Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 191940

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


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



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.


Issued at 138 PM EDT Tue Sep 19 2017


Another messy aviation forecast with high ambient near surface
moisture, convective shortwave remnants in vicinity, all under very
weak ridging aloft. First item is the glaciated remnants of prior
convective activity that lifted into southeastern Michigan today from
the southwest. All active areas of this convection ceased this
morning. So what remains is now showing signs of shearing or
streamlining out to the northwest as a sign that some slightly larger
scale deformation is providing more of an influence. The next item
to monitor is the upper level low that is stalled out south of South
Bend Indiana. Models drift this low to the north and east during the
course of the next 6 to 12 hours. Convective activity that has
initiated across portions of northern Indiana resides along a
secondary warm front and theta e gradient that is in place to the
east of this circulation. A weak but very persistent signal exists in
the model data that suggests shower activity may in fact increase
again this evening between the 00-03Z timeframe as this secondary
warm front lifts into the Detroit area. Farther north, model
soundings at least suggest that pre-existing activity over us now
will linger and bring potential for MVFR conditions right through
this evening. There is some question on whether or not the model data
is oversaturated with cloud and moisture across the north half of
the cwa in light of the brighter skies and mixing. However, given the
setup, a more pessimistic cloud solution is at least a reasonable
outcome. Did introduce some tempo grouping for showers this evening
at DTW.


* High for ceilings below 5000 ft today and tonight.

* Low in thunderstorm potential this evening.


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



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