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Area forecast discussion 
National Weather Service La Crosse WI
1149 PM CDT Thursday Aug 21 2014

issued at 908 PM CDT Thursday Aug 21 2014

The main focus for the overnight hours is with convective trends
and the potential for widespread fog.

So far this evening...the main convection has been tied to a
narrow corridor along and ahead of a surface cold front/low that
runs from northwest Wisconsin through south central Minnesota.
22.01z rap analysis shows that SBCAPE is on the wane with the two
main areas of convection along the northern gradient in
instability...northwest WI...and the other main cells along the highest
instability axis poking into northwest Iowa. The main convergence will
remain with this slow moving front through the night though
surface/low level winds will be decreasing through the night thus
weakening this convergence. While surface/ml instability will be
decreasing through the night...0-3km MUCAPE actually gets a bump
up per the 22.01z rap as the front approaches late. Thermal
profiles are fairly it will take quite the updraft to
get any of the convection to go severe which does not look like it
will happen based on the weakening convergence. Still may manage
to see a few showers...just not expecting anything that would get
out of hand.

With a humid air mass in place with surface dew points in the low
70s...fog still looks like a possibility overnight. The 21.21z
sref is locking onto northern Wisconsin where it appears that a
weak warm front is located and some locations have already started
to drop below 1sm. With winds lightening up as the low/front
approaches...some more sites could develop fog overnight if the
skies can clear out. High level cloud cover is blowing off of the
convection in southwest Minnesota and could initially slow things
down...but expect that skies will continue to clear and lead to
this fog development. Confidence is not high enough in the
when/where to issue a dense fog advisory...but some portion of the
area may need one later.


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 06z tafs through 06z Friday night)
issued at 1149 PM CDT Thursday Aug 21 2014

The main focus is on the fog overnight as much of the convection
is staying well off to the northeast of the taf sites. With a
humid air mass in place and winds lightening up...some widespread
fog is expected overnight...possibly dense. Conditions should
improve by late morning as temperatures Jump Up and cause the fog
to mix out. The main negative for the fog is with some middle level
clouds that are coming across the region from earlier convection
that occurred along the Minnesota/Iowa border. Once that clears
out...the fog should develop.

&& 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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