Flash Flooding and Flooding in Wisconsin, July 11 to 24, 2017

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
800 PM CDT Tue Jul 11 2017

.UPDATE...
The atmosphere is not cooperating. The high resolution guidance,
including the NAM, had been suggesting that we'd see rapid
redevelopment toward midnight along a NW/SE line from about
Baraboo to Milwaukee. This would be along or near the 850mb warm
front lifting through. However, satellite imagery shows an upper
level jet streak has helped to trigger early convection along that
850 boundary over far northeast IA. So, this new convection is
likely to just expand and roll across the area through the evening
hours. We have the moisture, with dewpoints well into the 70s,
effective deep layer shear is pushing 40-50kts. MUCAPE is around
2500J/KG. This is a rapidly changing evolution, remain flexible in
your expectations. Severe storms with large hail and strong winds
are the primary threat, but a few tornadoes are possible,
especially during the evening, west of Madison.

&&

.MARINE...
Winds and waves will remain below small craft advisory levels.
Mariners should be on the look out for strong storms tonight into
Wednesday morning along with areas of fog. The fog may be dense in
spots with a very moist airmass in place.

&&

.PREV DISCUSSION... (Issued 520 PM CDT Tue Jul 11 2017)

UPDATE...

Showers associated with a weak mid level wave are moving through
the area late this afternoon. A few tried to reach thunderstorm
status, but dissipated quickly. They still have a chance to
produce thunder, but this activity is expected to move off quickly
before about 8 pm. The NAM and the HRRR are suggesting we will
then be rather quiet, waiting for rapid redevelopment toward
midnight, along a northwest to southeast line from about the Dells
to Milwaukee. That's a very rough estimate and that location may
change going forward. The complex now developing over northwest MN
would then slide down this convective boundary. It all points
toward the potential for a lot of rain across parts of south
central and southeast Wisconsin. Thankfully, right now, that focus
may be across areas that didn't get hit hard Sunday night. But,
vigilance is required tonight. Keep up with the latest trends.

AVIATION(00Z TAFS)...

Mainly VFR conditions are expected through the TAF period. But, we
do anticipate showers and storms developing, especially for
KMKE/KENW/KUES around or shortly after midnight. The showers
moving through southern WI early this evening will move off to
the east by 8 pm or so. We should then be quiet until the rapid
development of activity toward midnight. As always, very low
conditions could accompany any of the stronger storms. The
overnight storms should move off to the southeast by mid morning
on Wednesday with more storms possible Wednesday evening. There is
the potential for severe thunderstorms, with the round of storms
tonight and again Wed evening.

PREV DISCUSSION... (Issued 300 PM CDT TUE JUL 11 2017)

TONIGHT AND WEDNESDAY - Forecast Confidence...Medium to High.

A weak wave will bring showers into the forecast area into the early
evening hours. Could be some thunder too given increasing moisture
and instability this afternoon.

Attention will then turn to widespread thunderstorm development late
this evening into tonight. Low level jet will strengthen tonight and
point into the forecast area. This will continue to pump deep
moisture into the area with precipitable water values increasing to
around 2 inches tonight. Models are suggesting persistent convective
development along a surface boundary as the boundary slowly lifts
northeastward. Given recent rainfall and the potential for heavy
rain tonight, have a Flash Flood Watch in place from 10 pm tonight
to 10 am Wednesday morning. Excluded the 3 southwest counties of the
forecast area, as most models suggest the bulk of precip to develop
and then advect to the northeast of this area. Some of the storms
could reach severe levels tonight too as 0-6 km shear is expected to
be around 30-40 knots. Large hail and damaging winds are the main
threats.

Storms are expected to wind down early to mid Wednesday morning.
Could be quiet then into the afternoon hours. Given enough clearing,
temps could approach or top 90 degrees. Dewpoints will likely be in
the low to mid 70s, so heat index values may climb toward 100
degrees. Some uncertainty with this setup though, given the
potential for convective debris and onshore winds near the lake for
at least part of the day. On the flip side, could get into the low
90s if it clears out.

Left a 20 percent chance for storms late morning and afternoon due
to the moisture and instability in place, though it will probably be
dry most of this time. Have somewhat higher chance precip chances in
the northwest toward evening as another round of storms may develop
as the next wave approaches.

WEDNESDAY NIGHT THROUGH FRIDAY - Forecast Confidence...Medium.

Expect scattered convection to be ongoing across southern Wisconsin
from Wednesday afternoon into the evening.  Latest GFS and NAM slow
southward progression of surface low pressure and cool front, mainly
due to slightly stronger and slower mid-level low pressure over
western Lake Superior area. Passing upper level jet places central
and southern WI in a period of upper level divergence and enhanced
synoptic scale forcing. NAM even carries a coupled jet across
central WI. Wl keep pops going through the night and have a period
of likely in the south.

Expect at least some breaks in the cloud cover by later Wednesday
afternoon into the evening.  Very moist and unstable airmass will be
in place with MUCape in the 2-4K joule range. Can not rule out
severe threat due to nearby boundary acting as focus for very
unstable air mass, and moderate bulk shear in the 30-35kt range.
Deep moisture in place, so can not rule out another bout of heavy
rain.

Due to slower eastward progress, will continue small pops even into
Thursday in the east.

