Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
338 am CDT Sat Aug 23 2014
Today and tonight...forecast confidence is high.
The middle/upper level ridge builds across Wisconsin today and
tonight. There is some weak upper divergence this afternoon and
evening...along with mainly weak upper motion at 700 mb. Little in
the way of a cap on the NAM forecast soundings...so the 2000
joules/kg of zero to 1 km mixed layer cape along with the upward
motion and upper divergence should be enough for scattered
showers/thunderstorms. There will be a lake breeze boundary that
may act as a triggering mechanism. With the weak flow the storms
could cause some isolated heavy rains.
The main concern is the moist air off Lake Michigan causing dense
fog and stratus. Lowest visibilities are in the counties bordering
the lake. The light east flow will persist tonight...so more fog
and stratus...especially near Lake Michigan is expected.
Another concern is heat index values. For the most part they
should be in the middle 90s...although a few areas...like the
Wisconsin River Valley could approach 100.
Short term...Sunday through Monday night...forecast confidence is
Models taking surface low from North Dakota into Manitoba Canada
Sunday and Sunday night. This allows for southeast winds to occur
over the area...with gradual warm air advection. This should allow
for a capped airmass over the area...NAM more aggressive with the
cap than the GFS. Thus...NAM/Canadian are dry Sunday...with light
quantitative precipitation forecast on GFS/European model (ecmwf) by 00z Monday. Opted to maintain dry forecast
Sunday given lack of decent forcing for upward vertical motion.
Cold front approaches the area from the west Sunday night...so
kept lower end probability of precipitation there. Models then differ with the frontal
boundary placement over the region Monday into Monday night.
GFS keeps stalled boundary north of the area...with the NAM and
European model (ecmwf) near or over the area...with the Canadian to the south. Low
level jet tries to point toward the area Monday into Monday
night...though there are differences here as well between the
For now...kept chance probability of precipitation going for Monday into Monday night.
Good amount of mean layer/elevated convective available potential energy during the Sunday to
Monday night period with relatively weak deep layer shear seen on
area forecast soundings. High precipitable water values suggest
heavy rainfall is possible with any of the storms...with some hail
and gusty winds possible in the strongest storms.
Highs Sunday and Monday should reach into the upper 80s to lower
90s well inland...with cooler values near Lake Michigan including
the Milwaukee metropolitan area with onshore winds. Dew points in the 70s
suggest heat index values in the 95 to 103 degree range
Sunday...with 95 to 99 on Monday. Clouds with possible
showers/storms may limit the heat somewhat on Monday. Still...a
heat advisory may be needed in later forecasts for mainly south
central Wisconsin for each day.
Long term...Tuesday through Friday...forecast confidence is
GFS/European model (ecmwf) trying to show quantitative precipitation forecast over the region within an area of
low level frontogenesis response Tuesday into Wednesday. GFS
taking surface low northeast through the area Wednesday into
Thursday...with drier air for the end of the week. The European model (ecmwf) keeps
the baroclinic zone near the area through the end of the week.
For now...will continue with consensus blend of models for probability of precipitation
and temperatures for this period. Looks like a good shot at
showers and storms for Tuesday night and Wednesday. Good mean
layer/elevated convective available potential energy with decent deep layer shear suggests a
possibility for strong/severe storms Tuesday night into
Wednesday. Precipitable water values remain high...so heavy
rainfall would also be possible during this time.
Areas of IFR/LIFR over south central Wisconsin...with widespread
LIFR over the southeast...especially within 40 miles of Lake
Michigan. The fog/stratus will gradually lift later this morning
with most areas expected to scatter out to VFR. However some lower
visibilities/stratus may persist along Lake Michigan with the
light east winds.
Chances for thunder increase this afternoon and evening with an
approaching warm front.
More IFR/LIFR conditions in fog are expected tonight...especially
near Lake Michigan.
Dense fog advisory in effect for all marine nearshore zones until
10 am for widespread fog with visibilities of 1 mile or less. The moist
flow will continue with light east winds and some fog is expected
in the afternoon and evening...then becoming more widespread and
dense again later tonight.
WI...dense fog advisory until 10 am CDT this morning for wiz052-059-
Lm...dense fog advisory until 10 am CDT this morning for lmz643>646.
Today/tonight and aviation/marine...hentz
Sunday through Friday...wood