Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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355
FXUS63 KDTX 211627
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1227 PM EDT Sat Sep 21 2019

.AVIATION...

Seasonably mild and moist environment will remain entrenched across
the area this taf period. Advancing warm frontal boundary arriving
coincident with peak heating will offer a limited window for a low
coverage of showers and thunderstorms early this evening. Forecast
will continue to employ a VCTS mention to appropriately highlight
this window. Otherwise, sct to bkn coverage of late day diurnal VFR
cu, fading this evening with the loss of heating. Plume of deeper
moisture will then orient from southwest to northeast tonight and
early Sunday, impacting mainly MBS with periodic showers/possible
embedded TSRA with lower cigs. FNT likely resides along the fringe
of this moisture during this time.

For DTW...Window for possible thunderstorm development remains
centered early this evening /21z-00z/. Coverage expected to remain
quite limited, supporting simply a VCTS mention.

DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings at or below 5000 ft late this afternoon and
  evening.

* Low for thunderstorms late afternoon and evening.

&&

.PREV DISCUSSION...
Issued at 339 AM EDT Sat Sep 21 2019

DISCUSSION...

We start the weekend located in between a positively tilted longwave
trough over the Plains and expansive ridge over the SE conus and up
the east coast. The ridge axis will actually pass east through lower
MI today allowing for deep layer southwesterly flow to commence. The
southwest flow aided by a 30-40 knot low level jet positioned more
over Lake MI and WI will pull a very moist airmass into the region
for the weekend with periods of showers and thunderstorms Saturday
afternoon through Sunday night.

The moisture advection has already begun as evident of the pockets
of mid and high clouds across lower MI and plume of moisture as seen
on current water vapor loops nudges into the state. The day should
start dry as surface ridge holds on but a mid level warm front will
lift north this afternoon allowing deeper moisture into the state.
PWATs are advertised to rise from 1.2" per the 21.00Z sounding, up
to around 1.7" this afternoon. Models continue to show the cap
breaking this afternoon concurrent with this moisture surge and the
heat of the day but tall skinny CAPE profiles and wind shear which
may struggle to hit 20 knots should lead to scattered pulse type
storms. Flow is unidirectional with speeds around 15 to 20 knots
through most of the column which will limit gust potential though
inverted V sounding will help some. Look for mostly heavy rain with
any showers/storms that develop with gusts around 40 mph.

Axis of moisture will then reside over the region overnight and
through Sunday as we await the passage of a cold front Sunday
evening. Upper level jet and low level jet will be strongest to our
west just ahead of the front keeping the bulk of storms and heaviest
rain to the west as well. Plume of moisture may become enhanced
further by shortwaves riding up the front as well as tropical
moisture from the remnants of Topical Storm Imelda. With the parent
low well to the north over Hudson Bay, and flow parallel to the
front, expect a slow and weakening frontal passage Sunday. Highest
confidence periods of precipitation will be Sunday morning and
evening as shortwaves look to lift through the region but the overly
moist airmass with prefrontal trough forcing and then the front
itself will likely continue to produce rain through the day.
Question is how far east does the early day precip occur? Models
show best coverage to the north and west closer to the front and
deeper moisture, with lower coverage south and east deeper into the
retreating ridge. So Metro Detroit might start off dry with
increasing chances through the day and overnight. There will again
be storm chances but the moist sounding will eat away at the already
low instability and there are hints of a cap around 800mb which will
also hurt chances for convection. With PWATs potentially exceeding
2" Sunday, heavy rain will again be a concern with training cells
and embedded thunder adding to the threat of possible flooding.
Other issue on Sunday will be the gusty gradient winds ahead of the
front peaking around 35 mph.

Showers will try to linger into Monday morning as upper level low
and cold pool slide across northern MI while southern extent of
deformation region of the surface low will try to scrape the Thumb.
Shortwave ridge aloft will slide in Monday night with surface high
building through the Ohio Valley bringing drier conditions although
cool northwest flow will keep temps down in the low 70s through mid
week. The next chance of precipitation will come Wednesday night as
the next cold front advances through lower MI.

MARINE...

A warm front will lift into Lake Huron by this evening. There will
be an increase in the south-southwest winds along and in the wake of
this warm front. Winds will turn southwest by Sunday and will
continue to increase ahead of a slow moving cold front.

Probabilities for wind gusts of 25 to 30 knots are moderate to high
from Saginaw Bay into the middle of Lake Huron late tonight through
Sunday. Probabilities for gusts to reach 25 knots outside of this
region are low to moderate. The strength of the southwest winds will
likely result in hazardous boating conditions on Saginaw Bay. The
winds will veer to the west-northwest Sunday night into Monday as
the cold front exits to the east of the region. There will be some
increase in the post frontal winds Monday before high pressure moves
in from the west and allows the gradient to relax.

Intense rain in showers and thunderstorms will be possible Sunday
and Sunday night along this front, likely to result in poor
visibilities at times.

HYDROLOGY...

A region of tropical moisture will advance into Southeast Michigan
today into tonight. Meanwhile, a slow moving cold front with
multiple waves of low pressure lifting along it Sunday and Sunday
night will interact with this moisture to produce a chance for
widespread heavy rainfall. Rainfall amounts are expected to range
anywhere between a half inch up to two inches. At this point, the
heaviest rainfall appears more likely across the Saginaw Valley.
There is certainly the possibility for locally higher rainfall
totals, particularly in areas which experience more thunderstorm
activity. So there is a chance for some localized flooding,
especially in low lying and/or urban areas.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....MR
DISCUSSION...DRK
MARINE.......SC
HYDROLOGY....SC


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