Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
711 am CDT Wednesday Sep 17 2014
Short term...(today and tonight)
issued at 318 am CDT Wednesday Sep 17 2014
Latest infared satellite imagery indicate stratocumulus deck across
southern Minnesota into Iowa. This is in association with a weak
impulse over southwest Minnesota and northwest Iowa evident on
latest water vapor satellite imagery. Latest 07z surface analysis
shows ridge of high pressure extending from northern Illinois
into northern Wisconsin. With clear skies and light winds east of
the Mississippi River...temperatures have fallen into the middle
to upper 30s across central Wisconsin per latest 07z metars.
Main forecast concern tonight is fog potential in river valleys
and cloud formation over northern Wisconsin.
The latest 17.00z GFS/NAM push weak impulse south of the forecast
area this morning and this will continue to advect the stratocumulus
deck over the forecast area through 18z. Any residual moisture
left with departing impulse will evaporate due daytime heating and
allow skies to become mostly sunny after 18z.
Focus turns to fog potential tonight. The latest 17.00z
GFS/NAM/ECMWF continue to push weak surface front north to south
across the forecast area this evening. All models indicate 850mb
moisture along weak surface front and produce clouds east of the
Mississippi River...mainly over central Wisconsin. The question will
be how far south and west the deck of clouds will be to inhibit any
River Valley fog formation after 06z Thursday. The 17.00z GFS/NAM
BUFKIT soundings show winds light enough under inversion after 06z
Thursday and near surface boundary layer moistening. This would
suggest fog is possible in river valleys and will depend on if any
clouds move over southwest and western Wisconsin to inhibit
formation of fog. For now...have left mention of fog in river
valleys in forecast.
Long term...(thursday through tuesday)
issued at 318 am CDT Wednesday Sep 17 2014
The 17.00z GFS/NAM/ECMWF linger the 900-850mb moisture over
central Wisconsin into Thursday per BUFKIT soundings. If clouds
form and linger over this area. Temperatures may not reach the
current forecast highs over central Wisconsin.
Main forecast concerns Thursday night through Saturday are
precipitation chances. The 17.00z GFS/NAM/ECMWF/Gem continue to
indicate timing differences with shortwave trough and associated
surface front moving into the Great Lakes region. The 17.00z European model (ecmwf)
is trending towards a broader and more progressive solution than the
GFS/Gem. The 17.00z GFS/Gem/ECMWF/NAM focus lift/moisture
transport and convergence over the northern sections of the
forecast area mainly after 18z Friday in association with the first
impulse. Hence...higher precipitation chances over this area.
Shortwave ridge remains entrenched over the southern forecast area
and allow for lesser to no precipitation chances over the southern
forecast area Friday.
Main shortwave trough digs into the Great Lakes region and surface
front moves across the forecast area Friday night into Saturday
morning. With the timing differences between the 17.00z
GFS/NAM/ECMWF/Gem...confidence in exact timing of thunderstorms
continues to be low. The 17.00z GFS/Gem/ECMWF/NAM indicate decent qg
forcing/lift/moisture transport and convergence associated with
trough/surface front Friday night into Saturday morning. Much of
the area should see showers and thunderstorms. Models continue to
forecast high precipitable water values...instability and shear with
the surface front/shortwave trough. A few of the thunderstorms
could potentially become severe.
Surface ridge builds into the Great Lakes region Sunday night into
Tuesday and provide drier weather conditions across the forecast
area. Cooler air will filter in behind the surface front/shortwave
trough and temperatures should be near normal early next week.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 711 am CDT Wednesday Sep 17 2014
VFR conditions are expected until the end of the period when clear
skies and calm winds will again cause the potential for fog. A
weak cold front moving southward out of the northern Great Lakes
will also bring an MVFR or lower VFR cloud deck toward the taf
sites by late in the taf period. Ahead of the front...clear skies
and radiational cooling will lead to valley fog and stratus
formation. Have introduced these conditions Thursday
morning...with the idea that future forecasts will refine the
details. If the MVFR clouds move in from the north...IFR ceilings and
visibilities would be more unlikely at klse...or shorter lived at least.