Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 251538 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1038 AM CDT SUN SEP 25 2016

.UPDATE...

An approaching cold front boundary will make its way through S WI
late this morning through the afternoon, which should support
showers and thunderstorms. Showers have already started developing
in southwest Wisconsin. High-res guidance shows more robust
convection developing this afternoon as instability builds ahead
of the front. GFS/NAM 0-6 km bulk shear values between 35-40 kts
this afternoon would support a marginal risk of severe weather
with the thunderstorms that develop. High temperatures for today
look to remain on track in the mid to upper 70s.

&&

.AVIATION/18Z TAFS/...

Showers and thunderstorms look to develop along a cold front
boundary later this morning into the afternoon. Expect a shift in
the wind direction from a southerly direction to a westerly
direction as the front passes through by mid to late afternoon.
Some reduced cigs to MVFR (down to below 2 kft), and isolated
vsbys down to IFR (down to 2SM), are possible with showers and
storms that develop along the front. Winds could gust up to 20 kts
with these storms as well.

&&

.MARINE...

Continuing the Small Craft Advisory from Port Washington north for
southeast to south winds gusting to between 22 and 25 knots along
with building waves. Cooler lakeshore waters should prevent stronger
gusts from mixing to lake surface farther south so no headline
there, however will be close and small craft should exercise
caution.

Tight pressure gradient over Lake Michigan as low pressure passes to
the north with gusty west to northwest winds behind the departing
cold front later tonight into Tuesday. Will assess latest wind
guidance as gusts look to get close to Gale levels on Monday. May
have to issue a Gale Watch if 12Z model runs support the higher
probability of frequent Gale force gusts.

&&

.PREV DISCUSSION... /Issued 330 AM CDT SUN SEP 25 2016/

TODAY AND TONIGHT...Forecast Confidence - Medium.

Will continue Flash Flood Watch across Sauk and Marquette counties
due to susceptibility to any additional rainfall today.  Short term
guidance in good agreement on surge of column moisture this morning
across southern WI as precipitable water increases one quarter to
one half inch, approaching 2 inches by afternoon.  Upstream front
now located in western MN expected to slowly progress ewd and move
into wrn CWA by late am, and exit ern CWA by early evening.  Best
chance for more numerous showers and storms will be this afternoon
as front and low level frontogenetical forcing moves thru srn WI.
However wl keep in lower pops for more showers this morning as mid-
level moisture increases and upstream weak mid-level short wave
clips srn wi.  Expect anywhere from a tenth to six tenths of an inch
as showers and storms expected to be more progressive over srn WI
compared to upstream overnight scenario, but may still be more
localized higher amounts so continuing Flash Flood Watch. Marginal
Risk warranted as bulk shear to increase to 35 to 40 kts ahead of
passing cold front while MUCape reaches around 1000 j/kg. Sfc
dewpoints likely to increase into the upper 60s this aftn, pooling
along cold front. Expect convection to end rapidly during the early
evening behind front as drier, more stable conditions spread across
srn WI.

MONDAY - Confidence...Medium
The surface low is proggd to be situated north of Lake Superior. A
much cooler and drier airmass will be set up as 1000/500
millibar thicknesses drop to near 546DM. The 925 temps will be
are coldest on the GFS...dropping to 7-8C by days end with the NAM
and ECMWF around 10-11c. West winds will make it feel even cooler.
Some of the MOS guidance suggests some parts of the northern CWA
may be lucky to hit 60 for daytime highs. A strongly cyclonic 500
millibar flow will become set up. The first vort lobe passes
through in the morning with the overall circulation becoming
entrenched across the Upper Midwest. Given this regime expect
cloud cover to be fairly widespread with cyclonic flow aloft and
low level thermal trough. For now will keep any shra chances
across the northern CWA but may need to expand these POPS further
south but will keep it in the north at the moment.

MONDAY NIGHT THROUGH WEDNESDAY - Confidence...Medium
The models are in pretty decent agreement on waffling this 500
millibar low/expansive circulation slowly south through Tuesday
evening and then eastward to the eastern Lakes region/Ohio Valley
on Wednesday. The low level thermal trough will continue to
amplify with 925 temps dropping to 5-8c this period. There will be
an increased chance of showers across mainly the northern and
eastern CWA as the surface/850 lows shift sse from Lake Superior.
This will set up lower level forcing in combination with the mid
level cyclonic flow. So the models are generating mainly light
precip amounts...mainly a tenth or two. The ECWMF shows a little
more enhanced low level troughing later Tuesday night into
Wednesday morning.

THURSDAY - Confidence...Low to Medium
The GFS and GEM solutions show the 500 millibar low across the
eastern Lakes and Ohio valley with the ECMWF showing much less
progression further south and west. In fact it would keep at least
our southern CWA on the northwest periphery of the expansive
circulation. Meanwhile the GFS and GEM has rising heights for us
with surface ridging dominating. Even the ECMWF solution still
keeps us dry though a bit more of a ne wind with a tighter pressure
gradient.

FRIDAY AND SATURDAY - Confidence...Low
The model divergence becomes more pronounced as it relates to the
positioning of the 500 millibar low. The ECMWF continues a trend
towards retrogression in contrast the more progressive scenario
of the GFS and GEM. The ECMWF drifts the low back towards lower
MI for Saturday and this results in shra being wrapped back into
srn WI. The GFS and GEM show the high dominating. At this time the
Superblend POPS are leaning towards the dry solutions, and will
stick with that route for the time being.

AVIATION/12Z TAFS/...Thickening clouds likely to spread eastward
this morning with the best chance for showers and a few storms
affecting TAF sites from late morning through the afternoon. Cigs
should be mainly VFR however may drop to MVFR with any showers or
storms. Better chance for lower cigs late tonight and Monday as
core of coldest air settles over southern WI in tight cyclonic
flow.

MARINE...Expect increasing south to southeast winds this morning as
pressure gradient tightens ahead of approaching cold front. Recent
MODIS imagery and nearby buoys measure near shore lake surface
water temperature in the upper 50s to lower 60s while warmer sfc
water in the lower 70s remain offshore. Will continue Small Craft
Advisory today and push start time to mid-morning from Port
Washington north as gusts exceeding 22 knots should start to be
felt at Sheboygan lakeshore later this morning along with building
waves. Counting on cooler lakeshore waters to prevent stronger
gusts from mixing to lake surface farther south today, so wl hold
off on expanding Small Craft Advisory south, however will be close
and caution is advised.

Breezy west to northwest winds will develop late tonight and
continue through Tuesday as much cooler air settles over the Great
Lakes behind passing cold front.  Tight pressure gradient over Lake
Michigan as low pressure passes to the north wl result in the gusty
winds. Wind gusts will get close to Gale levels on Monday, however
confidence in persistent gusts exceeding 33 knots low at this point.
If confidence increases, the first Gale Watch of the season may be
issued later today for Monday into Monday night.

&&

.MKX Watches/Warnings/Advisories...
WI...None.
LM...SMALL CRAFT ADVISORY until 7 PM CDT this evening for LMZ643.

&&

$$

UPDATE...JTS/REM
TODAY/TONIGHT AND AVIATION/MARINE...MBK
Monday THROUGH Saturday...Collar


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