Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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346
FXUS63 KMKX 270217 AAB
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
917 PM CDT Sun Mar 26 2017

.UPDATE...

Showers will continue to pull out of the northeast forecast area
through late this evening as low pressure slowly moves on.
Visibilities are behaving so far, but potential is still there for
some dense fog to develop overnight.

&&

.MARINE...

With low level moisture sticking around tonight and light winds in
place, see no reason to change the timing of the Dense Fog
Advisory.

Gusty north winds could bring Small Craft Advisory conditions back
into the area Monday night into Tuesday.

&&

.PREV DISCUSSION... (Issued 655 PM CDT Sun Mar 26 2017)

UPDATE...

Rain continues to pivot around the slowly departing low,
affecting roughly the north half of the forecast area. The rain
should slowly pull away to the north during the mid to late
evening hours.

Will monitor observations closely this evening, as still think
there could be a threat of dense fog given the recent rains and
light to calm surface winds.

AVIATION(00Z TAFS)...

Widespread rain is currently mainly north of Madison and
Milwaukee, with a couple showers or light drizzle to the south.
The rain will pull away slowly to the north during the evening
hours along with the departing low pressure system.

Low clouds will persist into Monday. Could see some low
visibilities again tonight into Monday morning given the recent
rainfall and light to calm winds at the surface. Will monitor
trends and amend the visibility forecast as needed if it looks
like dense fog begins to develop.

PREV DISCUSSION... (Issued 325 PM CDT SUN MAR 26 2017)

TONIGHT AND MONDAY - Forecast Confidence...Medium.

Light rain associated with a closed upper low drifting across
southern Lake Michigan will gradually diminish over east central
Wisconsin this evening. Visibility has improved across the area this
afternoon.

With light winds and ample low level moisture, I expect areas of
dense fog to develop once again tonight and persist into mid to late
Monday Morning. Still some uncertainty of where the dense fog will
end up developing, so will hold off on a dense fog advisory for now.
Thinking it will be the south half of the forecast area, including
Madison and Milwaukee. Onshore winds will give lakeshore areas a
better chance for fog off the lake as well.

The baggy surface pressure gradient will remain over southern
Wisconsin all the way through Monday afternoon as another surface
low approaches. Best chance for rain on Monday will be in southeast
WI during the afternoon.

MONDAY NIGHT THROUGH WEDNESDAY - Forecast Confidence...Medium
trending to High.

Still some uncertainty of track of second weakening low pressure
area expected to move across central MS Valley region Monday and
Monday evening.  However this second system will be taking a much
more southerly track than todays passing circulation. 12Z GFS has
now shifted back to a more southerly track with less threat of light
precipitation returning to southern WI later Monday.  In contrast,
12Z NAM has shifted synoptic scale forcing farther north with
greater QPF spreading into far srn/SE WI later Monday into the
evening. 12Z ECMWF also following GFS more southerly track with less
threat of precip returning. NAM layer Q-vector convergence does
spreads east of the area fairly rapidly late Monday afternoon and
evening. Meanwhile, GFS Q-vector convergence remains south of the IL
border. Low level frontogenesis remains across northern IL during
the period as does low to mid level deformation. Hence enough
confidence to go only with slt chc/low chc pops for the early
evening across southern CWA.

Upstream higher amplitude long wave troffing over western CONUS wl
result in downstream ridging across upper Midwest and quiet period
for Tuesday and Wednesday.  Onshore flow likely most of the period,
but low no significant push of colder air.  Hence no lake effect
expected and inland temperatures may be able to climb into the low
to mid 50s both days.

EXTENDED PERIOD...

WEDNESDAY NIGHT THROUGH SUNDAY - Forecast Confidence...Low.

Southwest low looking a bit stronger now as it ejects east and
northeast from the southern Plains towards the OH Valley region on
Thursday and Friday.  GFS has shifted much farther northward with
its latest solutions while ECMWF has been trending southward.  ECMWF
and Canadian had been spreading precipitation into the area on
Thursday, but now all guidance shows effects of warm air advection
ahead of low pressure system possibly reaching southern WI as early
as Wednesday night.  Other issue is that farther south track is
allowing slightly colder air to get drawn into this system which
would increase the possibility of a mixed ptype event.  Still too
early to guess timing and amounts as medium range guidance continues
to adjust...however surface low track passing just south of Chicago
Friday morning favorable for heavier precip in srn WI.  Wl need to
watch for possibly rain/snow mix, especially across northern CWA
beginning Wednesday night, continuing into Thursday night.  For now
have chances for rain and snow mix Wednesday night into Thursday
transitioning to all rain.  However, dynamic cooling of atmosphere
may result in all wet snow scenario for northern CWA on Thursday
into Thursday night. Due to model shifting and lack of run to run
continuity, confidence is low for this period.  March may end up
ending as a Lion as well.

Precipitation from this system will diminish later Thursday night
and Friday.  Medium range guidance diverges for the weekend as
upstream long wave trof over the western CONUS transforms into yet
another cut-off low in the southwest.  ECMWF already begins to shift
this system into the central and northern Plains later in the
weekend while GFS and Canadian show more zonal flow across northern
tier of CONUS, and weaker short waves moving across the region.
Hence a lot of uncertainty regarding the last days of March and
first days of April. It is safe to say there are no significant
warm-ups in sight as temperatures will likely remain near or
slightly above seasonal normal.

AVIATION (21Z TAF update)...

Low clouds, fog and drizzle will continue to be a problem across
southern Wisconsin tonight and all the way through Monday as weak
low pressure slowly crosses the area. After a brief period of
ceiling/visibility improvement this afternoon, conditions will go
back down this evening. Things should improve once again Monday
afternoon, but could go down again Monday night.

MARINE...

Ample moisture and relatively mild air over the cooler lake is
allowing for dense fog over Lake Michigan. Areas of dense fog with
visibility of 1 mile or less will persist through Monday morning. A
Marine dense fog advisory is in effect.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Dense Fog Advisory until 10 AM CDT Monday for LMZ643>646.

&&

$$

Update...DDV
Tonight/Monday and Aviation/Marine...Cronce
Monday Night through Sunday...MBK



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