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Area forecast discussion 
National Weather Service La Crosse WI
649 am CDT Friday Sep 19 2014

Short term...(today and tonight)
issued at 219 am CDT Friday Sep 19 2014

Forecast concerns include precipitation chances through
tonight...severe weather potential tonight...and winds this

Currently as of 07z...water vapor loop and rap 500mb height analysis
showed ridging over the Central Plains while a fast semi-zonal flow
was present along the U.S./Canadian border. Within this flow...a
shortwave trough was over southern Saskatchewan and adjacent
Montana. This trough has been responsible for... 1. Plenty of
middle/high clouds downstream over the Dakotas...some of which were
producing showers and a few thunderstorms. 2. Pulling more humid air
through the Dakotas on a 20-40 knots jet at 925mb with surface
dewpoints in the well as precipitable water of 1-1.5
inches. Finally 3...developing an unseasonably warm...capped airmass
over South Dakota characterized by 850mb temperatures of 18-24c and 700mb
temperatures of 10-13c. Over the forecast area...much cooler and drier air
exists with 850mb temperatures around 10c and precipitable water around 0.5
inch. A warm front separates the division of airmasses over western
Minnesota...which isentropic lift/mid-level moisture transport into the
front has led to middle clouds in Minnesota. Nothing yet falling out of

Models are in agreement tracking the shortwave trough over
Saskatchewan and Montana into the Dakotas by 00z. What this does is
help continue a warm advection regime downstream into the forecast
area. The big plume of warm 700mb and 850mb air over South Dakota
streams east-northeast across the forecast well as over
central Minnesota and northwest WI...effectively putting a nice cap in
place. Prior to this cap arriving this afternoon...a few days ago it
appeared that the isentropic lift and middle-level moisture transport
would be enough to initiate convection. However...that does not
appear to be the case anymore with most models suggesting a dry day
today. Have still left some low chances in Taylor and Clark counties
for the late afternoon...due to activity over North Dakota prognosticated
to March east across northern WI. Now the 19.00z GFS does produce
some precipitation from eau to lse during the early afternoon hours...but
appears to be the lone model out. Soundings too from the GFS at lse
look odd for producing rain...having just some small layer of
saturation below the cap.

Pressure gradient expected to be quite tight today over the forecast
area ahead of a cold front crossing the Dakotas. With 850mb winds
increasing this afternoon to 35-50 knots southeast to northwest across
the forecast area...and daytime mixing...we could see gusts upwards
of 25-35 knots. Highest gusts are anticipated over higher terrain of
southeast Minnesota. These would be just under advisory criteria. Regarding
the aforementioned mixing...periods of sun plus 925mb temperatures climbing
to 16-20c by 00z...most locations outside of Taylor/Clark should see
highs in the 70s.

For tonight...models bring the shortwave trough into Minnesota and northern
WI...dragging the surface cold front through the northwest half of
the forecast area by 12z Sat. All models suggest that the DPVA ahead
of the trough will help cool the cap...and force some convection to
develop. Appears much of the convection will be rooted above the
boundary layer with parcels being lifted between 850-900mb...or
about 1 km above the ground. Cape lifted from this level ranges from
1500-2000 j/kg while the 1-6km shear is only 10-15 knots. That shear is
certainly unfavorable for severe. If for some reason the parcels can
get rooted below 1 km...the 0-3km shear is 35-40 knots...much more
favorable. Given that equilibrium levels are up near 40000 feet...can
not rule out an isolated severe storm capable of hail/wind...which
looks most likely sometime in the late evening or early morning.
Considered raising precipitation chances up to 60...but both the
19.00z NSSL WRF-arw and NCEP hires-arw suggest anywhere from
isolated-scattered coverage. Appears the lack of decent convergence
between 900-700mb is the cause. All models do agree that the bulk of
the convection should be exiting the area by 12z. It should
generally be a warm night until the frontal passage due to a breezy
wind and higher dewpoints. After frontal looks like a low stratus
deck may form as models suggest a shallow cool airmass moving in.

Long term...(saturday through thursday)
issued at 219 am CDT Friday Sep 19 2014

Compared to the short term period...generally quiet weather
dominates the long term. There are a few time periods to focus on.

General models consensus is for the shortwave trough to move
into/over the forecast area by 18z Saturday...pushing the cold front
all the way through. It appears any convection re-firing along the
cold front on Saturday will occur southeast of the forecast area.
Still maintained some low precipitation chances over the southeast
half of the forecast area in the we wait for
the shortwave trough to fully move in. Then there should be a
break...ahead of a fairly potent shortwave dropping in on the back
side of the shortwave trough over Minnesota by 00z Sunday. This shortwave
dives through the forecast area Saturday night. The potent nature
could be enough to bring some showers across the forecast area...but
there is some uncertainty on moisture availability. A lot of models
have any precipitation with the shortwave diminishing before it
comes into the forecast area...or maybe holding onto it for I-94.
Some cold pool showers...enhanced by a secondary cold front dropping
through the forecast area...could occur on Sunday along and north of
I-94. Otherwise...Sunday looks significantly cooler compared to
Saturday with 925mb temperatures falling from 18-21c at 00z Sunday to 11-
13c at 18z Sunday.

Sunday night through Tuesday continues to be dominated by high
pressure at the surface...while very strong upper ridging marches
from the northwestern U.S. Into the region. This setup will favor a
cool night with potential for valley fog Sunday night...followed by
gradual airmass modification and warming through Tuesday.

For Wednesday and upper trough currently sitting off
the California coast is prognosticated to lift through the region. Tough to
say how much moisture this feature will have with it at this
time...and if its track ends up going through the region or staying
off to the west. Therefore...for now the forecast just has some low
precipitation chances with it. Temperatures could get knocked down a
little bit with this upper trough moving through.

Beyond Thursday...good model run to run consistency exists that
the upper trough will lift out of the response to strong
upper ridging builds over the eastern two thirds of the country
going into next weekend. This upper ridging should bring dry
conditions along with above normal temperatures.


Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 648 am CDT Friday Sep 19 2014

The surface pressure gradient will continue to gradually tighten
this morning. As this occurs...the sustained south winds will
climb into the 15 to 25 knot range. In addition with diurnal
heating...the winds will gust into the 25 to 30 knot range from
19.16z and 20.01z.

A cold front will produce scattered showers and storms at krst
between 20.03z and 20... at klse between 20.05z and 20.09z.
The thunderstorms may reduce visibilities to less than a mile at
times. However the timing of this is too uncertain to include in
the tafs at this time.


Arx watches/warnings/advisories...


Short term...aj
long term...aj

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