Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service La Crosse WI
1138 PM CDT Friday Apr 18 2014

Short term...(this evening through saturday)
issued at 351 PM CDT Friday Apr 18 2014

At 3 PM...a 1031 mb surface high extended from Western Lake
Superior southeast to Green Bay. This high kept skies mainly sunny
across much of Wisconsin and Minnesota arrowhead. Despite the
sunshine...temperatures were around 40 in the Minnesota arrowhead
and northern Wisconsin /due to the snow from Tuesday night into
Wednesday night/ and around 50 across southern Wisconsin...
southeast Minnesota...and northeast Iowa. Further to the west...a
warm front extended from Aberdeen South Dakota to Des Moines Iowa.

For tonight...the 18.12z models are in good agreement that the
warm front will move east across the area. The air mass across the
upper Mississippi River valley remains very dry outside of the 650
to 550 mb range...thus only expecting some middle and high clouds
with its passage.

On Saturday...the area will be located in the warm sector of a low
pressure system moving east across Manitoba. Southwest winds will
cause 850 mb temperatures to warm from around 6c at 19.12z to 10c
by 20.00z. With BUFKIT soundings mixing up to 800 mb expect...this
air will mix down to the surface allowing temperatures to warm in
the middle and upper 60s south of Wisconsin 29. Meanwhile
temperatures will only be in the middle 50s north of Wisconsin 29
where there is still up to 4 inches of snow on the ground. In
addition to mixing warm air to the surface...stronger winds from
aloft will mix to the surface. Due to this...expect sustained south
winds of 15 to 25 miles per hour with gusts to 35 miles per hour. These winds may
dramatically elevate fire weather concerns if the dew points do
not come up fast enough. See the fire weather section below for
more details on this.

On Saturday afternoon...the cold front associated with the
Manitoba low will move close enough to Taylor County to warrant a
15 to 30 percent chance of rain. In addition...with MLCAPES
approaching 100 j/kg there may be enough instability for an
isolated thunderstorm or two.

Long term...(saturday night through friday)
issued at 351 PM CDT Friday Apr 18 2014

From Saturday night into Sunday night...the cold front will push
south toward the Interstate 90 corridor and then stall as the
front becomes parallel with the winds aloft and another wave of
low pressure moves along this front. While there is weak 850 mb
moisture transport into the area...precipitable water values
between 1.00 and 1.25 inches will cause some brief periods of
moderate rain. With rainfall totals up to 1 inch over a long
period of time...this should just slow the fall on area rivers.

From Wednesday night through Thursday night...a negatively tilted
low pressure system will move through the region. Ahead of this
system an elevated mix layer will be brought into the area from
the central and Southern Plains. This will cause an elevated cap
across the region on Thursday and Thursday night ahead of the cold
front. This cap looks to be strong enough to prohibit surface
based convection from developing. While the 0-6 km shear is very
favorable for super cell thunderstorms to develop...the 1-6 km
shear is not favorable at all. With the Wisconsin and Minnesota
state severe weather tests being scheduled that day...we will just
have to keep monitoring that time period.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1138 PM CDT Friday Apr 18 2014

Winds will be on the increase today as the pressure gradient
tightens over the taf sites. Initially this increase is expected
above the ground early this morning...with 40-50 knots southeast to
south winds forecast near 1500 feet. With winds only 8-15 knots at the
surface...have continued low level wind shear. Look for the low
level wind shear to end around 12-13z as the winds begin to mix
down the surface...resulting in southeast to south gusts of 24-28
knots for much of the day. Winds should diminish quickly in the
evening due to loss of daytime heating and the pressure gradient
relaxing with the approach of a cold front. There is the potential
for low level wind shear late in the evening at lse...but marginal
enough to not mention at this time.

Outside of the winds...VFR conditions are anticipated as dry air
holds firmly in place. There could be some showers this evening
associated with lowering ceilings and the approaching cold front
from the west. Have only included a vcsh at rst at this time since
the front will be slow moving.


Fire weather...Saturday afternoon
issued at 351 PM CDT Friday Apr 18 2014

Expect sustained south winds of 15 to 25 miles per hour with gusts to 35
miles per hour. These winds may dramatically elevate fire weather concerns
if the dew points do not come up fast enough. Currently expecting
the dew points to climb into the 40s during the afternoon which
keeps the relative humidity values between 40 and 50 percent.
However if the dew points remain in the 30s...the relative
humidity values would drop into the 20s which would cause The
Grasses to dry out quickly...thus...allowing fires to spread
quickly. Burning is not advised due to the winds and the potential
for lower relative humidities.


issued at 351 am CDT Friday Apr 18 2014

Recent rainfall and snow melt is keeping river levels elevated
across the region...particularly along the Mississippi River. A
front is expected to stall out near the Interstate 90 corridor
from Saturday night through Sunday night. This front will produce
a prolong period of light to moderate rain with rainfall totals
ranging from a half inch to 1 inch. Since this rain is falling
over a prolonged time period...this rain should just keep river
levels higher longer.


Arx watches/warnings/advisories...


Short term...boyne
long term...boyne