Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 162255
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
555 PM EST Thu Nov 16 2017


.AVIATION...

There will remain some degree of uncertainty with respect to the
trends in the strato cu field during the night. Ongoing weak low
level cold air advection has attempted to lift bases, but
strengthening mid level subsidence is forcing a lowering of the
inversion. The result thus far has been cigs hovering around 3k ft.
Increasing mid level subsidence during the evening is expected to
force a subtle lowering in the clouds. Low level cyclonic flow will
be replaced by increasing difluent low level flow as sfc high
pressure nudges in from the west this evening. Model soundings and
RH cross sections suggest this will support substantial breaks in
the low clouds during the overnight. The quick departure of the sfc
high to the east Fri morning will quickly lead to the redevelopment
of southerly return flow. The extensive stratus across the Ohio
Valley raises some concern that the lower clouds will spread back
across Se Mi within the southerly return flow. With high clouds
increasing overnight, and given the current sfc/dewpoints spreads
and uncertainty in coverage of low clouds, the mention of fog will
be removed from the TAFs.

For DTW...Sub 5000 ft clouds should persist through the night to
some degree. The main uncertainty being the coverage of these
clouds.

//DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings below 5000 ft this evening. Low overnight and
  Friday morning.

&&

.PREV DISCUSSION...
Issued at 400 PM EST Thu Nov 16 2017

DISCUSSION...

A broad surface high pressure system and amplified upper-level
ridging aloft will continue to push east into the Great Lakes late
Thursday into early Friday, which will act to diminish wind gusts
from 20 - 25 mph to calm to light winds overnight as the pressure
gradient relaxes. Additionally, enhanced subsidence aloft and the
advent of dry conditions in the low levels (1000MB - 500MB relative
humidity values ranging between 20 - 30%) will help scour out low
level clouds tonight into the early morning hours on Friday,
providing a brief period of partly cloudy skies across SE MI. This
set-up does raise concerns for the possibility of patchy fog
developing overnight, taking into account both that ground
conditions remain saturated due to yesterday`s accumulating rainfall
(.25 - .5 inches) and due to the lack of sunshine seen throughout
the morning and afternoon that would have helped enhance
evaporation. The NAM/GFS and various high-res models (HRRR/RAP)
explicitly do not output problematic dense fog, however, opted to
leave patchy fog in the forecast as several synoptic signs promoting
fog are prevalent (calm winds at the surface, saturated low-level
moisture, dry-air aloft, large-scale subsidence.

Otherwise, any breaks in cloud cover will be short-lived as high
level clouds move in in response to an elevated warm front. Overcast
conditions are expected to persist through Friday and into the
weekend as a low pressure system develops across the Plains and
pushes northeast into the Ohio Valley Friday into Saturday. An upper-
level trough and embedded short-waves along with an impressive surge
of moisture (850mb dew points in the upper-single digits and PW
ranging between 1.00 - 1.20 inches) will bring likely rain chances
starting late Friday into Saturday. Thunderstorm chances remain in
place ahead of the low as conditions will become weakly unstable
(showalter neutral to slightly unstable and weak MUCAPE). Better
moisture advection and stronger forcing are expected to occur along
and south of the MI/OH MI/IN border, thus higher thunderstorm
chances will remain situated along and south of that area.

Likely rain chances are to continue through Saturday afternoon and
early evening as low pressure travels into the Ohio Valley and
eventually into Lake Erie/southern Ontario. A strong cold front and
strengthening pressure gradient will push and develop behind the
low, bringing gusty conditions to the CWA, especially along the
shoreline near the thumb as winds shift NW to N. The last detail to
mention regarding the low pressure system will be the possible
transition from rain to a rain/snow mix throughout early Sunday
morning. At this time temperatures are expected to plummet from a
high in the 50s to an overnight low in the low to mid 30s, which
will promote the possible transition from rain to a rain/snow mix,
especially across the Flint to Tri Cities area and into the Thumb.
The only caveat being that the bulk of the moisture will be off the
the east as the low enters New England, thus PoP values are low. Any
snow that is seen is not expected to impact travel as little to no
accumulation is expected, and ground temperatures remain above
freezing. Breezy conditions are then expected to hold steady into
Sunday morning as cold air filters into the region (850 mb
temperatures averaging -9C at 12 Z Sunday), tapping into stronger
winds aloft.

Cold cyclonic flow behind the departing low pressure system and some
trailing shortwave energy bring a slight chance for some lake
enhanced snow showers and flurries to the region on Sunday, though
the dry air moving in looks to prevent much of this from reaching
the ground. Sunday will otherwise be mostly cloudy, cold, and gusty
as a pressure gradient sets up between the departing low and a ridge
of high pressure building from the south. Highs will only reach the
mid 30s as brisk northwesterly flow allows cold air to take over the
Great Lakes.

Upper ridging to develop on Monday into Tuesday morning and keep
conditions calm for the early part of the week. Southerly flow will
return and help temperatures rebound back into the low to mid 40s. A
weak system passing over northern Ontario Tuesday will bring a
chance for showers or a light wintry mix over the Thumb Tuesday
evening as a cold front clears the area, though the majority of
forcing looks to stay to the north. Gusty winds will be ushered in
with this front on Tuesday as well. Surface high pressure and subtle
upper ridging then return and will result in calm and quiet weather
for Thanksgiving, although temps will be chilly with highs only
reaching the mid 30s both Wednesday and Thursday.

MARINE...

Strong winds will continue to veer more northwesterly throughout the
afternoon, but may still see winds elevated into the early evening.
Winds through the early afternoon will remain around 20-25 knots and
gusts around 30 knots. Cannot rule out occasional gusts to low end
gales in the open waters of Lake Huron. A Gale Warning is in effect,
but will be allowed to expire late this afternoon. Strongest winds
will begin to decrease this evening, though rough boating conditions
along the near shores will keep Small Craft Advisories in the Outer
Saginaw Bay and around the Thumb into the early overnight hours.
High pressure passing through the region will help bring a period of
lighter winds and lower wave heights for the early part of tomorrow
before the next system impacts the Great Lakes.

Tightening pressure gradient Friday evening ahead of an approaching
low pressure system will increase winds to around 20-25 knots with
gusts to around 30 knots. The winds will increase rapidly after the
low pressure system strengthens moving into the eastern Great Lakes
by Saturday afternoon and cold air surges into the region. Winds
will be out of the northwest at 25 to knots and there will be a
likelihood of gales late Saturday afternoon through Sunday. There is
increasing potential that some of these gales will be on the high
end around 45 knots. Wave heights late Saturday night and early
Sunday have potential to reach upwards of 12 feet in the open waters
of Lake Huron.

HYDROLOGY...

The next low pressure system will arrive in southeast Michigan this
weekend. Greater moisture will lift northward ahead of the system
Friday evening with PWAT values increasing above 1.00". Rain chances
will begin Friday night, but the majority of rainfall is expected to
fall Saturday morning through the Saturday evening. Precipitation
type will be mainly rain, but there is a chance for some snow
showers Saturday night as colder air funnels south. Forecast
rainfall amounts ranging between .75" to 1.50" will be possible for
this event.  The highest amounts of 1.25" or higher, depending on
the current track of the low pressure system, will be south of I-96.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Saturday evening through Sunday evening for
     LHZ361>363-421-441>443-462>464.

     Small Craft Advisory until 4 AM EST Friday for LHZ421-441>443.

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

$$

AVIATION.....SC
DISCUSSION...AM/TF
MARINE.......AA
HYDROLOGY....AA


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at www.weather.gov/detroit.



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