Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 240835
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
335 AM EST Sat Feb 24 2018

.DISCUSSION...

Dynamic mid level system now lifting across the southern Rockies
will steadily eject northeast over the next 24 hours, reaching
northern lake MI/eastern lake Superior by Sunday morning. Upper
heights will briefly build accordingly downstream today in response
to this mass adjustment. This process will reinforce the positioning
of a 1025 mb surface high now anchored over eastern Ontario, firmly
establishing a local environment characterized by a cool/moist but
rather shallow low level easterly flow beneath a stout inversion
layer. Cloud trends subsequently the primary issue today, with
recent hi res guidance suggesting some erosion of this moisture may
commence under ensuing diurnal processes. However, given the lack of
dry air entrainment, this environment certainly warrants a more
pessimistic outlook in retaining a high coverage of stratus. The
elevated thickness field already carries support for above normal
temperatures, despite the typically negative net result often noted
on temperatures under this particular pattern. Highs in the 40s. A
lead surge of gulf moisture lodged within the burgeoning downstream
warm sector will rapidly stream east-northeast today. Northern
expanse of this moisture advection carries some uncertainty yet, but
modeled moisture transport vectors generally project this plume to
largely remain south of the border.  Glancing shot of light rain
remain plausible closer to the Ohio border, simply worthy of a
chance pop mention.

Strengthening of the aforementioned upper system with time under
favorable upper jet dynamics will bring a corresponding deepening of
the attendant surface low, projected to eventually track due north
along the western spine of lake Michigan overnight. While the
strongest dynamic forcing remains confined to the west/northwest in
closer proximity to the main height falls and surface low track, an
extensive region of modest large scale ascent will emerge downstream
under a lead area of isentropic lift and eventually accompanying the
trailing frontal convergence. This will support a period of
widespread rainfall tonight. Main axis of higher theta-e advection
streaming across the Ohio valley may briefly sneak north overnight
/05z-10z/, providing a small window for possible elevated convection
to sneak into southern areas. An increasing wind field still offset
but sufficient depth to the low level stability, meaning non-
existent instability across the lowest 3000 ft. Some localized
pockets of heavier rainfall certainly plausible, but the scale of
upstream convection over the Ohio valley may ultimately deflect the
better northward moisture flux away from lower Michigan.

Attendant cold front blasts through mid morning Sunday /11z-13z/.
Deep layer drying commences within the ensuing cold air advection
during this time. Progressive onset of gusty conditions immediately
in the wake of this frontal passage, as the advective process and
ensuing growth to the diurnal mixed layer taps the existing strong
wind field just off the surface. Wind gust magnitude will carry
strong dependence on the degree of turbulent mixing, as greater
isallobaric forcing will be lacking given the surface low
trajectory. An immediate greater wind gust response may accompany
the frontal passage under brief stronger isentropic descent, but any
spike in downward momentum transport tied to the front appears
brief. Given the wind magnitude, looking at a marginal wind advisory
event /peak gusts 40 to 45 mph/ during the daylight period,
strongest to the north. Mixing will help offset the advective
process, maintaining a rather steady diurnal temperature curve
during the day. Highs again in the 40s.

A period of generally benign weather conditions for the early week
period, as lower amplitude flow of pacific origin maintains control.
Thermal profile of the post-frontal resident airmass will hold above
average, despite limited potential for a greater period of warm air
advection under this pattern. Highs generally averaging around 10
degrees above normal both Monday and Tueday. Dry conditions through
this period.

&&

.MARINE...

High pressure will drift eastward across the central Great Lakes
today leading to light flow across the local waters. Strong low
pressure developing over the central Plains will track northeast
across the northern Great Lakes late tonight and into Sunday as it
rapidly deepens. The flow will become east/southeasterly ahead of
the low pressure tonight and rapidly increase to gales over Lake
Huron into Sunday as the flow shifts southwest behind the departing
low pressure system. Gales will diminish Sunday night and into
Monday as high pressure ridges into the region from the southwest.

&&

.HYDROLOGY...

Flood Warnings continue to remain in effect for several primary
river basins across southeast Michigan as water levels continue to
slowly recede. A strong low pressure system will track northwest of
the region late Saturday night and into Sunday bringing the next
chance for widespread rainfall. Rainfall amounts look to range
between a quarter of an inch to a half of an inch. This rainfall is
not expected to have any significant impacts to local rivers and
streams.

&&

.PREV DISCUSSION...
Issued at 1150 PM EST Fri Feb 23 2018

AVIATION...

Shallow cold advection void of any tangible dry air entrainment
continues to lead to a moist environment in the lower troposphere
this evening. Satellite imagery this evening supports a solid MVFR
overcast throughout the night. Latest RAP and Nam3km are onboard
with that. Cool easterly flow will then takeover with time on
Saturday as center of surface anticylone drifts northward into the
northern Great Lakes. This will keep saturated conditions likely
underneath a well organized subsidence inversion.

For DTW...Expectations are now for lower MVFR cigs to hold
throughout the night. Stratus expecting to hold on Saturday as well.

/DTW THRESHOLD PROBABILITIES...

* High for cigs below 5 kft tonight Saturday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning from 7 AM to 7 PM EST Sunday for LHZ363-421-422-
     441>443-462>464.

     Gale Warning from 10 PM this evening to 7 PM EST Sunday for LHZ361-
     362.

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

$$

DISCUSSION...MR
MARINE.......IRL
HYDROLOGY....IRL
AVIATION.....CB

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