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Area forecast discussion 
National Weather Service La Crosse WI
326 PM CDT Tuesday Apr 21 2015

Short term...(this evening through wednesday)
issued at 326 PM CDT Tuesday Apr 21 2015

Currently as of 20z...water vapor loop and rap 500mb height analysis
showed ridging over the western U.S. Into Alberta and very deep
troughing over the forecast area into the Great Lakes. Within this
trough was an upper low over Eastern Lake Superior...a shortwave
dropping south towards I-90 in the forecast area...and another
shortwave over northeast Minnesota. Cool temperatures aloft reflected by
500mb temperatures of -27 to -33c per 12z radiosonde observations...daytime heating...near
saturated conditions between 700-850mb and the aforementioned
shortwaves helping to produce plenty of showers across the
region....especially along and north of I-94. Temperatures have
struggled to rise with clouds...precipitation and 850mb temperatures of -3 to
-7c. Readings have only climbed into the middle 30s to low 40s. The
cold temperatures have supported a mix of rain and snow...with the
snow confined moreso to higher elevations and Taylor/Clark
counties. On top of the cool air...a tight pressure gradient and
925mb winds of 25-35 knots has resulted in winds gusting to the 925mb
speeds...making it feel fall-like. These winds are just below
advisory criteria.

The overall upper pattern changes little in the next 24 hours with
only a slight drift eastward of the upper low and troughing. This
slight drift looks to be enough such that much of the shortwave and
instability induced showers on Wednesday will fall east of the
forecast area. However...portions of Taylor and Adams could get
clipped in the afternoon. Getting back to tonight...general model
consensus suggests the loss of daytime heating will allow shower
activity to become more isolated through the evening...eventually
ending by midnight.

Clouds look to hold over much of the forecast area tonight...aided
by a surge of 850mb moisture currently in northwest Ontario dropping
southward with the shortwave in northeast Minnesota. Subsidence in the wake
of this shortwave...combined with 925-700mb winds backing a bit more
westerly on Wednesday...will help to result in more sunshine.

Despite the clouds tonight...and a persistent breeze...temperatures
will still fall as that shortwave in northeast Minnesota drags colder air
into the forecast area. By 12z Wednesday...850mb temperatures are prognosticated to drop
to -8 to -11c...or almost 2 Standard deviations below normal.
Guidance lows in the middle 20s to low 30s seem reasonable. Having more
sunshine on Wednesday will help produce a warmer day than
today...albeit still well below normal given 850mb temperatures only
climbing to -2 to -7c by 00z Thursday. Most locations should climb into
the 40s...with low 50s likely in river valleys and the southwest
portion of the forecast area.

Long term...(wednesday night through tuesday)
issued at 326 PM CDT Tuesday Apr 21 2015

The upper flow pattern looks to become more progressive through the
end of the week in response to a deep upper trough approaching the
Pacific northwest. This will help to push the upper ridging and
warmer air over the western U.S. Towards the forecast area. 850mb
temperatures do not change a whole lot Wednesday night...but more impressive warm
advection comes in for Thursday with 850mb temperatures reaching 0 to -2c by 00z
Friday...and 0 to 4c by 00z Sat. Low precipitable water on
Thursday...around 0.1 inches...should help to tank dewpoints and
resulting rhs. Combined with a drying day on Wednesday and a 10-15
miles per hour northwest breeze...fire weather could be a concern. See fire
weather section below.

For Friday into Saturday...an upper low currently near Los Angeles
is prognosticated to lift through the Central Plains and track eastward
across Illinois. Although this track is south of the forecast
area...moisture advection spreads northward ahead of the upper low
to help produce some rain. The speed has slowed down a bit...and as
a result delayed the spreading of chances of rainfall eastward on
Friday. Further delays may be needed. Rain chances spread across the
forecast area Friday night and then shift to south of I-90 on
Saturday...resulting from easterly flow dragging more dry air into
northern sections. Overall appears to be a light rain...with the
heavier rain to the south tied to the upper low forcing.

Drier weather looks to follow Saturday night and Sunday as upper
ridging builds back into the forecast area from the west.
Temperatures should rebound closer to normal as well with more sun
on Sunday and 850mb temperatures in the 0-4c range.

For early next week uncertainty begins to increase. Not so much on
the upper pattern where models advertise troughing developing from
the northeast U.S. Into the southeast u... with precipitation
chances as this trough develops. The 21.12z GFS has a shortwave
trough dropping through the region to bring precipitation to the
area. Meanwhile the 21.12z European model (ecmwf) and Canadian suggest a dry
period as high pressure dominates at the surface...with the
aforementioned shortwave trough falling apart. For now have went
with the consensus and have 20-30 percent chances. How this
shortwave trough evolves also has an impact on temperatures. In
the case of the 21.12z GFS...it suggests 850mb temperatures plunge to
around -4c by 12z Tuesday...about 8c colder than the European model (ecmwf). Also
followed consensus here for now with temperatures just shy of normal.

&&

Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1201 PM CDT Tuesday Apr 21 2015

Expect windy conditions out of the northwest to continue through
the taf period with gusts of 25 to 35 knots this afternoon. VFR
should persist with a brief MVFR ceiling possible with a heavier
shower this afternoon. Some guidance hints at MVFR ceilings early
Wednesday morning as back edge of clouds move through.
However...with mix of VFR and MVFR upstream across Minnesota kept
ceilings at VFR with scattering out by middle Wednesday morning.



&&

Fire weather...
issued at 326 PM CDT Tuesday Apr 21 2015

After some recent cool/wet days...conditions look to dry out again
Wednesday and Thursday. Relative humidities are expected to drop
into the 20s on Wednesday and may even drop below 20 percent in a
few southeast Minnesota locations on Thursday. Figure much of Wednesday
will be a drying day for fuels given the wet weather...with a 15
to 25 miles per hour northwest breeze aiding in the drying. Therefore
Thursday would appear to be the more concerning day...though winds
look lighter in the 10 to 15 miles per hour range. Nevertheless...we\ll have
to keep a close eye on Thursday and see too where all the
vegetation stands.

&&

Arx watches/warnings/advisories...
WI...none.
Minnesota...Wind Advisory until 6 PM CDT this evening for mnz086-087-094-095.

Iowa...Wind Advisory until 6 PM CDT this evening for iaz008>010-018-019-
029-030.

&&

$$

Short term...aj
long term...aj
aviation...cirrocumulus
fire weather...aj

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