Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 191653

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1253 PM EDT Sat Aug 19 2017


Scattered to broken VFR diurnal cumulus embedded within northwest
flow best characterizes the weather through the balance of the
daylight hours. Winds will calm out tonight under clear skies.
Southwest flow will emerge during the surface layer mixing cycle
late Sunday Morning.

For DTW...scattered to broken clouds at or below 5kft is expected
through the balance of the afternoon hours. Varying winds in the 290-
320 window with speeds around 10kts will pose some challenges for
air traffic flow direction into the air field.


* Low for cigs aob 5kft through 22z.


Issued at 347 AM EDT Sat Aug 19 2017


An upper level trough passing through Lower Michigan will keep a
chance of showers and an isolated thunderstorm in the forecast
during the morning. This will be mainly supported by a corridor of
weak DCVA from south to north over the area downstream of the upper
level trough axis. Some late developing nocturnal elevated
instability will also contribute to forcing but will be limited by
underwhelming moisture transport. Model analysis fields indicate a
lack of backed flow in the mid levels compared to the otherwise
strong appearance of the wave in satellite imagery so far since
midnight. The relatively weak mid level theta-e advection is not
expected to enhance elevated instability before sunrise which
supports holding POPs in the chance category.

Timing remains solid on an early afternoon departure of the system
which will bring an end to rain concerns. The possible exception
will be some lake breeze and terrain induced instability showers
over northern Lower Michigan that could drift into the Saginaw
Valley and northern Thumb during late afternoon. These will be
strongly dependent on daytime heating and will fade around sunset. A
clearing trend will then progress during the night as the short wave
ridge aloft and surface high pressure exert a stabilizing influence
that will provide dry and warm weather through Sunday.

The passage of the upper level trough axis today will mark the
beginning of a larger scale pattern shift toward a more zonal
configuration to start the week. This is projected in model data to
be kicked off mainly by a 100 kt upper jet moving into the Pacific
Northwest which will break down the amplified flow over North
America that has been a preferred mode over the last few weeks. The
zonal flow is itself an unstable configuration and is shown to
quickly begin buckling during Monday. There is usually some
uncertainty in model performance during these transitions but the
latest NWP package is in good agreement regarding upper level trough
development over central Canada extending into the northern Plains
states. This will produce the next low pressure system that is
expected to move into the Great Lakes by Tuesday with rain potential
as early as late Monday afternoon into Monday night in SE Michigan.
The intensity and timing moisture return flow ahead of the front is
the main source of uncertainty for rain probability Monday and is
subject to slower timing adjustments as the upper trough and surface
low pressure system develop.

Moisture transport is expected to ramp up considerably later Monday
night into Tuesday. Surface high pressure linger over the Gulf and
Atlantic coast states will limit that source of moisture while the
frontal system taps instead some mid level monsoonal moisture from
the southwest. This is shown by modeled upper single digit 700 mb
dewpoint moving into the Plains states by Tuesday morning. The
plentiful moisture supply and supportive large scale trough and low
pressure system in the Great Lakes makes it difficult to make a case
against aggressive machine generated guidance on POPs through
Tuesday night. Some potential for thunderstorm organization will
also exist in this scenario, especially if afternoon timing holds on
the frontal passage.

Global model solutions are trending toward sharper trough
development over eastern Canada after the system moves across the
Great Lakes. This will open the door for stronger high pressure to
develop over central Canada and spread colder air into the Great
Lakes Wednesday into Thursday. The GFS offers 850 mb temperatures in
the 5-10C range by Wednesday evening which could limit highs to the
upper 60s in the Saginaw Valley. Modifying high pressure then holds
during the second half of the week and into next weekend.


Moderate northwesterlies will back to southwesterly by tonight and
persist into Tuesday. The next chance of thunderstorms late Monday
and again on Tuesday will be followed by winds veering back to
northwesterly behind a cold front Tues aftn.


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



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