Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
857 PM CDT Friday Aug 28 2015
things are progressing as planned. Water vapor imagery is
impressive...clearly defining an energetic middle level trough that
is somewhat out of character for late August. The models continue
to show the low level...below 700 mb...wind fields weakening with time
as the trough weakens...and this should lower the intensity of
the rainfall overnight. Storm Prediction Center mesoscale analysis shows the elevated
precipitable water axis...1.5 to 1.7 inches...running from Green
Bay back into the southwest corner of the state. As the trough
moves very slowly east into Saturday morning...this moisture axis
will drift along with it. All but the far southeast corner of the
forecast area has seen steady rain up to this point...but the
southeast will eventually get into it.
The total rainfall estimate off of our radar over
Grant/Iowa/Lafayette counties is running too high. This is likely
due to some bright banding at that beam elevation. Klot is
suffering the same overestimation...but karx is looking very good.
Only minor adjustments needed to the forecast this evening.
look for conditions to gradually lower overnight as the steady
rain continues southern Wisconsin. IFR ceilings/visibilities may hit kmsn
later tonight into Saturday morning with MVFR ceilings/visibilities for
kmke/kues/kenw. Winds will be light through the taf period. We
have conditions improving by middle to late morning on Saturday...but
given the weak wind fields and the potential for the rain to hang
on a bit longer...we may have to back off on the improving
ceilings/visibilities until afternoon.
Previous discussion... /issued 337 PM CDT Friday Aug 28 2015/
Tonight - confidence...medium
rap/water vapor combo shows well defined middle level shortwave
approaching from Iowa. All models weaken surface low as warm front across
Iowa/Illinois tries to lift northward towards southern WI with time. Individual cells
trying to get going west-northwest of kdvn though not expecting anything strong/severe
to evolve into southern WI. Combo of the warm front...increasing low level jet and
associated moisture advection induced forcing...will sustain large area
of rain across southern WI. Soundings show decent column moisture though
only limited elevated cape. Should still see some decent and much needed
rain totals in many areas. Radar mosaic suggests southeast 1/2 of County Warning Area may be in
line for a bit more rainfall closer to better moisture/instability and
quantitative precipitation forecast grids largely reflect this. However a couple of the mesoscale models are
showing heavier totals may be skewed further northwest...but current radar trends
suggest southeast 1/2 at this time. Will retain isolated thunder mention. Have boosted
probability of precipitation even more. No big diurnal dropoff in temperatures due to the cloud cover.
Saturday - confidence...medium
models show primary vorticity maximum off to the east of the area early. However
middle level flow remains cyclonic with secondary shortwave dropping in from
the northwest. While 850 jet pulls out early the moist axis lingers along
with an 850 trough. The Gem is the most aggressive on more appreciable
rain beyond 18z while the GFS/NAM/ECMWF lighter precipitation amounts. Will
gradually taper probability of precipitation from northwest-southeast as the day wears on. Cloud cover likely
to be very stubborn so already trimmed highs may still be too optimistic.
Saturday night through Monday...forecast confidence medium.
The shortwave trough will exit southeast WI by Sat evening with the surface
trough continuing to decay from northern Illinois into lower Michigan. Kept slight
chance probability of precipitation over southeast WI for very light rain or drizzle via the surface
trough. Otherwise expect stratus to continue Sat nt over southeast WI
although some clearing is possible...more so over south central
WI. However...believe fog and stratus would then develop within
the moist low level airmass.
Surface high pressure would then build into southern WI on sun with a high
amplitude upper ridge extending from the Great Plains to the Lake
Superior region. Morning stratus and fog is probable but partly
cloudy skies and warm temperatures would be expected for the afternoon
hours. Some fog would then redevelop once again late sun nt. The
upper ridge will then roll over into the entire Great Lakes region
for Monday. Warm and humid conditions are forecast.
The upper ridging will weaken some over the Great Lakes into New
England for the week while a mean upper trough will also remain
over western Canada and the Pacific northwest. Southerly flow and warm...humid
conditions will prevail over southern WI. A shortwave trough within the
southwesterly flow aloft will bring slight chances of thunderstorms at times for
the middle to late portion of the week.
Aviation/00z tafs/...cloud cover continues to thicken with rain
spreading in ahead of middle level wave and in conjunction with
increasing 850 moist advection. Consshort ceiling and sref ceiling/visible probs show
MVFR/IFR ceilings working into SC WI later tonight into Saturday morning
with higher confidence of IFR ceilings in SC WI than southeast. However low level relative humidity
forecasts and met MOS suggest caution with IFR ceilings all the way across
with the steadier rains lingering for at least Saturday morning per
tonight/Saturday and aviation/marine...collar
Saturday night through Friday...gehring