Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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000
FXUS63 KMKX 121941
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
245 PM CDT SAT AUG 12 2017

.TONIGHT AND SUNDAY - Forecast Confidence...High.

The upper flow backs westerly tonight as shortwave ridging
deamplifies ahead of shortwave energy progressing from the
northern Plains into the upper Mississippi valley. This will push
surface high pressure to the southeast, with its axis centered
over southern Wisconsin. Conditions appear favorable for some
ground fog tonight, with dense fog developing within the Wisconsin
River Valley. With near neutral low-level thermal advection, lows
tonight should be near, to perhaps a degree or two warmer, than
last night.

A shortwave trough will lose amplitude while moving into the upper
Midwest on Sunday. Downstream surface and mid-level ridging
centered over the Great Lakes should hold enough influence to keep
any precipitation associated with the wave to our northwest during
this time. Model relative humidity fields suggest we`ll see an
increase in mid to high level clouds during the day, although deep
layer moisture remains somewhat meager with precipitable water
values under 1 inch. With southerly return flow we`ll see some
weak warm advection with temperatures likely a degree or two
warmer than on Saturday.

.SUNDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Low to
Medium.

Short term guidance continues to shift gears in showing weaker
short wave ridging across the western Great Lakes Sunday night and
Monday. This is in response to slow moving but stronger mid-level
short wave that will be moving across WI on Monday into Monday
night.  Will continue to carry smaller pops for this period,
trending toward dry conditions later Monday night and for a time on
Tuesday as the weak short wave ridging temporarily builds into the
western Great Lakes.  Instability by Monday afternoon expected to
reach 500-1000 j/kg with favorable bulk shear values of 30-35 kts.
If this trend continues, SPC may eventually update parts of the area
to a Marginal risk.

Expect decreasing trend in any precipitation Monday night as
weak low level jet and upper wave slide off to the east.  Dry
conditions should continue into Tuesday morning before warm
front located to the south of WI begins shifting north. Not seeing
much upper level forcing but forecast soundings show plenty of
instability in the afternoon. Will keep pops low for now due to warm
front distance from southern WI.

.EXTENDED PERIOD...

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

The transition continues to more unsettled conditions looking
more likely for at least the first half of this period.  With weaker
short wave ridging in place early in the week, approaching upstream
trof wl require less time and energy in flattening ridge in the
Tuesday night/Wednesday time frame.  Hence increasing low level
southerly winds will pull warm front and deeper column moisture into
southern WI on Wednesday.  Due to rapid influx of moisture, wl need
to add shower and storm chance into Tuesday night and beef up chance
Wednesday morning with likely wording most locations in the
afternoon. May be a 1-2 punch of showers and storms as surface front
and upper level wave now looking to move through 12-24 hours
earlier, more later Wednesday into Wednesday night.  Both 00Z
ECMWF and 12Z Canadian speeding up features while even 12Z GFS
slightly faster and deeper with approaching low pressure and upper
level wave.  12Z ECMWF consistent in more progressive solution.
ECWMF and GEM move front well off to the east by Thursday, while GFS
a bit slower.  Wl be keeping in smaller pops for Thursday due to
these changes.

Uncertainty remains in later periods as medium range guidance having
trouble resolving strength of next short wave approaching from the
northern Plains and its placement.  WPC using a ECMWF/GFS blend with
brisk WNW steering flow.  For now, wl carry low pops for most of the
later periods.  Aside from the wishy-washyness of the extended pop
forecast, temperatures look to remain near seasonal normal for mid-
August.

&&

.AVIATION(00Z TAFS)...Prevailing VFR into Sunday. Scattered to
broken 4 to 5 kft CIGS should dissipate this evening, with some high
level clouds moving in. Patchy ground fog tonight, with IFR fog in
the WI river valley. Light and variable winds tonight with high
pressure overhead. Southerly winds on Sunday, except southeasterly
near Lake Michigan.

&&

.MARINE...No major marine concerns in the short term as both winds
and waves remain below small craft advisory levels. Light west to
southwest winds tonight will back southeasterly on Sunday as a lake
breeze develops. Wave heights should be around 1 foot or less for
tonight into Sunday.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...

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

$$

TONIGHT AND SUNDAY/AVIATION/MARINE...SPM
SUNDAY NIGHT THROUGH SATURDAY...MBK



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