tue evening.

Data analysis at 18z had high pressure centered over Lk Huron with
ridge westward into northern IA. Low pressure was located west of
Hudson`s Bay with a trough/cold front thru western Ont to NW MN and
and eastern SD. One more humid/summery day across the area. Temps in
the upper 70s-lower 80s over much of MN/IA/WI early this afternoon
with dew points in the upper 60s to lower 70s, more reminiscent of
July than getting ready to turn the calendar to Sept. WV imagery and
RA-OBs showing a deep, moist airmass over much of WI/IA/southern MN
with connections to a sub-tropical moisture plume coming up across
MX/west TX/eastern NM. This deep moisture, around 3K J/KG MU/SB CAPE
and a weak shortwave rippling thru the flow producing widely sct
SHRA/TSRA across much of IA early this afternoon.

No issues noted with 29.12z model initializations. Models in good
agreement as stronger troughing moving into the northwest CONUS
gradually buckles the flow, with ridging building over the western
plains into Alb/Saskat/Man. A stronger shortwave trough to track
across northern Ont, with northwest flow aloft developing over the
Upper Midwest between the plains ridging and eastern Can
troughing. Short-term fcst confidence is good this cycle.

For the short term, amplifying ridging over the plains and
developing NW flow aloft over the region pushes the weak sfc-850mb
trough southeast across the region tonight/Tue. Bulk of stronger/
deeper lower level forcing passes well north/east of the fcst area,
closer to the main shortwave. Little convergence/lift with this
lower level trough/front as it weakens/shears apart as it crosses
the area. PW values around 1.5 inches progged to accompany the
trough, along with MUCAPE as high as 1K to 2K J/KG. However, both
shallow and deeper layered shear quite weak with 500mb winds over
the area only in the 15-20kt range, and little sfc gradient flow
ahead of and behind the trough/front. Some of models do `spit and
sputter` some convection near/ahead of the front tonight, along with
continuing the weak forcing driving the sct convection across IA
today over the south 1/3 to 1/2 of the fcst area tonight. Inclined
to leave 20-30% SHRA/TSRA chances over much of the fcst area
tonight. Higher (20-40%) chances Tue would then be during the
afternoon with more CAPE and the boundary sliding southeast across
the fcst area. Boundary pushing south of the area Tue evening along
with loss of diurnal heating should put a rather quick end to any
SHRA/TSRA Tue evening. Dry/clearing later Tue night as deepening NW
flow behind the front spreads in low level cold advection and pushes
out the moisture plume. Blend of the numerical guidance looks to
have lows/highs tonight thru Tue night well in hand.

.LONG TERM...(Wednesday through Monday)
Issued at 232 PM CDT Mon Aug 29 2016

For Wednesday through Thursday night: main fcst concerns this period
are temperatures.

29.12z model runs in good agreement for the mid level ridging over
the plains to slowly narrow and move east Wed thru Thu night, with a
rather sharp mid level ridge axis to move into western MN/IA late
THu night. This leaves MN/IA/WI remaining under dry/quiet NW to N
flow aloft thru this period. Fcst confidence is good in the Wed thru
Thu night period.

Not much to talk about this period as Can high pressure at the sfc
builds in Wed then remains over the western great lakes into Fri.
Weak to mdt Low level cold advection continues over the area much of
the period as northeast to southeast winds out of the high filter
cooler air across the great lakes into WI and eastern MN/IA. PW
values thru the period along with 925-850mb temps a bit below normal
as drier low level air flows W-SW out of the sfc-850mb high. 925mb
temps in the 15C-18C range Wed/Thu, for highs mostly in the low-mid
70s. Dewpoints in the 50s to accompany these temps for a rather
comfortable end to Aug and start to Sept. Light winds/mostly clear
skies/drier airmass should allow lows to dip into the upper 40s to
mid 50s, giving air conditioners a break. With the sfc ridge axis
near/overhead Thu/Fri mornings, will have to watch both late nights
and mornings for fog development in the valleys and low laying
areas. Plenty of time to sort out those details and add late
night/early morning fog to the grids as needed. Blend of the
numerical guidance highs/lows for Wed thru Thu night looks good at
this time.

Friday through Monday (days 4 to 7): main fcst concerns this period
are SHRA/TSRA chances Sat night thru Monday, temperatures thru the
period.

Medium range model runs of 29.00z/29.12z in good agreement for a
rather strong mid level ridge axis to move across the region, ahead
of strong longwave troughing coming thru the Rockies. Models in good
agreement Sat thru Mon with the main longwave trough remaining over
the Rockies and pieces of energy ejecting northeast out of it.
One stronger piece of energy to lift into south-central Can Sat with
another moving thru the trough Mon. Fcst confidence is average for
Fri-Mon.

This period starts out with high pressure centered over the great
lakes and the mid level ridge axis overhead. The sfc high moves
little thru Fri/Sat, with PW and moisture transport axis west/
north of the fcst area until Sat night. Main lower level warm
advection remains west of the area thru Sat as well. 925mb temps
in the 16C-18C range Fri/Sat. Highs looking to be in the 70s under
what should be mostly sunny skies for Fri/Sat. Sat trends to be
the nicer of the upcoming holiday weekend days at this time.
Pieces of stronger shortwave energy lifting across south-central
Can Sat night and again Mon push the moisture plume and increasing
lower level thermo-dynamic forcing/lift east and into the area for
Sat night/Sun then even more so for Sun night/Mon. PW values
looking to be in the 1.5 to 1.75 inch range across the entire fcst
area by later Sun night/Mon, around 1.5 std deviations above
normal. This with the sfc-850mb trough/front pushing into the area
and increasing MUCAPE. Per reasonable between model and run-to-run
agreement, SHRA/TSRA chances spreading west to east across the
area Sat night/Sun, then in the 30-50% range Sun night/Mon looking
well trended for now. 925- 850mb temps warm for Sun/Mon, but trend
toward more clouds and potential SHRA/TSRA in the area looking to
keep highs thru Sun/Mon mostly in the 70s. If Sunday would trend
drier, potential for some highs in the lower 80s and it to be the
nicer of the holiday weekend days. For now stayed with the model/
ensemble consensus of highs/lows Fri thru Mon.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 636 PM CDT Mon Aug 29 2016

Quiet weather continues locally for the moment, with storms both
northwest and south of the TAF sites as we sit in a humid airmass
ahead of a cold front pressing southward across Minnesota. Can`t
totally rule out a very small risk for a shower or storm later
tonight as that boundary gets closer, nor can we do the same into
midday Tuesday as the front actually passes, though the overall
risk appears quite low with no support aloft so will continue to
forego any precip mention for this round of TAFs.

The bigger story will be the likelihood of another night of MVFR
to IFR conditions at times centered either side of sunrise with
some stratus and/or fog expected to develop in the sticky air.
With very light flow through the column and late afternoon
temp/dew point spreads already under 10 degrees in many spots,
don`t see why some fog and lower stratus won`t develop, with the
highest potential of the past several nights for LSE to see a
period of 1/4SM visibility. Any fog/stratus should lift back into
an MVFR cloud deck through the morning and early afternoon hours
in advance of the incoming front, with lower clouds scouring later
in the day as much drier air crashes in from the north while some
thicker cirrus remains.

&&

.ARX Watches/Warnings/Advisories...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...Lawrence



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