Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
920 PM CDT Friday Oct 9 2015
Aviation/06z tafs/...a period of scattered-broken high cloud cover will
move across southern WI late this evening but decrease after 06z.
Dewpoint depressions are currently fairly low so fog in low lying
areas is expected but probably not until the high clouds decrease.
Cannot rule out brief MVFR visibilities at kmsn...kues...kenw around
sunrise. Otherwise southwesterly winds and VFR conditions are expected Sat
into Sat nt.
Previous discussion... /issued / Friday afternoon
Short term...tonight through Monday
Tonight and Saturday...forecast confidence is medium.
Broken stratocumulus clouds are expected to slowly scatter out and
then dissipate by early this evening. There may be some middle to
high clouds that move southeast through the area later this
evening...but not sure if they will hold together.
High pressure is expected to move east across the area tonight...and
bring light/calm winds. Should see lows in the upper 30s to lower
40s in most areas per 925 mb temperature fields...if middle to high
clouds do not linger.
Pressure gradient will increase Saturday...as southwest winds
develop within warm air advection regime. Gusty winds are expected
during the afternoon. This should allow temperatures to rise into
the middle to upper 60s...perhaps a bit higher than currently
forecast given the mixing expected.
Saturday night and Sunday...forecast confidence high
Warm air advection on strong west to southwest winds ahead of a
vigorous short wave trough approaching from the plains raises 850 mb
temperatures 5c to 6c...and 925 mb temperatures 7c to 8c...between 00z Sunday and
00z Monday. Not much direct impact on nighttime temperatures due to
nocturnal inversion...but enough stirring to hold lows in the lower
to middle 50s.
Lower sun angle and shorter daylight hours reflected in NAM forecast
soundings which show more limited heating time/strength barely
allowing boundary layer mixing to reach the 20c to 22c 925 mb
temperatures with less than dry adiabatic lapse rates...especially
at eastern sites...with deeper mixing and steeper lapse rates on the
GFS. This leads to a 5f to 8f degree difference in high temperatures
for Sunday. Will go with blended temperatures that lean more toward
the warmer mav guidance...which brings readings to near 80 in the
west. Southwest winds will be gusty as top of mixed layer will be
tapping 25 knot winds.
Sunday night...forecast confidence high
Some surface frontogenetic response to approaching cold front...with
a stronger response in the 850-700 mb layer...as surface low sweeps
the cold front through the region overnight. Maximum isentropic
adiabatic Omega with the thermal ridge ahead of the trough reaches
the west by 00z Monday and overspreads the region by 06z...though
condensation pressure deficits are high...struggling to get down to
50 mb in the far northern forecast area...reflecting the overall
lack of moisture accompanying the short wave in the zonal flow. Lack
of moisture also shown on the forecast soundings. Models keeping
precipitation close to the surface low as it crosses southern Ontario so
expect a dry frontal passage...with the front nearing the Lake
Michigan shore by 12z Monday. Southwest winds stay up
overnight...and with thermal ridge
Monday...forecast confidence high
Subsidence and cold air advection spread in behind 500 mb trough
axis/vorticity channel which reach Lake Michigan around 18z. Cold
air holds off long enough for highs to reach the middle-upper 60s
around 18z before cooling through the afternoon as thicknesses
collapse as colder air spreads in on gusty west winds.
Long term...Monday night through Friday...forecast confidence medium
925 mb and 850 mb cold pockets move across state Monday night...then
shift east Tuesday as flow becomes west-northwest behind trough
through middle week with weak short wave energy staying north of the
area. This brings at...or above normal temperatures through
Thursday...then turning cooler as flow becomes northwest late in the
week with amplifying western ridge developing in response to
deepening closed low just north of Hudson Bay and a second deep
trough approaching the Pacific northwest. GFS and European model (ecmwf) at odds
about location/amount of precipitation with different timing of a cold front
dropping through state Wednesday night into Thursday...or Thursday.
Will keep minimal probability of precipitation along front in blended solution for now.
VFR ceilings should gradually scatter out by sunset across the
area...dissipating early in the evening. There may be some middle to
high clouds that move through the area later this evening...but did
not mention in tafs due to uncertainty.
North winds will gradually weaken and become light tonight...as high
pressure moves across the region. Southwest winds will develop
Saturday morning...becoming gusty by the afternoon hours.
Gusty north winds will gradually weaken and become light by early
this evening...as the pressure gradient weakens. High waves of 4 to
5 feet will subside to 3 to 4 feet by sunset. Thus...the Small Craft
Advisory will be allowed to expire at 00z Saturday.
A Small Craft Advisory may be needed at times from Sunday through
Monday evening...as the pressure gradient becomes tight with the
approach and passage of a strong cold front. Gusts to 25 knots are
possible at times during this period. Winds will be mainly
offshore...so high waves should be limited toward and over the open
Lm...Small Craft Advisory until 7 PM CDT this evening for lmz644>646.
Small Craft Advisory until 4 PM CDT this afternoon for lmz643.
tonight/Saturday and aviation/marine...wood
Saturday night through Friday...rem