Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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330
FXUS63 KMKX 200210
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
910 PM CDT TUE JUL 19 2016

.UPDATE...

Main change will be in lowering precipitation chances tonight and
into Wednesday as latest HRRR and now the new 00Z NAM is
precipitation free tonight and even through Wednesday on the NAM.

Heat Watch still looks likely to verify, as advisories and
warnings will likely be needed for Thursday.

&&

.AVIATION/06Z TAFS/...

May see mvfr fog form later tonight into Wednesday morning, with
a more humid airmass moving into the area. Brief IFR possible in
the river valleys.

Otherwise, expect mainly VFR conditions tonight and Wednesday.

Latest meso scale models and the new 00Z NAM keep the forecast
area precipitation free through the night. Will likely remove the
TRW VCNTY in the TAFS.

The thunderstorm chance will increase Wednesday afternoon but the
new NAM continues to keep precipitation from forming. IF so just
scattered late morning and afternoon cumulus.

&&

.MARINE...

Main issue will be fog over the waters. Area webcams show decent
visibilities, but with the increasing dewpoints fog will become
more likely later Wednesday and Thursday.

Latest meso models and the new 00Z NAM shows little in the way of
thunderstorms tonight and even into Wednesday.

&&

.PREV DISCUSSION... /Issued 331 PM CDT TUE JUL 19 2016/

TONIGHT AND WEDNESDAY...Forecast Confidence Is Medium.

Area of elevated showers/storms continues to progress east into the
far western counties, likely due to its proximity to the passing
differential cyclonic vorticity advection with the system to the
southwest. Corfidi vectors suggest this activity should start
sliding southeast and remain west and southwest of Madison into
early this evening. Brief heavy rainfall would be possible with
these showers/storms.

Mesoscale models, and the synoptic models for that matter, are more
uncertain with precipitation chances tonight into Wednesday across
the area. Most of tonight may end up dry across the area, until
scattered convection possibly moves southeast into the area later
tonight into Wednesday morning.

There will be steady warm air advection during this period, with the
low level jet gradually veering over the area. There will be plenty
of moisture in place, with some advection of moisture as well. Weak
elevated CAPE remains tonight, increasing on Wednesday. Deep layer
shear is rather weak to modest. Some mesoscale models suggest
scattered convection at best would develop and/or slide southeast
through portions of the area later tonight into Wednesday. However,
others are fairly dry.

For now, will go with higher end chance POPs for later tonight in
the far west, and across most of the area on Wednesday. High
precipitable water values suggest heavy rainfall would be possible
if storms do occur, which would be welcome. The southeast storm
motion may encourage training of cells as well. Certainly there is
some uncertainty with if and how widespread the convection will be.
This will impact highs on Wednesday, depending on how much cloud
cover there is. Went with more modest highs in the mid 80s for now.

May see light fog develop later tonight into early Wednesday
morning, depending on how much cloud cover there is over the area.

WEDNESDAY NIGHT...Forecast Confidence Medium

Upper ridging builds over region with jet stream shifting north to
the Canadian border. Weak vorticity maxima/convective vorticies
rotate through the area around core of broad 500 mb high over the
south-central U.S., with weak but persistent 700-850 mb warm
advection.

Models differ with location/timing of these weak disturbances, with
only the NAM bringing a chance to the entire forecast area while GFS
and ECMWF focus precipitation more toward the better low-layer Q-
vector convergence to the north and east, closer to the apex of the
850 mb thermal ridge and relatively weak 20 knot cross-isothermal
850 mb winds.

THURSDAY...Forecast Confidence High

Warm...moist advection on increasing 925-850 mb southwesterly winds
ahead of approaching southern Canadian short wave and surface trough
extending  raises 925 mb temperatures into the 25C to 27C range
around 18Z and 27c to 29C by 00Z Friday. A dry adiabatic mix out
indicated on the forecast soundings produces highs in the lower to
middle 90s.

Along with the expected mid 70 dew points, heat index values look to
reach the 105 to 109 range across the South Central WI portion of
the current Excessive Heat Watch area, and close enough in the rest
of the watch area to keep it going unchanged.

