Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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804
FXUS63 KMKX 200404
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1104 PM CDT Tue Sep 19 2017

.UPDATE...

The combo of a saturated boundary layer and light easterly winds
is starting to produce the anticipated low/clouds and fog across
the southeast. This will expand in coverage for a while, but it
shouldn`t get to far west due to more mixing and drier conditions
out there. So, expect it to generally remain over mainly the
lakeshore counties and one more tier inland. A dense fog advisory
is in effect starting at 3 am, but this may have to be started
earlier due to trends. An increasing southerly flow will quickly
blow this fog out of the area from south to north after sunrise on
Wednesday.

&&

.AVIATION(06Z TAFS)...

Look for an expanding area of IFR CIGS/VSBYS due to stratus and
fog across the southeast tonight. All of this will clear out
rather rapidly from south to north Wednesday morning as the
southerly flow increases ahead of the approaching frontal
boundary. That front will bring a band of showers and storms into
south central WI, including KMSN, toward early evening. This
activity will probably wipe out before reaching the southeast.

&&

.PREV DISCUSSION... (Issued 848 PM CDT Tue Sep 19 2017)

UPDATE...

The very persistent area of low stratus from Washington county
southeast through Milwaukee/Racine/Kenosha counties was showing
rapid clearing from the east. But, right at sunset, that clearing
line stalled and the status continues. The past 15 min of
satellite imagery does suggest the clearing from the east has
picked up again, so it should work out of the far southeast by 10
or 11 pm. However, redevelopment is expected after midnight. Most
of the high resolution guidance continues to show we will see
much of the southeast become socked in again with a low stratus
along with areas of dense fog from about 4 am through 9 am
Wednesday. The dewpoints remain elevated and with the easterly
flow off the lake, this will only be fortified. Have decided to
issue a dense fog advisory from 4 am to 9 am for mainly the lake
shore counties.

MARINE...

Be on the lookout for fog developing later tonight through mid
morning on Wednesday. Increasing southerly winds on Wednesday will
bring elevated waves north of Milwaukee, but they should remain
below small craft advisory levels. Small craft should exercise
caution.

PREV DISCUSSION... (Issued 618 PM CDT Tue Sep 19 2017)

UPDATE...

The lower stratus across the southeast should scatter out by 7-8
pm this evening, but admittedly, there is uncertain if it will
completely disappear. Even if it does clear, redevelopment is
expected after midnight. Most of the high resolution guidance
suggests we will see much of the southeast become socked in again
with a low stratus along with areas of dense fog from about 4 am
through 9 am Wednesday. The dewpoints remain high and with the
easterly flow off the lake, this will only be fortified. I beefed
up the fog and sky cover. I`ll also need to contemplate the need
for a dense fog advisory in the next few hours.

AVIATION(00Z TAFS)...

The low clouds that have been persistent across the southeast
should clear out by 00-01z this evening, but this isn`t a sure
thing. Even if it does, more redevelopment of IFR conditions is
expected after about 08z tonight and will continue through about
14-15z Wednesday. Widespread low stratus and fog is expected to
form as a moist marine layer flows in off Lake Michigan. Once this
lower stuff clears out by mid morning on Wednesday, VFR
conditions should prevail through the remainder of the TAF period.
We will be keeping an eye on an approaching cold front. That
front will likely bring a round of scattered showers and storms
from late afternoon into the evening hours.

PREV DISCUSSION... (Issued 330 PM CDT TUE SEP 19 2017)

TONIGHT - Forecast Confidence...Medium.

Main concern will be fog/stratus trends. Already seeing widespread
stratus/light fog near Lake Michigan. A moist easterly flow is
helping to sustain this within the lingering influence of the
departing high pressure. As surface high continues to pull away
should start to see some mixing especially in the western cwa. So
more concern for the fog/stratus in the eastern cwa. SREF/MOS both
show a high confident stratus situation with potential for some
thicker fog evolving with less mixing in the east. Meso model vsby
progs are trending towards some lower vsbys, so potential there for
some thicker fog to evolve. Winds off the deck do start to increase
even in the east per RAP/HRRR soundings so not confident in a
widespread dense fog scenario just yet.

