Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 130806
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
406 AM EDT Fri Oct 13 2017

.DISCUSSION...

High antecedent moisture in place over southeastern Michigan,
surface dewpoints in the mid to upper 50s, and shortwave ridging
aloft has resulted in a fairly long duration of grungy conditions
full of clouds and drizzle. Peak of the shortwave ridging component
passed to the east this evening with the 500mb ridge axis now over
the eastern Great Lakes and Northeastern U.S. early this morning.
All indications are the very weak inverted surface trough feature
that had been in place will dissolve. Surface analyses, including
MSAS, supports this is in fact happening due to the introduction
immediately upstream of sharp gradient to kinematics with upper
level jet streak. There has been an organized improvement in reduced
visibilities over far southeastern Michigan, within and south of the
Detroit Metropolitan area. Difficult to buy in completely that the
fog and drizzle has ended with surface winds calm at many sites.
However, improvement in conditions has been very uniform and with
ceiling height now as high as 4000 ft agl down in Detroit,
expectations are on the more optimistic end of the spectrum. Did
continue an areas of fog and patchy drizzle mention this morning,
but low impact weather is anticipated.

From a big picture perspective, the most notable item to discuss is
the positioning of southeastern Michigan relative to the jet streak
over the northern Plains, northern Mississippi River Valley, and
western Ontario. Specifically, southeastern Michigan is shown to be
situated in the middle of the jet streak on the anticyclonic shear
side. This is an area that is very supportive of downward vertical
motion. Forecast soundings increase confidence in active subsidence
given the overall look/magnitude of the inversion and high stability
at and above 5000 ft agl. Confidence is relatively high in going
with a dry forecast today. Only caveat, was some possible increase
in isentropic lift as the dead spot in flow exits, but plan-
view signal is practically non-existent below 600mb.

Strong height falls and shortwave, now lifting towards James Bay,
will continue to progress to the northeast today. This will
effectively knock the top off of the ridge and bring the main
westerly belt, zonal flow to the Great Lakes for the weekend. The
edge of the gradient flow will become to favored spot for developing
baroclinic zone and wet, active weather Saturday, Sunday.

Saturday...First weather of note will be main warm advection surge
that is expected to push along the developing baroclinic zone. Mode
data suggests that elevated thunderstorm activity will develop at
the nose of the warm advection over sections of eastern Iowa and
northern Illinois tonight, before lifting into northern portions of
the cwa, north of I69, after 12Z Saturday. Initial activity is
expected to be weak, decaying as it arrives with relatively higher
stability throughout the column.

Lower column warming, in as much as the lowest 7500 ft agl, will
allow lapse rates to become progressively steeper by the afternoon.
The data suggests the warm advection being a more significant factor
in developing some modest instability. Large amount of sky fraction
will limit surface heating which is expected to significantly limit
the development of instability. In fact, model soundings are in very
good agreement with high static stability in lowest 3000 ft agl
throughout the night. Did add thunder to the forecast for the
afternoon hours, but no strong or severe weather is anticipated at
this time.

Saturday night...Sharp midlevel trough axis will push towards the
central Great Lakes with strong shortwave timed to arrive over
portions of Wisconsin/Michigan between 06-12Z Sunday. Very strong
geopotential height fall response with this system will occur in
advance of the shortwave, favoring the greatest synoptic support
that will occur under an emerging coupled upper level jet structure.
Main forecast item that is being monitored is likelihood for
elevated thunderstorm activity, and potentially heavy rainfall that
will become established over the northern cwa. Favorable setup is
expected for overrunning with the exit region of a textbook +50-55
knot low level jet forecasted. Latest forecast suggests as much as 1-
2 inches of rainfall possible in the Tri Cities and Saginaw Bay
area.

Sunday...Deepening surface low pressure is then forecasted to rocket
to the northeast, pushing into Quebec already by 18Z Sunday. Will
continue to monitor the overall wind magnitude potential for Sunday.
Strong northwest gradient flow will put a relatively high floor to
the potential winds. Currently, the forecast will read wind gusts up
to 40 mph. The one area is the Thumb, particularly those areas
immediately downwind of Saginaw Bay. At this time, most favorable
time for strongest winds will be during the daylight hours Sunday
with growth of the boundary layer and increased mixing/downward
momentum transport.

&&

.MARINE...

A frontal boundary will drop southeastward today, eventually
stalling across the central Great Lakes. Winds will veer to
southwesterly today ahead of this boundary which will stall and
impact the lakes through the weekend bringing periods of showers and
thunderstorms. Winds will primarily stay below 20 knots today, but a
surge in winds will be possible Saturday night and Sunday as an area
of low pressure develops along the frontal boundary and tightens the
pressure gradient. Southwest gales look possible ahead of the low
Saturday night, with stronger northwest gales arriving behind the
passing cold front on Sunday.

&&

.PREV DISCUSSION...
Issued at 1148 PM EDT Thu Oct 12 2017

AVIATION...

While the better low level moisture convergence (and IFR status) is
now sliding into the thumb, a region of fog has enveloped much of
metro Detroit in its wake. Recent model soundings, aided by
observations just to the west and southwest suggest increasing
southerly flow may actually cause fog/low clouds to lift northeast
of the terminals just prior to daybreak, leading to a VFR cloud deck
through the remainder of the morning. Most likely, ceilings and
visibility will remain highly variable during the course of the
morning hours.

For DTW...Fog and low clouds may struggle to hold through into
daybreak as south-southwest low level flow increases and thick 4k ft
cloud deck lifts overhead from the south.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings below 5000 ft this morning and afternoon.

* Low for visibilities at or below 1/2 SM this morning.


&&

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

$$

DISCUSSION...CB
MARINE.......DRK
AVIATION.....SC


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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