Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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651
FXUS63 KMKX 211741
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1241 PM CDT Mon Aug 21 2017

.UPDATE...

&&

.AVIATION(18Z TAFS)...

VFR conditions will persist through the day before cigs begin to
gradually fall ahead of an approaching trough. Models generally in
good agreement showing thunderstorms arriving around the MSN area
by mid-evening and the eastern sites a few hours later. Cigs will
continue to fall into the morning hours, perhaps dropping into
IFR if a storms moves over a TAF site. Any storms that do move
through could potentially produce very heavy rainfall. Severe
weather is a lesser threat, but we can`t rule out hail and gusty
winds as well. Cigs will reach their minimum (near the VFR/MVFR
threshold) right around sunrise and then begin to increase well
into VFR late tomorrow morning.

As the trough moves through the region in the late evening/early
morning hours, 2kft winds will increase to around 35kts. This will
put us right on the threshold of LLWS at all TAF sites.

&&

.PREV DISCUSSION... (Issued 1006 AM CDT Mon Aug 21 2017)

UPDATE...

Continued cloud top warming is evident on IR satellite as an
upstream MCS weakens over Iowa. This is potentially good news for
would-be eclipse viewers. By no means will we be cloud-free, but
hopefully those pesky upper level clouds will thin enough to still
see some interesting effects from the partial eclipse.

The previously mentioned MCS has somewhat complicated the picture
for the potential for severe convection this evening. All these
clouds will really limit surface-based/mixed-layer CAPE, but then
again, surface based convection was never really much of a
concern for southern Wisconsin. At this point in time, it looks
like convection will break out this afternoon in northern
IA/southern MN as an upper level trough moves into the region
before gradually pushing ESE. Questions remain on exactly how
these storms will develop as the move toward our region, although
there has been a trend in some of the high-res models to push the
strongest storms just to our south where there will be better
mixed-layer CAPE.

All that said, elevated CAPE will still remain sufficiently high
through the evening to support at least isolated severe storms.
Moreover, 0-6 km shear will approach 40kts this evening as the
upper trough moves through the region, so I cannot rule out the
potential for a few storms to reach severe levels. One thing that
may help limit the severe potential is an extremely moist
atmosphere. Guidance is still pushing PWs to around 2 inches this
evening, and while there is some anecdotal evidence that that much
moisture may diminish severe potential, this does obviously
elevate the potential for locally heavy rainfall.

MARINE...

PREV DISCUSSION... (Issued 553 AM CDT Mon Aug 21 2017)

AVIATION(12Z TAFS)...

Areas of fog around the area will lift by mid morning as southerly
flow increases a bit ahead of an approaching trough of low
pressure. Southern Wisconsin looks generally dry through the day,
although convection across Iowa this morning is pushing into far
southwest WI and may clip areas south and west of Madison before
diminishing later this morning.

Our main round of showers and storms will arrive this evening and
continue through the overnight hours. Once the fog lifts this
morning, we`ll see VFR conditions into tonight, dropping to MVFR
and possibly IFR CIGS/VSBYS as any strong storms move through.
There is a slight risk for severe storms tonight, with strong wind
and large hail the main threat.

PREV DISCUSSION... (Issued 307 AM CDT Mon Aug 21 2017)

TODAY AND TONIGHT...Forecast Confidence Is High.

The large convective complex across Iowa is on the nose of the
low level jet. The HRRR seems to be doing a good job with it,
showing that the cold pool begins to dominate with propagation
vectors taking it east to southeast with time. Many of the lower
res models bring it into our western areas this morning, then wipe
it out diurnally as support fades. Additionally, there is a mid
level wave that will be exiting by midday, and the associated
subsidence should help to diminish any lingering activity. This
gives a little hope that we might see enough thinning of the cloud
cover to observe the partial eclipse around here.

