Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 140003

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
703 PM EST Mon Nov 13 2017


Weather throughout the taf period will be influenced by expansive
surface high pressure that is in place from James Bay clear down to
the Gulf coast. The surface ridge axis will slowly push through
southeastern Michigan tonight, setting up over the central and
eastern Great Lakes later Tuesday. Surface ridge will result in very
weak and variable surface winds with little to no tangible
advections. High confidence in clouds holding tonight as a slowly
lowering stratus deck. While some very light haze could be possible
at daybreak with center of surface ridge, not really seeing the
potential for any significant fog late for a couple of reasons. The
first is that plan view perspective of moisture on isentropic
surfaces offers focus that the Nam moisture profile is way overdone
owing from over moist boundary layer scheme. Rap soundings are
vastly different there, much more subsaturated. Two, forecast
soundings remain overwhelmingly mixed just off of the surface with
little support for impactful fog in latest HRRR surface condensation
pressure deficits. Models sounding do support a lowering of the
inversion heights which should cause cig heights to drop MVFR.
Little change tomorrow, expecting stratus.


* High for cig aob 5kft through tonight. Low Tuesday morning.


Issued at 326 PM EST Mon Nov 13 2017



As of 325 PM EST...A dreary and cool, but dry, day continues across
the region, despite high pressure ridging into the region and
currently centered over Illinois. A sharp subsidence inversion around
850 hPa continues to trap boundary layer moisture, leading to the
persistent stratus deck seen throughout the day. Enough daytime
mixing has allowed for a few peaks in sun from time to time, and even
some more substantial clearing across western Michigan over the past
few hours. High temperatures have reached into the lower 40s for
most places, although a few readings in the upper 30s were noted in
the Thumb.

Heading into tonight, there continues to remain some doubt as to the
potential degree of cloud clearing, especially after midnight.
Current thinking is that any clearing that does occur late this
afternoon will fill back in with stratus after sunset and loss of
daytime mixing. The degree to which the subsidence inversion erodes
remains in question, as the ridge axis will be directly over
southern areas overnight, with enough drier air potentially pushing
in closer to Saginaw Bay to allow for some eroding of the stratus.
Latest probabilistic guidance supports these observations, which
will play a critical role in fog formation later tonight. Boundary
layer analysis supports more of a low stratus event tonight for most
areas, but have gone with patchy fog in the grids as there will
likely be areas where visibility gets down into the 2-5 SM range. As
of now, the best shot at seeing more substantial fog appears to be
west of US 23 and north of I-69, where dense fog may be a
possibility. Any fog that does form will be a result more of
advection rather than radiation, as low-level warm air advection
begins late tonight and moves over antecedent coldish ground.

Low temperatures will also be dependent on degree of cloud cover,
with upper 20s to lower 30s where stratus persists, but any clearing
towards daybreak could allow lows to dip into the mid 20s. Will need
to monitor fog potential trends closely this evening into tonight.


Dry weather will continue into Tuesday as the surface high exits off
to the east. High temperatures will be a few degrees warmer than
today in the increasing low-level warm air advection regime, with
highs into the mid 40s for most places, and even some upper 40s
possible from the Detroit metro and points south.

Heading into Tuesday night, upper-level shortwave energy will eject
out of the northern Plains, following a similar wavetrain pattern
exhibited over the past several weeks. Modest dip in northern stream
PV looks to enhance the upper-level jet as a cold front approaches
the region. Modest isentropic ascent will preclude the arrival of
the front, with rain showers on the increase primarily across
northwestern areas by early Wednesday morning. Thermal profile is
generally warm enough for all rain at the onset, although cannot
rule out a few wet snowflakes mixing in where low temps (mid to
upper 30s) are closer to the mid 30s and enough evaporational
cooling in the preceding dry midlevels can cool the column enough.

Wednesday will feature the cold front moving across the region, at a
relative quick pace and exiting eastern areas by late Wednesday
evening. A warm front looks to also lift north through the region
placing all areas in the warm sector and for a rain frontal passage.
Rather impressive height fall field will accompany the frontal
passage and help to steepen midlevel lapse rates. Although moisture
will generally be limited with the shower activity (only around a
quarter of an inch anticipated), enough dynamical forcing looks to
be in place coupled with surface temps approaching 50F that a slight
chance of thunder has been added across far southern areas for
elevated thunderstorms Wednesday afternoon.

Good shot of cold air advection will occur behind the frontal
passage, with 850 hPa temps falling to -5 to -8 C after peaking
around 5 C earlier in the day. Another quick-moving impulse will
dive southeastward behind the cold front across northern areas,
helping to spark some additional rain/snow showers mainly across
Lake Huron. Low temperatures Wednesday night will be in the 30s.


Dry conditions expected Thursday as a ridge of surface high pressure
moves eastward through the Great Lakes. High temperatures look to be
seasonably cool in the low 40s. Return southerly flow and warm air
advection to commence Friday ahead of the next approaching low
pressure system. Moist air and a series of shortwaves moving into
the region will also lead to a chance of showers that will increase
in coverage from scattered Friday morning to widespread in the

The strongest forcing for this system will arrive Saturday as the
low deepens thanks to favorable left exit region of upper jet and
mid level PVA. The low is still progged to pass directly over lower
MI Saturday morning, which will lead to a prolonged period of
precipitation. Looking like an all rain event Saturday as SE
Michigan should stay on the warm side of the low, though snow
showers have the potential to mix in Saturday evening after the cold
front pushes through. The deepening low will lead to gusty
conditions from late Friday through Sunday, and 850 mb temps of near
-10C settling in will make for a cold Sunday with some flurries
lingering over the area. High pressure building in the wake of this
departing system will result in drier and calmer conditions for the
first part of the next work week.


Modest northwest winds will decrease tonight before shifting to
southwesterly Tuesday morning as high pressure slides east of the
region. South winds will then increase to 15 to 20 knots Tuesday
afternoon ahead of a cold front. South winds will further strengthen
to 20 to 30 knots Tuesday night into Wednesday, especially across
the open waters of central Lake Huron where waves will approach 8 to
10 feet. A period of showers will be likely as the cold front pushes
through on Wednesday before winds shift to the west on Wednesday
evening. Moderate northwest winds of 15 to 25 knots will then
develop in the post-frontal environment Wednesday night into
Thursday allowing waves to grow to 6 to 8 feet across southern Lake
Huron on Thursday. This will lead to small craft advisory conditions
for the nearshore waters of Lake Huron on Thursday.


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



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