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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
657 am EDT Wednesday Oct 1 2014



Aviation...

//discussion...

A two to three hundred foot ceiling with areas of dense fog has been
impacting ptk/fnt/mbs through the early morning and there are no
indications that this will change over the next 2 to 3 hours.
Daytime heating will eventually lift the bases. The rate at which
ceilings lift and extent of late day clearing carries a good deal of
uncertainty. Given the degree of low level moisture and time of
year...a gradual lifting of ceiling bases will be forecast in the
tafs.

For dtw...there has been a persistent area of clearing attempting to
nudge into metropolitan Detroit which greatly complicates the morning
ceiling/visibility forecast. The expectation is that any clearing this
morning will be brief with the onset of daytime heating leading to a
fast redevelopment of low clouds.

//Dtw threshold threats...

* medium confidence in ceilings below 5000 feet this morning and
afternoon.

* Low confidence in ceilings below 200 feet and/or visibilities below
a half mile Thursday morning.

&&

Previous discussion...issued 322 am EDT Wednesday Oct 1 2014

Short term...today and tonight

The upper low which impacted Southeast Michigan yesterday is shown on the latest
water vapor loop rotating into the middle Atlantic...tucked under the
upper ridge now expanding into southern Quebec. A middle level shear axis
extending from the departing upper low will linger over Southeast Michigan into
late afternoon before being washed out by short wave ridging. The
net result of the building middle level heights over lower Michigan will be
to sustain large scale subsidence through much of the short term.
The latest surface analysis shows a trough axis extending across The
Thumb and Saginaw Valley. Enhanced convergence along this boundary
is allowing some drizzle to persist. Expanding surface high pressure
from the north will continue to force this boundary south this
morning...with it expected to wash out completely by afternoon as
the surface high pressure takes hold.

The cloud and fog forecast will remain the more challenging aspect
of the short term. Aside from Lenawee and Monroe counties which
experienced a period of clear skies...clouds remained solid
across the forecast area overnight. East-NE low level winds during
the night advected very moist air in from southern Ontario /where surface
dewpoints were in the upper 50s yesterday afternoon/. The result
has been a 200-400 feet stratus deck blanketing most of the northern two
thirds of the forecast area. Weakening low level flow and a
moisture rich boundary layer may continue to lower the cloud bases
toward daybreak...which will sustain some fog /especially in the
higher terrain of the Irish Hills/. Model solutions suggest
daytime heating and low level anticyclonic flow will lead to
clearing this afternoon. However... the 00z DTX sounding showed a
secondary inversion /rather deep/ based near 800mb which none of
the model solutions resolved. With some thermal troughing near
850mb forecast to linger through the afternoon...daytime heating
will most likely lead to an extensive higher based strato cumulus
field. This will support leaning on the lower end of guidance maximum
temperatures /low-middle 60s/.

Warming in the 900-800mb layer this evening into the overnight
should erode any lingering inversion...offering the possibility for
some clearing prior to the arrival of the high clouds from the next
system. The low levels across the eastern Great Lakes will remain rather
moist today. A southeast component to the surface winds tonight and influence
from Lake Erie will thus provide a chance for some fog and/or
stratus redevelopment toward daybreak Thursday.

Long term...

Ill-defined warm front lifting into northern lower Michigan during
Thursday...with solid warm advection taking place over Southeast
Michigan...as 925 mb temperatures warm into the upper teens to around 20 c
toward 00z Friday. Still...with surface flow backed out of the
southeast and increasing/thickening clouds with moisture
advection/isentropic ascent...boundary layer mixing will probably
come up just short of 925 mb...and will continue with the more
subdued maximum temperature forecast in the low to middle 70s...with limited rain
chances...mainly along/west of U.S. 23 through the day.

An even deeper moisture surge Thursday night with burgeoning height
falls will allow for developing rain showers. The trend of a slower
and stronger storm system for the end of the work week has continued
with the 00z models...as merging height fall centers consolidate
over the central Continental U.S. On Thursday...with potent upper wave/pv
anomaly over northern British Columbia forecasted to track along or
south of the southern Michigan border Friday night per
GFS/Euro...which will serve as the anchor/Center Point of the
eastern North American longwave trough during the first half of the
weekend.

With slower passage of the cold front on Friday...maxes have a
reasonable shot of climbing past 70 degrees before we take Big Nose
dive for the weekend...as 850 mb temperatures fall below zero...potentially
as cold as -4 c (see 00z gfs). Even though instability looks modest
Thursday night-Friday...slightly negative surface-850 mb lifted indice's...about
500 j/kg of cape during Friday is a bit concerning with 50 knots
noted at 850 mb...and strong gusty winds appears likely with any
convective activity.

Cyclonic flow...steep low level lapse rates...Lake Michigan
contribution on Saturday should assure scattered to numerous rain
showers...as there also looks to be a pronounced surface trough
rotating through. Cold westerly flow off Lake Michigan on Sunday
still probably sufficient to generate scattered light showers...with
maxes holding in the low to middle 50s .

Marine...

High pressure situated over the central Great Lakes will provide
light winds and low waves into Thursday morning. Southerly winds
will begin to pick up Thursday afternoon...as a strong cold front
approaches. South winds up to 30 knots over the open waters of Lake
Huron are anticipated Friday morning...as the cold front moves
through during the afternoon hours. Warm air streaming into the
region is expected to lead to stable near surface profiles...and
thus keep wind speeds predominately below 30 knots. Any convective
activity with the cold front could lead to stronger wind speeds
however. Seasonably cold airmass following the cold front Friday
night into the weekend will be conducive to higher waves along with
numerous rain showers...as westerly winds gust up to 30 knots over
the open waters of Lake Huron.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.

&&

$$

Aviation.....SC
short term...SC
long term....sf
marine.......sf



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