Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 192305
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
705 PM EDT FRI AUG 19 2016


.AVIATION...

A region of mid level subsidence now overspreading Se Mi will keep
conditions free of convection at the terminals through the evening
and well into the overnight. The cluster of thunderstorms now north
of MBS look to maintain a track which holds them well north of the
airport. A little better influx of low level moisture and
instability will arrive from the southwest late tonight, which may
lead to some isolated morning convection. The better chances for
thunderstorms on Saturday will arrive during the afternoon/early
evening as a weak sfc warm frontal feature and instability axis
lifts across the area preceding the main sfc cold frontal passage
Sat night. Meanwhile, with sfc dewpoints hovering in the upper 60s
and clearing skies tonight, MVFR vsbys in fog toward daybreak look
probable. There will be a bit of a gradient increase overnight,
which should preclude dense fog development. The increase in
southwest flow and low level moisture advection may however support
some low stratus development. Confidence is rather low at this stage
in the forecast but certainly something to monitor through the
night.

For DTW...Some fog and/or a period of low clouds may impact the
airport in the 09Z to 14Z time frame. Air traffic is likely to be
more impacted by late afternoon/evening thunderstorms, which could
become rather widespread.

//DTW Threshold Probabilities...

* Low in ceilings below 5000 feet Saturday morning.

* Moderate in thunderstorms late Saturday afternoon and evening.

&&

.PREV DISCUSSION...
Issued at 320 PM EDT Fri AUG 19 2016

DISCUSSION...

Per spc mesoscale analysis, Mlcapes increased at or above 2000 J/kg
this afternoon over souther Michigan, coupled with mid level lapse
rates around 6.5 C/KM, which have been sufficient to generate
scattered to numerous (east) shower and thunderstorm activity, as
weak remnant circulation works through lower Michigan. Downdraft
capes at or above 1000 J/kg could allow for isolated wet micro-
bursts potentially producing wind gusts around 50 mph (note the
36 knots recorded 1820z at Flint) over far eastern areas despite
generally weak 0-6 km bulk shear. Should stabilize quickly in next
couple of hours, leading to quiet overnight period tonight as
heights build downstream of amplifying upstream trough. Dewpoints
will remain elevated around 70 supporting a warm and muggy night
with lows also likely hovering near 70.

Strong shortwave as seen on WV imagery is emerging over the northern
front range this afternoon. Decent agreement agreement between the
ECMWF/GFS/GEFS on attendant surface low lifting toward the straits
by Saturday evening as it deepens into the mid/upper 990s. High
resolution simulations all indicate a substantially deeper
low/slower arrival time/diminished severe threat. At this lead time,
and given the known behavior of high res guidance, the forecast will
continue to lead toward the EC/GFS. In any case, lowering cloud deck
through the afternoon and into the evening hours will likely put the
brakes on destabilization. Most likely scenario attm appears to be
one characterized by strong dynamic forcing within an environment
characterized by adequate 30-35kt shear and MLCAPE near 1000 j/kg.
Certainly adequate for likely pops/good coverage and a low end
severe threat for damaging wind gusts and isolated hail to one inch
in diameter.

Fropa Saturday night will usher in substantially cooler h85 temps in
the single digits and is the first autumn-like frontal passage of
late summer. Cross-sections reveal a tight/well-organized frontal
zone extending up to the mid-level PV anomaly. Highs on Sunday will
be accordingly slightly below average values in the low to mid 70s.
Scattered showers/iso tstorm still plausible for Sunday given
pattern recognition with post-fropa H5 cold pool directly overhead.
However, with temps around -12 to -13C it is not particularly cold.
Less-than-ideal lapse rates preclude increasing inherited low chc
pops at this time. The degree of cold air that is wrapped into the
trough will depend largely on the how deep Saturday evening low gets
as it lifts to our NW. Will therefore leave mention of thunder for
Sunday as is given plausibility of a stronger/deeper scenario.

The amplified upper level trough for August standards will be
sliding to the East Coast on Monday, with surface ridge axis still
slowly working through the Central Great Lakes underneath the
northwest confluent flow. Thus, another pleasant and cool day with
highs in the 70s expected as 850 mb temps hold in the upper single
numbers. Return flow around the departing high will allow for a warm
up Tuesday and Wednesday, as another solid upper level ridge (588
dam at 500 mb) builds overhead, per 12z euro. The forecasted high
temperatures of around 80 degrees/lower 80s could be a touch too
cool as 850 mb temps push into the mid teens and then upper teens by
Thursday, but increasing moisture and instability ahead of next cold
front looks to be triggering scattered showers and thunderstorms on
Thursday.

MARINE...

Modest southwesterly flow will develop this evening as low pressure
tracking to James Bay drags a surface trough through the area. A
strong low pressure system will then develop over the Mississippi
Valley on Saturday with southeasterly flow increasing to 15 to 20
knots over parts of Lake Huron. Winds will veer to the south and
southwest Saturday evening as the low lifts to Lake Superior with
gusts increasing to 25 knots or better. This system will also bring
an increased chance of showers and thunderstorms late Saturday and
Saturday night.

The strongest winds from this system will occur late Saturday night
through Sunday as this low pressure occludes to the north of Lake
Huron and a cold front surges east across the area. In fact, it
appears that west to northwest gales will be possible over the
northern third of Lake Huron as the cool airmass funnels over the
relatively warm waters (70F or better). Within this area, wind gusts
in the 35 to perhaps 40 knot range will be possible, peaking Sunday
morning.

HYDROLOGY...

Scattered thunderstorms will produce locally heavy rainfall of
one quarter to one half of an inch into early this evening. Better
coverage of showers and thunderstorms is expected late Saturday
afternoon through Saturday evening as a low pressure system lifts
from the mid Mississippi Valley to eastern Lake Superior. It
appears that rainfall of one quarter to one half of an inch will
be common as this system crosses the area with localized amounts
in excess of one inch possible. This activity will move through
the area rather quickly so any flooding will be limited to
localized minor flooding in poorly drained locations, especially
in urban areas.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late Saturday night through Sunday evening FOR
     LHZ361-362.

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

$$

AVIATION.....SC
DISCUSSION...JVC/SF
MARINE.......DG
HYDROLOGY....DG


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