We then will likely have to wait for the next round of convection
that will fire upstream of Wisconsin this afternoon and evening.
These storms will be assisted by a decent mid level trough pushing
in, coupled with a strengthening surface trough and leading low
level jet. Soundings show plenty of CAPE, though it looks elevated
for much of the time. 0-6km shear is good at 30-40kts, even 50kts
for a time off the NAM. Precipitable water values will be
approaching 2 inches, very high now for mid to late August. The
entire area is under a slight risk of severe storms.

By the time the convection gets into our area this evening, we
will likely be looking at primarily a wind and hail threat. Very
heavy rain is also likely where any storms merge or repeatedly
move over an area. Not unusual to see 2 to 3 inches in isolated
areas causing localized flooding in this type of airmass. The
heaviest rainfall should be west of Madison, diminishing in the
east, due to a gradual diminishing in activity as we get into the
later overnight hours.

TUESDAY THROUGH THURSDAY...Forecast Confidence Is Medium.

Models are in pretty good agreement with trends during this
period. They show northwest flow at 500 mb over the region, with a
deep 500 mb low well to the northeast. At the surface, the cold
front will exit the area early on Tuesday, with strong cold air
advection in its wake into Tuesday night. High pressure then
slides southeast into the region Wednesday into Thursday.

Kept some PoPs for showers going in the morning, highest in the
southeast, as the front exits the area. Dewpoints may take awhile
to fall on Tuesday, until drier air works in for later Tuesday
into Tuesday night. Breezy northwest winds are expected Tuesday.
Some cumulus development may occur or move into the area Tuesday,
with another round on Wednesday.

May also see more middle clouds and perhaps some showers for
Wednesday night into Thursday morning, as a fairly strong 500 mb
shortwave trough slides southeast through the region. The
GFS/Canadian are similar, with the ECMWF slower and the NAM
further to the east. Kept some PoPs in parts of the area for
Wednesday night into Thursday morning.

Temperatures look to be below seasonal normals by Tuesday night
and should linger into at least Thursday, with less humidity.

THURSDAY NIGHT THROUGH SUNDAY...Forecast Confidence Is Medium.

Models continue to keep the region in northwest flow at 500 mb
Thursday night into Friday night. High pressure at the surface
slowly moves off to the east during this time. This should keep
quiet weather going across the area. Temperatures will remain
below seasonal normals.

Models then generally show a 500 mb shortwave trough sliding east
across northern Wisconsin Saturday, perhaps extending toward the
area. The ECMWF is the only model to bring QPF toward the area,
with the GFS/Canadian further to the northwest. Kept low PoPs in
the far north Saturday, but could remove if later forecasts
continue a dry trend.

GFS/ECMWF are then trying to bring an amplifying 500 mb trough
toward the region Sunday. The ECMWF brings QPF into the area with
some warm air/moisture advection, with the GFS/Canadian keeping
this to the north and west of the area. Kept some PoPs in the area
for Sunday afternoon for now. Temperatures look to remain a bit
below seasonal normals into the weekend.

AVIATION(09Z TAFS)...

Light winds and ample moisture will lead to IFR/MVFR VSBYs around
southern WI this morning. It will lift by mid morning as the
southerly flow increases ahead of an approaching trough of low
pressure. The area looks generally dry through the day, though
convection across Iowa this morning could clip far southern WI
into the early afternoon.

Our main round of showers and storms will arrive this evening and
continue through the overnight hours. Once the fog lifts this
morning, we`ll see VFR conditions into tonight, dropping to MVFR
and possibly IFR CIGS/VSBYS as any strong storms move through.
There is a slight risk for severe storms tonight, with strong wind
and large hail the main threat.

MARINE...

Look for southerly winds today around 10kts. Winds will
eventually turn to the southwest and west later tonight as a
trough of low pressure moves through. West to northwest winds will
approach Small Craft Advisory levels late tonight into Tuesday
morning. Showers and thunderstorms are expected tonight into
Tuesday morning, with quiet weather returning for the remainder
of the week.

&&

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

$$

Update...BSH
Today/Tonight and Aviation/Marine...Davis
Tuesday THROUGH Sunday...Wood



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