Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1141 PM CDT Wednesday Oct 22 2014
Short term...(tonight through Thursday night)
issued at 250 PM CDT Wednesday Oct 22 2014
Main forecast concerns this period...rain showers chances tonight/Thu...
18z data analysis had high pressure over Michigan with ridging to Arkansas. Low
pressure was located over southern man with a trough/front south to
the Kansas/Colorado border. The high was holding clear skies across much of
WI/IL/eastern Iowa while clouds were increasing across Minnesota/western Iowa
ahead of a rather strong trough/shortwave energy seen in WV imagery
moving east across the Dakotas/neb. Rain showers/thunderstorms and rain early this afternoon
generally along/behind the surface-850mb trough/front in the eastern
Dakotas to central Nebraska.
22.12z models initialized well...except GFS with its 3-5f high bias
on surface dew points across the upper Midwest/northern plains. Solutions
trending toward a tighter consensus as the plains trough axis moves
into western Iowa/Minnesota 12z Thursday...into eastern WI by 00z Friday then into Michigan
and Ohio Valley by 12z Friday. Latest tighter consensus favors faster of
the earlier runs with the approach/passage of the trough tonight
through Thursday night. Short-term forecast confidence is good this cycle.
For the short term...bulk of the 500-300mb pv advection with the
approaching shortwave energy/trough moves across the west half of
the forecast area later tonight and across the east half Thursday morning.
This with MDT lower level Thermo-dynamic forcing/isentropic lift
with/ahead of the surface-700mb trough...precipitable water values of 1 to 1.25
inches and 85-100 percent saturation in the 850-500mb layer.
Present timing has the strongest/deepest lift/moisture moving
across krst around 09z... klse around 12z and kvok around 15z.
Used this to time 85-95 percent rain showers chances east across the forecast
area...mainly between 06z and 18z. NAM/GFS showing weak MUCAPE of
up to 150 j/kg when lifting 850mb parcels this evening into the
overnight hours. Added an isolated -tsra mention between roughly
07z-15z when the strongest/deepest of the lift passes. Most
precipitation amounts tonight/early Thursday continue to trend on the light
side with weakening 925-700mb moisture transport/Theta-E
convergence as the trough passes.
With the trend toward faster...subsidence/drying at and above 925mb
behind the surface-500mb trough spreads into the forecast area Thursday
afternoon. Locations along/west of the MS river once again have a
chance of seeing the partial solar eclipse late Thursday afternoon. With
another weak surface-500mb trough/shortwave into Minnesota/Iowa by 12z Friday...any
cooling behind the Thursday morning trough is weak/short lived. 925mb
temperatures in the +12c to +15c range at 12z Friday...under southwest flow/
warm advection aloft. Used a blend of the guidance lows/highs for
tonight/Thursday then trended toward warmer of guidance lows for Thursday
Long term...(friday through wednesday)
issued at 250 PM CDT Wednesday Oct 22 2014
Forecast concerns for Friday through Sat night...temperatures through the
22.12z models converging on a tighter consensus Friday/Friday night...as
one weak shortwave slides across the upper Midwest Friday and a second
stronger wave drops toward/across Lake Superior Friday night. Trend is
stronger with the Friday night wave. Good consensus for this wave to
continue quickly east Sat and for ridging/rising heights to build
across the northern plains into the region Sat night. With the good
model consensus through Sat night...forecast confidence in this period is
good this cycle.
Weak shortwave moving across the region Friday has a modest surface-700mb
trough with it. Ahead of this rough is a round of lower level warm
advection and moisture increase. Forcing/lift aloft is weak and the
system is moving rather quickly across the area. The moisture
increase is prognosticated mainly in the 925-850mb layer. Will leave any
mention of -shra out of the grids on Friday but did increase the cloud
cover much of Friday into Friday evening as the moisture axis passes.
