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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
717 am EDT Wednesday Apr 1 2015


IFR stratus between mtc and phn will be monitored for possible
impact at det but is not expected to reach the other terminals as a
weak surface pressure trough dissipates during the morning. The
resulting light wind field will gradually turn light from the
southeast during the afternoon with just some middle and high clouds
increasing mainly in the mbs to fnt region. VFR conditions will
continue tonight but the wind will increase from the south. A warm
front developing over the Ohio Valley will present a low level
wind shear scenario over Southeast Michigan by late evening through
sunrise Thursday.

For dtw... early sunshine will quickly dissipate MVFR fog and any
ceiling below 5000 feet will be equally short. That leaves no other
concerns outside of low level wind shear development tonight.

//Dtw threshold probabilities...

* low for ceiling at or below 5000 feet early this morning.


Previous discussion...issued 347 am EDT Wednesday Apr 1 2015

Short term... today and tonight

The upper level impulse sliding through the eastern Great Lakes will
continue to the Atlantic coast during the early morning. This will
lead to a clearing trend over Southeast Michigan as the modest surface
pressure reflection fills. The exception to monitor will be in The
Thumb where middle clouds will be last to exit and where some potential
for lake influenced stratocu will occur due to weak onshore flow. A
generous amount of sunshine is expected elsewhere to aid in warming
temperatures through the morning.

The Great Lakes and Southeast Michigan will come under the influence of
larger scale weather features through the rest of the day and
tonight. Satellite imagery and model analysis fields show the upper
ridge over the plains set to progress steadily eastward and
strengthen surface high pressure over the Appalachians. This will
bias the low level wind field toward an easterly direction until
late in the day which will add some complexity to the maximum temperature
forecast outside of the typical shoreline and downriver areas. The
Thumb shoreline will struggle in the 40s...but there is potential
for overachievement through interior areas based on low level
thickness warming compared to how well readings responded to
afternoon sun yesterday. There will be some middle and high clouds to
contend with during the afternoon associated with the elevated warm
advection pattern that is helping drive the ridge eastward. Model relative humidity
fields indicate these clouds will mainly affect the Tri Cities and
northern thumb leaving the door open for maximum temperatures to touch 60
interior toward the Ohio border.

The main theme for tonight will be continued development of
southerly gradient wind over the region driven by the large low
pressure system moving from the plains to upper Midwest. Model
depictions of precipitation remaining north of our area look
reasonable given activity will be tied to middle level isentropic
ascent. Middle and high clouds lingering over our area will develop
northward as well before thickening from the west late. The
evolution of the wind field then has the look of late evening min
temperatures in the upper 30s thumb to middle 40s interior rising overnight
toward 50 interior and well back into the 40s elsewhere by sunrise

Long term...

A busy end of the week is expected with the foci of the forecast
involving the potential for thunderstorms/heavy rainfall on Thursday
and the potential for accumulating snow on Friday.

The progressive high amplitude ridge axis will be directly over
southeastern Michigan at the start of the period Thursday. In
rapid succession...the ridge axis will push eastward allowing for
the influence of cyclonic flow to encroach upon County Warning Area from the West.
Point of contention for this period has been the timing of
precipitation into lower Michigan. Still poor inter-model
agreement on onset with models seemingly half and half on whether
or not precipitation will begin before noon. Still prefer a dry
morning before probability of precipitation ramp up substantially for the afternoon.
Reasons are: upstream elevated nocturnal precipitation is expected
to be back across Wisconsin at 12z...will need time to push out
residual near surface ridging/dry air...and eastern edge of jet
aloft and shortwave energy are not forecasted to arrive until the
afternoon. Things look good from a probability of precipitation
perspective as convective instability will be in place as well as
a synoptic scale trigger in a wide ranging occluded front
structure. NAM is still suggesting upwards of 700 j/kg of elevated
cape...however...raw data is also suggesting a surface dewpoint of
between 55 and 60 degrees. This is almost certainly overzealous
and stuck closer to the offered MOS values of the upper 40s to
near 50 degrees. Steep lapse rates between 800-600mb continues to
support a chance for thunderstorms. Depth of moisture (pwats of
1.25-1.30 inches) and low column stability is all lining up for
what could be a window for some heavy rainfall. One large negative
will be working on rainfall amounts will be storm motion to the
east northeast at 55 to 60 knots. Latest swody2 keeps much of
southeastern Michigan under a designation of marginal.

The main baroclinic zone is expected to become positioned southeast
of the area over portions of Ohio and Indiana Thursday night.

A trailing upper level jet streak and pv anomaly is also forecasted
to eject out of The Four Corners region Thursday night. Models
really do show an organized coupling of this trailing jet streak
with the main branch over the upper Midwest from the start.
Evolution of this coupling jet structure only increases in strength
moving into the day Friday as the lead/Great Lakes branch undergoes
a transition to anticyclonic curvature. Pv phasing over the middle
Mississippi River valley tips the scales into cyclogenesis along
the Ohio River on Friday. A good frontal wave setup with a swath
of deformation snowfall striping along the northern proximity of
the deepening low center. There still remains a high amount of
uncertainty within the model data on whether or not a significant
amount of precipitation will fall over semich. Pattern recognition
suggests the book has not been written yet as there is some
potential for precipitation to miss the area to the south due to a
greater southern pv contribution from convection/latent heat
release. The move this forecast package was to incrementally
increase probability of precipitation while introducing a rain/snow mention for the
daylight hours on Friday. Certainly a time period to watch for
forecast evolution.


High pressure today will provide light winds and low waves.
Southerly winds will increase tonight ahead of a cold front...which
is expected to move through late Thursday. Warm southerly flow will
increase to 20 to 25 knots during the day Thursday but stability
will limit wind gust potential.


DTX watches/warnings/advisories...

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



long term....cb

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