Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
600 PM CDT Friday Aug 28 2015
Short term...(tonight through Saturday night)
issued at 300 PM CDT Friday Aug 28 2015
Main forecast concerns this period...rain/rain chances exiting
tonight...clouds Saturday...valley fog potential late Sat night.
Data analysis at 18z had low pressure centered over west-central
Iowa...moving slowly but steadily east. WV imagery depicted a rather
vigorous middle/upper level circulation moving east across Iowa. Moisture
transport and lower level thermodynamic forcing/lift ahead of/under
these features and the divergence aloft producing a broad area of
light to MDT rain over much of WI/southeast Minnesota and northeast Iowa. Few
thunderstorms and rain across north-central Iowa in the strongest/deepest of the lift.
Temperatures on the cool side under the cloud/rain shield ahead of the surface-
middle level low...with warmer temperatures in the region today over central/
northern Minnesota with some sunshine.
28.12z models initialized well for the most part. However GFS was 3f-
5f too low with surface dew points over the west 1/2 of the region and
that much or more too high with surface dew points over eastern WI/IL.
Models in good agreement as the trough/shortwave bringing rain to
much of the area today weakens and moves east of area tonight. Trend
favors faster of the earlier models with the trough continuing to
exiting east Sat and with the ridging aloft building in behind it
Sat/Sat night. Short-term forecast confidence is good this cycle.
In the short term...bulk of the middle/upper level energy with the main
shortwave moves east of the area by 06z. However with the surface-850mb
low/trough axis moving into the west side of the forecast area around
06z...some surface through about 750mb thermodynamic forcing/lift and
moisture transport continue over at least the southeast 2/3 of the
forecast area to about 09z. This aided by a secondary/northern stream
shortwave now set to move south across the area late tonight/early
Sat morning. Continued the downward trend of rain chances across the
northwest 1/2 of the forecast area this evening while raised evening
rain chances across much of the southeast half. Lingered a bit more
rain chance across the southeast 1/2 of the forecast area in the 06-09z
period...then diminished/ended them with passage of lower level
trough axis 09-15z. With minimal to no cape...and the forcing/lift
slowly waning...limited isolated thunderstorms and rain mention to the evening hours.
Dry after 15z Sat and Sat night with rising heights/high pressure
building in behind the departing trough. The 850mb circulation
remaining just southeast of the area Sat...and model soundings
showing moisture trapped under an inversion near 900mb. Lower clouds
in the wake of the system may be slow to clear on Sat...especially
over the southeast 1/3 to 1/2 of the forecast area. Will remain
optimistic that diurnal warming/mixing will work to erode the clouds
through the afternoon. Surface ridge axis nearby/overhead...light through the
lower 2km of the column...clear/mostly clear skies and the recent
rains...late Sat night/early Sun morning appearing very favorable
for radiational valley fog formation. For now added patchy to areas
of fog in the river valleys and fog-prone low laying areas after 06z
Sat night to 14z Sun morning. Plenty of time to detail this one the
current system exits. Used a blend of the guidance lows/highs for
tonight through Sat night.
Long term...(sunday through friday)
issued at 300 PM CDT Friday Aug 28 2015
For Sunday through Monday night...
Main forecast concerns this period...late night/early morning valley
28/12z models in good agreement on rising heights/ridging aloft to
continue building into the upper Midwest sun/Sun night. Trend at 12z
Monday is toward the less robust of the earlier models with the
ridging/heights over the region. Good consensus for the middle level ridge
axis to quickly push east of the region for Monday/Monday night. This
results in more broad southwest flow aloft as some of the energy
ejects out of the northwest Continental U.S. Troughing and across the northern
plains...with some lowering of heights over the region by 12z Tuesday.
Given the reasonable model consensus...forecast confidence in the sun
through Monday night period generally remains good this cycle.
Sun through Monday night continues to shape up as a dry/quiet/warming
period under the ridging aloft building across the region. Southwest
lower level flow between Lee troughing in the western/northern
plains and high pressure over the eastern Continental U.S. Returns warmer and
moist air northeast across the upper Midwest sun through Monday night.
925mb temperatures in the 20c-24c range by 00z Monday and 23c to 27c range by
00z Tuesday. Highs looking to be near normal sun then above normal Monday.
Models do try to bring a weakening trough/front into Minnesota Monday...but
bulk of moisture transport and forcing ahead of/along this front
remains well west/north of the forecast area. Otherwise...broad ridging
northwest of the high centered over the Appalachians holds over the
area for sun through Monday night. With light winds through a greater depth
of the column...mostly clear/clear skies and the moist lower level/
boundary layer airmass...will have to watch for valley/low laying
area fog development again late Sun night/Monday morning. Did not at
this to the forecast grids at this point. Appears stronger winds just
above the nocturnal inversion would limit radiational fog potential
later Monday night/early Tuesday morning. Trended toward warmer of guidance
highs sun/Monday and used the model consensus lows for sun/Monday nights.
For Tuesday through Friday...days 4 to 7...
Main forecast concerns this period...warm temperatures...potential small
rain showers/thunderstorms and rain chances most of the period.
Medium range model runs of 28.00z/28.12z in pretty good agreement
for ridging aloft over eastern noam and troughing over western noam
Tue/Wed...with the ridge axis well east of the upper Midwest. Trend
is for a little flatter ridging over the region Tue/Wed...with
shortwave energy coming out of the western Continental U.S. Trough rippling
trough through the flow. Reasonable agreement for the flow to amplify
and the ridge axis to retrograde to be over the western Great Lakes
Thu/Fri. Forecast confidence in the day 4-7 period remains good for
above normal temperatures but is below normal with the small rain showers/
thunderstorms and rain chances.
With surface high pressure over the Ohio Valley and Lee troughing in the
central/northern plains most of the period...the area remain under
deep layered southwest flow. Airmass over the region in this flow
prognosticated to be rather moist /pw values 1-2 Standard deviations above
normal/...with 500-2k j/kg MUCAPE over much of the upper Midwest
during the afternoons. This with a few weaker and difficult to time
shortwaves to ripple through the southwest flow aloft and across
Iowa/MN/WI through the period. Depending on how much cin there may be
any one afternoon...cannot rule out the weak shortwaves bringing
at least small chances of rain showers/thunderstorms and rain to the area on any of the days.
Also potential for one of the waves to initiate a convective
complex well northwest of the forecast area one or more evenings...
with corfidi vectors indicating potential for any complexes turn
and track southeast across the region as they mature overnight.
Not much for confidence in the details next week but numerous
small rain showers/thunderstorms and rain chances per the model consensus okay for now. More
consistent signal for 925mb temperatures this period to be in the 24c to
28c range...supporting highs in the middle 80s to around 90...some
10f-15f above normal. Again raised highs fro Tuesday-Friday a bit over
the consensus blend...mainly in the river valleys. Consensus blend
of the guidance lows continues to look well trended.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 600 PM CDT Friday Aug 28 2015
Rain shield making steady progress east...and looks to be done at
krst for the 00z taf issuance. -Shra threat likely lingers to 03z or
so for klse.
Expect a lowering in ceilings as the night wears on...with an increase
in the low level moisture per rap/NAM relative humidity fields. Sub 1 kft for the
taf sites still looks on track as the surface low slides south of the
area. The lower ceilings will hold through Sat morning...with some
increasing heights/breaking up indicated via BUFKIT soundings later
in the afternoon.
Visibilities improve outside of the rain...but should drop back into MVFR
territory...accompanying the lowering ceilings. Some suggestions in
the models that visibilities could drop into the IFR to LIFR
categories...but confidence not high in that yet quite yet.