Excessive Heat Warning 1, NWS La Crosse, WI

Allamakee County, IA, Chickasaw County, IA, Clayton County, IA, Fayette County, IA, Floyd County, IA, Howard County, IA, Mitchell County, IA, Winneshiek County, IA, Dodge County, MN, Fillmore County, MN, Houston County, MN, Mower County, MN, Olmsted County, MN, Wabasha County, MN, Winona County, MN, Adams County, WI, Buffalo County, WI, Crawford County, WI, Grant County, WI, Jackson County, WI, Juneau County, WI, La Crosse County, WI, Monroe County, WI, Richland County, WI, Trempealeau County, WI, Vernon County, WI

Area Forecast Discussion
National Weather Service La Crosse WI
1142 PM CDT Wed Jun 27 2018

.SHORT TERM...(This evening through Thursday)
Issued at 343 PM CDT Wed Jun 27 2018

Over the past hours, clouds have dissipated ahead of schedule with
a bit more sunshine and mixing depth. This deeper mixing and
better dewpoint depressions will help reduce fog chances
overnight. Clear skies and deep light winds layer through about
5kft will promote the fog development. A majority of shorter term
models are suggesting some fog development but overall not
impactful (< 1 mile visibility). Have included a brief period of
patchy fog in the sunrise hours.

.LONG TERM...(Thursday night through Wednesday)
Issued at 343 PM CDT Wed Jun 27 2018

Main forecast concerns for this period remain heat on Friday and
shower and storm chances, mainly Saturday.

Models still in good agreement of upper level ridging building in
Thursday night into Friday, with surface pressure gradient
tightening leading to strengthening south/southwesterly flow. Models
do show good moisture transport into northwest portions of the
forecast area Thursday night as a warm front moves through (around
the 7-9Z timeframe), but warm air aloft looks to be a big hindrance
on any showers/storms forming. In addition, 850 mb warm air
advection really ramps up shortly after the moisture transport,
preventing the lower levels from saturating. Bottom line, not
impossible for a few showers and storms to form across northwest
portions of the area, but thinking it's not very likely. Slight
chance PoPs from Superblend seem to make sense.

Strong warm air and moisture advection will continue on Friday and,
combined with good mixing and 925 mb temperatures in the 27-30 C
range, will lead to a very hot and humid day. High temperatures look
to be in the 90s area-wide, with heat index values in the 100-110
range. Will likely need at least a heat advisory for much, if not
all, of the area, with an excessive heat warning possible for some
spots. A degree or two in temperature and/or dewpoint could make a
difference in which counties get what headline, so will hold off on
issuance until forecast can be refined a little more. For what it's
worth, the GFS has been a bit cooler than the NAM and ECMWF, as it
hangs onto low clouds near the warm front for longer during the day.
Thinking this is probably due to overdone low level moisture
(surface dewpoints nearing 80 degrees), so tend to lean toward the
NAM and ECMWF solutions. Friday also looks to be pretty breezy, with
around 35 kt winds at the top of the mixed layer. Could certainly
see a few gusts over 30 kts at the surface.

A cold front will then slide through the area Saturday evening and
into the overnight hours. Some question of how much instability can
be built up with increasing clouds ahead of the front, but ECMWF has
consistently brought in 2500-3000 J/kg of MUCAPE. The GFS has had
well over 5000 J/kg, but also has dewpoints approaching 80 degrees,
which is way too high. Regardless, plenty of instability ahead of
the front to get at least strong storms to develop. The strongest
deep layer shear still looks to be mainly behind the front, and 0-3
km shear is parallel to the front, so not sure how widespread any
damaging wind/large hail threat would be. It does look like heavy
rain will be a threat, with precipitable water values approaching
2 inches and warm cloud depths near 4 km.

Looking like precip should be out of the area by late Sunday
morning. A brief period of more zonal upper level flow is expected
for the first part of the work week, though models differ on precip
chances. The GFS continues to bring various shortwaves and
associated precip through the entire area from Monday night into
Wednesday, while the ECMWF has held precip to only the western parts
of the forecast area. Otherwise, looking like high temperatures are
expected to return to the upper 70s to mid 80s Sunday and into the
beginning part of the work week. Then there are hints at more upper
level ridging and some warmer weather returning by the middle of the

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1142 PM CDT Wed Jun 27 2018

Despite mostly clear skies, light winds up to 8000 ft agl, and
favorable surface dew point depressions, short-term high
resolution forecast guidance has backed off on fog development
overnight. Not ready to completely bite on this given other
parameters, so will keep some MVFR reductions in visibility
between 28.10Z and 28.13Z. Thereafter, expect VFR conditions
through the period with mostly periods of cirrus. Winds overnight
will be light and variable, increasing only slightly Thursday
morning from the south and then turning a bit to the southeast
Thursday evening.

&&

.HYDROLOGY...
Issued at 342 PM CDT Wed Jun 27 2018

A super-loaded air mass is in place Saturday night for heavy
rainfall rates. We must keep our eyes open for the potential of a
Maddox synoptic-archetype flash flooding event - a slow-moving
cold front with prolonged moisture feed. If there is even a slow
progression east of the convection Saturday night, with the
rainfall rates being high, it may still cause flash flooding. The
short term soil moisture will recover /drying/ from Tuesdays
rain, but longer term soil moisture is still high. Obviously river
flooding could occur as a result as well.

The Mississippi river flood wave continues to move downstream.

&&

.CLIMATE...(For Friday)
Issued at 320 AM CDT Wed Jun 27 2018

Below are the record high temperatures for June 29th (Friday)...

Northeast Iowa...

Charles City 102F in 1931
Cresco 96F in 1970
Decorah 105 in 1910
Elkader 100F in 1913
New Hampton 100F in 1931

Southeast Minnesota...

Austin 95 in 1988
Caledonia 96F in 1970
Grand Meadow 99F in 1931
Preston 96F in 1970
Rochester 99F in 1931
Theilman 99F in 1962
Winona 103F in 1931

Western Wisconsin...

Alma Dam 4 97F in 1970
La Crosse 101F in 1931
Medford WI 98F in 1910
Mauston 94F in 1966
Mondovi 100F in 1931
Neillsville 104F in 1910
Prairie du Chien 104F in 1931
Sparta 94F in 1991
Trempealeau Dam 98F 1939

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

SHORT TERM...Baumgardt
LONG TERM...CA
AVIATION...Rogers
HYDROLOGY...Baumgardt
CLIMATE...Boyne