Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 190145
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
845 PM CDT Tue Jul 18 2017

.UPDATE...
Scattered to numerous showers and tstorms are developing along
and ahead of the cold front from central WI to far nrn IA. The
showers and storms will move esewd through the evening but reduce
in coverage after midnight. Still small probs of a SVR storm with
MLCAPE of 1000-2000 J/KG and effective shear of 25-30 kts. Then
dry on Wed with the front becoming stationary from north central
IL into ern IA.

A weak shortwave trough with strong warm, moist advection via a
swly 40-50 kt LLJ will then likely trigger a long track SVR MCS
or Derecho that will track along the nwd moving warm front from
the ern Dakotas to srn WI for Wed aft and night with arrival time
likely after midnight for srn WI. Widespread damaging winds would
be the main threat. At this time it appears the LLJ will veer
enough that back building storms will not occur on the tail end of
the MCS, thus the flash flood threat would be small.


&&

.AVIATION(00Z TAFS)...Sct storms will move across srn WI this
evening into the overnight but completely dissipating before
sunrise. Patchy fog and stratus may form during the early morning
hours especially in areas that had rainfall. Otherwise drier air
will move into the region for late Wed AM with few-sct030-040
cumulus but becoming clear near Lake MI. A line of severe tstorms
is then expected to move rapidly across srn WI late Wed nt, mainly
after midnight.

&&

.Marine...Relatively light winds and low wave heights for tnt
through Fri.

&&

.PREV DISCUSSION... (Issued 337 PM CDT Tue Jul 18 2017)

SHORT TERM...

Tonight and Wednesday...Forecast Confidence...Medium

There is a fast zonal upper flow across the Dakotas to the upper
Great Lakes. One jet max is exiting the upper great Lakes this
evening, as a second stronger 110 knot jet max exits North Dakota
before reaching lake Superior Wednesday afternoon. Upper level
divergence and 700 mb upward motion increases mainly tonight.

Zero to 1 km CAPE is around 2000 joules/kg in areas northwest and
west of the Dells early this evening, The area of greatest
instability moves southeast later tonight, but decreases with loss
of heating, and is around 500 Joules/kg southeast around midnight.
There is a slight risk of severe thunderstorms this evening
mainly north of a line from lone Rock to Fond du lac.

CAPE increases to around 2000 again southcentral and around 1500
far southeast Wednesday afternoon.

The latest high resolution HRRR model drops a line of
thunderstorms from the Dells to Fond du lac at 10 pm, then sags
it south as it weakens as it arrives toward Milwaukee at 2 am.

Not expected an excessive heavy rain potential with these storms.

LONG TERM...

Wednesday night through Saturday...Forecast Confidence...Medium

This is the period with the greatest heavy rain potential. There
is also a severe potential.

The zonal upper flow becomes a bit more northwest by Friday, as a
trough forms near the Hudson Bay area. Several jet max`s will
move across the U.S. Canadian border area. The best chance of
heavy rain is late Wednesday night into Thursday morning, but 3
rounds are likely during this period with another Friday and Friday
night. The storms would likely produce heavy rain. Precipitable
water values are around 2 inches for much of the period.

Zero to 1 km mixed layer CAPE rises to 3500 Joules/kg most areas
Thursday. Zero to 1 km shear is around 17 knots with zero to 3 km
shear around 25 knots.

Sunday and Monday...Forecast Confidence...Medium

A rather strong shortwave moves across the Lake Superior region
saturday night and Sunday. The ECMWF builds an upper ridge across
Wisconsin Monday night, but the GFS has a more northwest flow.

The ECMWF is precipitation free, but the GFS still has some
shower possibilities Sunday, with both models fairly dry Sunday
and Monday.

AVIATION(18Z TAFS)...

The narrow band of stratus will diminish shortly. mainly VFR until
the chance of thunderstorms begin this evening from the northwest.
MVFR CIGS and IFR VSBYS in any storms. A brief period of MVFR cigs
and vsbys around sunrise tomorrow. Then VFR.

MARINE...

Chance for thunderstorms tonight as a weakening cold front moves
through. South winds will veer to the west to northwest late
tonight. Look for mainly northeast winds Wednesday afternoon. The
winds and waves are expected to remain below Small Craft Advisory
levels.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Update...Gehring
Tonight/Wednesday and Aviation/Marine...Hentz
Wednesday Night through Tuesday...Hentz



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