Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
305 PM CDT Monday Sep 1 2014
Short term...(tonight through Tuesday night)
issued at 305 PM CDT Monday Sep 1 2014
Main forecast concerns this period...potential small -shra chances
late tonight and Tuesday...temperatures.
Data analysis at 18z had low pressure over north-central. Trailing
cold front extended across the region near a krhi-klse-kdsm line...
making steady progress southeast at 25-30kts. The trailing Minnesota/Iowa
shortwave and the front producing a broken line of rain showers/isolated thunderstorms and rain
across WI into northeast Iowa early this afternoon. Heavier of these
producing a quick 1/4 to 1/2 inch of rain as they pass. Behind the
front/shortwave/line of rain showers visible imagery showing generally scattered cumulus/
strato-cumulus across much of Minnesota/northwest Iowa/WI with dew points falling
into the middle 50s to lower 60s.
01.12z model initializations not as tight as the past couple days
but no glaring errors noted. Initializations showed early models
were all on the weak side with the secondary shortwave over southern
Minnesota/northern Iowa this morning. This with little impact beyond this
afternoon though. Models offer a tight consensus as a shortwave over
Wyoming tracks east tonight...passing mainly south of the forecast area late
tonight/Tuesday morning. Tight consensus for Tuesday afternoon/night as
heights rise behind this feature and ahead of the next tough into the
Pacific northwest. Trend favors faster of the earlier runs with the
energy into northern Idaho/western Montana by 12z Wednesday. With the tight
model consensus and a quieter weather period...short-term forecast
confidence is good this cycle.
Trailing southern Minnesota/northern Iowa shortwave will pass east of the
area this afternoon...taking the scattered -shra along/ahead of it with
it. Distance speed tool has this area of -shra east/south of the
forecast area around 23z. Model soundings show some persistent 250-500
j/kg of cape over the area this evening/overnight...especially when
lifting 800-700mb parcels. This ahead of the Wyoming shortwave due in
around 12z...and the 300-500mb pv advection/divergence aloft ahead
of it. This wave producing scattered -shra over northeast Wyoming/western South Dakota
this afternoon and several of the hi-res/WRF models indicating it
will produce a few -shra as it passes across Minnesota/IA/WI late tonight/
early Tuesday morning. Thus have added isolated -shra to roughly the
southwest 2/3 of the forecast area in the 06z-15z time-frame. Increasing
middle clouds later tonight should keep any late night/early morning
radiational fog to a minimum. 500-1000 j/kg of cape indicated across
the northeast 1/2 of the forecast area Tuesday...with little if any capping
by Tuesday afternoon. This with some weak pv advection ahead of a weak
shortwave tracking across central Minnesota/ northern WI and weak 850mb
moisture transport and 700-500mb fn convergence. Continued a 20
percent -shra/thunderstorms and rain chance across mainly the northeast 1/3 of the
forecast area much of Tuesday given the consistency of this signal. Rising
heights/waning cape late Tuesday afternoon/night... for what should finally
be a dry period with a weak ridge of high pressure over WI/MN.
Model/ensemble consensus of lows/highs for tonight-Tuesday night looks
to have temperatures well trended.
Long term...(wednesday through monday)
issued at 305 PM CDT Monday Sep 1 2014
Forecast concerns for Wednesday through Thursday night...shra/tsra chances with
the next approaching/passing low/cold front...temperatures.
01.12z model consensus favors faster of the earlier runs with the
Idaho/Montana shortwave east to northern ND/southern man by 12z Thursday. The
faster trend continues Thursday/Thursday night as the main shortwave lifts to
north of Lake Superior by 12z Friday. Trend also favors a stronger
secondary shortwave to drop into Montana/Wyoming by 12z Friday. With the tight
and improving model consensus...forecast confidence in the Wednesday through
Thursday night period is average to good this cycle.
The middle level troughing into Montana on Wednesday deepens the Lee low into
Easter Wyoming/western South Dakota. Southerly low level flow strengthens in the
plains with 850-700mb moisture transport/increase quickly spreading
northeast across the region on Wednesday. Precipitable water values back in the 1.5 to
1.75 inch range by 00z Thursday. Cape increases as well. Surface-850mb warm
front ahead of the low looks to lift across the area Wednesday...to
central Minnesota to near kgrb by 00z Thursday. Stronger of the lower level
convergence lift along/north of this front on Wednesday. Limited small
rain showers/thunderstorms and rain chances Wednesday to the north end of the forecast area closer to
the warm frontal zone as appears airmass south of the warm front is
likely to become capped. Lower level thermodynamic forcing over the
area increases Wednesday night ahead of the surface-850mb low/trough/front.
