Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 211759

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
159 PM EDT SAT MAY 21 2016


Mid to upper level moisture associated with the central great lakes
closed circulation is in the process of stripping out. The primary
axis to the light rain and sprinkles has already pushed into
southwestern Ontario. Some question exists as to whether or not this
upper level moisture will thin out completely with additional
moisture and energy pushing due southward into the state from Canada
overnight in response to a weakness in the height field. This will
conveniently allow the corridor of cloud over Ontario to slip due
southward with time. Given the amount of mixing that will occur
later this afternoon suspect cloud tonight will be the 6-9 kft agl
layer. There is some model data that supports a pocket of near
surface moisture tracking over and across Lake Huron, providing
enough boundary layer moisture to result in a potential robust
cumulus response late Sunday morning. At this time, confidence in
the cumulus response is low.

The mixed north to northeasterly winds this afternoon are
expected to become light overnight.

//DTW Threshold Probabilities...



Issued at 353 AM EDT Sat May 21 2016


Low pressure moving through the southern Great Lakes today is not
very deep but is well organized. The observed pattern of showers
could even be considered overachieving if not for the alignment of a
few important characteristics. Most notable is a hybrid of weak
isentropic ascent/deformation and moisture transport occurring
around the 700 mb level that is under an adequate cold pool aloft
associated with the upper level wave. Temperature at the 500 mb
level running near -18C is allowing showers to develop increased
convective nature and improve the chance of overcoming dry NE low
level flow originating from high pressure over the northern Great
Lakes. That being said, the northward progress of showers into SE
Michigan will be more dependent on how quickly the system shears
eastward during the morning through early afternoon. A greater
concentration of vorticity over Illinois before sunrise will pivot
through the larger scale upper level through toward the Ohio valley
and this will start the shearing process and halt the northward
progress of showers into our area. There is some forecast error
sensitivity to this process but model agreement looks better than
average among the 00z runs. Overall, just enough evidence to hold
the line on northward extent roughly from Jackson to Port Huron
while nudging POP up into entry level likely for Lenawee and Monroe
counties through early afternoon. Points north through Flint, the
Tri Cities, and northern Thumb will remain dry but with substantial
high cloud coverage that will help keep max temps in check. Guidance
temperatures look good showing warmest reading north and coolest
toward the Ohio border.

As the low pressure system exits into Ontario and the eastern Great
Lakes tonight, it will begin to absorb a short wave from central
Canada. This feature is moving through northern Ontario early today
and will carry some mid level warm advection clouds into lower
Michigan after a brief evening clearing trend. Open sky during the
evening will get temperatures falling through the 50s before the
associated cold front helps readings settle into the mid and upper
40s most locations overnight. Low prospects for rain with this
backdoor front as it will be moving into the dry air mass that is
battling back rain today over most of SE Michigan. Some lingering
stratocu is likely for a while Sunday, in a typical response to
low level cold advection and morning acceleration of surface
heating, followed by a sunny afternoon as broader high pressure
takes control.

The short wave carrying the backdoor front Saturday night into
Sunday will merge with the eastern Great Lakes upper wave before it
reaches the mid Atlantic coast Monday. Both systems being imbedded
within the highly amplified larger scale flow will easily lead to a
closed low over the mid Atlantic, especially with the big upper low
out west. The corresponding full latitude ridge and related surface
high pressure will then settle over the Great Lakes to start next
week. We will see how fast this long wave blocking pattern actually
takes to break down. The latest medium range model solutions suggest
soon enough for an active period of showers and thunderstorms mid
to late week.


High pressure and a weak pressure gradient will sustain light winds
across the marine waters this weekend. Light southerly winds and
warmer temperatures arrive early next week...leading to a chance of
showers and thunderstorms by mid week.


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



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