Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 192031
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
330 PM CDT TUE SEP 19 2017

.TONIGHT - Forecast Confidence...Medium.

Main concern will be fog/stratus trends. Already seeing widespread
stratus/light fog near Lake Michigan. A moist easterly flow is
helping to sustain this within the lingering influence of the
departing high pressure. As surface high continues to pull away
should start to see some mixing especially in the western cwa. So
more concern for the fog/stratus in the eastern cwa. SREF/MOS both
show a high confident stratus situation with potential for some
thicker fog evolving with less mixing in the east. Meso model vsby
progs are trending towards some lower vsbys, so potential there for
some thicker fog to evolve. Winds off the deck do start to increase
even in the east per RAP/HRRR soundings so not confident in a
widespread dense fog scenario just yet.

.WEDNESDAY - Confidence...Medium

Mid level energy rides to our north with upper jet also favoring
locales to our north. The sharp 850 trough axis dampens as it comes
across. There will be an uptick in lo level convergence arriving
into the cwa especially during the afternoon as surface front draws
closer. So this will result in increasing pops especially in the
west and northwest cwa. Bufkit showing appreciable CAPE with some
mid level lapse rates aob 7.0c. Best 0-6km shear to our north though
axis of instability to be across the area with the approaching low
level boundary. Toasty 925 temps expected into the 20-25c range so
quite warm and humid with a southerly breeze. Onshore component
likely to keep some lakeshore areas cooler.

.WEDNESDAY NIGHT THROUGH FRIDAY - Forecast Confidence...Medium.

Weakening cold front progresses slowly eastward across southern WI
Wednesday night, further weakening into Thursday morning.  The front
will not make much progress to the east and south due to low to mid
level moderate southwest wind flow.  Convergence will be weak along
the front, but instability in the late afternoon and early evening
fairly impressive.  MUCape of 1500-2500 j/kg will result in at least
a few storms.  Bulk shear on the lower end, around 25-30 knots, but
may be just enough to generate an isolated severe storm. Weak front
settles just to the south of WI by Thu mrng and then begins to move
back north as a warm front through Thursday night. Atmospheric
moisture drops briefly but PWAT returns to around 1.5 inches on
Thursday.  Fortunately, there is little synoptic and mesoscale
forcing.  Hence wl keep conditions dry as warm front moves northward
across the area on Thursday.  Also, some concern about low clouds
lingering in light easterly flow for much of the day on Thursday.
Drizzle not out of the question.

In contrast, southern WI should be well into the warm sector on
Friday with at least partial sunshine and breezy, warm south winds.
Low confidence on max temperatures on Thursday, but should return
to the 80s most areas on Friday, even close to Lake Michigan.

.EXTENDED PERIOD...

FRIDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium.

Continued agreement in medium range guidance in continued
unseasonably warm weather affecting southern WI for much of this
period.  Tropical disturbances meandering northward over the western
Atlantic will be partially responsible for creating a nearly
stationary, high-amplitude pattern across the CONUS this week, and
it appears this scenario will persist through much of the extended
period. Large amplitude ridge will continue over the eastern CONUS
while long-wave trof brings colder weather to the west, with
accumulating snow in the MT/WY Rockies. Warm southerly breezes are
expected for much of the period.  Sprawling downstream high pressure
should suppress cloud development.  With 925H temps in the 21-25C
range, expect daytime temps to peak mostly in the 80s, even with the
daylight hours continuing to shrink.

ECMWF and GFS bring stronger mid-level short wave across northern
plains into southern Canada Monday night and Tuesday, with resultant
cold front slipping across Wisconsin.  Medium range guidance has
been backing off and slowing up cold front eastern progress.  With
tropical disturbances still expected to be churning through western
Atlantic, think this trend will continue.  For now, wl have pops in
the Mon night/Tue time frame.

&&

.AVIATION(21Z TAFS)...Meso models coming into decent agreement
with fog development overnight across the eastern cwa. This is due
to lingering influence of surface high with a light onshore wind
regime. With some faster wind speeds evolving aloft not confident on
widespread dense fog at the moment. Any lingering fog/stratus to
burn off Wednesday morning with potential for showers/storms most
likely later afternoon/evening across southern WI.

&&

.MARINE...Three concerns. First fog trends overnight into early
Wednesday. Vsbys ok at the moment but area is still under influence
of lingering surface high so some fog may evolve/lower from the
stratus as the night goes on. expecting southerly winds to pick up
on Wednesday ahead of a frontal boundary. At this time conditions
look to remain below small craft levels. Then thunderstorm chances
will increase later Wednesday afternoon into Wednesday night as the
front comes through.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...

WI...None.
LM...None.

$$

TONIGHT THROUGH WEDNESDAY/AVIATION/MARINE...PC
WEDNESDAY NIGHT THROUGH TUESDAY...MBK



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