Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 202120

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
420 PM EST Fri Jan 20 2017


Fog, periodic chances of light rain, and waves of drizzle are the
primary forecast items for a good chunk of this upcoming weekend.
Air mass to changeover beginning Sunday with shrewd easterly winds
developing late.

A very convoluted upper level pattern exists over much of North
America with as many as 4 separate jet impulses aloft providing an
influence. The impetus behind the noisy pattern has been the
reintroduction of the massive early week, southwestern US closed low
system back into the mean flow. That massive upper level low has
been able to hold on and maintain some integrity over the upper
Midwest. The circulation is resulting in a larger influence of
cyclonic flow within the heart of a much larger amplitude/wavelength
upper level ridge. This initial upper level low and synoptic scale
forcing is fading by the second and is currently undergoing an
tremendous amount of shearing. As a result, forcing now has
collapsed down to just last gasps of stretching deformation. This
has played out with the band of rain that lifted northward today
with nothing much left then a few pockets of light rain near the
area around Saginaw Bay.

Tonight...The narrative for the rest of the weekend will be one
revolving around a slow to develop upper level trough over the
eastern United States. What this means for tonight is a straight
persistence forecast. Virtually zero mixed flow exists overhead
behind the earlier band of precipitation under calm winds (nil
pressure gradient) and surface T/Td depressions at a degree C or
less. Surface observations the past hour have taken a concerted
movement in the negative direction and traffic cams across all of
southeastern Michigan show a shroud of fog hanging in the air. Given
the structure of the near surface inversion forecasted tonight...the
decision was made to issue a dense fog advisory that will be in
effect for all of southeastern Michigan between 00Z this evening and
15Z Saturday morning.

Another jet exit region aloft will push toward the state overnight
from the mid Mississippi River Valley. Just enough larger scale
support will exist with this feature to support another spoke of
midlevel convergence and thermal, theta e advection lifting into the
area. The nam suggests that frontogenetical support with this
activity will be much lower in the column which could yield more
drizzle activity than rain showers. Timing of this chance pops is
across the Ohio state line near Toledo just before 09Z...along M59
corridor...just before 12Z...and the northern cwa and Thumb by 15Z.

Saturday...Another grungy day of persistence from a cloud
perspective. Model data continues to support a very saturated near
surface air mass that will be supportive of reduced visibilities
right until the noon hour. However, the big difference will be
surface temperatures as the slowly developing cyclonic flow  will
support increased low level theta e advection with a burgeoning warm
sector. Latest guidance suggest a warm front becoming draped across
the northern cwa with middle 50s in place over southern 2/3rds of
the cwa. Definitely have some reservations about getting there with
all this cloud in January. However, middle to upper 50s are in
place across southern IN and OH today. It will not take much at
all to get those readings here.

Sunday...A very dynamic and perfectly configured upper level jet
structure will scream across the southern United States Saturday.
Very strong upper level jet dynamics will then lead to a high end
cyclogenesis event over the southeastern United States, Tennessee
River Valley Sunday. Southeastern Michigan will remain well enough
to the north that predominately dry weather will be experienced
Sunday and Sunday night. What will happen is a steadily increasing
easterly pressure gradient that will ratchet up winds particularly
heading into Sunday night. The feeling is that the available
guidance may be too mild both late Sunday and Monday. What is
becoming more into focus is that categorical rain may be likely for
Monday as deformation forcing increases substantially and a slug of
deep moisture is forecasted to backdoor into the state from Ontario.

Mild, unsettled weather will continue through mid-week, as a series
of surface and upper level waves rotate through the region. Chances
of rain or rain/snow mixed will generally persist, barring a brief
break in conditions scheduled for Tuesday. Temperatures will remain
above average through Thursday with the warmest temperatures
occurring during the overnight hours. During the day on Thursday, a
decent cold front pushes through; dropping the temperatures through
the day. By Friday, weak chances for snow return, along with
temperatures more in line with January norms.



Broad low pressure will bring light winds through Saturday night.
Warmer air funneling into the region behind a warm front will
promote the development of fog, however. East winds will strengthen
on Sunday and Monday as a stronger area of low pressure tracks
through the Ohio and Tennessee Valleys. The persistent easterly flow
will allow wave heights to grow, potentially necessitating small
craft advisories for any ice-free areas by Sunday night.


Issued at 1253 PM EST Fri Jan 20 2017


Moist easterly flow north of a warm front will continue to provide
LIFR conditions until a warm front lifts through during the early
evening (roughly 20-22Z). This will cause light winds to shift more
southerly, and should briefly increase visibilities for several
hours. Confidence has increased that ceilings and visibilities will
then fall to low-end LIFR or VLIFR this evening and overnight as a
mild and moist airmass remains in place with stagnant conditions.
There is high potential for ceilings and vis to drop to 200 ft and
1/2 mile, with a period of 1/4 mile visibilities certainly possible.
The most likely timeframe for vis at 1/4 mile will be between 08-
13Z. Improvement tomorrow will be slow, and expect conditions to
only improve to low-end MVFR at best.

For DTW...Medium confidence with visibility rising to 3-5SM in the
20-22Z window as a warm front lifts through the area. High
confidence overnight with ceilings/vis falling to at least 200 feet
and 1/2 mile. 1/4 mile fog is possible.

//DTW Threshold Probabilities...

* High for cigs below 5000 feet through the TAF period

* High for cig/visibility aob 200ft and 1/2SM late tonight


MI...Dense Fog Advisory until 10 AM EST Saturday FOR MIZ047>049-053>055-

Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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