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000
FXUS63 KDTX 200355
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1155 PM EDT Tue Sep 19 2017

.AVIATION...

Low level moisture remains high across southeast Michigan, with
surface dew pts holding in the 60s. Mid/high clouds have thinned out
in spots, which has allow for lower ceilings toward LIFR in spots.
However, still not totally confident the ceilings will drop below
400 ft or in the development of dense fog, as a light easterly wind
remains and transient mid/high clouds persist. Will continue to
monitor trends before 6z issuance, but anticipating going with IFR
conditions overnight into early tomorrow morning. Ceilings will then
be slow to lift on Wednesday, and probably going to take most of the
day to dissipate the High MVFR clouds during the afternoon with weak
surface winds persisting.

For DTW...Difficult ceiling and visibility forecast overnight, and
likely to dip into IFR with LIFR possible by sunrise.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings below 5000 ft through early tomorrow afternoon,
  then decreasing confidence late tomorrow afternoon.

* Low for cigs/VIS to fall below 200ft and/or 1/2sm 8-14z.

&&

.PREV DISCUSSION...
Issued at 756 PM EDT Tue Sep 19 2017

UPDATE...

Showers have dramatically waned in the past couple of hours, and
have updated zones to reflect the low/slight chance as upper level
PV center/mid level cold pool (-11 C at 500 MB) along central
Illinois/Indiana border is sinking southeast (away from area). With
dew pts running in the 60s, there is some concern for fog development
as subsidence and dryer air begins to arrive 8-12z tonight with
negative upper level PV advection as Upper level ridge builds through
the Central Great Lakes tomorrow. However, preference at this time
is for a low stratus deck to develop, and visibilities more in the
2-4 SM range, generally not low enough to include in zones. Will be
watching for signs of the mid level dry air arriving quicker than
expected, which would than lead to greater fog potential.

PREV DISCUSSION...
Issued at 340 PM EDT Tue Sep 19 2017

DISCUSSION...

The open wave that broke off from the tail end of the northern
stream jet will continue to drift over the area this afternoon and
into the evening. A stalled boundary that was south of the Michigan
border will drift into southern Michigan this afternoon and evening.
There is already scattered convection forming along this boundary as
of 2:00pm. This along with a circulation currently southeast of Lake
Michigan will drift northeastward this afternoon and provide an
additional focus for scattered showers and thunderstorms to stick
around in the forecast into the evening and overnight. Highest
precipitation chances look to remain along and south of the I-69
corridor where the boundary will reside, but a few showers could
make it into the Thumb and around Saginaw Bay as the circulation
slowly spins over southeast Michigan. Trends show this wave weaken
over the next 24 hours as increasing 500mb heights begin to take
over the open wave and flip the upper pattern over the central and
eastern Great Lakes. This should help put a damper on any shower
activity remaining overnight by tomorrow morning.

Overnight low temperatures will remain in the 60s for many locations
with increased moisture and cloud cover. Clouds will begin to
scatter out through the late morning into the afternoon with weak
ridging and high pressure returning. On Wednesday, an upper trough
will lift into Ontario and in response lift the boundary across
southern Michigan to the north as a warm front. This will help to
maintain moist southerly flow at the surface in Michigan.
Temperatures at 850mb and 950mb will be on the rise resulting in
highs climbing into the upper 70s and low 80s. Wednesday evening and
overnight we could see increasing rain chances in western and
northern Michigan supported by a theta-e axis drifting east along
the eastern edge of the surface high pressure. Mid level lapse rates
being brought over the area with this instability axis would be
enough to support thunderstorm chances. However, it remains unclear
what the coverage of these showers and thunderstorms will be as the
move closer to eastern Michigan. Will keep chances on the low end
for the for now.

An amplifying upper trough over the western CONUS will
only cause more amplification of the upper ridge over the eastern
CONUS through midweek. The temperatures at 850mb by Thursday will
increase into the upper teens and even warmer to above 20 degrees at
950mb. This will support temperatures in the mid to upper 80s for
Thursday, which is well above normal for this time of year.

A broad surface high pressure system and ridging aloft will help
support above normal temperatures and relatively dry conditions
through the extended period. 850 mb temperatures averaging 17 - 18
degrees C, southerly flow, and clear conditions will help push
daytime highs into the mid to upper-80s Friday and into the weekend.
For reference, record highs at Detroit, Flint, and Saginaw range
between 89 - 92 degrees. Above normal temperatures with daytime
highs peaking in the 80s will continue through at least Tuesday,
before Hurricane Maria travels northeast into the northern Atlantic,
turning upper-level flow more zonal. The next chance for possible
rain and thunderstorms will return Wednesday, as a cold front pushes
though Michigan, which will bring more seasonal temperatures to the
region.

MARINE...

Surface winds will veer to the southeast tonight and Wednesday in
response to surface high pressure expanding into the eastern Great
Lakes. Surface high pressure will remain in place over the eastern
Great Lakes through the weekend, resulting in generally light south-
southeast winds.

&&

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

$$

AVIATION.....SF
UPDATE.......SF
DISCUSSION...AA/AM
MARINE.......CB


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