Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 300216 AAB
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
916 PM CDT Sat Apr 29 2017

.UPDATE...

Rain continues east of Madison this evening. Models are showing
another surge developing over the next hour or two and beginning
to see signs of it in the southwest forecast area. Thus kept high
pops in going in the east for a few more hours and bumped up pops
west a bit as this next round fills in. Should see widespread rain
wind down by 2-4 am. Still some uncertainty beyond that, as a few
solutions have it mainly dry until Sunday afternoon, while others
keep the rain going through Sunday morning (especially in the
east).

&&

.MARINE...

Seeing occasional higher gusts this evening, so decided to keep
the Gale Warning going. After the warning winds down later
tonight, will need a Small Craft Advisory possibly into Tuesday
given persistent breezy winds. Waves will eventually ease a bit
by Mon/Tue as winds become along shore to offshore.

&&

.PREV DISCUSSION... (Issued 636 PM CDT Sat Apr 29 2017)

UPDATE...

The back edge of the rain is currently pushing in from the west,
but will probably not get much farther than Madison as another
surge arrives from the south. Will see the potential for a sleet
mix at times over the next couple hours. Model soundings suggest
this mix will end by mid-evening or so.

AVIATION(00Z TAFS)...

Rain will persist into tonight...slowly winding down by the early
morning hours. A little sleet may mix in through mid-evening.
Latest meso model show a break in the widespread rain arriving by
09Z, though will have to keep an eye on radar trends due to a
couple wetter model solutions. More widespread rainfall is
expected to develop by Sunday afternoon.

Ceilings will likely lower tonight, with low ceilings expected
through the day Sunday. Some lower visibilities of 1-2 miles
possible at times with the moderate rainfall based on upstream
obs.

A little thunder is possible this evening, though lightning
continues to remain well to the south of the WI/IL border. There
will be thunder chances with the rain tomorrow.

PREV DISCUSSION... (Issued 340 PM CDT SAT APR 29 2017)

TONIGHT AND SUNDAY - Forecast Confidence...Medium.

There is currently an amplified upper trough situated near the 4
corners regions and an associated surface low over the Southern
Plains. The northern stream jet is just to the north of the Great
Lakes regions while the southern stream jet is near the base of the
trough. The elevated warm front extending from the low center is
situated over Wisconsin, sloping to the south toward the surface.
Widespread rain along the warm front has been pushing toward
southern WI all afternoon, and has finally reached north of the
state line.

The upper trough will continue to deepen as it takes on a slightly
negative tilt and progresses eastward. Accordingly, the southern
stream jet will develop on the eastern flank of the trough by Sunday
afternoon, setting up a coupled jet situation over southern
Wisconsin. Meanwhile, the surface and mid level low will continue to
deepen and move to the northeast. The elevated warm front will push
to the north, remaining over southern Wisconsin through tomorrow
afternoon. Strong moisture advection will take place as the upper
levels of the warm front gradually push north of the area, with
PWATS reaching as high as 1.5" by Sunday afternoon.

This all sets the stage of an extended period of precipitation.
However, many of the convection allowing models have been showing a
consistent signal of rain this afternoon and evening, followed by a
considerable lull in the precipitation from late this evening into
tomorrow morning. Given the synoptic set up, I have a hard time
seeing why this prolonged dry period would occur, but with the
consensus and continuity of the CAMs, I can`t rule it out. I
therefore have lowered pops and QPF from late tonight and tomorrow
morning considerably from the previous forecast. Needless to say,
uncertainty in my pop and QPF forecasts is relatively high.

Precip will reintensify tomorrow afternoon as the surface front
pushes through the CWA. Most of the guidance is showing at least
some MUCAPE, so left the chance mention of thunder in the forecast
for tomorrow afternoon.

SUNDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium to
High.

Expect showers and a few embedded thunderstorms to be ongoing at the
beginning of the period.  Low pressure moving along occluding front
will have considerable synoptic lift as it passes across northern IL
and southern WI Sunday evening. Layer Q-vector convergence briefly
reaches 20 units in the evening with a period of sloping
frontogenetical forcing.  Despite weak elevated instability, due to
strength of lift, will continue to include a chance of tstorms
during the evening as low level jet pivots across southern WI
where low level thermal ribbon will be draped. Expect showers to
diminish in areal coverage overnight as mid-level drier air wraps
northward around parent low circulation in the central Plains.
This will end the thunder threat by late evening or shortly after
midnight.

Expect showers to spread back across southern Wisconsin on Monday as
strong low pressure occlusion passes across the area.  Column
moisture will remain deep and several strong mid-level short waves
will rotate around the main low pressure system.  Low level thermal
profile looks to remain warm enough to keep the returning
precipitation as all rain Monday.  Parts of the northwest CWA may
see a mix as the colder air deepens across southern WI Monday night
but the showers will be waning by that time.

Possible a few showers could linger into Tuesday due to lingering
cyclonic curvature and steep low level lapse rates.  Temperatures
will remain cool for late April with daytime readings remaining in
the 40s to low 50s.

EXTENDED PERIOD...

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

The period should start out quiet and cool as weak ridging passes
through the western Great Lakes.  Frost may be a threat Tuesday
night and early Wednesday as 925H temps will be around 1-3C and
boundary layer winds will be lighter.  Will depend heavily on
lingering cloud cover.

Upper level steering flow will back slightly to more zonal to
northwest around mid-week allowing slightly warmer air to return.
Developing split flow looks to keep upper Midwest and Great Lakes
drier through the end of next week while more active southern stream
keeps precipitation south of Wisconsin. While dry conditions are
expected for the most part, several weakening short waves caught in
the northern stream of the westerly flow may bring a few showers to
the area, especially Thu and Fri.

While the coldest morning may be Wednesday, night-time temperatures
will remain cool through the rest of the period, with lows in the
upper 30s to lower 40s.  The large negative 500H anomoly over the
region at 00Z/Thursday quickly shifts off to the east, replaced by a
weakly positive anomoly by 00z/Sunday. CIPS Analogs continue to
show the potential across southern WI of near or below freezing
temperatures in the day 6-8 period (next weekend) so holding off
on planting tender vegetation still prudent.

MARINE...

Strong, onshore winds will persist through Sunday as low pressure
moves toward the state. Winds will be the strongest this evening and
into tonight, with gusts approaching 34 kts. This is a borderline
Gale Warning event, so have chosen to stick with the Gale Warning we
already have out there. Winds will begin to diminish late tonight,
but remain elevated. Moreover, the onshore winds will result in high
waves through Sunday, and an additional SCA will be needed for much
of the day Sunday. Winds will briefly weaken on Monday morning as
the the low moves through the state. Then winds will swing around to
the south southwest and reintensify, perhaps prompting an additional
SCA.

HYDROLOGY...

Issued hydrologic outlook across southeast Wisconsin.  Parts of the
southeast may receive 1 to 2 inches of rainfall tonight, and another
1 inch Sunday afternoon into Sunday evening.  This 1-2 punch of
heavy rainfall will push several rivers above flood stage, and
possibly approaching Moderate Flood.  The most susceptible rivers
will be the Fox and Root Rivers in western Kenosha and Racine
counties.  Can not rule out some minor street and rural field
flooding, especially in low, susceptible areas.  Still can not rule
out the need for an Areal Flood Watch for the area, but will wait to
see how much rain occurs tonight before deciding.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Gale Warning until 4 AM CDT Sunday for LMZ643>646.

&&

$$

Update/Aviation...DDV
Tonight/Sunday and Marine...BSH
Sunday Night through Saturday and Hydrology...MBK


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