Flash Flooding and Hail in Wisconsin, July 9 to 10, 2017

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
901 PM CDT Sun Jul 9 2017

.UPDATE...

Severe convection has developed in central Minnesota. There is
some uncertainty if the severe convection will take more of a
south southeast turn and move along the differential
heating/moisture gradient, mainly into northeast Iowa, later this
evening into the overnight hours.

There is an area of greater mean layer CIN and lower dewpoints to
the east of that gradient, which extends into southwest
Wisconsin. This casts some doubt as to if the convection will
remain severe when some of it moves into the western portions of
the area, mainly after midnight.

The latest HRRR run seems to bring convection southeast through
the area during the overnight hours, but it does not appear to be
very strong. We will still have forcing for upward vertical motion
from the veering low level jet nose, and a passing 500 mb
vorticity maximum with some upper divergence.

It seems to be a matter of will there be enough elevated CAPE and
low enough CIN to sustain severe convection. For now, will
continue to bring high PoPs southeast through the area later
tonight. Severe chances may be more in question. Will continue to
monitor things into the overnight hours.

Wood

&&

.MARINE...

It looks like an area of showers and thunderstorms will move
southeast through the area later tonight, exiting during the
morning on Monday. Strong storms are possible, though there is
some question as to how strong they will be once they make it into
the nearshore waters of Lake Michigan. Still, gusty winds, hail
and cloud to surface lightning are possible later tonight.

Wood

&&

.PREV DISCUSSION... (Issued 650 PM CDT Sun Jul 9 2017)

UPDATE...

AVIATION(00Z TAFS)...

Middle to high clouds will continue to push southeast across the
area this evening. Winds will become light and southerly this
evening as well.

A complex of thunderstorms is forecast to slide southeast into
south central and then southeast Wisconsin between 05Z and 12Z
Monday. Severe storms with damaging winds, heavy rainfall and some
large hail is possible across the area during this time. The best
chances will be west of a Wisconsin Dells, to Madison, to
Janesville line between 05Z and 09Z Monday.

Ceilings and visibilities down to or below alternate minimums are
possible in any storms. Light south winds later tonight into
Monday morning will become north to northeast by later Monday
afternoon.

Chances for showers and storms will linger into the rest of
Monday, though confidence in timing and intensity are still rather
low. It will depend on what happens overnight into early Monday
morning.

Wood

PREV DISCUSSION... (Issued 205 PM CDT Sun Jul 9 2017)

DISCUSSION...

Tonight...Forecast Confidence is Medium.

Low pressure across central ND/SD continues to be the focus of the
forecast period. There is strong WAA along and south of the
attendant warm front, which reaches ESE across central MN. Aloft, we
will remain firmly entrenched in NW flow with and embedded shortwave
moving south into northern ND/MN. As that wave continues to push
south, the surface low will follow suit, eventually moving over
southern WI, bringing a good shot at thunderstorms to SE WI
overnight. While the guidance is in good consensus regarding the
general synoptic pattern over this period, considerable differences
exist with respect to the details.

The primary difference is born from the fact that the GFS/ECMWF
break out warm sector convection across MN this afternoon and grow
it upscale into an MCS. Meanwhile, the NAM/GEM either keep
convection limited to the warm front farther north, or fire warm
sector convection but then do not grow it upscale. With that
backdrop, the GFS/ECMWF solution drives the MCS roughly along the
Mississippi overnight, limiting overnight convection along the warm
front which will be draped across central WI. Meanwhile, the NAM/GEM
solution break out widespread overnight convection along the warm
front in the absence of the MCS farther south and west.

Therefore the question becomes: will we see thunderstorms initiate
away from the warm front in southern MN? The cap will remain quite
strong through most of the afternoon, but will weaken toward the
evening. The previously mentioned short wave coupled with mid to low
level WAA and some weak surface convergence should be enough to kick
off some warm sector storms. This scenario is well supported by most
of the meso models. Therefore, I am favoring the GFS/ECMWF solution,
with an MCS barreling along the Mississippi River valley and its
eastern flank reaching into our CWA.

Thanks to moderate to strong flow aloft, 0-6km bulk shear values
remain in the neighborhood of 40kts through the night, and all
synoptic models are showing MUCAPE remaining as high as 1000 j/kg
through much of the night in SW WI. Given the strong wind profile
and potential for a significant bowing line of convection, strong
winds will be the primary threat with overnight storms. However,
given the good shear and sufficient instability, large hail is also
possible. Furthermore, 0-3km shear will be in the 30-35 kt range,
and depending on how the bow becomes aligned with respect to the 0-
3km shear vectors, while not likely, a few spin up QLCS tornadoes
cannot be ruled out.

Finally, precipitable water values will continue to increase ahead
of the approaching convective system, reaching to around the 90th
percentile per the SPC sounding climatology. Fortunately the MCS
lends itself to fast moving storms. Therefore while heavy showers
are expected, the flood threat should be fairly low.

