Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 220036 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
736 PM CDT Wed Jun 21 2017

.UPDATE...

Our focus is on the 850mb warm front this evening. It is on the
move northeastward and is currently over southwest WI per the
latest SPC Mesoanalysis page. This is the region of increasing
Most-Unstable CAPE. Bulk shear will be a respectable 40 knots
within this region.

There are no storms developing yet with this warm front since we
are lacking a trigger. We are waiting for the nose of the
strongest part of the low level jet (LLJ) to arrive which looks
like it will be late evening. Once that races to meet up with the
warm front, then storms will fire. That warm front will probably
be up in central WI by late evening, so it`s looking like southern
WI will be dry tonight. The better chance for storms is north of
I-94. The latest couple of HRRR model runs suggest that the whole
MKX forecast area will remain dry all night.

There could still be a few elevated showers/storms as the LLJ
makes its way through southern WI this evening. I am not bold
enough to remove precip chances for us all night until I see where
the storms develop. This will probably not be until between 10
and midnight.

By the way, that convection over eastern Iowa is tied to a warm
front in the 925-850mb layer and supported by a shortwave. These
showers/storms will head east and gradually weaken tonight.
Southern WI will just see the high clouds associated with them
during the evening hours.

&&

.AVIATION(00Z TAFS)...

Please see the discussion above for an explanation of thunderstorm
potential for tonight. VFR conditions are expected through tonight
and Thursday except within storms. Look for gusty southwest winds
Thursday. Best chance for storms will be Thu evening.

&&

.PREV DISCUSSION... (Issued 355 PM CDT Wed Jun 21 2017)

TONIGHT AND THURSDAY...Forecast Confidence Is Medium.

Mesoscale models suggest that the best chances for elevated
convection with the low level jet nose this evening will be across
the far northern and northeastern portions of the forecast area.
However, last few runs of the HRRR are trending northward into
central Wisconsin with most of the convective development.

This trend may continue and keep us dry this evening. Uncertainty
here with where convection will initiate. For now, kept this area
in the highest PoPs later this evening into the overnight hours.
Area forecast soundings are showing enough elevated CAPE with
fairly strong effective shear to bring a large hail risk to the
area. SPC Slight Risk looks good for now, given this combination.

There is some potential for convection to move southeast through
the area later tonight, though confidence is shaky as the best low
level jet forcing moves out of the area. Kept PoPs going for now
across the area.

Mesoscale models suggest another round of strong to severe storms
is possible later Thursday afternoon. Strong upward vertical
motion with frontogenesis with the cold front is expected to
initiate storms just to the northwest of the area by the middle to
late afternoon hours. Adjusted PoPs upward in the northwestern
portions of the area later in the afternoon, as storms may be
developing or pushing into that area.

Given the expected mean layer CAPE and deep layer/low level
shear, all hazards are possible. Heavy rainfall is also a concern,
given the favorable moisture parameters expected. SPC Slight Risk
looks good for now.

THURSDAY NIGHT AND FRIDAY...Forecast confidence is medium.

The main focus for this period is the risk for severe thunderstorms
and heavy rainfall on Thursday night.

A low amplitude wave will progress east-southeast from the northern
Great Plains into Wisconsin. An increase in westerly mid to upper
tropospheric flow will accompany the wave, with jet-level divergence
maximized by Thursday evening. Respectable height falls will precede
the wave`s arrival with synoptic scale forcing for ascent maximized
early Thursday night. Deep layer moisture will be drawn poleward
ahead of the wave, with low-level forcing provided via frontogenesis
along a southward sinking cold front. All of this suggests a good
environment for thunderstorm development, but there are a few more
things to consider with regards to our severe risk.

First, let`s look at wind shear. We already have good mid to upper
flow with our close proximity to the mid/upper jet. Flow will also
increase in the lower troposphere due to a low-level jet impinging
on southern Wisconsin from the southwest. This will provide around
35-40 knots of speed shear in the 0-6 km layer, with around 25 knots
occuring in the lowest kilometer. Some directional shear will also
be present in the low levels, as the flow backs south-southwesterly.
This shear profile would potentially support both multicell and
supercell storm modes, with a risk for damaging winds and hail. The
combination of low level directional and speed shear also presents a
tornado risk, especially along any remnant boundaries leftover from
prior convection. Speaking of that, let`s also consider the
instability.

