Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181917

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
317 PM EDT Mon Sep 18 2017


Its been a slow process to scour out the low level moisture/clouds
today, as the drier air in the low level just slowly bleeds into
southeast Michigan. Fortunately, with the elevated temperatures to
start the day, maxes have still been able to rise into the 70s for
the most part. Surface high pressure to move over northern Lower
Michigan tonight, and have elected to add patchy fog (mainly northern
areas) to the forecast as surface dew pts still running on the high
side, and light northeast flow off lake huron does not give a whole
lot of confidence in continued drying out/lowering of the dew pts.
Still, wild card will be if there is sufficient moisture in the
2000-4000 foot layer to generate clouds, which would mitigate fog
threat, in addition to potential high clouds as upper level wave
tracks into Western Ohio Valley overnight. This upper wave trending
to slow down and potentially cutoff over the central sections of Ohio
Valley tomorrow (see GFS), but more than likely the 700-500 MB
circulation/low slips just north of Michigan border (see 12z Euro).
Thus, surface ridging/low level anti-cyclonic flow looks to be
weakening over Lower Michigan, with inverted surface trough and
deeper moisture backing in along the southeast Michigan/Canadian
border. This trough should provide the focus for showers, especially
across far eastern and southern areas, along with the moisture
advection/isentropic ascent. In fact, a few showers or light rain
could develop/arrive late tonight across the south if deeper moisture
does not push much farther east late this afternoon/evening.
Instability tomorrow looks marginal at best, and was tempted to
remove thunder mention all together as mid level lapse rates are
weak, but Euro still indicates showalter index at or slightly below
zero. Clouds, scattered showers, and light easterly flow expected to
hold maxes in the 70s once again.

Upper level ridge axis building over the Central Great Lakes on
Wednesday should suppress/cap any possible diurnal instability
buildup, but not totally out of the question a shower or two goes
up, mainly over the Thumb Region on the outer fringe of the ridge
and in the vicinity of the warm front.  925 MB in the 20-22 C range
suggesting highs climbing back into the lower 80s.

No major changes to the extended forecast... A broad surface high
pressure system sitatued over the eastern U.S. and an expansive
upper-level ridge aloft will support hot and dry conditions across
Michigan Thursday through the weekend. Went more aggresive compared
to the in-house blend with daytime highs as 850 mb temperatures
averagine 16 - 18 C aloft and generally clear skies will help
support highs in the mid to upper 80s. The next possible chance to
see more seasonal temperatures will be next week during the mid-week
period as Hurricane Maria moves off into the northern Atlantic,
allowing upper-level flow to turn more zonal across the Great Lakes.



Surface winds will veer toward the northeast tonight into Tuesday as
the center of surface high pressure slides into Quebec. A slight
uptick in the gradient will sustain northeast winds generally in the
10 to 20 knot range on Tuesday. Persistent high pressure is forecast
to take hold across the eastern Great Lakes during the latter half
of the weak, supporting generally light and variable winds across
the region.


Issued at 234 PM EDT Mon Sep 18 2017


Extremely messy setup across southeastern Michigan as old frontal
boundary coasted into the central Great Lakes from the west with
virtually zero upper level forcing remaining. Models show this old
surface trough stalling out over far southeastern sections of the
cwa at least for the next 24 hours all the while large scale
trajectories aloft become increasingly anticyclonic. As a result,
will continue to impacted by the amount of pre-existing clouds and
saturation that will hold on in a neutral advection regime. First
item will be watching afternoon showers re-initiate across the far
southeastern cwa. This is currently happening and thus far as
remain southeast of the DTW. Given these showers activated along the
800-675mb theta e gradient and data supporting some sight drift
eastward of the boundary will leave out of the TAF forecast this
afternoon. Second item of interest for aviation forecast is the
eventual northward movement of main theta e gradient as a pseudo
warm front tonight. Cloud from today is not expected to erode out
completely which should lead to reestablishment of ceilings again
after sundown. Easterly gradient flow is expected to remain high to
discourage fog formation.

For DTW...Postfrontal stratus will be slow to erode this afternoon
with front now stalling out. Main midlevel frontal boundary
providing focus for showers is pushing east. Question for late
tonight distills down to potential for low stratus or fog. Given
persistent light easterly gradient flow, preference was to err with
status. Otherwise, potential does exist for a shower late tonight as
moisture begins to lift back northward. Introduced a Prob30 group
late tonight.


* Moderate in ceilings below 5000 ft today.


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



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