Tomorrow...forecast confidence is low

Uncertainty is quite high for Monday given the lingering question
regarding tonight. At this point, it seems likely that southern WI
will be situated between a warm front to our north and a outflow
boundary from the MCS to our south. Most guidance keeps the low
levels moist as the low moves through southern WI tomorrow afternoon
and at least some elevated instability will be in place.
Furthermore, the upper level flow remains fairly strong, so 0-6
bulk shear will remain near 40kts. Given these ingredients, there
is at least a chance at additional thunderstorms, some of which
may become strong. However, this is all extremely dependent on how
tonight plays out.

Monday night and Tuesday - Confidence...Medium
Weak ridging takes hold for Monday night into Tuesday morning. A
low level thermal trough settles in during this period so cloud
cover may be stubborn in some areas. Later Tuesday some WAA
reloads with the models showing some QPF. Bufkit soundings show
mid level moistening with some elevated CAPE but overall not much
instability-wise with this initial return flow. there will be some
500 millibar shortwave activity riding through in the fairly swift
nearly zonal flow. So have some POPS in place for some initial WAA
induced TSRA especially in the southwest CWA during the afternoon
hours.

Tuesday night and Wednesday - Confidence...Medium
Models are in fairly good agreement with the idea of a low
pressure system moving through WI this period. It appears some WAA
related storms may strengthen as the LLJ ramps up Tuesday night
into Wednesday morning. As the trailing cold front pushes in for
Wednesday there may be additional storms along that boundary.
However this may hings on how the airmass recovers from the
earlier WAA/LLJ related convection. Will have to watch this period
for some heftier storms. SPC has a hint of this in their day 4
outlook.

Thursday - Confidence...Low to Medium
Mid level energy is proggd to pivot from the Lake Superior region
into the eastern Lakes. The 00z ECMWF showed more amplification of
this feature into eastern WI while the latest GFS tends to keep
the influence more to our northeast. Some uncertainty here with
respect to potential for some SHRA development, so will let
the dry look of the Superblend POPS ride for now.

Friday through Sunday - Confidence...Medium
High pressure looks to dominate for much of this period
especially per the ECMWF. However the GFS and GEM suggest a
frontal passage on Saturday with potential for some SHRA/TSRA.
Superblend handles this with some chance POPS and will follow this
for now.

AVIATION(18Z TAFS)...Forecast guidance is split on thunderstorm
activity overnight tonight. The most likely scenario appears to
be a situation in which a strong area of thunderstorms quickly
moves down the Mississippi River valley, with its eastern flank
extending into southern WI. This would bring thunderstorms to the
TAF sites, some of which may be potentially severe storms,
especially closer to the Mississippi River.

However, there is smaller chance that the thunderstorm activity
would be more focused over southern Wisconsin. If that were to be
the case, we would likely see more rain over a more extended
period, but the storms may be less severe. In either case,
thundestorms seem like a good bet in the early morning hours.

The forecast for tomorrow remains quite uncertain due to the
lingering questions regarding tonight. Details will become more
clear as tonight's events play out, but it does appear as though
we'll see more thunderstorms tomorrow.

MARINE...Winds will continue to swing around to the south this
afternoon and evening ahead of an approaching storm system. These
storms will move into the area overnight, some of which may be
severe. Storm chances will linger into tomorrow before finally
tapering off tomorrow night. Other than any gusts from thunderstorms,
winds should remain largely below 15kts until Wednesday when a
low pressure system moves through the area and winds approach
small craft advisory criteria.

&&

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

Update...Wood
Tonight/Monday and Aviation/Marine...BSH
Monday Night through Sunday...Collar

Hazardous Weather Outlook...UPDATED
National Weather Service Milwaukee/Sullivan WI
941 PM CDT Sun Jul 9 2017

WIZ046-047-051-052-056>060-062>072-110245-
Marquette-Green Lake-Fond Du Lac-Sheboygan-Sauk-Columbia-Dodge-
Washington-Ozaukee-Iowa-Dane-Jefferson-Waukesha-Milwaukee-
Lafayette-Green-Rock-Walworth-Racine-Kenosha-
941 PM CDT Sun Jul 9 2017

This Hazardous Weather Outlook is for portions of east central
Wisconsin, south central Wisconsin, and southeast Wisconsin.

.DAY ONE...Tonight

An area of thunderstorms is expected to push southeastward across
southern Wisconsin during the overnight hours. Severe storms with
damaging winds are possible, with the best chances west of a
Wisconsin Dells to Darlington line.

.DAYS TWO THROUGH SEVEN...Monday through Sunday

There are chances for thunderstorms Monday into Monday evening.
More thunderstorm chances are forecast for Tuesday night into
Wednesday. A few storms could be severe during this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed overnight tonight.

LMZ643>646-110245-
Sheboygan to Port Washington WI-
Port Washington to North Point Light WI-
North Point Light to Wind Point WI-
Wind Point WI to Winthrop Harbor IL-
941 PM CDT Sun Jul 9 2017

This Hazardous Weather Outlook is for portions of Lake Michigan.

.DAY ONE...Tonight

An area of thunderstorms is expected to push southeastward across
the nearshore waters of Lake Michigan during the overnight
hours. Strong storms with gusty winds are possible.

.DAYS TWO THROUGH SEVEN...Monday through Sunday

There are chances for thunderstorms Monday into Monday evening.
More thunderstorm chances are forecast for Tuesday night into
Wednesday. A few storms could be severe during this time.

Wood
www.weather.gov/mkx