This is where things become a bit trickier, as any leftover debris
clouds from Wednesday night`s convection could influence where the
greatest instability develops. For now, will defer to a consensus of
the global forecast models, which suggest the effective cold front
(and primary low-level forcing mechanism for thunderstorm
development) will be located near our northern areas Thursday
evening, with a relatively uncontaminated environment over southern
Wisconsin (free from clouds and showers). Given this scenario,
moderate instability would build, with dewpoints well into the 60s
and mixed layer CAPEs reaching 1.5 to 2.5 kJ/kg. Given the
previously mentioned shear profiles, along with around 0.9 kJ/kg
of MLCAPE in the hail growth region, and theta-E differences
around 25K, this environment would be supportive of both large
hail and damaging wind gusts (in addition to the tornado threat
mentioned earlier).

The caveat with this scenario is where the greatest instability
develops. If debris clouds temper our heating on Thursday, it`s
possible that our risk for severe thunderstorms would decrease.
While this is a possible scenario, it`s not the one favored at this
moment among the guidance. So for this reason it`s not a good idea
to let down your guard just yet. Keep up with the forecast, and
we`ll fine tune it after seeing how tonight`s round of storms
progresses.

Last, but not least, is the concern for heavy rainfall. MRMS radar
estimates suggest southern Wisconsin has received 1 to 3 inches of
rain on average over the past 7 days. Additional heavy rainfall is
possible Thursday night, which may set the stage for flooding/flash
flooding concerns. This is highlighted via the WPC Day 2
Excessive Rainfall Outlook, which places our area in a slight
risk. Working in favor of heavy rainfall is good southwesterly
moisture transport via the low-level jet, deep warm cloud depths,
and precipitable water values climbing to around 2 inches. Global
models do suggest that the front will be progressive, which could
help us by limiting this threat. However, we`ll have to watch for
any possible training convection, and for heavy rain over urban
areas. Will highlight this concern in the HWO.

Cyclonic flow will prevail aloft on Friday with scattered to broken
cumulus clouds developing during the afternoon. The GFS tries to
spit out a few showers, but for now kept the forecast dry.

SATURDAY THROUGH TUESDAY...Forecast confidence is medium.

The upper level wave train continues, bringing periodic chances for
showers and thunderstorms. Keep an eye on the forecast if you have
weekend activities. Cool temperatures are expected.

AVIATION(18Z TAFS)...

Southeast winds are expected near Lake Michigan this afternoon
with lake breeze. Area of showers may clip Madison and areas to
the southwest of there for a few hours early this afternoon.
Middle to high clouds will linger at times into this evening. Some
diurnal cumulus clouds may develop as well.

Best chances for storms mainly from middle to late evening will
be toward the Montello, Fond du Lac and Sheboygan areas. They may
be severe with large hail possible. These may linger for a time
later this evening, before moving to the northeast by later
tonight.

Low level wind shear is forecast mainly later tonight into early
Thursday morning, with 2000 foot southwesterly winds around 40 to
45 knots.

Chances for storms linger into Thursday, with the best chances for
a round of severe storms later Thursday afternoon and Thursday
night. A strong cold front will slide southeast across the area
during this time. Severe storms with all hazards are possible
during this time, with heavy rainfall possible as well. Ceilings
and visibilities will be greatly reduced in any storms.

Gusty southwest winds are expected Thursday, with gusts to around
20 knots possible at times.

MARINE...

Storms may develop across the area this evening, with the best
chances north or North Point Lighthouse. Severe storms with large
hail are possible in this area. There is a possibility that storms
may develop further to the north than currently anticipated.

Chances for storms are lower to the south. These storms should
linger for a time later this evening and overnight, before moving
to the northeast.

Chances for storms linger later tonight into Thursday. The best
chances for another round of severe storms will be Thursday night,
as a strong cold front moves southeast through the area. All
hazards of severe weather are possible Thursday night as the
storms move through.

Gusty southwest winds may reach Small Craft Advisory levels at
times Thursday, with a tight pressure gradient and good low level
mixing.

HYDROLOGY...

Southern Wisconsin has averaged 1 to 3 inches of rain over the
past 7 days, and there is some concern for flash flooding from
late Thursday afternoon into Thursday night. The greatest threat
would be from training convection, or if heavy rain sets up over
urban areas. Not enough confidence in coverage/impacts for a flood
watch, but have highlighted this concern in the HWO.

&&

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

$$

Update...Cronce
Tonight/Thursday and Aviation/Marine...Wood
Thursday Night through Wednesday...SPM



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