Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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931
FXUS63 KDTX 260106
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
906 PM EDT Thu May 25 2017

.UPDATE...

A band of steadier light rain now extending across the nrn and ern
thumb region is associated with a ribbon of mid level deformation to
the west of a 500mb trough axis. This trough axis will reside over
srn Lake Huron through the night, weakening as a closed mid level low
takes hold across the Mid Atlantic. The mid level deformation will
weaken as it slowly contracts during the night, exiting east of the
thumb region prior to daybreak. The net result will be a gradual
diminishing trend to this steadier rainfall across the thumb during
the overnight.

Farther west, slightly heavier scattered showers managed to develop
from late day heating. These will continue to wane this evening with
the onset of nighttime cooling. Lingering low level moist cyclonic
flow will however lend itself to areas of drizzle, possibly at fairly
good intensity at times, through the night. A minor update will be
made to the forecast to reflect these trends.

&&

.PREV DISCUSSION...
Issued at 651 PM EDT Thu May 25 2017

AVIATION...

There has been an increase in ceiling heights to MVFR at the Detroit
terminals and MBS as late day diurnal heating has eroded the back
edge of the IFR stratus deck. A band of low stratus and drizzle
persists across the thumb region, with the back edge still hovering
around FNT and PTK. Broad low level moist cyclonic flow will persist
through the night, holding stratus in place. There are indications
in the latest HRRR and RAP that the IFR stratus deck to the
northeast will advance back across the terminals during the night.
This seams highly probable as the low level trajectories remain from
Lake Huron and Ne Lower Mi.

For DTW...The low level winds will weaken as they back to the west-
northwest this evening with the ongoing departure of the sfc low
across the ern Great Lakes. The subtle downslope into metro tonight
casts some degree of uncertainty as to the coverage and persistence
of any IFR stratus that will be able to slide back across DTW. This
uncertainty will warrant just a TEMPO group overnight for lower
ceilings.

//DTW THRESHOLD PROBABILITIES...

* High for cigs below 5000 ft through Friday afternoon.

PREV DISCUSSION...
Issued at 348 PM EDT Thu May 25 2017

DISCUSSION...

Satellite imagery confirms the accelerated progression of the upper
level trough during the afternoon. This is occurring as mid level
deformation matures and expands up the spine of Lake Huron with a
ragged western fringe as depicted in radar composite. This will
allow some for some refinement to the POP forecast for the rest of
the afternoon through mid evening when matched up with hi res model
guidance. The main adjustment will be to sharpen the gradient from
categorical POPs along the Thumb shoreline down to chance interior.
A shower component will remain to be accounted for there as daytime
heating converts drizzle production to scattered showers within the
inverted surface trough extending over southern Lower Michigan. Both
the showers over interior sections and the primary area of
deformation will have to be accounted for through mid evening before
the upper trough moves far enough eastward to bring an end to
rainfall in our area. That will leave broken to overcast cloud cover
to persist overnight through Friday morning as boundary layer
moisture remains high within the lingering cyclonic low level flow.
The clouds will help keep morning lows on the mild side despite
northerly surface wind trailing the system, and also keep readings
on the cool side of normal during Friday, although expect to see
highs touch 70 as some breaks develop in the clouds mid to late
afternoon.

Today`s model runs continue to trend toward some consensus on the
central Rockies system as it moves into the Plains and possibly into
the Great Lakes Friday night into Saturday. There is some better
agreement in terms of mean RH areal coverage while solutions on
intensity have considerable spread due to varying amounts of upscale
convective growth. This alone is cause for a conservative approach
to POPs over southern Michigan with low end chance POPs north to
around 40 percent toward the Ohio border. The ECMWF and Canadian
solutions appear more trustworthy managing convective growth and
show the wave sliding on a more southward track and giving the Ohio
border region the best chance of a glancing shot of rain or
convective remnants. The larger scale upper ridge centered directly
over the Great Lakes, anchored by upper lows over central Canada and
the New England coast, will do a lot to steer the small scale low
pressure system southward should the intensity of the wave prove
weaker than shown by the NAM and GFS.

As the long wave pattern evolves through the end of the week, the
upper low over central Canada will extend all the way into the
central Plains by Saturday. This will provide a more favorable
environment for cyclogenesis associated with short wave energy that
is projected to move into the Great Lakes by Sunday. There is more
confidence in the model solutions at this time frame compared to
earlier in the Friday night-Saturday period. A large scale upper
trough guiding short wave energy and associated surface features
through the Great Lakes will bring a chance of showers and
thunderstorms given afternoon timing in current global model
solutions. Passage of the cold front will bring a modest cooling
trend from highs near 80 to readings closer to 70 to start next week.
Another large cut-off low is then projected to develop over northern
Ontario by Tuesday and keep conditions cool and unsettled over the
Great Lakes through next week.

MARINE...

Moderate northeast winds will ease into this evening and back to the
north as low pressure progresses into the eastern Great Lakes.
Thumb. Hence, the Small Craft Advisory from the tip of the Thumb
through Saginaw Bay will be cancelled with this forecast package.
Modest northwest flow then takes hold on Friday. A weak gradient
will keep wind and wave activity on the lower side heading into the
holiday weekend. That said, several low pressure systems will bring
unsettled conditions at times. The first will bring some potential
of rain, generally south of Lake Huron, late Friday night into early
Saturday. Meanwhile, the second low will bring the potential for a
more widespread area of rain on Sunday with some activity perhaps
lingering into Monday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...BT
MARINE.......DG


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