Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 190931
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
531 AM EDT Tue Mar 19 2024

.KEY MESSAGES...

- Reinforcing shot of arctic air for Wednesday and Thursday with
  scattered snow flurries.

- Increasing likelihood for accumulating snowfall for Southeast
  Michigan late Thursday night into Friday.

&&

.AVIATION...

Low clouds have mostly dissipated and pushed east of the TAF sites.
However, mid clouds (6-12 kft) will be persisting through the day.
Southwest gradient will quickly increase this morning. Model
soundings indicate steepening low level lapse rates, aided by
diurnal heating, with 25 to 30 knots winds within the mixed layer.
This will support ample gustiness to the sfc winds during the
daytime hours. A sfc cold front is forecast to track across the area
late Tues afternoon, leading to a veering of the winds to the west-
northwest tonight. These winds will usher in colder temperatures and
a return of the low clouds (3-6 kft).

.DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings aob 5000 feet Wednesday morning.

* Low in crosswind thresholds being exceeded this afternoon.

&&

.PREV DISCUSSION...
Issued at 355 AM EDT Tue Mar 19 2024

DISCUSSION...

The next upper level jet packet and associated favorable left exit
region dynamics will allow a deep cyclonic circulation to track from
the northern Lake Superior shoreline today to far southwestern
Quebec by Wednesday morning. For the early portion of today large
scale forcing will be dominated by differential geopotential height
rises off of the exiting trough; for the afternoon the upper level
jet axis will push directly overhead. Positioning is such that
Southeast Michigan will largely miss out of the cyclonic shear side
of the jet as well as the ageostrophic circulation. The exception
will be the Thumb where flurries will remain a possibility into the
afternoon. The proximity of Southeast Michigan to the south of the
passing low will yield warm advection. With some sunshine during the
late afternoon temperatures are expected to rise into the 40s.

A strong cold front will push across Southeast Michigan by daybreak
Wednesday. 850mb temperatures are forecasted to drop some 10 to 12
degrees C which will place the thermodynamic setup perilously close
to ice nucleation zone. However, the main forecast narrative for
Wednesday will be an extremely dry air mass despite some
trajectories downwind of the Great Lakes. The forecast has a mention
of flurries for Wednesday and that seems reasonable. Did lower high
temperatures for Wednesday as readings should remain a couple of
degrees of the freezing mark. The other notable aspect will be the
breezy west northwest winds likely reaching and exceeding 30 mph.
Windchills will struggle to climb to 20 degrees Wednesday afternoon.

Surface high pressure is in store for Thursday as broadly
anticyclonic flow trajectories build over the region. Loss of
gradient wind will result in a more noticeable comfort to daytime
conditions with highs in the 30s and windchills in the upper 20s.

A significant inflection/trough in the planetary vorticity reservoir
will drive a wave of low pressure through the region on Friday. Main
trends in the numerical weather prediction guidance have been a
convergence of the operational ECMWF run towards the GFS solution
which suggests greater phasing between shortwave energy tracking
through the northern Plains and the shortwave tracking across the
southern to mid Mississippi River Valley. D(prog)/dt has suggested
greater amplification of both the northern and southern shortwaves.
The net result of the setup suggests the area of greatest lift
associated with the northern wave will track directly through Lower
Michigan Friday while the richest thetae content will be shunted
ahead of the southern Mississippi wave and miss Southeast Michigan to
the South. There are two big unknowns. 1. What portion of the
elevated front will observe the greatest lift due to frontogenesis
(current model data suggests invof of Saginaw Bay). 2. How long dry
air will hold on and when exactly saturation will occur in the
lowest 6.0 kft. The uncertainty with the low level dry air suggests
it could vw  problematic for how much precipitation falls across the
southern forecast area. Model consensus suggests most of the
precipitation will fall during the daytime Friday which also brings
uncertainty to precipitation type as rain could mix in across
Detroit. Too early for any expected snowfall amounts of the system
but liquid equivalent of around 0.25 inch has been the consensus.

Models suggest a significant pattern change is possible for next
weekend as very strong ridge amplification occurs over the eastern
United States. Lower confidence with regards to sensible weather
here in Southeast MIchigan exists as the edge of the blocking ridge
builds very near Southeast Michigan. At this time, the timing of
surface high pressure supports favorably dry conditions this
weekend.

MARINE...

Southwest flow is developing early this morning in response to an
approaching clipper system. Winds quickly strengthen through the
morning as the influence of overnight high pressure wanes resulting
in rougher conditions in the Saginaw Bay and MI waters of Erie.
Small craft advisories remain in effect for these waters until this
afternoon. Clipper drops through the region during the day today
bringing additional rounds of snow showers. This system eventually
tracks over the Northeast while deepening resulting in strengthening
cold air advection into the central Great Lakes as an arctic airmass
sinks south. Low-end NW gales still look possible daytime Wednesday
over the northern portions of Lake Huron so have continued to run
with a Gale Watch. Winds weaken into Thursday as the East Coast low
slides into the Canadian maritimes.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...Gale Watch from late tonight through Wednesday evening for
     LHZ361>363.

     Small Craft Advisory until 4 PM EDT this afternoon for LHZ421-422.

Lake St Clair...None.
Michigan waters of Lake Erie...Small Craft Advisory until 4 PM EDT this afternoon for LEZ444.

&&

$$

AVIATION.....SF
DISCUSSION...CB
MARINE.......CB


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