Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
637 am CDT Tuesday Sep 23 2014
Short term...(today and tonight)
issued at 345 am CDT Tuesday Sep 23 2014
At 3 am...a 1026 mb surface high was centered over northern
Indiana. This high was producing light surface winds across the
region. The combination of light winds and clear skies has
resulted in dense fog forming at Black River Falls...Medford...
Sparta...and Volk Field. From the office...we can see fog forming
in the root... La Crosse...black...and Mississippi River valleys.
Meanwhile the winds at the office and Granddad Bluff are in the 10
to 20 miles per hour range. The latter is based off the flag. This is in good
agreement with what the NAM soundings are showing. Both the GFS
and rap are too low with their wind speeds at least at 550 to
700 feet. With the stratus and fog already developing in the
local River Valley... expect that the nocturnal inversion now
extends at least up to 500 feet. With the NAM continuing to show
light winds below the inversion...the fog will continue to develop
in the river valleys and become dense by 5 am to 530 am....and
then persist through 930 am. NAM soundings also show light winds
and shallow inversion across central and north central Wisconsin.
So expect the fog to continue to become more widespread there too.
This is in good agreement with many of the mesoscale models. However
with such a shallow inversion /less than 200 feet/...there is
concern that the dense fog could come and go. We have already seen
this at several sites...so no plans to issue a dense fog advisory
at this time.
Tonight a short wave will move slowly east toward the area...weak
925 mb and 850 mb moisture transport develops west of the
Mississippi River valley during the evening...and then continues
through the night. This causes the soundings to slowly saturate
the column between 800 and 600 mb. As this occurs...expect isolated
to scattered showers to start to develop across southeast
Minnesota and northeast Iowa between 10 PM and 3 am. Rain amounts
will be under a tenth of an inch.
Long term...(wednesday through monday)
issued at 345 am CDT Tuesday Sep 23 2014
From Wednesday into Thursday morning...a trough will reside across
the western and northern portions of the forecast area. With weak
925 to 850 mb moisture transport into the region along and ahead
of this trough...expect the isolated to scattered showers to
continue across northeast Iowa...southeast Minnesota...and west
central and north central Wisconsin. On Thursday this trough gets
sheared apart with the northern part of the trough being absorbed
by the polar jet across southern Canada.
Meanwhile...the southern portion of this trough will dig
southwest as a 90 knot 250 mb jet moves around the western portion of
this trough. This system eventually forms into an upper level low
late this week over Kansas. This system then open up and gets
kicked northeast into the middle Mississippi River valley somewhere
between Sunday and Monday. With timing uncertainties just kept the
forecast dry for now.
Even though there is a strong upper high over the region from
Thursday night through Saturday night. The 850 mb temperatures
remain around 14c /which is 2 Standard deviations above normal/.
This will result in temperatures in the middle and upper 70s /maybe a
few lower 80s/ for this time period. This is about 10 degrees
Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 637 am CDT Tuesday Sep 23 2014
Valley fog and stratus has developed in lse...with LIFR ceilings
and visibilities observed. There is potential over the next 2-3
hours for the visibility to drop to vlifr...but predominantly LIFR
conditions should be observed. Look for the valley fog and stratus
to dissipate between 9-10 am just like yesterday.
Otherwise...the rest of the forecast is VFR...due to dry air in
the lowest 5000 feet above the ground provided by high pressure.
Note...though...there will be an increase of altostratus tonight
as a low pressure trough approaches from the west. There could
even be a few showers impacting rst.