Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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393
FXUS63 KDTX 211102
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
602 AM EST Tue Nov 21 2017

.AVIATION...

Surface pressure gradient has maintained higher wind speeds through
the early morning hours, especially away from the influence of the
glacial ridge. Expect gusty southwesterly winds through the early
afternoon hours. Respectable cold front will sweep through in the
20z-00z time window. A ribbon of rain showers is expected to develop
along the front along and south of Flint. The column cools very
quickly within the frontal zone, which should allow the rain to
transition to wet melting snow before precipitation ceases.

For DTW...Gust in excess of 25 knots from the southwest during much
of the day. The cold frontal wind shift to westerly is expected
around 21z and a full shift to northwesterly around 00z - which may
flirt with traffic flow considerations. Rain showers may transition
to brief period of wet/melting snow, while surface temperatures
remain above freezing.

//DTW THRESHOLD PROBABILITIES...

* Medium confidence in cigs aob 5000 feet after 20z today.

* Moderate confidence in precip type falling as a rain transitioning
  to melting snow late this afternoon into this evening.

&&

.PREV DISCUSSION...
Issued at 305 AM EST Tue Nov 21 2017

DISCUSSION...

Strong surface pressure gradient in place early this morning as the
narrow MSLP depression associated with 986mb low pressure tracking
just north of Lake Superior impinges on expansive anticyclone
anchored to the south and east. SW wind is commonly gusting in
excess of 20 kts preventing decoupling and ensuring temps struggle
to fall much lower than about 40 degrees. As the parent PV anomaly
begins to pivot south and east through the morning, modest
relaxation of the gradient will be countered by mixing within the
deepening boundary layer. The existing warm column characterized by
850mb temps int he low single digits and ongoing gusty SW flow will
ensure temps warmer than yesterday, at least for locations that
don`t experience cold fropa until later in the afternoon.
Strengthening dynamics as the PV anomaly wraps into the western
Great Lakes will strengthen the cold front as it approaches the area
with deep, but modest, fgen supported by developing right entrance
dynamics. Dynamic forcing will be strong but short-lived. With
little time to saturate the well-mixed boundary layer and high
static stability through the column to begin with, precipitation
will be quite light with 1-2 hour rain totals expected to average
just a couple hundredths of an inch. Fropa will bring falling temps
to the Saginaw Valley beginning afternoon with the remainder of SE
Michigan to follow by 4pm. Nothing more than a few flurries behind
the front. No argument with with consensus low temps falling into
the mid 20s by Wednesday morning, perhaps upper 20s in the lake
plume over Washtenaw/Lenawee Counties as well as Detroit City.

Subtle shortwave ridging will build in during Wednesday ahead of a
trailing low amplitude wave to arrive Wed night. Warming low and mid-
levels will degrade overlake instability while high pressure builds
in at the surface. Any lingering lake clouds affecting parts of the
area early Wed will diminish by afternoon giving way to mostly sunny
skies and temps recovering into the mid/upper 30s. Antecedent dry
column will greatly limit snow potential late Wed night into Thurs
morning as the aforementioned weak shortwave transits the Great
Lakes. Potential compounded by the poor thermodynamic environment
for dendrite production, expect nothing more than a few hours of
flurries mainly over the Saginaw Valley/Thumb early Thurs.

Pattern remains largely unchanged through the weekend as temps
moderate back into the 40s before the next, and rather dynamic, wave
tracks north of Lake Superior Friday into Saturday. Strong NWP
consensus on a considerably stronger low than today`s suggests
another round of strong southerly winds and a round of warm
advection-type showers Fri evening.

MARINE...

Firm southwest winds will transition to west/northwest through the
course of the day as a cold front sweeps through the region. Gusts
to gale force will continue across Lake Huron, especially within and
downstream of Saginaw Bay during the first half of the day. The cold
front will pass clear the Straits prior to noon and the south end of
Lake Huron, Lake St Clair, and the west end of Lake Erie around
sunset. Gale gusts are expected across most of the open waters of
Lake Huron behind the front, with the best chances for a prolonged
low-end gale within the long fetch regime downstream of the north
arm. Wind potential will maximize during the late evening hours with
a secondary surge of cold air across Lake Huron. Lake-effect snow
showers will be common in the cold air behind the front. Winds will
relax early Wednesday morning as high pressure builds into the
region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning until 10 AM EST this morning for LHZ363-421-422-
     462>464.

     Small Craft Advisory until 4 PM EST this afternoon for LHZ442-443.

     Small Craft Advisory until 1 PM EST Wednesday for LHZ441.

     Gale Warning from 10 AM this morning to 10 PM EST this evening for
     LHZ361.

     Gale Warning until 4 AM EST Wednesday for LHZ362.

Lake St Clair...Small Craft Advisory until 4 PM EST this afternoon for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 PM EST this afternoon for LEZ444.

&&

$$

AVIATION.....Mann
DISCUSSION...JVC
MARINE.......Mann


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