Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
318 PM CDT Thursday Aug 28 2014
Tonight and Friday...forecast confidence is medium.
Area of rain showers currently moving east northeast into the far
western counties and weakening. This is being driven by remnant 500
mb mesoscale convective vortex. Mesoscale models are showing this
area of showers moving east northeast through most of the area
through the late afternoon hours...continuing to weaken as it moves
into drier air.
There is a more organized area of rain showers in northern
Illinois...which could also move into the area late this afternoon
or early this evening. Little to no elevated cape seen on area
forecast soundings...so pulled mention of thunder late this
afternoon. Have highest probability of precipitation west with lower probability of precipitation in the east until
Main 850 mb low level jet nose will focus to the northwest and north
of the area this evening and overnight...which should keep the
strongest upward vertical motion and moisture convergence in that
area. Synoptic and mesoscale models are in general agreement with
Still should see some showers and storms within focused 850 mb warm
air advection zone and passing warm front tonight into early Friday
morning. Highest probability of precipitation are in the northern half of the area...closest
to best upward vertical motion. Left chance probability of precipitation in the south. Area
forecast soundings do show elevated cape slowly increasing through
the night. Precipitable water values remain in the 1.80 to 1.90 inch
range...but corfidi vectors remain on the high side. So...just
localized heavy rain is anticipated.
Models showing area getting into warm sector air on Friday...as warm
front moves north of the area. Again...best upward vertical motion
will be focused north and west of the area...so there may be a lull
in the precipitation. Left chance probability of precipitation going on Friday for now.
Still...no capping with 1500 to 2000 plus j/kg mean layer cape and
weak to modest deep layer shear suggest some strong storms possible
if they can get going. Precipitable water values around 1.50 to 1.75
inches with weak corfidi vectors...so localized heavy rainfall is
again a possibility. South winds and some sun in the afternoon
should bring highs into the lower to middle 80s across most of the area.
Friday night and Saturday...forecast confidence is high
The middle/upper level shortwave pushes into Wisconsin. Somewhat of a
coupled jet structure Friday night...but the jet maximum is rather
weak at 50 to 60 knots. Upper level divergence is moderate during
the evening to around midnight before weakening. 700 mb upward
motion is rather strong as it approaches south central Wisconsin
but weakens as it moves across the south. 700 mb relative humidity increases...
but is not quite saturated. The 850/700 mb speed maximum is across the
upper Great Lakes...with southern Wisconsin in the entrance
region. 850 mb dewpoints are around 16 celsius during the early
evening...but then decreases to around 11 celsius by sunrise. The
surface cold front reaches south central Wisconsin Friday night
and exits the southeast Saturday morning. Evening zero to 1 km
cape values around 2000 joules/kg during the early evening...
decreases to around 600 joules/kg over the southeast late. The
higher cape is then to the south on Saturday with weak high
pressure building into Wisconsin. The NAM/GFS and 00z European model (ecmwf) bring
heavy precipitation into south central Wisconsin Friday
evening...with diminishing mainly moderate amounts over the
southeast later Saturday night. The local severe weather parameter
increases to around 70 percent over south central Wisconsin Friday
evening and around 70 percent over the southeast overnight. The nsharp
forecast soundings gives a severe potential south central and
marginal severe potential southeast.
Saturday night and Sunday...forecast confidence is high
The main shortwave moves east but a secondary weaker shortwave
moves into Wisconsin late Saturday night and Sunday morning ahead
of the next strong shortwave moving into the plains.
Upper divergence across southern Wisconsin is weak...as is the 700
mb upward motion. The NAM does have scattered mainly light
precipitation Saturday night and Sunday morning.
Sunday night and Monday...forecast confidence is high
The GFS and European model (ecmwf) takes a strong shortwave across Wisconsin
during this period. The GFS/European model (ecmwf) and dgex bring a band of heavy
rain into south central Wisconsin later Sunday night...and across
the southeast Sunday morning. The GFS is a little quicker than
the 12z European model (ecmwf).
Monday night and Tuesday...forecast confidence is high
The GFS and European model (ecmwf) takes a second shortwave across Wisconsin
Tuesday. The GFS/European model (ecmwf) and dgex have just light
precipitation...mainly more north toward central and northern
Wednesday and Thursday...forecast confidence is high
The GFS and European model (ecmwf) have a zonal flow across the U.S./Canadian
border area as another trough pushes across the northwest U.S.
Weak high pressure moves across the Great Lakes Wednesday with a
warm front and precipitation moving into southern Wisconsin
Wednesday night and Thursday on the 12z European model (ecmwf) but is dry on the
Expect band of showers to push east northeast through Madison and
possibly the eastern taf sites between 21z Thursday and 00z Friday.
MVFR ceilings are possible. East southeast winds will linger during
Main area of showers and thunderstorms will remain north and west of
taf sites tonight into early Friday morning. Still should see some
showers and storms at times...as warm front moves north through the
area. For now...will use vicinity thunder wording in tafs due to low
confidence in specific timing. MVFR ceilings and MVFR/IFR
visibilities are possible in any storms.
May be a period of quiet weather later Friday morning into the
afternoon...as best precipitation area will remain to the north of
the area. Some uncertainty here with the precipitation trends...thus
the vicinity thunder wording continues. Southeast winds tonight will
shift to the south Friday and increase in magnitude.
East southeast winds tonight will increase and become southeast to
south on Friday across the nearshore waters. Winds will increase
into the 10 to 20 knot range on Friday. This will build waves to 2
to 4 feet toward Sheboygan. Winds and waves will gradually subside
Increasing southeast to south winds of 10 to 20 knots later tonight
and Friday should build waves to 2 to 4 feet along beaches in
Sheboygan County. Thus...a moderate swim risk is expected to occur
later tonight into Friday.
Tonight/Friday and aviation/marine...wood
Friday night through Thursday...hentz