Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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676
FXUS63 KMKX 210246 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
946 PM CDT Fri Oct 20 2017

.UPDATE...

High clouds overspread southern WI this evening. The clouds and
steady south winds will keep temperatures mild. Raised min temps
another notch.

&&

.MARINE...

Tightening pressure gradient ahead of approaching strong cold
front will cause south winds to become gusty Saturday into
Sunday. Strong winds just above the ground should be able to
occasionally reach the lake surface later tonight. A Small Craft
Advisory will begin later tonight and continue through mid-
afternoon Sunday. Mariners should expect the winds to switch from
the south to the West Sunday morning. Some showers will accompany
the frontal passage, along with isolated thunder.

&&

.PREV DISCUSSION... (Issued 645 PM CDT Fri Oct 20 2017)

UPDATE...

High clouds will stream over southern tonight. No change to the
forecast.

AVIATION(00Z TAFS)...

Plan on non-convective low level wind shear across southern WI
tonight and again Saturday night. There are strong south-southwest
winds just above the surface due to a tight pressure gradient as
low pressure approaches from the Plains. This will cause gusty
southerly winds during the day Saturday. Showers and a few
thunderstorms associated with a cold front will hold off until
late Saturday night.

PREV DISCUSSION... (Issued 315 PM CDT FRI OCT 20 2017)

TONIGHT AND Saturday- Forecast Confidence...Medium to High.

Low level mixing and increasing high clouds will keep overnight
temperatures a little milder, with readings falling mostly into the
low to mid 50s.  Broadscale ridging slipping off to the east toward
the east coast will result in increasing south winds.  These south
winds will gradually pull deeper column moisture into the region.
Air mass does not saturate enough to react with warm air advection
to produce shower threat until Saturday night, so wl be keeping
Saturday mild and dry most areas.

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

Persistent southerly winds ahead of approaching plains short wave
trough will continue to pull deepening column moisture into the area
Saturday night.  Precipitable water values increase to over 1.25"
Sat night.  Layer Q-vector convergence increases to 10 units later
Saturday night through Sunday morning across the area while
impressive layer frontogenetical forcing also passes across the
area.  Hence wl continue to have high pops for mostly showers from
after 06Z Sunday through 18Z Sunday with QPF in the quarter to half
inch range.  While low to mid level shear will be impressive later
Saturday night, elevated instability remains negligible, well above
low level inversion.  In addition, mid-level lapse rates remain
mostly in the 5.5 to 6 degree range.  Will include small chances for
thunder but difficult to get excited about isolated severe threat.

Showers will end from west to east on Sunday with cooler air
settling into the region.  Short term guidance showing some
discrepancies in handling southern Plains low pressure early next
week.  GFS and NAM had shown more of a split flow with southern
system remaining southeast of the area while earlier runs of the
ECMWF moved system quickly northward into the central Great Lakes.
With 12Z guidance, the short term guidance in better agreement on
amplifying short wave troffing over the Great Lakes early next while
southern system gets absorbed into expanding troffing over the
eastern CONUS.  Digging short wave will result in the chance for
showers returning to the area later Monday into Monday night.

Cooler temperatures are expected over the weekend, but should still
average out above seasonal normal for mid-October.  That will change
later next week.

EXTENDED PERIOD...

MONDAY NIGHT THROUGH FRIDAY - Forecast Confidence...Medium.

Colder air will settle over Wisconsin and the region early this
period as we say goodbye to high temperatures in the upper 60s and
warmer, perhaps for the remainder of the season. Medium range
guidance in fair agreement on amplifying long-wave troffing
developing over the western Great Lakes and Midwest early this
period.  Colder air will settle into the region as 925H temps fall
into the single digits Monday night and Tuesday.

Increasing zonal flow across the northern Plains and southern Canada
and embedded upstream trof will nudge Great Lakes long wave troffing
to the east around mid-week.  This will allow for at least some
temperature recovery on Wednesday and Thursday before another surge
of cold air surges in around Friday.  The latest GFS is quicker with
this surge of cold air and cold front passage for Thursday while the
ECMWF has been trending about 12-18 hours slower. WPC blended
guidance using an even blend of GFS/ECMWF and ensembles.

Temperatures likely to drop into the mid to upper 30s Tuesday night
but low level mixing and likely cloud cover should prevent
widespread frost. Will include mention of patchy frost in the west
where lighter winds and more clearing are expected. Better chance
for freezing temps and frost formation Friday night into Saturday
morning and looking even more likely around the first of the month.

AVIATION(21Z TAFS)...VFR conditions expected for the period with
increasing mid-high clouds overnight into Sunday morning. Increasing
low level winds will result in low level wind shear developing this
evening, persisting into Saturday morning.  May see some sct-bkn
lower VFR cigs develop over southern Wisconsin during Saturday. Bulk
of showers will hold off until later Saturday night into Sunday
morning.

MARINE...Tightening pressure gradient ahead of approaching
strong cold front will cause winds to become gusty Saturday, lasting
into Sunday.  Strong low level winds will begin tonight, and should
be able to occasionally reach the lake surface later tonight. Hence
will post Small Craft Advisory to begin later tonight and continue
it through mid-afternoon Sunday.  Mariners should expect the winds
to switch from the south to the West Sunday morning. Some showers
will accompany the frontal passage, along with isolated
thunder.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory from 1 AM Saturday to 4 PM CDT Sunday for
     LMZ643>646.

&&

$$

Update...Cronce
Tonight/Saturday and Aviation/Marine...MBK
Saturday Night through Friday...MBK



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