Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service La Crosse WI
307 PM CDT Sat Oct 25 2014

Short term...(tonight through Sunday night)
issued at 307 PM CDT Sat Oct 25 2014

Main forecast concerns this period...shra/tsra chances Sun night...
temperatures.

Data analysis at 18z had 1021mb high pressure centered over the
Dakotas. The high was bringing sunny skies and drier air to the
northern plains/upper Midwest. Gradient between the high and a
987mb low west of James Bay along with diurnal warming/mixing
resulting in northwest winds 10-20kt g20-25kts across much of
Minnesota/WI/eastern Iowa early this afternoon. Temperatures again above normal in
the middle 50s to upper 60s across much of MN/IA/WI. WV imagery
showed the next system to impact the upper Midwest coming on shore
over Washington/or/northern California.

25.12z models initializations quite good this cycle. Solutions very
similar for tonight/sun as heights rise and the middle level ridge axis
moves over the upper Midwest. Trend favors stronger of earlier runs
with the ridge axis and a compromise on the troughing/energy moving
through The Rockies. Good consensus as the trough moves in the central/
northern plains Sun night. Sun night trend favors faster/stronger
of the earlier runs with the trough/energy at 12z Monday. Given the
tight consensus...short-term forecast confidence remains good this cycle.

For the short term...quiet weather continues tonight as the surface
ridge axis slides over the area. Clear skies/light winds/dry
boundary layer airmass favorable for strong radiational cooling with
lows dipping into the low-middle 30s...near the normals. The high slips
off quickly on sun with southeast to southwest surface-850mb gradient
flow increasing across the area. Soundings show mixing to 925-900mb
Sun afternoon with around 20kts of wind in the top of the mixed
layer...especially west of the MS river. After brisk northwest winds
today...brisk/gusty southeast winds expected across the higher
terrain of southeast Minnesota/northeast Iowa Sun afternoon. Stronger of the
925-850mb warm advection and increase of surface-850mb moisture remains
South/West of the forecast area sun...until very late in the day. Bulk
of moisture increase Sunday is at/above 700mb in the increasing
southwest flow ahead of the middle level trough. Bulk of the leading
low level warm advection GOES into warming with 850mb temperatures climbing
some 5c to 6c on Sunday. However warmer low levels Sunday will be
offset by the increasing middle/high clouds in the afternoon with
highs similar to those of today.

Will continue to leave Sunday dry for now buy may yet need a small
rain showers/thunderstorms and rain chance over the far west/northwest end of the forecast area
late in the afternoon if more robust models with moisture/lift are
more correct. Surface-850mb moisture increase spreads across the area
Sun night along with stronger of the 925-850mb warm advection/
isentropic lift. Stronger of the lift/greater low level saturation
spread across the northeast half of the forecast area centered on about
06z Monday...ahead of the approaching surface-850mb front/trough. This with
MUCAPE prognosticated to be at least 500 j/kg lifting elevated parcels.
Raised rain showers/isolated thunderstorms and rain chances in the 03z-09z Monday timeframe into
the 50-75 percent range over the northeast half of the forecast area and
mostly 20-50 percent across the southwest half. Used a blend of the
numerical guidance lows/highs for tonight through Sun night.

Long term...(monday through saturday)
issued at 307 PM CDT Sat Oct 25 2014

Main concerns for the Monday through Tuesday night period...shra/tsra chances
Monday into Monday night...lingering rain showers chances Tuesday...temperatures through
the period.

25.12z models offer a slowly improving consensus Monday/Monday night as
the trough/energy lifts toward/into the upper Midwest. Trend favors
faster of the earlier runs with more of a positive tilt trough as
it moves in. The faster trend continues Tuesday as the middle level
trough axis moves east of the area and Tuesday night as heights rise/
shortwave ridging builds into the upper Midwest. Forecast confidence
is average to good in the Monday through Tuesday night period.

