358
FXUS63 KDTX 251042
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
642 AM EDT Wed Sep 25 2024

.KEY MESSAGES...

- There is a low chance for showers today, with dry weather forecast
 Thursday and Friday.

- Daytime highs will mainly be in the 70s.

&&

.AVIATION...

Very moist low level conditions left in the wake of an exiting low
pressure system this morning. This will maintain an extensive
coverage of IFR to lower MVFR stratus throughout the first half of
the day. Improvement in boundary layer depth with daytime heating
will work to lift ceilings into VFR with time this afternoon. Early
day southwest wind becoming westerly with modest speed today. Some
widely scattered showers possible. Drier air arrives this evening
and overnight as winds become northwesterly. This will bring a
decrease in cloud coverage through the night. Some potential for
patchy fog toward daybreak Thursday morning.

For DTW/D21 Convection...Thunderstorms are not expected this period.

DTW THRESHOLD PROBABILITIES...

* High for ceiling at or below 5000 feet through this afternoon. Low
  this evening.

&&

.PREV DISCUSSION...
Issued at 239 AM EDT Wed Sep 25 2024

DISCUSSION...

Several wave interactions get underway today as an incredibly
amplified (and progressive) long wavelength ridge extending from New
Mexico to the Northwest Territories releases eastward. This leads to
a more defined split in the downstream vorticity field aloft,
resulting in the eventual distinction between two cyclonic systems;
a closed low over Southeast Missouri and a shortwave trough over
Ontario. Locally, waning CVA between these two features still
supports a few additional isolated low-topped showers through the
morning hours.

The occluded (southern) low is forecast to hold position over the
Mid-Mississippi Valley today while a secondary height fall response
resulting from the merger of yesterday`s departing low and upstream
a shortwave troughing the periphery of the inbound ridge brushes the
northern half of the forecast area. Cannot rule out some light
showers/sprinkles during the afternoon hours today based on
steepening boundary-layer lapse rates and a couple hundred J/kg of
SBCAPE, but weak capping inversion near 6 kft AGL should suffice in
preventing any meaningful precipitation (or convective) response.
Latest guidance does favor areas along/north of I-69 which are
better situated with respect to the next lobe of vorticity generated
from the composite cyclonic wave.

Formidable stratocumulus clouds start to scatter out early this
evening with the approach of inbound ridging. Given H8 temps of 10-
12C (more in-line with GFS/ECMWF) and a brief late day diurnal
boost, highs warm into the low 70s for most with mid 70s more likely
for the urban areas. Effective clearing and realized sky fraction
will dictate the degree of nocturnal cooling tonight. A modest
gradient in overnight lows is expected between the Metro Detroit and
the Tri-Cities (upper 50s to lower 50s, respectively) as a bit of
low cloud and perhaps some stray cirrus linger later into the
overnight period for the Metro area.

Lower column anticyclone emerges through H7 over the Upper Midwest
Thursday morning while southern Lower Michigan becomes the center
point of a narrow split-flow configuration. Surrounding synoptic
evolution involves the composite Canadian upper trough closing off
as it moves into Quebec, becoming increasingly ageostrophic as the
basal curvature interfaces with Mid-Atlantic ridging. To the
southwest, the cut-off low retrogrades and turns counter-clockwise
back into the northern Gulf states as Hurricane Helene slams into
the Florida Big Bend Thursday evening, colliding with the orphaned
low. Fortunately, all of these moving parts will be of little impact
for the local forecast area Thursday given statically stable
thermodynamic profiles and subsidence which yield mostly sunny
conditions and highs in the mid-upper 70s.

The sequence of the aforementioned wave interactions will have an
impact on how conditions unfold over Southeast Michigan Friday night
once the post-tropical wave slingshots into the Ohio Valley via the
vertically stacked low. Current expectations roll the remnant Gulf
system into the Lower Ohio Valley Friday evening marked by a 993 mb
surface low quickly rolling northwest across southern Indiana (21Z
Fri to 03Z Sat). This lends potential for some light showers, mainly
south of I-94, as mass gets shove up adjacent isentropic surfaces.
Wide range of outcomes in-play this point given the conditional
nature of how the overall pattern unfolds as evidenced by 0.75
inches of rainfall by 12Z Saturday per 25.00Z GDPS versus zero via
ECMWF/ICON. Auto-populated WPC/NBM QPF of a few hundredths of an
inch of seem reasonable at this time, as long as trends maintain a
more southerly track. Given the proximity to the system, areas
closer to the Michigan/Ohio border can expect a more notable gust
response. East-northeast winds will be capable of gusting to around
30 mph Friday afternoon/evening.

Non-zero rain chances continue Saturday and into Sunday for the
southern half of the CWA as the coupled lows merge near the
confluence point of the Ohio and Mississippi Rivers. There is some
longitudinal spread between the ECMWF and GFS as where this
unification occurs, but the ThetaE field will be well-established by
this point in time with higher likelihood for additional rainfall
correlated with areas that received rain Friday night and/or
Saturday morning. Temperatures will be a few degrees above
climatological averages during this time with highs in the mid 70s
and lows in the upper 50s. A progressive Pacific wave-train forces
the system eastward Sunday or Monday which precedes a polar airmass
by the middle of next week.

MARINE...

Low pressure clearing the Central Great Lakes this morning,
with mostly light northwest winds around today, except over northern
Lake Huron where winds look to briefly top out around knots. Winds
then dropping at or below 15 knots Wednesday night into
Thursday as high pressure builds back into the region. Winds
becoming northeast to end the work week as the high moves off into
Eastern Canada and low pressure lifts north out of the Southeastern
United States. The pressure gradient will be tightest over Lake
Erie, and gusts of 25 to 30 knots appear likely Friday-Saturday
morning.

&&

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

$$

AVIATION.....MR
DISCUSSION...KGK
MARINE.......SF


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at www.weather.gov/detroit.