000
FXUS63 KMKX 170125
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
825 PM CDT Wed Aug 16 2017

.UPDATE...
Our overall lack of decent CAPE and mid level lapse rates has kept
this activity more as rain showers, though heavy in spots, vs
thunderstorms. We`ve had a few storms goose up to produce
lightning, but it`s been very quiet. Our main concern is the combo
of low LCLs and increasing low level wind shear along and north
of the approaching warm front to the south. This has caused the
storms to rotate, raising the concern for funnels or a quick weak
spin-up tornado. Or, at least a focused downburst of winds.
Any tornado that develops would be short lived and weak. The
chance is low, but it is there. RAP analysis now has 30kts of
0-1km shear poking into southern Rock/Walworth counties, and 25kts
over a larger area to the north. The SR Helicity increases this
evening as well...along that warm front. Despite the lack of
thunder, this activity needs to be watched closely. Things should
settle down as the evening progresses, especially toward midnight,
with CIN increasing and CAPE continuing to fall.

&&

.MARINE...

Strong south to southwest winds develop by Thursday afternoon in
the wake of a passing surface trough. Those winds will continue to
veer to the southwest and west into Thursday night. A small craft
advisory will be issued for the entire area to capture these
increasing winds.

&&

.PREV DISCUSSION... (Issued 534 PM CDT Wed Aug 16 2017)

UPDATE...
Convection along the sfc warm front continues to lift north across
the area. A secondary, lake effect boundary that stretched from
Chicago, northwest into Walworth county has caused the warm
frontal convection to rapidly increase for a time over Rock and
Walworth counties late this afternoon. These may die back down a
bit as it moves away from that feature, into less favorable air.
The previous discussion is excellent with respect to the overall
threats for tonight. We are in a rather weak CAPE/Lapse Rate,
higher shear environment. These does lend itself to weaker storms
with potential rotation. These have to be watched closely for any
quick weak tornado spin-ups. It`s very low end stuff, but a
threat.

AVIATION(00Z TAFS)...

Look for conditions to gradually lower to MVFR/IFR CIGS/VSBYS this
evening as storms rotate north across the area overnight. These
lower conditions will persist through much of the TAF period. The
east to southeast winds will shift to the south after midnight,
then southwest Thursday afternoon.

PREV DISCUSSION... (Issued 310 PM CDT Wed Aug 16 2017)

SHORT TERM...

Tonight and Thursday...Forecast Confidence is medium.

Synoptic forcing for ascent is maximized tonight and includes both
jet-level divergence and increasing DCVA. A TROWAL (trough of warm
air aloft) will bring some focused mid-level theta-E advection
and a frontogenesis response. Thunderstorms have already initiated
along the low-level warm front across northeast Iowa and northern
Illinois, and they will continue to push north into the area this
evening as the front lifts north. Localized heavy rainfall
remains possible in any thunderstorm, given the favorable moisture
parameters. A secondary concern for this evening is the low-end
tornado potential associated with any thunderstorms located near
the warm front. The front will provide just enough increase above
the ambient low-level shear to provide 100 to 200 m^2/s^2 of
effective helicity this evening, which could be enough to induce
rotation in any robust updrafts that may develop. This is
considered to be a very low risk, but one worth paying attention
to nevertheless. The upper trough will be working through the
western Great Lakes on Thursday, resulting in mostly cloudy skies
with chances for showers and perhaps a thunderstorm. Went a bit
below guidance for high temperatures considering this.

Thursday night through Saturday...Forecast confidence High.

The occluded low will be centered over Upper MI at 00z Fri and
will continue newd into Ontario, Canada for Fri. A few instability
showers could linger Thu eve. The stratocumulus field may continue
through the night and decrease Fri AM. A shortwave trough will
then approach in wnwly flow aloft with the most focused area of
PVA and lift to pass across IA and IL, thus only slight chances of
showers over far srn WI for Fri aft-eve. Weak high pressure will
then settle over the region for Sat-Sat nt. Seasonal temps and
humidity is expected during this period.

LONG TERM...Sunday through Wednesday...Forecast confidence
Medium.

Swly sfc flow will support warm and humid conditions for Sun-Mon
followed by the cold front Mon nt-Tue AM. Nly sfc flow and
comfortable temps and humidity for Wed.

AVIATION(21Z TAFS)...

The first round of showers and thunderstorms will push from
southwest to northeast this evening into early tonight. Briefly
lower CIGs and VSBYs are possible in the heavier storms.
Additional chances for showers and storms are expected around or a
little after midnight. Ceilings will drop later tonight, likely
lowering to IFR levels across northern and western portions of the
area. Visibilities may lower a bit as well, but there is
uncertainty. A low-level jet at 2 kft should increase to around 25
knots tonight, bringing us close to low-level wind shear
criteria. Southwest to west winds will increase late tomorrow into
tomorrow night with surface gusts to around 25 knots.

MARINE...

Small craft conditions are anticipated north of Port Washington
later tonight into Thursday afternoon. Small craft conditions are
likely once again for Thursday night into Friday, when gusts of 25
to 30 knots are possible. Another small craft advisory will likely
be required during this time.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory from 4 AM to 4 PM CDT Thursday for LMZ643.

&&

$$

Update...Davis
Tonight/Thursday and Aviation/Marine...SPM
Thursday Night through Wednesday...Gehring