Area Forecast Discussion
Issued by NWS Gaylord, MI

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FXUS63 KAPX 211742
AFDAPX

Area Forecast Discussion
National Weather Service Gaylord MI
142 PM EDT Mon Aug 21 2017

.UPDATE...
Issued at 1105 AM EDT Mon Aug 21 2017

It took all...night...long, but isolated/sct convection finally
developed toward dawn over far northern lower MI. That convective
cluster is now pushing into Georgian Bay. Associated outflow
boundary tails off toward APN, and a few wanna-be sprinkles/
showers are seen percolating along said boundary. This is also the
primary location of low/mid clouds, along with some marine stratus
in and near the south shore of upper MI. Otherwise, cirrus from
upstream convection is entering northern lower MI. That (and cu)
will be our primary cloud coverage issue during the eclipse this
afternoon.

The boundary hanging back across ne lower MI (and interactions
with the Lk Huron lake breeze) is an obvious trigger mechanism to
get convection going this afternoon. Last couple of runs of the
HRRR have picked up on this, developing vigorous convection just w
and sw of APN beginning between 19z and 21z. Recent RAP runs are
vaguely similar. Further west, decaying convection from WI and
associated forcing will support a shower/maybe tsra threat, but
the lake shadow from Lake MI will result in less instability
there (especially west of US-131).

Greater APN area actually has a considerable amount of shear
available, with 45kt west winds at 500mb late today, and 10kt sw
winds below 850mb. 0-6km shear is a very healthy 50kt near APN,
tailing off rapidly to the sw. Pooling moisture near the
boundaries, and surface dew points running well into the 60s, will
support MlCape values of 1-1.5k j/kg. That is more than enough
shear/instability for supercells, with a wide gamut of svr types
possible (though over a small area spatially and temporally).

Have coordinated with SPC, who anticipate expanding the marginal
risk area across much of northern lower MI with the 1630z update
to the day 1 svr outlook.

&&

.NEAR TERM...(Today through Tonight)
Issued at 336 AM EDT Mon Aug 21 2017

High impact weather potential: Chances for thunderstorms. Maybe a
severe storms south of M-32 late this afternoon into this evening.

Pattern synopsis and forecast:

Early this morning, a cold front was poised just NW of the CWA,
draped from eastern Lake Superior back through Wisconsin. Only some
isolated showers were seen along and ahead of this front, as forcing
was minimal with weak WAA under less than expected low level
winds/LLJ, which were more focused back into western Iowa. This is
where the better showers and storms resided. Further upstream, a
shortwave trough was seen working SE through Saskatchewan with
additional showers and storms in the Dakotas.

The front is expected to sneak into the CWA this morning, but will
likely get masked by developing lake breezes in light low level
flow. Dew points are already in the lower to middle 60s, with
additional low level moisture set to move in through afternoon and
evening. This moisture will be drawn in by increasing winds out
ahead of developing low pressure on the front in Wisconsin. This low
pressure then crosses across the nrn CWA/eastern upper by Tuesday
morning with winds in the middle of switching from SW to NW rather
quickly. The difficult part of the forecast is in the evolution of
showers and storms, and the potential for a severe storm or two. Can
see an isolated shower drifting into nrn Michigan with the front
this morning, then it`s looking like pretty good daytime heating at
least through the morning hours, especially considering the current
diminishment of clouds. Cumulus will be firing off by eclipse time,
but maybe, possibly, there will be better viewing than earlier
anticipated. We would also need for higher level blowoff from Iowa
thunderstorms to not spread into nrn Michigan.

Now for the showers and storms. Chances will be increasing into this
afternoon and especially tonight as the low pressure passes by. We
actually generate as much as 900j/kg in eastern upper this afternoon
and 1700j/kg in nrn lower. We will have the front in the vicinity,
along with lake breezes for low level wind convergence, albeit weak.
Also, we will have steeper mid level lapse rates of 6.5-7.0 c/km,
but the kicker for seeing showers and storms at least for the
afternoon, will be the need of some wiggle/shortwave/MCV in the flow
aloft, which we do start to get as the Canada upper troughing
sharpens and dives into the upper Mississippi valley. The GTV Bay
region remains under a marginal risk for a severe storm or two with
the main threat damaging winds. With the amount of instability
around, which is fairly good for nrn Michigan this summer, later
this afternoon and evening is probably the most likely time. This
despite strongest forcing (increased frontal convergence, upper
divergence, DPVA and LLJ) arriving tonight, due to loss of heating
and expectation for MUCAPES going down through the night.

Highs in the lower to middle 80s most areas and muggy. Truly a
summer day. Lows in the 60s again tonight as the more cooler air
will not arrive until through the day Tuesday.

&&

.SHORT TERM...(Tuesday through Wednesday)
Issued at 336 AM EDT Mon Aug 21 2017

...Cooler and less active weather returns...

