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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
115 PM EDT Tuesday Sep 30 2014



Aviation...

//discussion...

For the most part drizzle is coming to an end across Southeast
Michigan. However...recent radar imagery still is showing some
spotty drizzle trying to force into the region. Therefore...have
kept a tempo of drizzle in the forecast until 22z. Ceilings will
remain overcast and below 2000 feet through the Wednesday
afternoon as low level moisture continues to filter across the taf
sites.

For dtw...lingering drizzle will be possible at metropolitan this
afternoon. Currently expecting the chance for drizzle development to
end by 22z...however low clouds /below 2000 feet/ will stick around
through Wednesday morning.

//Dtw threshold threats...

* high probability that ceilings will remain below 5000 feet through Wednesday.

&&

Previous discussion...issued 335 am EDT Tuesday Sep 30 2014

Short term...today and tonight

Low level cold air advection will prevail through the morning behind
the surface cold front which exited the forecast area shortly after
06z. Lingering Post frontal showers will exit to the southeast of
the state before daybreak. The showers overnight developed within a
ribbon of strong large scale ascent along the lead edge of an
amplifying upper wave. Recent water vapor imagery already shows the
evolution of this system into a closed middle level low...which is
forecast to rotate across The Thumb this morning and into NE Ohio
this afternoon. The associated surface low...now located over southern
Ontario...will in turn track into western PA/WV this afternoon.

Residual low level moisture and a deepening Post frontal inversion
has led to a stratus deck now extending all the way to Southern Lake
Superior. This inversion will only deepen during the course of the
morning as 925mb temperatures fall through the single digits. Low clouds
will thus remain in place into at least late evening. This will
limit any diurnal recovery and hold daytime temperatures in the 50s. The
evolving compact middle level low will support the development of an
enhanced region of middle level deformation /trowel feature/ targeting
northern/cntl Lake Huron and NE lower Michigan. Model solutions then indicate
this feature falling apart as it advances into Southeast Michigan tonight as the
upper low weakens and quickly advances east. So the better forcing
will remain north of the area today. However...model soundings are
very supportive of drizzle today /radar and surface observation over northern lower
Michigan overnight have also been showing a lot of drizzle/. This is due
to a moist boundary layer...low cloud bases...substantial dry air
above the inversion...and good directional shear near the top of the
moist layer. The drizzle is expected to be quite widespread over
much of The Thumb today as north-northwest flow over Lake Huron adds a little
better moisture to the boundary layer and enhances convergence along
the shoreline.

Surface ridging will build across lower Michigan tonight...while enhanced
thermal troughing will reside over Lake Huron. The flow above the
boundary layer will turn toward the east which will actually support
some low level warm air advection. This will weaken /possibly even
erode/ the low level inversion and may support some clearing across
at least the western portions of the area as the surface high builds in from
the north. Clearing potential still however carries a low confidence
as the easterly flow will also advect some higher moisture in from
the east. So fog and/or additional stratus development seem
possible. The potential cloud cover and low level moisture advection
will warrant a limited drop in temperatures tonight /lows from the middle 40s
to low 50s/.

Long term...

Confidence is not very high in low clouds scouring out completely
on Wednesday...as NAM/GFS/Gem/Euro all suggesting some 925-850 mb
residual moisture underneath the modest upper level
ridge...especially back across The Thumb region. With just partial
insolation expected...looking at maxes coming up just short of 70
degrees. We should however have no problem climbing into the 70s on
Thursday ahead of a cold front...with good warm advection allowing
925 mb temperatures to climb into the upper teens to lower 20s per 00z
Euro. However...with increasing and thickening clouds...backed
southeast flow at the surface...boundary layer mixing will probably
come up just short of 925 mb and will hold maxes in the middle to
upper 70s.

With a very good moisture feed...precipitable water values in excess of 1.5
inches...coupled with good height falls and modest instability
Thursday night...rain showers can be expected...with even colder air
on tap for the weekend as upper level energy over the Gulf of Alaska
and off the coast of British Columbia/Pacific northwest consolidate
over the Great Lakes region...leading to deep 500 mb low. In
fact...the longwave upper level trough axis will be attempting to go
slightly negative Friday morning...which is yielding a slower
frontal passage compared to last night...and it appears showers will
likely linger into Friday. Cold cyclonic flow and 850 mb temperatures
falling below zero over the weekend will likely translate to at
least scattered lake enhanced showers and mostly cloudy skies...with
forecasted 850 mb temperatures of -2 c yielding highs only in the low to
middle 50s on Saturday.

Marine...

A strong cold front will clear the central Great Lakes this
morning...with relatively short lived winds of 15 to 20 knots with
gusts to 25 knots across central and southern sections of Lake
Huron. This will prompt an increase in wave heights...with heights
topping out slightly above 4 feet over much of Southern Lake
Huron...and the previous small craft advisories will continue
through the day. Winds will diminish by tonight as high pressure
builds back into the region...but the northeast direction will
maintain some modest waves over the nearshore waters. Light winds
and low waves will persist Wednesday into Thursday.
However...southerly winds developing ahead of a cold front late
Thursday could result in winds up to 30 knots over the open waters
of Lake Huron Thursday night.

&&

DTX watches/warnings/advisories...
Michigan...none.

Lake Huron...
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Port Sanilac...until 6 PM Tuesday.

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

&&

$$

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



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