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Area forecast discussion 
National Weather Service La Crosse WI
340 PM CDT Thursday Aug 21 2014

Short term...(tonight through Friday night)
issued at 340 PM CDT Thursday Aug 21 2014

Water vapor imagery showing the strong middle-level wave that gave the
area mainly morning showers and thunderstorms now over eastern WI.
Subsidence in the wake of this wave provided some partial sunshine
to the arx forecast area with temperatures this afternoon in the middle
70s to the lower 80s. Also...dew points were creeping upward into
the middle 60s to the lower 70s for a bit more muggy feel to the

Model consensus shows warm frontal boundary remaining just to our
south overnight with models indicating some 850mb moisture transport
into/over it. This is in advance of a weaker middle-level Washington
deterministic models look bit overdone with quantitative precipitation forecast based on this weaker went with smaller-end shower/thunder chances. With
recent rainfall and relatively weak winds overnight...any breaks in
the cloud could also lead to some fog development.

Friday is looking mainly dry across the area with middle-level ridge
amplifying overhead. However...the boundary will remain just to our
south building instability through the afternoon. No real trigger
mechanism to focus convection other than weak convergence along the
boundary. As such...will go with a 20-30 percent chance of
showers/thunderstorms across portions of northeast Iowa/southwest WI.
Otherwise...plan on highs in the 80s with heat indices creeping into
the 95-100 degree range across far northeast Iowa/far southwest WI.

For Friday night...looking for that boundary to our south lift north
as a warm front. This is in response to surface cyclogenesis over
the Central Plains. Models in good agreement in keeping best 850mb
moisture transport focused more across western IA/MN. As such...kept
higher probability of precipitation focused more across northeast Iowa/southeast Minnesota...tapering
eastward of the Mississippi. Precipitable water values will be quite
high in the 1.5-2 inch any storms will be capable of some
heavier rainfall.

Long term...(saturday through thursday)
issued at 340 PM CDT Thursday Aug 21 2014

Look for the warm front to continue to push northward across the
area Saturday through Saturday night for additional shower and
thunderstorm chances. Will have to keep an eye on the potential for
pockets of heavier rainfall given higher-end precipitable water
values. Saturday afternoon into Saturday evening may also have to be
watched for severe thunderstorm potential as NAM indicating 0-3km
MUCAPE in the 2000-4500j/kg range with bulk shear in the 20-25kt
range. Looks like the highest threat area would be along and west of
the Mississippi River.

Sunday looking hot/muggy/capped as the area becomes entrenched in
the warm sector of surface low lifting north across the Dakotas into
Canada. NAM drawing in 27-31c 925mb air into areas along/west of the
Mississippi River in this warm sector which would dictate
temperatures nearing 100. Took a model consensus for now...yielding
highs in the 90s. With dewpoints in the 70s...resulting heat indices
will be in the 100-105 degree range for areas along and south of I-
90. Will keep an eye on this over the next day or two for a possible
heat headline.

Shower/thunderstorm chances return Sunday night into Monday as a
cold front pushes into the area. This frontal boundary looks like it
will linger across area through at least Wednesday for a continued
chance of showers and thunderstorms.


Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1235 PM CDT Thursday Aug 21 2014

First complex of rain showers/thunderstorms and rain and local MVFR/IFR ceilings/visibilities moving
south/east of the taf sites...but took a while to do so. Much of the
early/middle afternoon now looks to be VFR as the delayed exit of the
first round of convection has lowered confidence in the evolution/
redevelopment of convection for later this afternoon/evening...which
depends on where a surface warm front ends up. Surface pressure trough has
jumped north of the taf sites to near kmsp and north of klse...while
weak convective outflow and boundary layer cooling with clouds/rain
has kept another boundary in central/southern Iowa. Dew point
discontinuity is across the northern trough/boundary. Better lower
level moisture transport/Theta-E convergence is still aligned along/
northeast of the MS river later this afternoon/evening. Will
continue with the thunderstorms in the vicinity/cumulonimbus mentions at krst/klse in the 21-01/02z
time frame for now until a clearer signal is seen. Once this
convection would slide east/south of the area...attention for
tonight/early Friday morning turns to br/fg. Weak surface pressure
gradients over the area tonight as a weak surface low slides into the
area. This along with the recent rains/radiational cooling sets the
area up for br/fg...especially in the 09-13/14z period. Given the
weak gradient and it is now late will be slower to burn
off in the morning...with some 4-6sm haze expected to linger through much
of the morning.

&& into tonight
issued at 340 PM CDT Thursday Aug 21 2014

A warm front will lift northward through the area Friday night
through Saturday night and serve as a trigger for
shower/thunderstorm development. Precipitable water values will be
in the 1.5 to 2 inch range. As such...any storm will be capable of
heavy rainfall. Not thinking any flood headlines will be needed at
this time but certainly warrants a watchful eye.


Arx watches/warnings/advisories...


Short term...das
long term...das

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