Main cape pool and lower level convergence/lift appears focused over
central Minnesota Wednesday evening...where a convective complex looks to form.
Corfidi vectors would then take this complex east-southeast across
northern into eastern WI. Carried highest rain showers/thunderstorms and rain chances later
Wednesday night over the northeast 1/3 of the forecast area. 850-700mb
warming continues Thursday...with capping strengthening over the forecast
area. Limited rain showers/thunderstorms and rain chances Thursday to mainly the northeast side of
the forecast area and in the morning. This even as the low lifts to
north of Lake Superior Friday and the trailing cold front is dragged into
the area. Model soundings showing cap erosion Thursday night as the 850-
500mb trough axis/cooling move into the area. Surface front to push
across the area Thursday night...with a scattered-broken band of rain showers/thunderstorms and rain along/
behind it. Consensus rain chances around 50 percent Thursday night
reasonable. Used a blend of the guidance highs/lows for Wednesday and
Wednesday/Thursday nights. With the capping and 925mb temperatures again in the +22 to
+26c range Thursday...favored warmer of guidance highs in the middle/upper
80s for Thursday.
For Friday through Monday...days 4 to 7...
Main concerns this period...cooler temperatures Friday through sun...
rain showers/thunderstorms and rain chance Monday.
Medium range model runs of 01.00z and 01.12z continue the trend of a
stronger middle level trough to pass across southern can/northern Great
Lakes Friday. Decent model consensus lingers some broad troughing/
northwest flow over the upper Midwest Sat...progressing this east
with nearly zonal flow across the northern Continental U.S. For Sunday. The
nearly zonal flow appears short-lived once again with models showing
more energy/troughing to dig into the western Continental U.S. Monday and falling
heights over the north-central Continental U.S.. with reasonable agreement among
the latest model runs...forecast confidence in the day 4 to 7 period is
average this cycle.
Trend is a bit faster with the trough moving northeast of Lake
Superior Friday...which drives the surface cold front well south of the
area more quickly on Friday. Drier/cooler can high pressure builds in
more quickly behind it on Friday with Friday now trending mainly dry.
This drier/cooler high then slated to dominate the upper Midwest/S
weather through the weekend. 925mb temperatures fall to the +12c to +16c
range Friday...remain there Sat then moderate a bit to +14c to +17c
sun. Friday- Sat-sun looking to be 3 cooler...more fall-ish days
with highs in the upper 60s-middle 70s and lows in the upper 40s-middle
50s. Lee troughing develops in the plains Monday in response to
the western Continental U.S. Troughing and a lead piece of energy ejecting
into the northern plains. The can high moves east Monday with
southerly flow... warm advection...moisture transport returning
northward through the plains ahead of the low/Lee trough. GFS more
progressive with the low/moisture return and would spread rain showers/
thunderstorms and rain chances into the forecast area late Sun night/Mon. Small rain showers/
thunderstorms and rain chance Monday per the model/ensemble consensus okay for now.
Model/ensemble highs/lows appear to have Friday-Monday highs/lows well
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1239 PM CDT Monday Sep 1 2014
Main aviation forecast concerns is fog potential in the Mississippi
River valley tonight. Latest karx radar indicates last line of
showers along surface cold front extending from western Wisconsin
into northeast Iowa per latest 17z surface analysis. Latest 17z
metars are indicating ceiling heights rising above MVFR conditions
and becoming VFR conditions at taf sites this afternoon. Weak
surface ridge builds into northern Iowa behind cold front and allows
skies to clear west to east across the area this afternoon into this
evening. The 02.12z NAM BUFKIT sounding shows inversion developing
at lse taf site and winds become light within the lowest 500
feet...however BUFKIT sounding show 15 knots just above 500 feet
plus a weak impulse produces a middle level cloud deck over the region.
This would inhibit any fog development in the Mississippi River
valley and at lse taf site. At this time...did not introduce any fog
at lse taf site and later shifts will have to monitor if skies
remain clear through the night time hours. VFR conditions will
prevail at rst taf site through much of the period.