Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1026 am CST Tuesday Dec 1 2015
Aviation/18z tafs/...vsbl imagery shows dry slot wrapping into
southern WI though filling in with more MVFR stratocu. Upper low proggd
to shift eastward from northwest Iowa into WI later tonight through Wednesday
morning. Surface/850 trough works across later tonight. In addition
500 millibar speed maximum will enhance vertical motions with southeast WI in
left front portion of the speed maximum. 12z GFS and 12z NAM show the
highest quantitative precipitation forecast with these features to our east and
southeast...however we can still expect a period of light snow
tonight into Wednesday morning. Highest probability of precipitation in the southeast look good
and any accums under an inch. Expect to see some IFR ceilings/visibilities
in southeast WI.
Previous discussion... /issued 323 am CST Tuesday Dec 1 2015/
Today and tonight...forecast confidence is medium.
Strong frontogenetical forcing will continue to generate areas of
-ra as it passes across southern WI next few hours along with an isolated
rumble of T. Frontogenetical forcing associated with passing surface
occlusion also teaming up with enhanced synoptic forcing from left
exit region and strong differential vorticity advection. Seeing
some increase in returns to the southwest on upstream radars so
would not be surprised to see scattered -shra return to southwest County Warning Area
next hour or so.
Otherwise...drier colder air surges across County Warning Area this morning
behind occlusion...resulting in a break from the precipitation for a few
hours. Upstream stacked low pressure system will be getting nudged
eastward next 24 hours. Several short wave troughs rotating around this
system will likely bring scattered snow showers and a few rain
showers back to southern WI later this afternoon and evening...and
again later tonight. Some of the snow showers late this afternoon
and evening may be more vigorous and briefly drop visibilities with a
rapid accumulation of a couple tenths of an inch on grassy
surfaces. Low level lapse rates fairly impressive...reaching 7-8
degrees as area of potential vorticity rotates around upper low.
Short term...Wednesday through Thursday night...forecast
confidence is medium.
Models are in pretty good agreement with the closed upper low
track on Wednesday. It is mainly right over southern Wisconsin at
12z Wednesday...then shifts southeast of the area during the
afternoon hours. The models have a strong vorticity maximum
rotating through the area during the morning hours...before
sliding to the southeast with the low.
Area forecast soundings are saturated in the low levels...within
a somewhat deep dendrite snow Crystal growth zone. Thus...looks
like a good shot for the light snow showers to continue into
Wednesday morning...before slowly tapering off to flurries west to
east in the afternoon.
For now...have the highest probability of precipitation in the southeast counties...but
probability of precipitation should be raised across the area in later forecasts...given
the trends in the models. Could see light accumulations under an
inch across the area on Wednesday.
Northwest flow at 500 mb then ensues for Wednesday night into
Thursday night in the models...with a 500 mb vorticity maximum
trying to pass southeastward through the region Thursday night.
The models are in some disagreement with the exact track of this
feature. However...area forecast soundings are fairly dry during
this time...so not expecting much impact from this feature.
Looks like a dry period for Wednesday night into Thursday
night...with a dry airmass in place. Steady warm air advection
during this period should keep highs near seasonal normals.
Long term...Friday through Monday...forecast confidence is
GFS/European model (ecmwf) models are showing an amplified 500 mb ridge axis
building into the western Great Lakes region Friday into Friday
night...with this feature shifting east on Saturday. South winds
will continue to usher a dry and mild airmass into the region.
Highs should rise into the 40s during this period...above seasonal
GFS/European model (ecmwf) then bring a 500 mb shortwave trough east across the
region Saturday night into Sunday. However...the airmass remains
fairly dry during this time...so left dry forecast going. Mild
temperatures are expected to continue.
Dry conditions may linger into Monday...before a slow moving 500
mb shortwave trough or closed low moves northeast toward the
region. There is some disagreement among the models this far
out with this period...so will go with the consensus blended
forecast and keep things dry. Mild temperatures should remain
during this time.
IFR ceilings should rise to MVFR as drier...colder air surges into
southern WI this morning behind passing surface frontal occlusion. Ceilings
may even briefly become VFR but mostly MVFR ceilings are expected
later today into tonight as scattered -shsn and areas of -sn develop
with passing upper low pressure system.
Gusty southeast winds will veer to the southwest early this
morning as surface frontal occlusion passes across Southern Lake Michigan.
Despite change in direction...winds are expected to remain gusty
through the afternoon as colder air surges in from the west. Wave
heights should gradually settle due to the winds becoming more
offshore. Small Craft Advisory will remain in effect this afternoon.
Lm...Small Craft Advisory until 6 PM CST this evening for lmz643>646.
Today/tonight and aviation/marine...mbk
Wednesday through Monday...wood