Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 220353
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1153 PM EDT Thu Sep 21 2017


.AVIATION...

Strong upper ridging will provide dry conditions during the forecast
in terms of rainfall. However, with the long early fall night`s and
light/variable winds, expect patchy fog given the warm, humid
airmass in place with this ridge. Will maintain MVFR to occasional
IFR vsby conditions overnight and introduce for late in the forecast
Friday evening.

For DTW...Fog is expected to form again tonight in the warm, moist
environment. Dense fog is not expected at this time as some degree
of cirrus arcs over area within upper level ridge. Fog will occur
again Friday night as well and may begin to form in the 03z-06z time
frame per various guidance output. Area will be directly under the
ridge Friday night, so calm winds may lead to a better chance at a
period of dense fog.

//DTW THRESHOLD PROBABILITIES...

* None.

&&

.PREV DISCUSSION...
Issued at 313 PM EDT Thu Sep 21 2017

DISCUSSION...

High amplitude upper ridging will govern conditions across the great
lakes right through the upcoming weekend. The elevated upper heights
over prevailing low level southerly flow ensures this extended
stretch of seasonably warm/humid conditions continues for the
foreseeable future.  Convective potential through the evening the
outstanding question moving forward in the near term. Resident
summerlike environment will certainly leave a high degree of
instability for the back end of the heating cycle, given
temperatures of mid-upper 80s and a dewpoint near 70F. SPC
mesoanalysis now showing sbcape in excess of 2500 J/KG. Convective
initiation will remain conditional however, as the presence of a
mature elevated mixed layer maintains a tenuously capped profile.
Convective temperatures projected to remain just above actual
readings, suggesting some degree of forced ascent remains necessary.
Hi res guidance seemingly keying on a weak surface trough now
extending from the Tri-Cities back into Lansing. This convergence
zone the most likely source to support a low coverage of convection,
primarily north of M-59. While the overall wind field remains paltry
and greater convective organization seems unlikely, stronger pulse
cells worthy of gusty winds and small hail are plausible given the
magnitude of instability/mid level lapse rates.  Window for
convection quickly closes with the loss of diurnal assistance
shortly after 00z.  A mild/muggy night thereafter, with lows solidly
in the 60s.  The elevated near surface moisture content within the
background of weak flow and larger scale subsidence will support
some pathcy fog development by daybreak.

Conditions largely a carbon copy both Friday and Saturday, as the
590dm upper ridge becomes firmly entrenched. Little variation in
temperatures across the 700-925 mb layer, suggesting a similar
diurnal response with afternoon readings again averaging a good 15
to 20 degrees above normal.  Highs mid-upper 80s. Heat indices in
the vicinity of 90 degrees. Convective potential again limited by a
capped mid level profile and lack of tangible forcing. A
localized/isolated chance may ultimately emerge, but existing
limitations and subsequent uncertainty precludes a mention.

The streak of above normal temperatures will continue Sunday into
Tuesday as a broad surface high pressure system and expansive upper-
level ridge hold steady across the eastern third of the United
States. Generally clear skies, southerly flow, and 850 mb
temperatures averaging 17 - 18 C will continue to support daytime
high temperatures in the mid to upper-80s during this time frame.

A drastic change in temperatures is poised to take place next week
during the midweek period, however, some uncertainty does exist
regarding the passage of the cold front that will lead cooler air
into the region. The ECMWF model swings the front through Michigan
throughout Wednesday, seen nicely on 850 mb thermal maps, as
temperatures drop from 17 C to 7 C Wednesday morning into Thursday
morning. The GFS bring the cold front in one day later, as
temperatures drop from 15 C to 10 C Thursday into Friday, with the
GEM falling somewhere in between these two solutions. All scenarios
bring the chance for showers and thunderstorms as the front moves
through Michigan, but due to timing uncertainties, elected to leave
PoP values sub-30 Wednesday through Thursday. In either scenario,
daytime highs are expected to drop into the upper 60s to lower 70s
after the cold front moves through.

MARINE...

Upper ridge is anchored across the Great Lakes region today through
weekend. South to southeast winds will persist over the lakes today
through Friday as a surface high pressure settles in across the
eastern Great Lakes. Winds across the open waters of Lake Huron will
remain below 15 knots and winds will even lighting up a little more
over the weekend with the high pressure drifting slowly over the
region.

&&

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

$$

AVIATION.....DG
DISCUSSION...MR/AM
MARINE.......AA


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