Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 260902

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
402 AM CDT Mon Jun 26 2017

.TODAY AND TONIGHT...Forecast Confidence Is Medium.

The main 500 mb shortwave trough will slide southeast from the
western Upper Peninsula of Michigan to central Lower Michigan
today. There appears to be good differential cyclonic vorticity
advection across the area later this morning and this afternoon
with this feature.

Area forecast soundings are showing weak mean layer CAPE
developing this afternoon, though not as much as in the past two
afternoons. Given that some of the models are not showing much if
any instability above the -10 degree Celsius level, thunder may
not occur. For now, will continue chance PoPs for showers and
small chances for thunder for mainly this afternoon across the
area, with the highest PoPs in the northeast counties.

Another cool day is expected, with broken to overcast skies
moving into the northeast counties limiting highs to the lower
60s. Middle to upper 60s are expected elsewhere, with more
scattered to broken sky cover. Gusty northwest winds are expected,
highest in the eastern areas near Lake Michigan.

High pressure passing by to the southwest and south tonight will
bring quiet weather, weakening winds and clearing skies to the
area. Lows will be cool once again, mainly in the upper 40s.

.TUESDAY...Forecast confidence is high.

Persistent upper troughing finally slides east into New England,
with building heights progressing from the western Great Lakes into
the Ohio Valley. This will support surface high pressure as it
slides southeast from southern Minnesota/northern Iowa towards the
central Appalachians by Tuesday night. Anticyclonic flow around the
high will bring southerly winds and some modest warm advection
during the afternoon and evening, allowing high temperatures to warm
into the mid to upper 70s. Still expecting some clouds as mid to
high level moisture streams in ahead of an approaching wave over the
Great Plains.

.WEDNESDAY AND THURSDAY...Forecast confidence is medium.

A mid-level wave will progress from the northern/central Great
Plains into the western Great Lakes. The wave will escort a
gradually filling surface cyclone as it tracks from the western
Dakotas into northern Wisconsin. Showers and thunderstorms are
expected, and the primary forecast concern for this period is the
risk for severe thunderstorms and heavy rainfall from Wednesday
afternoon into Wednesday night.

The synoptic picture hasn`t changed much from yesterday, with
southern stream energy forecast to advance from the Four Corners
region into the mid Missouri valley. Northern stream energy will
concurrently feed southeast from the Pacific northwest into the
central Great Plains. As the mid-level wave arrives, a 70 to 90 knot
upper speed max should develop northeastward from Iowa into
central/southern Wisconsin by Wednesday night. This will serve to
both increase mid/upper tropospheric wind speeds, and to provide a
forcing source for large-scale ascent (via upper divergence and
increasing differential cyclonic vorticity advection from the
arriving wave).

A warm front attendant to the surface low should lift north during
the morning hours, resulting in a warm and humid environment
characterized by dewpoints well into the 60s. Thereafter, showers
and thunderstorms should develop in at least a few batches. This
results in a somewhat complex convective scenario, where mesoscale
details are key to how things unfold. Current deterministic model
output suggests one round reaching the area during the late morning
to afternoon hours. These storms would occur ahead of the better
thermodynamic and kinematic parameter spaces (i.e. before the
best CAPE and shear have arrived), and would likely be of the sub-
severe variety. The second round of scattered storms should
arrive during the evening, and these stand the better shot at
becoming severe. Provided the atmosphere isn`t already worked over
from the first round, at least modest instability would be
present, with MLCAPE in the 1000 to 1500 J/kg range. Deep layer
shear would also be on the increase owing to both the speed max
aloft and to a strong southwesterly low-level jet. Around 35 to 45
knots of west to southwesterly deep layer shear will be present
Wednesday evening, before becoming more unidirectional and
westerly later Wednesday night. This near-storm environment would
be supportive of all modes of severe weather. However, we`ll need
to see exactly how worked over things are after the first round
of showers and storms. The day 3 Slight Risk from the Storm
Prediction Center seems prudent given the factors outlined above.
Heavy rainfall also remains a concern with good precipitation
efficiency and precipitable water values nearing 2 inches. MBE
velocity vectors are progressive, on the order of 20 to 25 knots,
but we`ll have to keep an eye out for any localized hydro
concerns. Keep up with this evolving forecast.

The cold front should finally slide south into northern Illinois by
Thursday morning, ending the rain chances for southern Wisconsin.

.FRIDAY THROUGH SUNDAY...Forecast confidence is medium.

A mid-level trough will escort a surface wave as it tracks northeast
along the stalled frontal zone. This will bring renewed shower and
thunderstorm chances on Friday, as the front remains just to our
south. Another chance for showers may occur on Saturday night, with
model disagreement regarding rain chances on Sunday. Temperatures
will remain near, to perhaps slightly below, seasonal averages.



Gusty northwest winds are expected to develop by middle to late
morning, lingering into early evening before weakening. Frequent
gusts of 20 to 25 knots are expected. Winds will weaken tonight,
as high pressure passes by to the southwest and south.

Scattered to broken stratocumulus clouds are expected to move into
the area later this morning and linger into early this evening.
The most sky cover will be toward the Sheboygan area, with the
lowest southwest of Madison. Any ceilings should be above 3000
feet. Mostly clear skies are then expected tonight.

Chances for showers are forecast for mainly this afternoon across
the area, with the best chances toward the Sheboygan area.
Vicinity shower wording will linger in TAFs this afternoon. There
are small chances for thunderstorms this afternoon, but any
development should be isolated at best.



A Small Craft Advisory is in effect from middle morning into the
early evening hours across the nearshore waters of Lake Michigan.
Gusty northwest winds are expected during this period, with good
low level mixing. Frequent gusts to around 25 knots are expected.
Any high waves will remain over the open waters of Lake Michigan.
Winds will weaken tonight, as high pressure passes by to the
southwest and south.

Gusty south winds and resultant waves may reach Small Craft
Advisory levels Wednesday into Wednesday night. A tight pressure
gradient develops over the region during this time, with warmer
and more humid air moving in.


LM...Small Craft Advisory from 9 AM this morning to 7 PM CDT this
     evening for LMZ643>646.



Today/Tonight and Aviation/Marine...Wood
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