Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 251636
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1236 PM EDT Sat Mar 25 2017

.AVIATION...

Cold northeast flow off Lake Huron maintaining frontal boundary
south of TAF sites at the moment, with showers and IFR ceilings
north of the front. The problem is the cold air is shallow (under
925 mb per 12z DTX sounding), and with surface winds coming around
to more easterly direction late today and further warming in the low
levels, a south to north breakout of the low clouds into VFR during
early evening hours seems plausible (yip/dtw) based on drier air
observed over Eastern Ohio Valley. As the evening wears on and near
surface cools, difficult call whether or not IFR ceiling re-develop
or holds, as there remains a strong inversion just below 1000 feet.
However, models usually tend to be a bit aggressive with the near
surface moisture, and winds should stay up a bit, but ultimately
decided on IFR deck holding over northern TAF sites, with redevelopment
of IFR deck overnight across southern TAF sites with lower confidence.

For DTW...VFR/Ceilings above 5000 FEET are near the southern
Michigan border, but easterly flow off Lake Erie complicates the
forecast, otherwise confidence would be higher in VFR conditions
developing late this afternoon into the Evening hours. Overnight
forecast is also difficult as we warm conveyor belt resides
overhead, but preference was to allow for IFR ceilings to redevelop
and persist into tomorrow morning, with showers returning during
Sunday.


//DTW THRESHOLD PROBABILITIES...

* High for cigs early this afternoon, low late this afternoon and
  early this evening, then medium overnight.

* Low for cigs/vsby aob 200 FT or 1/2SM late Tonight.

&&

.PREV DISCUSSION...
Issued at 1012 AM EDT Sat Mar 25 2017

UPDATE...

Sustained strong mid level frontal forcing solidified by favorable
right entrance region upper jet dynamics continues to support an
elongated widespread band of rainfall.  Axis of deepest forcing
largely holding position over the M-59/I-69 corridors, but with a
general northward drift as ascent works up the frontal slope
allowing light rainfall to expand into the tri-cities/thumb. This
moisture simply reinforcing an already much colder northeast flow
locked in areawide, north of the surface boundary firmly positioned
across the northern Ohio Valley. Where rainfall is ongoing, any
meaningful diurnal temperature recovery will remain a struggle, with
readings likely capped in the upper 30s/lower 40s. Points south of
the I-696 corridor will witness some semblence of a modest recovery,
as precipitation chances drop off considerably with southward extent
thanks to increasing stability as upper riding noses northward behind
a slowly exiting upper jet. This process will work to diminish the
existing fgen as the boundary continues to drift north through the
afternoon, but not progressively enough to suggest points along/north
of the I-69 corridor won`t continue to see periodic bouts of
rainfall throughout the latter half of the day. With that, will nudge
pops upward for these locations, while trending expected highs
downward a shade. Forecast remains in good shape elsewhere heading
into the afternoon.

PREV DISCUSSION...
Issued at 306 AM EDT Sat Mar 25 2017

DISCUSSION...

There is a tight low to mid level baroclinic zone draped across the
Great Lakes early this morning. Meanwhile, the circulation
associated with the upper low now churning over Oklahoma is feeding
a plume of deep layer moisture into Illinois. During the night, the
frontal boundary across Lower Michigan has remained active in two
locations; across northern Lower Mi within the 700-600mb layer and
across the central portions of the forecast area (M59 to I-69
corridors) closer to the 925mb portion of the frontal zone.

The hi res model suite all suggest the 925-850mb layer will become
more active during the early morning, aided by the ongoing deep
layer moisture feed across Illinois. This will support increasing
coverage of showers across the forecast area (aside from perhaps
locals south of the I-94 corridor). Weak elevated instability
along/south of the I-96 corridor has supported some thunderstorms
overnight. The overall persistence of this weak instability will
continue to support some additional thunderstorms. Enhancing the
forcing along the front is the ageostrophic response within a 140+
knot upper jet streak centered over srn Quebec. This forcing will
persist through the first half of the day today. The upper jet
streak will then become focused much farther east this afternoon. In
fact, some mid level ridging will develop across the ern Great Lakes
today as the upper low rotates into Missouri. These factors will
fracture/weaken the frontal forcing, leading to a diminishing trend
to the coverage of showers during the course of the afternoon and
evening.

Despite the approach of the associated sfc low pressure into
Missouri today, strengthening sfc high pressure across northern
Ontario drove the sfc cold front across the forecast area last
night. Marine modification under the post frontal northeast winds
have resulted in a rather shallow but cold near sfc layer across Se
Mi. Temps actually dropped a good 20 to 30 degrees following fropa
last night. The easterly component of the sfc winds will persist
through the day, while post frontal stratus continues to expand
across Se Mi. This will keep the front south of the area today and
will only support temps in the 30s and 40s, with the coolest
readings downwind of Saginaw Bay and Lake Huron with the NE winds.

The upper low will lift into the nrn Ohio Valley and srn Great Lakes
on Sunday but will dampen considerably as it does so. There are
differences among the 00Z model suite with respect to timing and
location of this system as it approaches the region. Some upper
level diffluence and mid level deformation forcing will support some
regions of more persistent rainfall, the locations of which carries
a high degree of uncertainty at this time. Indications among the 00Z
model suite are that the better rain chance overnight into Sun
morning will be to the west of north of Se Mi, with rainfall
probabilities over the forecast area increasing Sun afternoon. The
low level wind fields will turn more southern Sunday as the
associated sfc low lifts across Lower Mi. This will allow temps to
make a run into the 50s.

Mid level subsidence will overspread the region by Monday as the
remnant upper wave lifts off the northeast. There will not be any
cold air intrusions in the wake of this system, so Monday highs will
possibly rise into the 60s, dependent upon the degree of lingering
low clouds. There has been a noted trend toward a weaker and farther
south mid level short wave feature Mon night into Tuesday. Precip
chances have therefore been lower accordingly. Mid level ridging and
sfc high pressure will dominate during the mid week period. Easterly
sfc flow will however keep temps somewhat on the cool side. The
approach of yet another upper low by the medium range suite will
warrant the next chance of rain the end of the work week.

MARINE...

The slow moving frontal boundary has dropped south of Lake Erie
overnight bringing a wind shift to the northeast across all of the
eastern lakes. Winds will increase this morning reach 25 to 30 knots
across central Lake Huron which will lead to waves in excess of 4
feet impacting the nearshore waters over the northern Thumb region
during the day. Small craft advisories are in effect for Saginaw Bay
and a portion of the nearshore waters through most of the tonight
period. High pressure building across Ontario and Quebec with a low
tracking into the western Ohio Valley will then cause the winds to
veer more easterly which will take the higher winds into Northern
Lake Huron for the rest of tonight and Sunday. The weak low then
looks to slowly pass through Lake Huron on Monday, with generally
light winds right into Tuesday, but from the north behind the low.

HYDROLOGY...

Periodic showers will affect the region through the weekend as a
frontal boundary stalls just south of the state line. Though a
lengthy period with on and off showers, rainfall totals are only
expected reach 0.5 to 0.75 inches through the duration of the event.
The resultant runoff will likely increase flow rates and water
levels in local streams and rivers but flooding risks are mitigated
by the long duration of this rainfall.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 1 AM EDT Sunday for LHZ421-422-441.

     Small Craft Advisory until 8 PM EDT this evening for LHZ442.

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

$$

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


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