THURSDAY NIGHT AND FRIDAY...Forecast Confidence High

A muggy night Thursday night with lows only in the mid 70s. Good
signal on the models of an MCS developing Thursday afternoon over
the U.P. of Michigan/northern WI on the nose of increasing
southwesterly 850 mb winds ahead of short wave moving along the
U.S./Canadian border Thursday/Thursday night. MCS dives to the
southeast over lower Michigan Thursday night, with northeast and
eastern sections of the CWA catching the western flank of the
complex. Then GFS and ECMWF show convective development Friday along
surface front sagging down into the state. If these showers and
storms do develop, it will definitely limit ability to heat out to
warning criteria Friday.

Will limit chance PoPs to the southeast per consensus blend for now.
Will not adjust watch for now as forecast apparent temperatures for
Friday are close to warning criteria with some limited areas still
hitting 105. Later shifts will be able to fine tune areal extent and
timing of any headlines with, hopefully, a more consistent
solution to precipitation coverage/timing.

LONG TERM...FRIDAY NIGHT THROUGH TUESDAY...Forecast Confidence
Medium.

Front sags south and stalls Friday night as northern Plains short
wave and associated surface low approach from the west. How far and
fast the front pushes south leads to differing precipitation chances
in the guidance, and track of expected MCS that develops over North
Dakota Friday night and follows the boundary, now lifting back north
as a warm front, across Wisconsin Saturday.

Faster GFS has low north of eastern Lake Superior by 12Z Sunday with
the trailing cold front bisecting the CWA from ne to sw. Slower
ECMWF has low over central WI at the same time. Will keep PoPs in
the chance category for now.

Both models clear precipitation out of the forecast area Sunday
night with high pressure building in Monday. ECMWF holds 500 mb
trough back over the region and almost closes it off over the
northern Great Lakes Tuesday, with diurnally-driven showers
developing underneath. GFS has another northern Plains short wave
trough moving towards the region Tuesday, passing across Lake
Superior Tuesday night. Will acknowledge potential of the ECMWF
solution and have slight chance PoPs during the day Monday and
Tuesday, with slight chance PoPs Tuesday night in the far west with
approaching short wave on the GFS.

After a toasty Saturday with highs in the upper 80s and a still
muggy Sunday, temperatures fall back toward seasonal normal levels
for Monday and Tuesday.

AVIATION/00Z TAFS/...

Expect mainly VFR conditions tonight and Wednesday across TAF sites.
Light easterly winds are expected to shift south tonight into
Wednesday. May see light fog form later tonight into Wednesday
morning, with a more humid airmass moving into the area.

There is more uncertainty regarding thunderstorm chances for later
tonight into Wednesday across the area. For now, may continue to use
vicinity thunder in TAFs for this period, until better confidence on
timing and areal coverage is found. MVFR/IFR ceilings and
visibilities would be possible in any storms.

MARINE...

Light easterly winds will shift south tonight and remain so on
Wednesday. Waves should remain low.

Main issue will be fog over the waters, and dense fog chances
tonight into Wednesday and beyond. Area webcams already showing
areas of fog over the nearshore waters. Some of it is rather thick
and shallow in depth. Light winds into Wednesday with higher dew
points working into the area suggest adding areas of fog during this
time to the forecast. Could see dense fog at times overnight with
the light winds.

The fog may linger into the rest of the week, with the very moist
airmass in place over the region. Dense fog could occur at times.

There is more uncertainty with thunderstorm chances later tonight
into Wednesday. Reduced the chances for storms a bit during this
time, until better confidence on trends can be attained. Any storms
would produce gusty winds and locally high waves.

&&

.MKX Watches/Warnings/Advisories...
WI...EXCESSIVE HEAT WATCH from Thursday afternoon through Friday
     evening for WIZ046-047-056>058-062>072.

LM...None.
&&

$$

UPDATE...Hentz
TONIGHT/Wednesday AND AVIATION/MARINE...Wood
Wednesday NIGHT THROUGH Tuesday...REM



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