Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
245 PM CDT Sat Aug 1 2015
Short term...(tonight through Sunday night)
issued at 245 PM CDT Sat Aug 1 2015
Main forecast concerns this period...shra/tsra chances tonight into sun
evening...any severe threat tonight and again Sun afternoon/evening.
Data analysis at 18z had high pressure over the middle MS valley with
ridging north into MN/WI. Winds lighter across the region today with
the surface-850mb ridge axis over the upper Midwest. Moisture gradually
increasing over Iowa/southern Minnesota as lower level flow becomes more west
and southwest. WV imagery depicted a shortwave moving southeast
across the Dakotas...with increasing clouds/few rain showers ahead of it
across western Minnesota. Early afternoon temperatures near normal for the first
No big issues noted with 01.12z model initializations. Solutions
quite similar for tonight through Sun night...and very similar to their
runs of 31.12z. This as one piece of shortwave energy drops Iowa/
southern Minnesota tonight then northern stream energy dropping out of man/
western ont lowers heights across the region as it passes sun/Sun
night. With the tight consensus...short term forecast confidence remains
generally good this cycle.
In the short term...shortwave approaching/passing across Iowa/southern
Minnesota tonight spreads a round of lower level Thermo-dynamic forcing/
lift across the area...ahead of the surface-700mb trough/front. Model
soundings continue to show the bulk of the moisture/saturation is at
or above 800mb. MUCAPE values in the 500-1500 j/kg range are mostly
lifting 800-700mb parcels...where the cin values are more in the 0
to 25 range. Sounding show lifting surface based parcels tonight will
generate some cape...but not before encountering 100-400 j/kg of cin
that is 200-300mb deep. Surface-850mb layer involvement in convection
tonight...where most of the shear lies...looks minimal with mainly
elevated rain showers/thunderstorms and rain at or above 8k-10k feet. Bulk shear in the
effective 2km-8km layer more in the 115-20kt range. Given all
this...confidence in any severe storm threat tonight is low. Will
maintain an isolated severe thunderstorms and rain mention tonight for continuity
sake. Tonights round of convection if it develops...as there are a
few hi-res/WRF models that keep the area dry tonight...would exit
the area early Sun morning. Surface front works its way southeast across
the forecast area sun but model soundings showing considerable capping/
cin between 900-850mb much of the day. Lowered/removed rain showers/thunderstorms and rain
chances across much of the forecast area for later Sun morning.
Continued rain showers/thunderstorms and rain chances across mainly the east 2/3 of the forecast
area Sun afternoon with the front dropping through...erosion of the
capping after 20-21z and approach of the stronger shortwave from
the north. If any storms later Sun afternoon into sun evening are
surface based...0-6km bulk shear would be more in the 35kt range along
with MUCAPE in the 1.5k to 3k j/kg range. Cannot rule out a severe
thunderstorms and rain or two over the east/south ends of the forecast area later Sun
afternoon/early sun evening. Front drops quickly south of the forecast
area sun evening...with drier high pressure/subsidence/low level
cold advection spreading in behind it and the middle level trough axis.
Used a blend of guidance lows/highs for tonight through Sun night.
Long term...(monday through saturday)
issued at 245 PM CDT Sat Aug 1 2015
For Monday through Tuesday night...
Main forecast concerns for Monday through Tuesday night...temperatures.
01.12z models remain in good agreement as the east can middle level low
and troughing over the Great Lakes slowly progress east Monday through Tuesday
night. Heights gradually rise this period...with the region remaining
under northwest flow aloft. Models in reasonable agreement on some
shortwave energy coming out of the West Coast troughing and moving
into the central/northern plains by 12z Wednesday. Forecast confidence for Monday
through Tuesday night is good this cycle.
Monday through Tuesday night shaping up to be a dry...quiet period with temperatures
a bit below normal. Drier can high pressure in the wake of sundays
cold front builds slowly east across the upper Midwest Monday through Tuesday
night...under the slowly rising heights aloft. 850mb temperatures trending to
be 0.5 to 1 Standard deviation below normal through the period. Normal
highs for the first week of Aug are in the 79f to 84f range. Model/
ensemble consensus highs mostly in the 75f to 82f range Monday/Tuesday
looking well trended. With a drier airmass...light winds and clear/
mostly clear skies...Mon/Tue nights favorable for stronger
radiational cooling. Consensus lows mostly in the low to middle 50s
both nights appear well trended for now. However will have to watch
the normally cooler/sheltered locations along/northeast of I-94 for
some lows potentially dipping into the low to middle 40s both nights.
For Wednesday through Saturday...days 4 to 7...
Main forecast concerns this period...shra/tsra chances later Wednesday night
into Sat...temperatures through the period.
Medium range model runs of 01.00z and 01.12z in reasonable agreement
on Wednesday with troughing near the coasts and a middle level ridge axis
over central noam. Plenty of detail differences with shortwave
energy coming through the ridge on Wednesday. Quite a bit of between model
and run-to-run variability on Thursday as stronger shortwave energy would
top/flatten the ridging and start to drop into the upper Midwest.
Some consensus between GFS/European model (ecmwf) for a stronger shortwave trough to
move across the region Thursday night/Fri...weak shortwave ridging Friday
night then another stronger wave to top the ridge and drop southeast
into the region Sat. Given the larger detail differences Wed/Thu...
forecast confidence in the day 4-7 period is below average this cycle.
Any shortwave energy to approach/move in Wednesday appears weak. A good
model consensus to hold drier surface high pressure over the upper
Midwest Wednesday...with the main lower level moisture/cape axis well
west/south of the forecast area until later Wednesday night into Thursday. Moisture
increases Thursday...ahead of the first shortwave to top the ridge and
low pressure advancing east across the plains toward/into the
middle/upper MS valley. Increasing rain showers/thunderstorms and rain chances Thursday into Friday as
this surface-middle level trough axis would pass reasonable at this point.
Timing differences by Friday night/Sat...but with another shortwave
looking to top the ridge and lower heights over the region...small
consensus rain showers/thunderstorms and rain chances these periods okay for now. Northwest
flow aloft and/or more clouds /especially Thursday/Fri/ looking to keep
temperatures near to below normal in the day 4-7 period. Model/ensemble
consensus with highs/lows a few degrees below normal looks to have
them well trended for now.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1130 am CDT Sat Aug 1 2015
Expect scattered rain showers/ts to move across the region this
evening/overnight...but how widespread and timing is not clear. Mesoscale
models Don/T have a consensus...although middle evening into the early
overnight is favored. Confidence not high enough to include a tempo
period for ts...and will hold with thunderstorms in the vicinity until trends become clearer.
Storms look to be high based/VFR...but the stronger ones could
produced some enhanced wind gusts and small hail. Expect all
activity to clear by 09z with a clearing trend for the rest of the
morning. More rain showers/ts likely to fire along a cold front on
Sunday...but this boundary will likely be east of krst and possible
klse. Something to watch...mostly after 21z Sunday.