925mb temperatures in the +13c to +17c range Friday afternoon. Fully realized
these 925mb temperatures would allow for highs in the upper 60s to middle 70s
on Friday. Given the lower strato-cumulus cloud potential...trended highs
more in the low-middle 60s for now. Wave dropping across Lake Superior
Friday night sends a round of subsidence/ drying and stronger low level
cold advection across Minnesota/IA/WI Friday night/Sat. Warmest 925mb airmass
moves across the area Friday evening ahead of the cold front. Even with
the frontal passage...lows Friday night to be on the mild side with
some winds for boundary layer mixing ahead of/behind the front. Sat/
Sat night continue dry/quiet as can high pressure builds in. 925mb
temperatures still in the +8c to +12c range Sat afternoon...for above
normal highs. Surface ridge axis is over the area Sat night...with clear
skies/light winds for strong radiational cooling. Model/ensemble
consensus of the highs/lows for Friday through Sat night look good...
though did raise Friday night lows degree or 2 above these values.
For Sunday through Wednesday.../days 4 to 7/...
Forecast concerns this period...shra/tsra chances late sun into
Tuesday... temperatures through the period.
22.00z and 22.12z medium range models in good agreement Sunday for
ridging aloft to move across the region...ahead of stronger/longwave
troughing moving into The Rockies. Improving consensus as The
Rockies trough/energy moves into the central/Southern Plains Monday.
Trend sun/Monday leans toward the slower the earlier runs with The
Rockies/plains trough. Models have the pattern remaining progressive
Tuesday...but plenty of detail differences as the trough moves into the
middle/upper MS valley. Still a better/slow consensus than the previous
runs on Tuesday. Considerable model differences with the flow across
noam by Wednesday...with run-to-run consistency on the low side by day 7.
Forecast confidence above average sun/Monday then below average by Wednesday.
High pressure holds over much of the region sun...sliding east of
the region Sun night. Increasing warm advection and 850-700mb
moisture transport spreading across western/central Minnesota/Iowa Sun
afternoon then increase across Minnesota/IA/WI Sun night. Initially a
rather deep dry airmass over the area sun...with mainly increasing
middle clouds. Precipitable water values increase to an inch or more Sun night. This
with some MUCAPE of 200 to 500 j/kg in the inflow airmass Sun
night...then MUCAPE of 200 to 500+ j/kg over the area Monday/Monday night.
Moisture/lift continue to spread across the area Monday into Tuesday...but
difference with timing/track of lower level features and stronger
forcing by Monday...then with timing of the middle level trough axis
passage Tuesday. Did add an isolated thunderstorms and rain mention for Sun night through Monday
night given the cape and what should be at least broad forcing for
ascent over the region. Lingering -shra chance Tuesday with the middle
level trough still in the region. Tuesday night/Wednesday should be a dry/
cooler period in the wake of the passing trough and rising heights over
the upper Midwest...however models at odds with the timing of one
trough exiting and the next one approaching. Will leave the model/
ensemble consensus as is for Tue/Wed.
Sun/Monday continue to trend toward a seasonably mild period with 925mb
temperatures in the +7c to +11c range at 00z Monday and +8c to +15c range at
18z Monday...1 to 1.5 Standard deviations above normal. Clouds/rain showers
likely to impact highs Monday but both sun/Monday should see highs in the
upper 50s to middle 60s. Low level cold advection spreads across for
Monday night/Tue... but temperatures for Tuesday/Wednesday look to only trend back
toward late Oct normals. Model/ensemble consensus of highs/lows for
sun through Wednesday has this well trended for now.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1141 PM CDT Wednesday Oct 22 2014
Confidence on how the rest of tonight into Thursday morning is
going to play out is not very high. The showers to the west with
the front have become scattered...which has been handled quite
nicely by the hrrr this evening. The 23.02z hrrr continues this
trend through the rest of the night into Thursday morning of
moving the showers steadily east but keeping them scattered as
opposed to the 23.00z NAM which continues to insist on an
increasing coverage trend despite it showing a decrease in the
isentropic up Glide and frontogenesis with the front. Have trended
toward the hrrr and have gone with some showers but for the most
part not expecting much if any visibility restrictions to occur.
What the ceilings will do is another concern. VFR ceilings are
predominant across Minnesota with some MVFR conditions mixed in.
All the models still insist that these will go down to IFR
overnight but have doubts about this if there will be less showers
around to help moisture the lowest levels of the atmosphere and
have opted to only go down to MVFR. These ceilings should hang on
through the morning and then scatter out during the afternoon.