More positive tilt of the trough shears more of the shortwave energy
west/north of the forecast area Monday. The surface-850mb trough axis and
stronger low level Thermo-dynamic forcing still pushes east of the
area by middle-day Monday...with drying westerly 850-500mb flow spreading
across the forecast area. Some weak MUCAPE lingers Monday morning...but
given the lack of any focused forcing signals...appears Monday morning
will be dry across much of the forecast area. Some increase of deeper
layered forcing/lift Monday afternoon/evening with approach of the 700-
500mb trough axis and some shortwave energy/pv advection. This with
an increase of moisture in the surface-500mb column centered on 00z
Tuesday. Some MUCAPE of 200-500 j/kg still indicated over forecast area at
18z Monday then over the southeast half at 00z Tuesday. Increased -shra
chances through Monday afternoon then carried 30-50 percent chances Tuesday
evening with the deeper moisture signal and passage of some of
the stronger pv advection. 850-500mb subsidence/drying rather
quickly spreads across the area later Monday night. Trended -shra
down and out for much of the forecast area after midnight Monday night
and removed any thunderstorms and rain mention with cape pushed well south/east of
the area. Continued to trend much of the area dry Tuesday morning with
faster eastward movement of the middle level trough and any forcing.
Question Tuesday is will any deformation band -shra clip the north
end of the forecast area Tuesday afternoon. Consensus shows an increase of
925-700mb moisture to wrap south into the north end of the forecast
area Tuesday afternoon but forcing/lift signals weak. Will continue a
small -shra chance north of Highway 10 Tuesday afternoon for now...mainly
to blend with neighboring grids. Middle level heights rise with shortwave
ridging Tuesday night. However low level ridge axis remains west of
the area with some weak 925-850mb cyclonic flow and moisture
remaining over mainly the WI portion of the forecast area. Lower
clouds may be tough to get rid of over the northeast 1/3 of the
forecast area Tuesday night but will remain optimistic in the forecast grids.
Used a blend of the guidance highs lows for Monday through Tuesday. With
potential for a drier day and some sunshine Monday...highs still
looking to be some 10-15f above normal to start the work week.

For Wednesday through Saturday.../days 4 to 7/...

Medium range model runs of 25.00z/25.12z in good agreement Wednesday as
one trough exits and heights fall again across the northern plains with
the next trough/energy. Improving signal for a stronger secondary
piece of northern stream energy to drop into/across the western
Great Lakes Thursday into Friday. 25.12z European model (ecmwf) back in line with other
models/previous runs at 12z Friday vs. Its strong...outlier looking
25.00z run. Good consensus for rising heights/middle level ridging to
build into the upper Midwest Sat. Forecast confidence is average to
good for the Wednesday-Sat period.

The region is generally between systems Wednesday with surface high pressure
and weak shortwave ridging aloft moving across MN/IA/WI. Weak low
level warm advection spreads over the area Wednesday afternoon but 925mb
temperatures only in the +4c to +8c range at 00z Thursday. Round of lower
level warm advection ahead of a surface low through middle level trough/
shortwave energy to slide across the area Wednesday night/Thu. Bulk of
the moisture with the stronger/deeper of the forcing/lift Wednesday
night looks to be at/above 700mb...with more surface-700mb moisture/
saturation arriving Thursday after the stronger/deeper of the lift has
passed. Small rain chances Wednesday night/Thursday per the model consensus
reasonable until details become clearer. Column too warm to
support -sn later Wednesday night/Thursday morning and removed that mention.
Another chance of light precipitation would spread south across the east
side forecast area Thursday night with the shortwave energy dropping through
the western Great Lakes. Column by late Thursday night would be cold
enough to support precipitation as -ra/-sn or -sn. System would exit
south/east of the area by late Friday...with Friday evening still
looking dry but cool for the trick-or-treaters. Cold/dry can high
pressure looking to settle into Minnesota/IA/WI Friday night/Sat east of the
building longwave ridge axis. Some of the coldest air of the
season looks to settle across the area Friday night. Those locations
that have escaped a hard freeze so far...that is looking to come
to an end Sat morning. Dry/quiet/cool day Sat under the can high
pressure. Model/ensemble consensus of the highs/lows for Wednesday through
Sat looks well trended at this time.

&&

Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1100 am CDT Sat Oct 25 2014

High pressure building in from the plains tonight will keep skies
sky clear/scattered through the better part of Sunday. Generally light surface winds
tonight but not anticipating widespread fog or valley fog. Drier air
continues to funnel into the region while winds just off the surface...
per BUFKIT soundings...will keep the atmosphere stirred.

Warm air advection ahead of an approaching shortwave trough Sunday
night should usher in some initial middle level cloudiness...and
scattered showers. Model trends favoring the better rain chances
along and east of the Mississippi River. Expect a lowering in
the ceiling heights...and a dip into MVFR ceilings Sunday night is likely.




&&

Arx watches/warnings/advisories...
WI...none.
Minnesota...none.
Iowa...none.
&&

$$

Short term...rrs
long term....rrs
aviation.....Rieck

National Weather Service Glossary of Abbreviations