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Area Forecast Discussion

000
FXUS63 KMKX 292040
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI 
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 
stength 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 postive 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...Small Craft Advisory until 4 PM CDT this afternoon for 
     LMZ643>646.
     Gale Warning from 4 PM this afternoon to 4 AM CDT Sunday for 
     LMZ643>646.

$$ 
TONIGHT AND SUNDAY/MARINE...BHS 
SUNDAY NIGHT THROUGH SATURDAY/HYDROLOGY...MBK