Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 292349

Area Forecast Discussion
National Weather Service La Crosse WI
649 PM CDT WED JUN 29 2016

.SHORT TERM...(This evening through Thursday)
Issued at 314 PM CDT Wed Jun 29 2016

Currently as of 20Z...Water vapor loop and RAP 500mb height analysis
showed strong ridging over the western U.S. into Alberta, with
northwest flow extending from this ridge into the Ohio Valley. In
the northwest flow a weak shortwave was evident in southeast MN
while a more potent looking trough was dropping south through
Manitoba. That weak shortwave has not had much impact on the area
besides aiding in some diurnal cumulus development. High pressure at
the surface, the lack of any low level convergence and plentiful low
to mid level dry air as seen on the 12Z MPX sounding has prevented
the cumulus from doing much. The combination of sun and 850mb temps
around 10C helping to push temperatures in the mid to upper 70s. To
our northwest, a cold front was evident approaching the
international border in North Dakota and MN. Scattered showers and
thunderstorms were present ahead of this front where dewpoints are
pooling in the upper 50s to low 60s.

The main focus for the short term period is on that cold front
coming into northern MN, as well as the more potent looking upper
trough behind it. Models are in good agreement dropping the upper
trough across MN on Thursday, pushing the cold front through the
forecast area during the afternoon hours. Really no forcing exists
ahead of the front tonight so kept the nighttime hours dry. Thursday
is a bit more interesting, with 2 forcing mechanisms for
precipitation. First, there is a strong signal for a mid level
frontogenesis zone to come out of the Dakotas tonight and slide
across the forecast area along and behind the front as it is passing
through. The second is the front acting on 500-1000 J/kg of MLCAPE
building ahead of the front with what looks to be an uncapped
atmosphere. Between these 2 forcing mechanisms most of the region
should see some rain, especially from the late morning into the
afternoon. Both synoptic qpf and high resolution models radar progs
agree with this assessment. Therefore chances are high.
Precipitation should be moving out of northwestern sections of the
forecast area late in the afternoon.

1730Z Day 2 outlook introduced a 5%/Marginal severe risk area for
the forecast area. This seems reasonable as an area of higher 0-6km
shear overspreads the area ahead of the front. The degree varies,
from 25-30 kt in the NAM, 30-35 kt in the ECMWF and 35-40 kt in the
GFS. This shear could support a few stronger storms. Think the
chance of a severe storm is extremely low given the weak CAPE.
Precipitable water up around 1.5 inches could support some heavier
rain in the storms, but definitely far from any flooding concern.

Increasing southerly flow and clouds tonight will result in a warmer
night than last night. 850mb temps around 12C tomorrow plus some sun
intermixed should push highs into the mid 70s.

.LONG TERM...(Thursday night through Wednesday)
Issued at 314 PM CDT Wed Jun 29 2016

Any lingering post frontal showers/storms from Thursday will quickly
exit the area Thursday evening. Things get quiet on the
precipitation front after that through the 4th of July holiday. The
area pretty much stays in northwest flow until Monday when finally a
flattened upper ridge axis moves overhead. Subsidence, high pressure
and dry advection in that northwest flow will combine to produce the
dry conditions. Note that there will be a system moving across
Kansas and Missouri during the weekend which should spread some
showers and storms up into Iowa. However, anticipating them to stay
well south of the area. The next chance of rain does not come until
late Monday night or really Tuesday through Wednesday. This is when
a weak trough looks to slowly drop down into the area. Note the
chances are low given plentiful model disagreement.

Temperatures for the long term period will definitely start on the
cool side. In the wake of the upper trough crossing Thursday night,
850mb temps fall to 6-8C by 18Z Fri. Despite plenty of sun, mixing
will only support highs in the low to maybe mid 70s at best. High
pressure sets up over the area Friday night which will help with
cooling. Precipitable water is progged to drop to or below 0.5 inch,
which is getting close to the observed minimum of all MPX soundings.
Therefore, have leaned towards the colder side of guidance,
especially in our typical cold spots. Valley fog is certainly a
possibility, especially outside the main channel. Warming occurs
from Saturday and heading right through Wednesday of next week with
the approach of that flattening upper ridge. We should be back
towards or even above normal by the 4th of July.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 648 PM CDT Wed Jun 29 2016

A cold front will bring showers and thunderstorms to the TAF sites
Thursday afternoon with MVFR conditions possible. In addition,
some of the stronger storms may produce strong gusty winds and
hail. Before the storms move into the region, we may see some
scattered rain showers during the morning hours, but conditions
should remain VFR. Forecast guidance suggests we may see some
MVFR to IFR visibilities develop tonight in with light winds and
mostly clear skies in place. Confidence wasn`t high enough to
include this in the TAF but will continue to monitor trends


.ARX Watches/Warnings/Advisories...


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