Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 191920

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


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



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



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.


Issued at 1236 PM EDT Fri AUG 19 2016


While MCS to the west will continue to weaken, forcing from its
remnants will act upon the modestly unstable airmass over area to
produce hit/miss shras/tsras. Best coverage, scattered at best, will
be along the enhanced convergence of lake breezes. With SSW ambient
flow, expect better coverage just east of terminals this afternoon
as boundaries do not migrate too far from lakeshores. Otherwise,
shra/tsra chances are minimal until just beyond forecast period as
much better forcing surges into area late Saturday. There will most
likely be light, patchy MVFR fog again tonight, but do not expect
much impact from it.

For DTW...Very low confidence in thunderstorms directly impacting
the terminal this afternoon as isolated convection briefly fires in
20z-24z time frame. SSW flow will peak around 10 knots today with
S/SSE flow Saturday pushing 15 knots within tightening pressure
gradient in advance of deepening low pressure to the west.

//DTW Threshold Probabilities...

* Low confidence in ceilings aob 5000 feet this afternoon.

* Low confidence in thunderstorms today and tonight.


Lake Huron...Gale Watch from late Saturday night through Sunday evening FOR

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



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