Mid-level short wave gradually shifts east of the area later
Thursday into Thursday night.  This will nudge frontal boundary and
deeper moisture farther south into central MS Valley and OH Valley
region, allowing drier and more stable conditions to sag into
southern WI.  Hence pleasant conditions expected to finish off the
work week.

EXTENDED PERIOD...

FRIDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium.

Medium range guidance has come into better agreement on placement
and strength of Canadian low pressure and southward extending low
pressure trof over eastern Great Lakes and eastern Conus early in
the period. Wisconsin and the western Great Lakes will be initially
under a drier, slightly cooler NW steering flow between this
downstream trof and building upstream blocking ridge over the Plains
into south central Canada. Plains ridge looks to flatten slightly
but expand ENE into the western Great Lakes by 12Z/17.  925H temps
warm into the mid 20C range by Tue which would push high temps into
the upper 80s to lower 90s pending cloud cover and wind direction.
Warm and dry atmosphere should act as cap preventing updrafts and
convection for the later periods, possibly through Wednesday
depending on when flattening ridge finally allows northern
convection to penetrate farther south. GFS 5day 500H Height
anomolies continues to show large 100meter positive anomoly centered
over the western Great Lakes at 00z/Wed.

AVIATION(18Z TAFS)...An area of showers will likely roll through
the forecast area into the early evening hours. Coverage will
probably be scattered at best. Could be a storm or two as well.

Focus then turns to the thunderstorm potential from late evening
into Wednesday morning. Multiple rounds of thunderstorms are
possible, with locally heavy rainfall possible. Some of the storms
may reach severe limits as well, with damaging winds and large hail
the main threats.

Expecting mainly dry weather from late Wed morning into the
afternoon. Another round of storms may develop by the evening hours.

Lower ceilings and visibilities are likely with thunderstorms.
Otherwise mainly VFR conditions are expected through Wednesday.

MARINE...

Another round of fog is possible tonight into Wednesday under
southerly winds and increasing moisture.

May see gusts to around 20 knots Wednesday, but winds are expected
to remain below Small Craft Advisory levels.

Thunderstorms are expected by later this evening into tonight, with
more storm chances on Wednesday.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch through Wednesday morning for WIZ046-047-051-
     052-056>060-063>066-069>072.

LM...None.
&&

Update...Davis
Tonight/Wednesday and Aviation/Marine...DDV
Wednesday Night through Tuesday...MBK

Hazardous Weather Outlook
National Weather Service Milwaukee/Sullivan WI
319 PM CDT Tue Jul 11 2017

WIZ046-047-051-052-056>060-062>072-122030-
Marquette-Green Lake-Fond Du Lac-Sheboygan-Sauk-Columbia-Dodge-
Washington-Ozaukee-Iowa-Dane-Jefferson-Waukesha-Milwaukee-
Lafayette-Green-Rock-Walworth-Racine-Kenosha-
319 PM CDT Tue Jul 11 2017

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

.DAY ONE...Tonight

Thunderstorms are expected to affect southern Wisconsin tonight
and exit the area Wednesday morning. A few of these storms could
become severe producing damaging winds to 60 mph and one inch
hail.

Heavy rainfall is also expected, possibly producing flash
flooding. A Flash Flood Watch remains in effect for most of south
central and all of southeast and east central Wisconsin from this
evening through mid-morning Wednesday.

.DAYS TWO THROUGH SEVEN...Wednesday through Tuesday

There is a chance for thunderstorms Wednesday afternoon and
Wednesday night. A few storms could be severe.

The Heat Index is expected to climb into the mid to upper 90s on
Wednesday.

There is a slight chance of thunderstorms Saturday night.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed Tonight into Wednesday night.

LMZ643>646-122030-
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-
319 PM CDT Tue Jul 11 2017

This Hazardous Weather Outlook is for portions of Lake Michigan.

.DAY ONE...Tonight

Thunderstorms are expected to affect the near shore waters
tonight and exit the area Wednesday morning. A few of these storms
could be severe producing damaging wind gusts to 50 knots and one
inch hail.

Areas of fog are expected to redevelop over the near shore waters
tonight.  The fog may become locally dense.

.DAYS TWO THROUGH SEVEN...Wednesday through Tuesday

There is a chance for thunderstorms Wednesday afternoon and
Wednesday night. A few storms could be severe.

Breezy southerly winds will approach Small Craft Advisory levels
for a time on Wednesday.

There is a slight chance of thunderstorms Saturday night.

www.weather.gov/mkx

MBK

Special Weather Statement
National Weather Service Milwaukee/Sullivan WI
842 PM CDT TUE JUL 11 2017

WIZ062-067-120245-
Lafayette-Iowa-
842 PM CDT TUE JUL 11 2017

...A STRONG THUNDERSTORM WILL AFFECT NORTHWESTERN LAFAYETTE AND
SOUTHWESTERN IOWA COUNTIES...

At 841 PM CDT, a strong thunderstorm was located near Lancaster,
moving east at 30 mph.

Winds in up to 40 mph and half inch hail are possible with this
storm.

Locations impacted include...
Belmont, Linden, Rewey, Platte Mounds, London and Leslie.

Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

LAT...LON 4290 9042 4293 9024 4272 9015 4270 9043
TIME...MOT...LOC 0141Z 260DEG 26KT 4278 9063

BSH