Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 131716
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
116 PM EDT Thu Jul 13 2017

.AVIATION...

Deep low level moisture entrenched across southeast Michigan will
maintain a high degree of lower level cloudiness through the latter
half of the day.  Continued diurnal mixing will work toward lifting
ceiling heights through this time.  Weak instability in place to
potentially support TSRA development.  However, overall coverage
expected to remain low, with greater potential focused east and
south of the region.  Brief window for diminished cloud coverage
this evening, before the high moisture content translates into an
expanding low stratus tonight.  IFR cigs possible Friday morning,
particularly from PTK northward.

For DTW...Low potential for thunderstorms remains through the
evening hours.  Lack of greater instability given the persistent
cloud cover will favor a very low coverage, too low to mention at
this time.

//DTW THRESHOLD PROBABILITIES...

* Low for thunderstorm occurrence through this evening.

* High in ceilings below 5000 feet this afternoon, Medium tonight.

&&

.PREV DISCUSSION...
Issued at 1008 AM EDT Thu Jul 13 2017

UPDATE...

Moist and humid airmass lingering across southeast Michigan behind
the early morning convection, with surface dew pts still hovering in
the lower 70s. Broad surface low over southeast Michigan
this afternoon, allowing for weak low level convergence zones,
especially with any Lake Huron push inland. Clouds thinning today,
which should allow temps to climb into the low to mid 80s,
generating MLcapes in excess of 2000 J/kg. With 0-6 KM Bulk shear of
35 to 45 knots, isolated severe storms are certainly possible if any
go up, and agree with the Day 1 marginal risk expansion as remnant
colder 500 colder temps (-8 C) hold on across thumb region into
early afternoon, with 6z 3-KM NAM and 11Z HRRR advertising a few
storms in that region. There is a cap and dry air evident in the 700-
800 MB layer which is expected to limit coverage, and thus just
holding onto chance/scattered pops.

PREV DISCUSSION...
Issued at 228 AM EDT Thu Jul 13 2017

DISCUSSION...

Incoming steep mid-level lapse rates are supporting widespread
convection this morning. Two primary foci of activity exist: a
northern complex which has developed out of last evening`s
convection over Wisconsin and a southern complex more directly tied
to a strengthening low-level jet. In between, there is exists widely
scattered less organized activity. All of this will move through the
area during the early morning hours, exiting to the east around 12z.
Effective layer shear of around 30 knots will be sufficient for some
organization of more vigorous updrafts, and this has already been
observed on radar this morning. With isolated rotating updrafts
noted on KAPX and KGRR and dewpoints over southeast Michigan well
into the 70s, there is concern that the heavy rain and marginal wind
threat may also be accompanied by the threat for an isolated weak
tornado.

Positively tilted upper low approaching the Arrowhead of Minnesota
will shift toward the Keweenaw during the day today. This will be
sufficient to eventually shunt the deeper mid-level moisture axis to
the east by this evening. In the meantime, some diurnal backbuilding
of exiting activity expected, especially within the surface trough
over the northern half of the CWA. Storm motion will be on the
slower side, west around 30 mph, and PWATS, while diminishing during
the afternoon, will still be sufficient to generate locally heavy
rainfall. Nonetheless, coverage/duration/intensity will not justify
continuation of the Flood Watch, and much of the activity is
forecast to be focused north of the watch anyway. Thus, cancellation
of the headline is expected upon the passage of this morning`s
convection.

Oppressive humidity characterized by Tds in the low to mid 70s will
hang tough through the daytime hours and, with the thermal ridge
overhead, temperatures will reach well into the 80s, a noteworthy
increase over the inherited forecast highs and a consequence of a
slightly slower-than-anticipated evolution of the mid-level wave.
Heat indices will top out close to 90 outside of the Saginaw Valley
and Thumb and possibly in the mid 90s in the Detroit urban heat
island.

Dry westerlies will advect a much drier airmass into the area
tonight as PWATS/Tds fall from 2 inches/low 70s this morning to
under 1 inch/low 60s by Friday morning. Upper trough axis is progged
to pivot through during the day Friday. Although surface-based
instability will be marginal-at-best, entrained mid-level moisture
and cyclonic flow should at least contribute a healthy cu field if
not some isolated shower development. Even in the latter case,
expect the bulk of the day to be characterized as a dry for all
locations with high pressure becoming increasingly dominant at the
surface.

Another reamplification of the western US longwave ridge will
support confluent northwest flow and high pressure through the first
half of the weekend. Strong model consensus that an amplifying
shortwave will dig into the western and central Great Lakes regions
sometime during Sunday. Sufficient, though not particularly
impressive, low-level moisture and steep mid-level lapse rates are
modeled to be drawn eastward into Lower Michigan from the northern
Great Plains. Timing of this wave during the diurnal cycle will be
important, but it should be noted that unlike the present scenario,
any convection associated with this wave would be supported by
stronger dynamic forcing. As such, it appears that Sunday will
feature a conditional severe threat.

MARINE...

Northeast winds across northern Lake Huron will continue to gust
around 20 knots this morning. This will be a result of this region
being located north of the elongated area of low pressure across
central Lower MI and southern Lake Huron. This region of low
pressure will stall over southern Lower Michigan today. As a result,
the northeast winds will build slight farther south across Lake
Huron. The winds will undergo slight weakening tonight as the
surface low expands across Lake Huron and southern Ontario. The
system will slowly exit east of the region on Friday, leaving
generally light northerly winds in its wake. High pressure will then
expand across the region over the weekend, sustaining relatively
light winds.

Ample moisture and instability across the region will support some
thunderstorms this morning through tonight. The high amount of low
level moisture will also support some marine fog today, mainly
across southern Lake Huron.

HYDROLOGY...

Convection is beginning to make its way through portions of
Southeast Michigan this morning. Although thunderstorms will be
progressive in nature, a quick 1 to 2 inches will be possible
potentially leading to some minor flooding issues. Because of the
long period between prior rainfall yesterday afternoon, cancellation
of the Flood Watch is expected to occur either on schedule or
slightly earlier upon the passage of this morning`s heavy-rain-
producers. Additional shower and thunderstorm development is
anticipated today...mainly north of the M59 corridor. Storm motion
will be sufficiently slow to potentially cause some minor flooding
issues such as ponding on roadways on an isolated basis. Dry weather
anticipated tonight through at least the first half of the weekend.

&&

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

$$

AVIATION.....MR
UPDATE.......SF
DISCUSSION...JVC
MARINE.......SC
HYDROLOGY....SC


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