WEDNESDAY - Confidence...Medium

Mid level energy rides to our north with upper jet also favoring
locales to our north. The sharp 850 trough axis dampens as it comes
across. There will be an uptick in lo level convergence arriving
into the cwa especially during the afternoon as surface front draws
closer. So this will result in increasing pops especially in the
west and northwest cwa. Bufkit showing appreciable CAPE with some
mid level lapse rates aob 7.0c. Best 0-6km shear to our north though
axis of instability to be across the area with the approaching low
level boundary. Toasty 925 temps expected into the 20-25c range so
quite warm and humid with a southerly breeze. Onshore component
likely to keep some lakeshore areas cooler.

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

Weakening cold front progresses slowly eastward across southern WI
Wednesday night, further weakening into Thursday morning.  The front
will not make much progress to the east and south due to low to mid
level moderate southwest wind flow.  Convergence will be weak along
the front, but instability in the late afternoon and early evening
fairly impressive.  MUCape of 1500-2500 j/kg will result in at least
a few storms.  Bulk shear on the lower end, around 25-30 knots, but
may be just enough to generate an isolated severe storm. Weak front
settles just to the south of WI by Thu mrng and then begins to move
back north as a warm front through Thursday night. Atmospheric
moisture drops briefly but PWAT returns to around 1.5 inches on
Thursday.  Fortunately, there is little synoptic and mesoscale
forcing.  Hence wl keep conditions dry as warm front moves northward
across the area on Thursday.  Also, some concern about low clouds
lingering in light easterly flow for much of the day on Thursday.
Drizzle not out of the question.

In contrast, southern WI should be well into the warm sector on
Friday with at least partial sunshine and breezy, warm south winds.
Low confidence on max temperatures on Thursday, but should return
to the 80s most areas on Friday, even close to Lake Michigan.

EXTENDED PERIOD...

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

Continued agreement in medium range guidance in continued
unseasonably warm weather affecting southern WI for much of this
period.  Tropical disturbances meandering northward over the western
Atlantic will be partially responsible for creating a nearly
stationary, high-amplitude pattern across the CONUS this week, and
it appears this scenario will persist through much of the extended
period. Large amplitude ridge will continue over the eastern CONUS
while long-wave trof brings colder weather to the west, with
accumulating snow in the MT/WY Rockies. Warm southerly breezes are
expected for much of the period.  Sprawling downstream high pressure
should suppress cloud development.  With 925H temps in the 21-25C
range, expect daytime temps to peak mostly in the 80s, even with the
daylight hours continuing to shrink.

ECMWF and GFS bring stronger mid-level short wave across northern
plains into southern Canada Monday night and Tuesday, with resultant
cold front slipping across Wisconsin.  Medium range guidance has
been backing off and slowing up cold front eastern progress.  With
tropical disturbances still expected to be churning through western
Atlantic, think this trend will continue.  For now, wl have pops in
the Mon night/Tue time frame.

AVIATION(21Z TAFS)...Meso models coming into decent agreement
with fog development overnight across the eastern cwa. This is due
to lingering influence of surface high with a light onshore wind
regime. With some faster wind speeds evolving aloft not confident on
widespread dense fog at the moment. Any lingering fog/stratus to
burn off Wednesday morning with potential for showers/storms most
likely later afternoon/evening across southern WI.

MARINE...Three concerns. First fog trends overnight into early
Wednesday. Vsbys ok at the moment but area is still under influence
of lingering surface high so some fog may evolve/lower from the
stratus as the night goes on. expecting southerly winds to pick up
on Wednesday ahead of a frontal boundary. At this time conditions
look to remain below small craft levels. Then thunderstorm chances
will increase later Wednesday afternoon into Wednesday night as the
front comes through.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory from 3 AM to 9 AM CDT Wednesday for WIZ052-
     059-060-065-066-071-072.

LM...None.
&&

$$

Update...Davis
Tonight/Wednesday and Aviation/Marine...Collar
Wednesday Night through Tuesday...MBK



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