Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210125

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
925 PM EDT TUE SEP 20 2016


The region of elevated showers/thunderstorms extending across nrn
Wisconsin into north-central Lake Mich are occurring within an
axis of higher mid level theta e air and just north of a region
of steep mid level lapse rates. The mid level moisture will
largely remain north of the forecast area tonight. The 00Z APX and
especially the 00Z DTX soundings show considerable dry air below
600mb. This will inhibit any stray showers/sprinkles from entering
the Saginaw Valley tonight. Upstream mid/high clouds are actually
dissipating as they head toward Se Mi.

Convection is expected to develop across the upper MS Valley
tonight and grow upscale as it pushes across Wisconsin late
tonight. This system will decay as it crosses Lower Mi on
Wednesday, especially across Se Mi where the low levels will
remain quite dry and stable. Steep mid level lapse rates and the
potential for a decent influx of mid level moisture, especially if
an MCV develops, does raise some concern that some high based
showers may survive into Se Mi. The current forecast does account
for this north of the I-69 corridor. At this time no changes will
be made to the forecast, allowing the night shift to monitor
upstream convective trends.


Issued at 652 PM EDT Tue SEP 20 2016


High pressure will extend from srn Lower Mi into the eastern Great
Lakes tonight. The rapid drop in dewpoints following the weak
frontal passage earlier today indicates the degree of low level dry
air now enveloping the region. This low level dry air and the
advection of some high level moisture overnight will preclude fog
development at the terminals, supporting light winds and clear skies
below 10K ft through Wed morning.

For DTW...The later fropa this afternoon allowed some diurnal cu
development, especially across the southern portions of the
airspace. Over the last couple of hours, dewpoints have finally
begun dropping around Detroit. The continued dry air advection into
the night should also inhibit fog development and prevent additional
clouds below 5k ft.

//DTW Threshold Probabilities...

* None

Issued at 320 PM EDT Tue SEP 20 2016


A few upper level features of note will guide the forecast for the
coming days. A large, stacked, low pressure system over Hudson Bay
will be drifting northeast through midweek while the next upper
level trough is already taking shape over the west coast out of the
nearly zonal jet laid out across the US/Canada border. The
development of the new upper low over the west coast, combined with
the exiting low over Hudson Bay will allow the ridge to amplify
across the Central Plains while building slowly westward into
the lower Great Lakes.

The jet will lift slightly northward today as broad and weak low
pressure slowly churns over the Northern Plains. This will allow the
tail end of a warm front to lift northward through lower MI later
tonight through Wednesday. A very dry airmass in place across the
region, with only weak southerly flow and really no frontal forcing
should keep the area pretty dry through Thursday. The fly in the
ointment seems to be a lead shortwave currently working across MN
which should reach central MI early Wednesday. This could be the
forcing needed to utilize the isentropic ascent and elevated
moisture to produce a shower or thunderstorm. Soundings suggest even
this wave will struggle to break through the dry air in the boundary
layer which suggests cloud bases up near 10kft through the day, even
as far north as MBS which would be in the region of interest (north
of I69). So will keep a slight to low chance pop going for mid MI
while trying to adjust for frontal placement and shortwave track.

Wednesday night through Thursday night will feature a continuation of
the going forecast with chance pops up across the Saginaw Valley
and Thumbs with decreasing chances southward. With the front
stalling over northern MI, precipitation chances will hinge quite
a bit on any smaller scale waves riding along the front. There
does appear to be a boost in the low level jet Thursday night
which coincides with a shortwave so that could be a timeframe to
keep an eye on for possible expansion of shower activity.

On Friday, the circulation around the strong surface high pressure
taking shape over western Ontario will begin to displace the warm
airmass in place over Southeast Michigan. This process may result in
some shower/thunderstorm activity. Low-level stability as this high
builds southward is expected to result in dry and cooler conditions
this weekend.


Light south to southwest winds will veer to northeasterly across the
northern half of Lake Huron from Thursday through the end of the
week as a warm front settles over the waters. A chance of
thunderstorms will accompany the warm front during this period.


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



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