Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Milwaukee/Sullivan WI
903 PM CDT Thursday Aug 27 2015
Watching bands of altocumulus castellanus showers off to the west on eastern edge of
better 850 mb warm advection. Hi-res/rapid update models keep
majority of precipitation back to the west with best forcing associated
with focused warm advection on increasing low-level jet ahead of
deepening low that points into north central Iowa/southern Minnesota
overnight. They do indicate far western counties might see an
isolated sprinkle after 06z...but dry layer between 900 mb and 750
mb would have to saturate before anything would reach the ground.
Current forecast as a good handle on chances for precipitation moving in
VFR conditions expected through the overnight...though some
patchy ground fog possible...especially in the east where cloud
blanket will be thinner. Warm...moist airmass advects in ahead of
surface low that will drop ceilings to MVFR levels by midday in the
west...and late afternoon in the east. Some indications that the
far southeast may remain at VFR levels until showers move in that
will drop both ceilings and visibilities to MVFR levels in the evening. Low-
level jet moving into the region after 00z brings surface to 2k
feet wind differences close to but not quite to low-level wind shear
criteria. Will take a look at full 00z data set but plan is to
leave out for now.
Previous discussion... /issued 318 PM CDT Thursday Aug 27 2015/
Thursday night and Friday...forecast confidence high.
Tonight winds will be light out of the south as high pressure drifts
over the eastern Great Lakes and low pressure approaches from the
southwest. Cumulus clouds should dissipate this evening as cirrus
clouds move in from the west. Expect lows around 60. Low level
moisture increase overnight but cirrus overhead should prevent any
On Friday strong middle level shortwave trough moves across
central/southern WI ahead of low pressure moving across Iowa.
Strong upper divergence also moves across southern WI during this
time. European model (ecmwf) lags behind with these features and is slower to bring
precipitation to southeast WI. Have probability of precipitation working in from the northwest
part of the County Warning Area during the late morning as low level moisture
convergence and low level warm air advection ramps up in southwest
/south-central WI during the afternoon. The latest mesoscale models
have shifted the greatest precipitation across the northern half of the
County Warning Area. Kept a chance of thunder going with cape up to 100 j/kg at
Friday night through Sunday...forecast confidence medium to high.
A slow moving shortwave trough and weakening surface trough will move
across southern WI late Friday nt and Sat but linger Sat nt as it gets
reinforced from another vorticity maximum dropping southeastward from
Ontario Canada. A southerly low level jet will accompany these features
and produce well organized low to middle level warm...moist advection
and frontogenesis for Friday after and night. Precipitable waters will rise to
1.60-1.70 inches with a moist adiabatic atmosphere expected. Thus MDT to
heavy rainfall rates will occur at times Friday after and night. Total
rainfall of 1.00-1.50 inches is forecast with the lesser amounts
over far southeast WI. The low level jet and stronger forcing will have moved east
by Sat but the trough passage will maintain cloudy skies and
showery conditions. The clouds will likely linger for much of Sat
nt and into the morning on sun over eastern WI. High pressure will
slowly build across southern WI on sun with a return of some sunshine
and warming temperatures.
Long term...Monday through Thursday...forecast confidence medium.
A very warm and humid week is expected as a large upper ridge
forms over much of the central and eastern USA with a polar trough
over western Canada and the Pacific northwest. No precipitation is forecast.
Diurnal cumulus will dissipate this evening. High to middle cirrus will
continue to move in from the west becoming overcast overnight.
Precipitation chances increase late Friday morning and early afternoon in
south-central and Friday evening in the southeast. IFR conditions
possible with the rain showers.
tonight/Friday and aviation/marine...marquardt
Friday night through Thursday...gehring