High Impact Weather Potential: Lingering thunderstorms and
pockets of heavier rain diminishes Tuesday morning.

Pattern Forecast: Low pressure progged to be across the northern
Great Lakes Monday night will continue to rapidly deepen as it treks
northeastward toward Quebec Tuesday morning. Chances for showers/
storms continue locally early Tuesday morning before the system`s
attendant cold front exits...ushering in a cooler and much drier air
for Tuesday afternoon.

Primary Forecast Concerns/Challenges: Focus revolves around
lingering showers and thunderstorms Tuesday morning. Fairly decent
guidance consistency in progging the aforementioned cold front to be
roughly along a line extending from Alpena to Manistee by mid-
morning Tuesday. As is typical, a sharp instability/moisture
gradient along the front will lead to widely varied PoPs across
northern Michigan. Highest across the southeastern half of the
area...diminishing to the north/west. Lingering rumbles of thunder
ahead of the front seems feasible as guidance continues to prog 300-
700 J/kg of MLCAPE across the southeastern tier of the CWA...in
conjunction with deeper moisture (mid-60s dew points), area of
steeper mid-level lapse rates and western periphery of the low-
level theta-e ridge. However, the threat for thunder (and showers)
quickly diminishes as the front rapidly pushes east, clearing the
forecast area by midday/early afternoon. Would not be all too
shocked to see a stray post-frontal afternoon shower develop as a
result of diurnal processes, but by in large, Tuesday afternoon is
trending much less active than the previous 24 hours. Noticeably
less humid and cooler conditions for the remainder of Tuesday with
increasingly gusty northwest winds and afternoon high temps topping
out in the low 70s for most.

Early Wednesday, a well-defined lobe of energy pinwheels around the
stacked low now over Quebec, pushing a secondary cold front and
associated band of enhanced moisture across northern Michigan. A few
scattered/light showers could result, mainly over eastern Upper, Tip
of the Mitt, and far northwest Lower. Otherwise, even cooler
temperatures will headline the midweek portion of the forecast with
high temps varying from the low-mid 60s north to the upper 60s to 70
degrees elsewhere...some 10 or so degrees below normal for the third
week of August.

.LONG TERM...(Wednesday night through Sunday)
Issued at 336 AM EDT Mon Aug 21 2017

High impact weather potential: None.

Little in the way of sensible weather is anticipated for the
majority of the extended forecast period as sprawling surface high
pressure dominates the mid-section of the CONUS. One final upper
level wave rounds the base of the trough anchored across the eastern
third of the CONUS on Thursday, but locally with little more than a
reinforcing shot of cool air aloft. Heights begin to rise Thursday
night through the weekend ahead of the next system ramping up across
the northern/central plains. Plenty of guidance spread this far out,
but it seems as if the next threat for precip won`t arrive until
late in the weekend or early the following week week.

Continued cool temperatures are certain to be primary focus/
challenge with highs in the mid-upper 60s Thursday before gradually
warming into the low-mid 70s to end the week. Chilly overnight lows
Wednesday night through Friday night look like a good bet as high
pressure overhead leads to mainly clear skies/light winds and decent
radiational cooling. Some upper 30s and an early taste of fall seems
feasible in the typically cooler/interior spots.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 142 PM EDT Mon Aug 21 2017

Showers widespread/few t-storms overnight. MVFR to IFR late
tonight/early Tue morning.

Low pressure in Nebraska will move ne along a front that is
currently draped across northern MI. A few showers/t-storms will
fire thru mid evening, especially near APN. More widespread
activity expected overnight as the low approaches. Cigs/perhaps
vsbys will eventually lower to MVFR-IFR levels. Rain ends nw to se
as low departs and cold front goes thru Tue morning.

Light w to sw winds today/tonight. Nw winds becoming gusty toward
midday Tuesday.

&&

.MARINE...
Issued at 336 AM EDT Mon Aug 21 2017

A frontal zone will remain draped out across the region today
resulting in a weak pressure gradient. Winds increase tonight, out
ahead of low pressure which rides east along the front and across
nrn Michigan. Some low end advisory speeds are possible as early as
late tonight, but a more solid chance at advisory level winds will
be for Tuesday into Wednesday morning. Chances for showers and
storms start mainly this afternoon, but become more likely tonight
into Tuesday morning. There is a chance for a severe storms south of
M-32 late this afternoon and this evening with damaging winds being
the main threat. The front will be crossing late tonight into
Tuesday morning and a fairly quick shift from SW to NW is expected.


&&

.APX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LH...NONE.
LM...NONE.
LS...NONE.
&&

$$

UPDATE...JZ
NEAR TERM...SMD
SHORT TERM...MG
LONG TERM...MG
AVIATION...JZ
MARINE...SMD



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