National Weather Service United States Department of Commerce


976
FXUS63 KDTX 211920
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
320 PM EDT Wed Aug 21 2019

.DISCUSSION...

Surface dew pts have begun to slowly drop this afternoon as mixing
depths increased, and surface winds have already veered around to
west. 45-50 knots of effective shear tracking through Saginaw
Bay/central Lake Huron has been able to support some weak low level
rotation with the low topped thunderstorms moving through southern
Saginaw County and the northern Thumb region, and this activity will
have to be watched as it exists through the Eastern Thumb Region, as
MLcapes are still hovering around 1000 J/kg. None-the-less, the
window for activity and certainly coverage is dwindling as dew pts
lower, along with peak heating passing.

Good consensus amongst NAM/RAP/Regional GEM/HRRR with the MCV coming
out off southern Iowa will dampen/track far enough south tonight to
miss southeast Michigan, and will just carry low chance pops for
showers toward the Ohio border. The mid/high clouds spilling north of
the border should at least alleviate fog concerns tonight.

Upper level Low (552 DAM at 500 MB) near James Bay will track into
Quebec tomorrow, with trough axis extending southwest into the
Great Lake region, right into the day on Friday. Cooler Canadian
air spilling in, with 850 MB temps lowering into mid to upper single
numbers will lead to pleasant and dry weather as sprawling high
pressure (1025 MB) builds in through the first half of the weekend.

Southern periphery of a surface high pressure system and advancing
upper-level ridge will bring a continued period of dry weather
through the weekend as northeast to easterly flow holds temperatures
to below seasonal norms. Long range models are advertising sunny to
partly cloudy skies due to the increased subsidence from high
pressure, however, flow off of Lake Huron may produce more low-level
cloud cover relative to what is advertised, especially over the
Thumb into the Tri-Cities. Low-levels of the atmosphere will however
remain very dry as cool Canadian air is pooled into Michigan with
1000-500 MB RH values sitting around 30 percent.

Push of a warm front/strengthening LLJ will pool in excess moisture
into Michigan and will bring warm and humid conditions marked by
temperatures in the 80s and dew points climbing back into the 60s to
potentially lower 70s across the Metro region. Increase in moisture
will bring the chance for showers and thunderstorms Monday through
Wednesday. Too much divergence is noted in long-range models
regarding potential PoPs on Thursday, tied to the passage of a cold
front, but higher confidence to see a cool off Thursday onwards.
&&

.MARINE...

Window for possible thunderstorms exists mainly over southern lake
Huron through this evening. This risk ends as a cold front lifts
across the region into tonight. Post-frontal northwest winds will
strengthen during this time as cooler air arrives. This will bring a
period with wind gusts of 25 to 30 knots over Lake Huron. The
combination of the gusty winds and building waves will lead to
hazardous conditions for small Craft on the nearshore waters of Lake
Huron through early Thursday. Northerly winds will gradually
diminish on Thursday as the gradient eases. High pressure building
into the region will then maintain modest winds and wave action
Friday into the next weekend. The colder air moving across the warmer
waters may prove favorable for waterspout development, particularly
late Thursday night and early Friday.

&&

.PREV DISCUSSION...
Issued at 112 PM EDT Wed Aug 21 2019

AVIATION...

A cold front on pace to track across southeast Michigan this
evening. A gradually destabilizing pre-frontal late day environment
will maintain a low potential for showers and thunderstorms to
develop in the vicinity of this boundary. Greater potential will
generally exist to the north/northeast, but may offer a brief window
mainly near MBS late this afternoon. The expected limited coverage
will preclude a defined mention at this time. Otherwise, simply a
high based sct/bkn cu for the latter half of the day, before fading
with the loss of heating this evening. Gradually drying low level
conditions within the post-frontal northerly wind tonight. This
process will tend to discourage renewed low stratus/fog development
again Thursday morning, although a more localized response remains
plausible. A mid level wave traversing the Ohio valley will also
bring a period of thicker mid cloud during the overnight period,
another limiting factor for fog.

For DTW...Thunderstorm chances late this afternoon/evening are
expected to be confined to the far northeastern portions of the
Detroit airspace with stronger capping at DTW likely inhibiting
convection at the airport.

DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings at or below 5000 ft this afternoon and
evening.

* Low in thunderstorms this evening.

* Low in crosswind thresholds being exceeded this evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 8 PM this evening to noon EDT Thursday
for LHZ441>443.

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

$$

DISCUSSION...SF/AM
MARINE.......MR
AVIATION.....MR


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