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Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1002 am CDT Thu Jul 20 2017

Update...

Stationary frontal zone remains draped across far southern
Wisconsin into northern Illinois. While the better upper forcing
has shifted off to our east, there is still a bit of weak warm
advection and frontogenesis response along the front from
southwest Wisconsin into northeast Iowa and northwest Illinois.
Scattered showers were occuring south of the Wisconsin state line
this morning, and this is where the better chance for rain should
remain today. However, given the continued weak forcing along the
front, can't rule out a few showers or storms through this
afternoon. The best chance for this will be in our south along the
Illinois border.

Look for the frontal zone to lift north into southern Wisconsin on
Friday, with moisture transport increasing from late afternoon
into evening. Deep layer moisture will rapidly increase from the
southwest as the front lifts north, with precipitable water
values climbing from around one inch Friday morning, to nearly two
inches late Friday afternoon. Available moisture parameters point
towards high precipitation efficiency, with near parallel mid to
upper flow along the frontal zone. Our concern for heavy rainfall
remains from Friday evening into Saturday morning, and we will be
assessing the need for a Flash Flood Watch today.

&&

Marine...

Offshore flow this morning will turn north to northeasterly later
today, with speeds generally under 15 knots. Slight chance for a
shower or storm today, but the better chances for storms arrives
Friday afternoon into Friday night. Prevailing winds and waves
look to remain below Small Craft Advisory levels, although higher
winds and waves are possible in thunderstorms on Friday.

&&

Previous discussion... (issued 658 am CDT Thu Jul 20 2017)

Update...showers and T-storms continue to diminish as low level
jet pivotes eastward away from southern WI. Weaker warm air
advection lingers this morning, so wl continue small chance for
T-storms redeveloping later this morning.

Aviation(12z tafs)...not seeing much indication of MVFR/IFR cigs
yet so postponed low cloud developing. May yet develop later this
morning due to moist low levels and weak frontal boundary passing
through.

Previous discussion... (issued 309 am CDT Thu Jul 20 2017)

Short term...

Today and tonight - confidence...medium.

Strong warm air advection and low level frontogenesis will continue
to carry thunderstorms across southern WI through the early morning.
The low level jet pivots off to the east after 12z so expecting a
rapidly diminishing trend through mid-morning. Main threat is heavy
rainfall as mesoscale analysis reveals considerable mlcin across the
area, but can not rule out an isolated severe. Several urban and
small stream flood advisories in affect from the Madison area.
Portions of Sauk County has already received 2-3 inches of rainfall,
so may need to issue a Flash Flood Warning depending on what
areas these upstream storms affect.

Will continue small chances for convection redeveloping across
mainly southern County Warning Area later today and then again later tonight. Weaker
low to mid-level warm air advection continues across parts of
southern WI north of the outflow boundary weak front that will be
settling southward across northern Illinois. Clouds will stick around
this morning but then gradually thin and clear but southern areas
may not see the clearing until late in the day.

Long term...

Friday through Saturday... forecast confidence is high.

Heavy rainfall possible this period. Southern WI is in a prime
area for showers and storms over a fairly long period of time late
this week. There will be zonal flow aloft with a few
ripples/shortwaves, the right entrance region of the upper jet
tracking over central WI, warm air advection, and moisture
transport on the nose of the low level jet from Friday afternoon
through Saturday morning. Precipitable water values are expected
to be around 2 inches during this time, which is around 175% of
normal. Showers with isolated thunder should be very efficient
Friday night and we will continue to monitor this period for a
flash flood potential.

There is still uncertainty about where the front, storm
evolution, and axis of heaviest precip will be, but the favorable
heavy rain set-up warrants the extra attention.

Showers and storms will be possible Fri morning with general
upward motion over the area. The higher precipitable water will
begin to spread into southern WI by the afternoon with the warm
front. This will allow for more instability and we could see a few
strong storms Fri afternoon. Storm Prediction Center has our area in a slight risk for
severe storms. The forecast soundings show more on a moist
adiabatic profile for the overnight hours, thus showers with
scattered/chance of thunder is in the fcst for this period with
the heavy rain potential.

We will lose the low level jet and other sources of stronger
forcing Saturday morning, but there could still be a few lingering
showers and storms with continued moisture over the area.

Saturday night looks dry as high pressure briefly passes by.

Sunday through Wednesday... forecast confidence is medium.

A more amplified upper trough is expected to roll across Lake
Superior on Sunday. The associated surface low should move across
northern WI that drags a cold front through southern WI midday
Sunday. There could be a few showers and storms associated with
this, but moisture and instability are still questionable.

High pressure will keep US dry and relatively cool for the first
part of next week. Our next chance for storms will arrive with a
warm front Tuesday night.

Aviation(06z tafs)...numerous showers and thunderstorms will
continue to affect southern WI through the early morning. Cigs will
be temporarily lower vcnty of storms. MVFR or possibly lower cigs
could linger for a time this morning in moist, light flow after
showers and storms diminish. Then clearing skies will affect
northern areas, but may have trouble clearing in the southern County Warning Area
until late today.

Marine...

Thunderstorms will affect the near shore waters early this morning
and then diminish or become more isolated by late morning and
afternoon. Mainly offshore winds will turn to the north to
northeast later this morning as thunderstorm outflow and weak front
slide southward. Winds look to remain less than 20 knots except in
the vicinity of thunderstorms. Next chance for more widespread
thunderstorms is Friday night.

Hydrology...

The time period to watch for excessive rainfall is Friday
afternoon through Saturday night. This active period of weather
has the potential for training of storms over the same areas to
bring the risk for heavy rainfall.

The recent heavy rainfall and additional rain would aggravate
ongoing flooding across far southeast Wisconsin and create
flooding issues elsewhere, especially in urban areas. Once more
confidence is gained with timing of these rounds of storms,
expected rainfall amounts will become more clear. Keep up with the
latest forecasts into this weekend.

&&

Mkx watches/warnings/advisories...
WI...none.
Lm...none.
&&

$$

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