Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 202246

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
646 PM EDT Wed Sep 20 2017


Low level moisture will be notably shallower overnight. However,
with surface dew points in the mid/upper 60s, would still expect
shallow fog to develop as SSE flow drops under 5 knots. There will
also be some chance of an sct-bkn mvfr deck late as weak low level
jet develops and veers with time, allowing higher theta-e air
to shift into area. Otherwise, only shra chances appear to be at
KMBS attm, but will leave out of going forecast and re-evaluate
based on evening trends in WI convection.

For DTW...Shallow moisture should lead to at least patchy fog on the
edge of MVFR/IFR late. Some chance of a period of sct-bkn MVFR
clouds in the +/- 3500 foot range during this time frame (09z-13z).


* Low for ceilings aob 5kft late tonight, ~09z-13z.


Issued at 409 PM EDT Wed Sep 20 2017


Most important item to communicate is long stretch of summerlike
warmth and humidity that will begin tomorrow and run through Monday.
High confidence exists in daytime temperatures running in the middle
to upper 80s with dewpoints in the 60s to near 70.

With negative PNA in place, the central Great Lakes will remain
nestled right along the interface between equally large wavelength
eastern Conus ridging and deep PAC NW and Western Canada PV
reservoir. The peculiar aspect of this particular setup, and what
provides a reasonable amount of uncertainty during the next 24 to 36
hours, is that while the upper level ridge will begin to reamplify
aggressively in response to at least partially all of the tropic
activity offshore, the ridge itself is one that does not have an
overwhelming amount of support for downward vertical motion below
500 mb. Rather, much of the model data suggests that pesky low
shower and thunderstorm chances Thursday and Thursday night will
hinge upon the ability of pre-existing convective shortwave energy
to survive downstream off of Rossby wave breaking.

This afternoon and tonight. NWP matches up well with most recent
Water Vapor imagery in showing corridor of deep drying associated
with strong anticyclonic vorticity that is now pushing into
southeastern Michigan this afternoon. Associated subsidence bubble
in the 3.0 to 9.0 kft agl layer has been very efficient in eroding
low cloud today. All that remains is a bkn boundary layer cumulus
that resides in much of the eastern cwa. Some congestus exists in
the lake convergence axis along the Lake Huron shoreline. So while a
sprinkle or brief shower will be possible in the eastern Thumb this
afternoon, there is very little signal to support anything more than
that. Regarding fog potential tonight. Plan view perspective of
forecasted theta e fields strongly supports that higher lower
tropospheric moisture content that had been in place last night as
been squeezed out of the cwa to the east. Surface pressure
condensation pressure deficits out of the HRRR suggests that best
potential for fog will exist within this air mass over southwestern
Ontario. There is some signal that suggests fog could bleed back
westward around daybreak and impact portions of the eastern thumb
down into Macomb County. Otherwise, additional drying from the
subsidence mentioned above should limit formation. Time of year and
warmer dewpoints still supports carrying a patchy fog mention for
shallow radiational fog potential.

Thursday. Interesting forecast with alot of moving parts. First
thing will be watching for is the relatively strong elevated mixed
layer plume that will fold over from Wisconsin/Northern Plains today
into southeastern Lower Michigan Thursday. This folding over of the
higher theta e ridge will be caused by incredibly strong 1000-500mb
geopotential height rises occuring  over the upper Mississippi River
Valley and Western Lake Superior tonight. The increasing lower
column high pressure, complete with a mesohigh, and rapid change to
anticyclonic curvature will shove the warm and humid air into the
region. Model data continues to support some convective shortwave
energy and pre-existing shower and thunderstorm activity surviving
within the EML across northern Lower Michigan and pushing into the
Saginaw Bay after daybreak. With the strong ridging, only be looking
at remnant elevated thunderstorm activity above 8-10 kft agl layer.
Overall, just looking at some shallow isentropic lift wash with no
upper level jet support, no low level jet. Confidence in showers and
thunderstorm during the morning across the north is low.

The second item to watch for will be exactly where the exact edge of
an internal surface ridge structure develops. Model guidance
suggests the main axis will develop to the east of the cwa with at
least a part of the western edge back into the area. This local
maximum to the ridge appears to set up directly under the corridor
of the strong anticyclonic vorticity that was discussed earlier. The
importance of this ridge is twofold as it will directly impact to
what degree temperatures will reach Thursday, but also set a hard
edge to the capping of any thunderstorm chances. Definitely will be
very warm and humid with temperatures pushing middle to upper 80s.
One certainly needs to stress the warmth as heat indices in many
areas will likely push to and exceed 90. Really dont think we will
see runaway highs in here in southeastern Michigan with such rich
moisture, some residual southeasterly flow trajectories, and lack of
mechanical mixing or southwesterlies.

Thunderstorm chance for Thursday is the tricky forecast. Models
including the ECMWF and regional GEM remain steadfast in keeping a
persistent scattered shower and potential thunderstorm chances in
play across the north particularly in and near Saginaw Bay. This
positioning is curious because this would be along the west edge of
the surface meso-ridge. The uncertainty is a result in the complete
lack of any synoptic scale lift. The eye-raising aspect of the
thermodynamic environment Thursday is the steep 800-600mb lapse
rates that will result in SBCAPES in the 2000-2250 J/KG range. There
will be a complete lack of shear with 0-6km bulk shear at less than
10 knots. As a result, not expecting any severe weather potential.
Expectations for any thunderstorm activity would be pulse type
activity with at least an entry chance for hail. Heavy rainfall will
also be possible with very little cell movement.

Friday...Large scale pattern will become very conducive for
retrograding upper level high pressure closing off over the western
and central Great Lakes. High confidence exists in dry weather
Friday. Temperatures Friday will again be very warm in the middle to
upper 80s, readings some 15 degrees above averages.

An expansive surface high pressure system sitting over the eastern
third of the U.S. along with an amplified upper-level ridge will
continue to bring hot, dry conditions to Michigan throughout the
weekend and into early next week. Long range models continue to
support 850 mb temperatures that average 17 - 18 degrees Celsius.
Warm air aloft along with southerly flow and generally clear skies
will support daytime highs that peak in the mid to upper-80s. For
reference, record highs range from the upper 80s to low 90s across
Southeast Michigan for this time frame.

The next chance for precipitation and closer to seasonal
temperatures will return on Wednesday, as a cold front acts to
produce the slight chance for rain and thunderstorms. Cooler air
behind the front will act to keep daytime highs capped in the mid to
upper 70s. While too early to say with high certainty, seasonal
temperatures will look to stick around into next weekend.


High pressure will dominate the area through the next several days
with south to southeast winds generally under 15 kts. Lake St Clair
and Lake Erie winds will be more generally out of the southeast and
lighter than the winds in the open waters of Lake Huron. Winds are
expected to remain generally under 15 kts through the weekend as
high pressure drifts in over Lower Michigan.


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



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