539
FXUS63 KDTX 242252
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
552 PM EST Sun Nov 24 2024

.KEY MESSAGES...

- Low pressure will bring rain and milder air tomorrow.

- There is low chance (~35%) for accumulating snow Wednesday into
Thursday.

- There is extremely high confidence for below normal temperatures
and periods of lake effect snow showers Friday into early next week.

&&

.AVIATION...

Southerly flow atop a shallow nighttime boundary layer this evening
will erode the deep inversion which has been in place through much
of the weekend. This will result in an erosion of the status deck
during the evening hours. The better thermal advection will remain
south of MBS tonight, possibly supporting a more prolonged period of
stratus across the Saginaw Valley. Low clouds will redevelop and
expand back across Se Mi during the morning Monday in advance of an
approaching warm front. This warm front is associated with low
pressure, forecast to track across the Saginaw Valley Monday
afternoon. Ample low level moisture associated with this system will
result in falling ceilings during the course of the morning Monday,
with prevailing IFR and some LIFR conditions in light rain expected
by Monday afternoon.

DTW THRESHOLD PROBABILITIES...

*  High in ceilings at or below 5000 ft Monday.

*  High for ptype of rain Monday.

&&

.PREV DISCUSSION...
Issued at 338 PM EST Sun Nov 24 2024

DISCUSSION...

Surface analysis reveals a col now shifting east of SE MI which has
resulted in a weak pressure gradient as evident by the light and
variable winds. This setup will contribute to stagnant conditions
and limited surface thermal advection through the early evening
hours. A shortwave ridge will then briefly fill in over the state
late tonight which will maintain dry weather for the remainder of
the evening.

Attention will then turn to the area of low pressure now centered
over Kansas, which will push northeast into the Ohio Valley and
southern Michigan within the next 24 hours. Low-level theta-e
advection will commence in advance of the low, which will initially
result in stagnant to slowly rising overnight temperatures, before
weak and subtle isentropic ascent brings the likely chance for light
rain showers. Orientation of the system relative flow becomes a
little more favorable through the late morning and afternoon hours
for all locations, but has more orthogonality over the Tri-Cities and
northern Thumb (and more-so across northern lower Michigan), which
will coincide with better coverage of showers. Still, theta-e
advection should still support periods of widespread to numerous
showers for locations south. This will also bring above normal
temperatures with highs in the mid 40s to low 50s.

Low pressure will then progress into the eastern Great Lakes which
will veer low-level flow to the west-northwest, which will be
concurrent with the passage of a trailing upper-level trough. A
boost in cold air advection will release lake moisture inland and
will have mid-level support, which will carry rain shower chances
through Tuesday morning. Colder air also elevated mixing depths,
bringing gusts on the order of 25-35 mph, which will last through
the day Tuesday. Otherwise, a transition to flurries or light snow
on Tuesday morning is unlikely but cannot be ruled out. The limiting
factor is the moisture-starved dgz which favors drizzle/rain, but
some soundings do support moisture depths extending through -10C.
This brings the low-end chances for ice nucleation. In any case,
surface temperatures will above freezing, so any impacts would be
null to minimal.

Upper-level flow turns zonal as dry air works into Michigan which
will end all lake effect precipitation by Tuesday afternoon. Late
Wednesday-early/midday Thursday will be the next chance for
precipitation (snow), but notable divergence continues to be noted
within mid-range models. Cluster analysis highlights the potential
strength of the amplification of a Midwest trough as the region of
highest uncertainty (40% of the variance), which will have
implications of the resulting strength and track of a low pressure
system.

Two camps have been observed within the ensemble guidance -- Less
amplification of the trough will lead to a more progressive and
weaker low pressure system which will travel well south of the Great
Lakes and would essentially bring a dry forecast, but perhaps an
earlier arrival to lake effect snow (see last paragraph). This camp
is dominated by GEFS members (74%) and a subset of CMC (35%) and EPS
(28%) members. The second camp exhibits greater amplification of
said trough which produces a less progressive and stronger low
pressure system that brings the northern precipitation shield to
portions of SE MI, which would be cold enough to support snow. The
majority of EPS members fall in this category, but interestingly,
the machine-learning models are in better agreement with the GEFS.
PoP values will be held at 30-40%, noting the bimodal solution space
that is observed at this time. Otherwise it is still too soon to
give credence to any one solution space at this time.

Of much higher confidence, regardless of the final track of low
pressure, there is excellent agreement between the EPS and NAEFS
ensemble system regarding below normal temperatures fill in over the
Great Lakes Friday, lasting through at least early next week. The
95th percentile of temperatures within the EPS members all fall
below normal values during this time frame, leading to extremely
high confidence for this event to materialize. Extended periods of
lake effect snow showers leading to light accumulations will be
likely, as early as Thursday evening (but more likely initiating
Friday) given the warm lake waters.

MARINE...

A defined deformation axis over the Great Lakes is noted by
northerly flow over Lake Huron and southwest flow over Lake Erie.
This flow field exists in the wake of departing low pressure over
New England and incoming low pressure over the Plains. The upstream
Plains system will gain influence tonight and especially Monday
morning as rain overspreads the area. Low pressure is on track to
reach Lake Huron Monday night, which will initiate cold advection
across the Great Lakes Tuesday and Wednesday. A transition to rain
and snow showers will accompany the colder air, while also
increasing mixing depths to generate gusty westerly flow. Small
Craft Advisory conditions will be likely starting early Tuesday
morning, although gusts look to fall just short of gales. For
reference, 34+ knot gusts fall in the 90th percentile (upper range)
of long range ensemble guidance. This is in combination with west
flow which is not particularly favorable for overachievement across
the western half of Lake Huron/Lake Erie. For now, will advertise 30
knot gusts with sporadic gusts to gales possible. Unsettled weather
then continues through the rest of the holiday week.

&&

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

$$

AVIATION.....SC
DISCUSSION...AM
MARINE.......MV


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