Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 270205 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
905 PM CDT FRI AUG 26 2016

.UPDATE...

Broad surface high remains in control keeping surface warm front
well south of the region, with leading edge of better surface to
850 mb moisture roughly along the I-80 corridor and only 10-15
knot southerly winds at 850 mb trying to push it northward.

Have trimmed PoPs prior to 06Z and lowered chances of thunder
with minimal instability on area forecast soundings. Hi-res/rapid
update models still bringing a good slug of QPF across southern Wi
after 07Z with warm advection and a surface wave of varying
strength so kept categorical PoPs during the overnight but
altered the alignment and timing based on latest meso-model
depiction.

&&

.AVIATION/06Z TAFS/...

Will await full set of 00Z data to see if NAM MOS guidance is
still showing a period of IFR cigs with warm front passage during
the late overnight and early morning hours. Higher probabilities on
21Z SREF for cig heights less than 3k ft are mainly west of KMSN
and lift off to the north during the early afternoon. Instability
on model soundings showing less of a chance of thunder overnight.
Should see a break in the precipitation late morning into the
early afternoon as weak mid-level ripple moves off with chances of
scattered coverage showers and storms in the late afternoon and
evening. Expect VFR conditions once the warm front lifts north
with MVFR cigs/vsbys with any of the scattered showers and storms.

&&

.MARINE...

Weak gradient keeps winds, and thus waves, relatively low through
the weekend and into next week. Wind direction will go from east
and southeast tonight, to southeast and south tomorrow with
passage of a warm front, then west tomorrow night behind a weak
surface wave of low pressure. Showers likely with the warm front
and subsequent passage of the surface low but only scattered to
isolated coverage thunderstorms with lesser instability on area
forecast soundings.

&&

.PREV DISCUSSION... /Issued 335 PM CDT FRI AUG 26 2016/

TONIGHT AND SATURDAY - Forecast Confidence...Medium.

Extensive area of middle to high clouds will continue to slide
northeast into the area into this evening. Radar showing an area of
light sprinkles or light showers shifting northeast across southwest
Wisconsin. There is a good amount of dry air below 10000 feet above
ground level. Thus, not expecting much in the way of measurable
precipitation to affect the far western and northwestern counties
into the early evening. May add sprinkles to this area instead.

12Z model runs differ somewhat with the location of the low level
jet nose and focused warm air advection later tonight into Saturday
morning. The NAM is the furthest to the northwest of the area, with
the Canadian/ECMWF focused more over the forecast area, and the GFS
between them. All models do bring QPF across the area later tonight
into Saturday morning.

The mesoscale models are showing a similar trend, with bringing a
round of showers and storms through the area later tonight into
early Saturday morning. Thus, will go with high end POPs for this
period.

Area is in a marginal risk for excessive rainfall tonight.
Precipitable water values and some other parameters get favorable,
and mesoscale models suggest some training of storms are possible.
However, think these storms will be moving quick enough to limit
heavy rainfall potential to localized areas.

Precipitation chances Saturday afternoon are still rather murky, as
it will depend on clouds clearing the area after the warm front
moves to the north. There will be modest deep layer bulk shear with
weak to modest mean layer CAPE, assuming some clearing of the
clouds.

There will still be some upper divergence from the right rear
quadrant of the 250 mb jet streak lingering. Mesoscale models have
trended drier on Sunday afternoon, as there is a lack of a decent
low level boundary to focus storm development.

For now, will maintain the chance POPs for the afternoon hours. A
marginal severe risk and marginal heavy rainfall risk exists in
parts of the area Saturday, but this will be conditional on clouds
clearing out and allowing for sufficient instability.

Warm and increasingly humid conditions are expected tonight into
Saturday. Lows in the mid 60s tonight, with highs in the mid 70s to
lower 80s, are anticipated for Saturday. It may be cooler if clouds
do not move out during the afternoon.

SATURDAY NIGHT THROUGH MONDAY - Forecast Confidence...Low to
Medium.

Will continue pops for showers and thunderstorms Saturday night due
to potential redevelopment of afternoon convection lingering into
the evening.  In addition, short term guidance in agreement on right
entrance region of upper jet affecting southern WI overnight. Weak
boundary in the area also results in some frontogenetical forcing
for a time during the evening.  Not enough confidence at this point
for likely wording so lowered pops during the evening to high
chance.  Expect considerable cloud cover to stick around into
Monday.  Low level moisture and light winds may produce occasional
low clouds and fog later Saturday night into Monday.

Passing mid-level short wave will have enough moisture to work with
to continue the chance for storms on Monday.

EXTENDED PERIOD...

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

Medium range guidance in reasonable agreement on more amplified
pattern developing across Conus early in the period. Amplifying long-
wave troffing developing off west coast of U.S. and Canada will
result in upstream broadscale ridging expanding across central
Conus. As a result, increasing northwest steering winds will carry
a cold front through srn WI Monday night or Tuesday morning.
Enough column precipitable water expected to be in place to
warrant keeping a small chance for precipitation during this
period.

ECMWF and Canadian show sharper northwest flow pulling noticeably
cooler air into the region by Wednesday while 00Z GFS had been
showing core of cooler air remaining to the east.  Latest 12Z GFS
now in better agreement with ECMWF and 925H temps cool to around 15-
17C by midweek. However, due to lingering uncertainty, will keep
daytime temperatures near normal with dry conditions expected for
much of the later periods.  Daytime temps could end up several
degrees cooler later next week. GFS 5-day 500h means show blocking
warm ridge across central CONUS as September begins with 50 to
100meter above normal heights. Hence September looks to begin fairly
benign with near to slightly above normal temps.

AVIATION/00Z TAFS/...

VFR conditions are expected into this evening across TAF sites.
Southeast lake breeze will linger at Milwaukee and Kenosha, though
not sure if it will reach Waukesha as well. Light and variable winds
are expected at Madison.

Otherwise, middle to high clouds will gradually thicken through the
evening. Should see a period of showers and thunderstorms later
tonight into early Saturday morning across TAF sites, as warm and
moist air pushes north into the area with a warm front. Should see
MVFR ceilings and visibilities at times, down to around 1000 feet
and 3 miles.

Winds will become easterly by later tonight, gradually veering south
to southeast on Saturday. Should see a lull in the precipitation
later Saturday morning into the afternoon. May see more scattered
showers and storms later Saturday afternoon into the evening, but
too uncertain at this time to mention in TAFs.

MARINE...

Light winds and generally low wave heights are expected through the
weekend. An area of showers and thunderstorms will move northeast
through the area later tonight into Saturday morning. Locally higher
winds and waves are possible with any storms. Expecting lake breeze
and/or onshore winds Saturday and Sunday afternoon.

The light winds and low waves should linger into early next week as
well.

MKX WATCHES/WARNINGS/ADVISORIES...

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

$$
TONIGHT AND SATURDAY/AVIATION/MARINE...Wood

&&

.MKX Watches/Warnings/Advisories...
WI...None.
LM...None.
&&

$$

UPDATE...REM
TONIGHT/Saturday AND AVIATION/MARINE...Wood
Saturday NIGHT THROUGH Friday...MBK


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