306
FXUS63 KDTX 210748
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
348 AM EDT Sat Sep 21 2024

.KEY MESSAGES...

- Areas of locally dense fog may drop visibilities below a half mile
  through 9 AM this morning.

- Warm weather continues through the weekend with higher humidity and
  rain returning Sunday evening.

- Lower confidence in rain chances next week, but higher confidence
  in temperatures cooling off to seasonable values.

&&

.DISCUSSION...

Final lingering showers have dissipated/exited stage left into the
southern Ontario peninsula early this morning with the departing
warm sector (and ThetaE ridge). This gives way to a drier forecast
for today as height rises permeate from southern stream longwave
ridging, but the overall response will be somewhat muted. The basal
portion of an upper level trough brushes North Dakota, shearing the
embedded shortwave ridge folding into Lower Michigan which is
located along the northern end of the synoptic ridge. Did remove
previous low-end PoPs over eastern Lower through the rest of the
morning. Latest obs and GOES composite imagery suggest accelerating
morning fog development in the wake of nocturnal cloud cover. Will
monitor the need for a CWA-wide SPS given the rather isolated nature
of advisory worthy sub-half mile visibilities, but those locally
dense areas may persist until approximately 9 AM. For the rest of
today, weak westerly winds persist with increasingly clear skies as
isentropic downglide further dries the column. H8 temps remain
elevated today in the absence of post-frontal thermodynamic relief,
in addition to some net-positive advective processes. This lends a
similar temperature forecast today with highs in the low to mid 80s.

Attention then turns upstream to a consolidating PV anomaly over
south-central Canada that strengthens through the weekend and
eventually drives a frontal boundary (and cooler airmass) across the
Upper Midwest. Lots of moving parts to address regarding when the
front actually reaches Southeast Michigan while a closed low over
the desert Southwest opens up into a belt of seasonably strong
southwesterly winds from the Texas Panhandle into northern Ontario.
This promotes a steady northeast shove of higher specific humidity
values that eventually spill eastward into the local area marked by
a two-fold increase in PWATs from 18Z today to 18Z Sunday. Showers
and storms will likely be on-going Sunday morning along the
dynamically active corridor, arcing into the primary closed/occluded
1000 mb surface low tracking into southern Hudson Bay. This noses
the frontal zone eastward Sunday evening with perhaps a secondary
surface convergence response over southern Lower which converts
hours of top-down moistening into a cluster of pre-frontal showers
and non-severe storms. Too soon to call specifics on timing, but NWP
support is largely lacking on anything developing before 18Z Sunday.
Consensus of deterministic and ensemble QPF for the 24-hour period
ending at 18Z Monday averages to around a half to three quarters of
an inch of rain with some indications of the better FGEN and/or left-
exit region ascent for the northern half of the forecast area which
could see closer to an inch. Did lower highs by a few degrees Sunday
(low 80s) to align with more restrictive cloud field.

Low confidence carries into the early week extended forecast period
as the lower portion of the frontal slope stalls out and waffles
over the western Great Lakes as evidenced by a quasi-static H8
temperature gradient. Surface winds come full-circle with a
northwesterly feed ushering in a more seasonable airmass with highs
in the low-mid 70s. Steady feed of CVA and saturated low-levels
maintains persistent Chance PoPs through midweek before several wave
interactions get underway. This includes an incredibly amplified
(and progressive) ridge over the western half of NoAm, the release
of the south-central Canadian low, and a potential tropical system
traversing the Southeast.

&&

.MARINE...

A ridge of high pressure will hold across the Great Lakes today,
bringing the continuation of light winds with the return of dry
weather. The ridge of high pressure will hold through tomorrow
morning, but rain and embedded thunderstorms will fill in over the
Great Lakes starting tomorrow afternoon and evening, initially from
a low pressure system with added support from a cold front. Wind
direction will veer from SSE to NNW following the passage of the
cold front. Dry conditions are expected Monday after the passage of
the front, with additional rain chances entering on Tuesday as a
second low pressure system pushes from the Ohio Valley into Lake
Erie/Ontario.

&&

.PREV DISCUSSION...
Issued at 1143 PM EDT Fri Sep 20 2024

AVIATION...

Lower VFR clouds will shift east with weak frontal boundary passage
leaving mainly clear skies overnight. This will encourage some fog
late as winds become light/variable in general. Will keep the MVFR
vsbys in terminals, but will monitor for the potential of dense fog
given some formation within clearing well to the west late this
evening.

For DTW/D21 Convection...No thunderstorms are expected through
Saturday.

 DTW THRESHOLD PROBABILITIES...

* Low for ceilings aob 5kft through 08z this morning.


&&

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

$$

DISCUSSION...KGK
MARINE.......AM
AVIATION.....DG


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