Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
350 PM EST Thursday Dec 5 2013
The Arctic airmass residing over the upper Midwest will continue to
gradually advance into Southeast Michigan this evening through tonight. Despite
the westerly flow...the degree of dry air is limiting the moisture
flux off Lake Michigan. This has been allowing a fair amount of
clearing...especially across the Saginaw Valley. Although clearing
of the low clouds will continue through the evening given the
persistent low level dry air advection...there is a considerable
amount of middle/high clouds to the southwest. These clouds are
occurring within a region of middle level frontogensis which will
overspread lower Michigan this evening and persist into Friday morning. The
more active portion of this frontal zone will remain well to the
south tonight...keeping Southeast Michigan dry. The middle/high level clouds will
however advance into the area during the course of the night.
Although the cloud cover and westerly gradient will limit the degree
of radiational cooling tonight...the low level cold air advection
will be strong enough to drop temperatures well down into the 20s. Wind
chill readings will drop into the teens.
Long term...Friday through next Thursday
First issue in the extended will be to determine if a low pressure
system tracking up through the Tennessee Valley will spread snow as
far north as Southeast Michigan Friday afternoon and evening.
Euro/GFS/NAM all show the 850 mb front on the north side of the
system just barely clipping extreme Southeast Michigan...and show
the 700mb front becoming situated overhead. Isentropic ascent within
these layers (290k and 295k) looks relatively weak and Theta-E
advection is pretty much non-existent this far north. By
comparison...ascent and moisture advection look very strong to our
south along the low-level front...particular from southern
Indiana/Ohio into Kentucky/West Virginia. Situation looks more
promising for lower-levels of the front to activate and think we
will see little snow if any over far Southeast Michigan from this
system. Best potential for a dusting of snow will be over Monroe
County. Low probability of precipitation do remain in the forecast from Detroit southward to
cover the potential for some light snow to spill into
Michigan...further shifts in the storm track...and to account for
any effect a weak upper wave tracking through Michigan could have.
Models all show this wave tracking through lower Michigan during the
afternoon and evening. Frontogentic response could be just strong
enough to squeeze out some light snow over far southern Michigan and
flurries for the remainder of the area. Overall...expect plenty of
cloud cover around with the increase in middle/high level moisture.
Cool airmass will also remain in place confining high temperatures
in the middle 20s to low 30s.
Upper wave now over Saskatchewan will track across the northern
Great Lakes Saturday into Saturday night...while strong surface high
pressure builds in from the west. GFS/NAM forecast soundings show
much drier air pushing into the region Saturday and especially
Saturday night...with only a little moisture trapped under a falling
inversion (all below 850mb) by Saturday morning. A little hesitant
to accept the atmosphere will be this dry given continued westerly
flow off Lake Michigan. Kept some clouds around for Saturday with
clearing overnight as moisture wanes further and high pressure drops
inversions quite low over the lake. Overnight min temperatures for the
weekend will be quite cold...with most areas in the teens both
Friday night and Saturday night.
A low pressure system will be approaching the Great Lakes region
late Sunday with models currently showing the low tracking across
the central Great Lakes. Southeast Michigan looks to be in the cold
sector...so snow will be the main precipitation type. One caveat with this
system however is the limited moisture as the better moisture
appears to stay to the south. This system will continue to push off
to the northeast on Monday bringing cold air advection to the area. Ridge of high
pressure then builds in behind the departing system just as a weak
disturbance looks to dig down across the northern Midwest and Great
Lakes. With westerly flow ongoing...potential will exist for snow
showers given enough moisture. A reinforcing shot of Arctic air is
then set to overtake the Great Lakes region Tuesday/Wednesday...with
highs only reaching into the upper teens to lower 20s.
Strong westerly winds will continue over Lake Huron into tonight as
a low pressure system deepens over St James Bay. Gale force wind
gusts to around 35 knots will continue north of Alabaster before
winds gradually subside from south to north tonight as the area of
low pressure starts to weaken and move into Quebec. Wind gusts
between 25 and 30 knots are expected over the remainder of Lake
Huron tonight...and small craft advisories remain in effect for the
High pressure will bring calmer conditions to most
of the area for Friday and into the weekend. Several upper level
disturbances moving through Ontario and Quebec will keep winds more
elevated over Lake Huron however. The strongest winds are expected
on Saturday...but look to remain just shy of gale force.
Aviation...issued 1158 am EST Thursday Dec 5 2013
West-SW winds will remain sustained around 15 knots into the
evening...with gusts around 25 knots expected. A weakening gradient
and onset of nocturnal cooling will allow the winds to diminish in
the 22z-00z time frame. The aspect of the forecast which carries
more uncertainty is timing and degree of clearing of the Post
frontal MVFR strato cumulus field. The ongoing shallow Arctic air
overspreading southern Michigan is very dry. The low level flow is also
expected to become slightly divergent this evening. These factors
should allow a clearing trend during the course of the evening.
For dtw...the westerly flow off Southern Lake Michigan will give a boost to the
moisture depth this afternoon. This may be able to hold the low
clouds in metropolitan past 00z this evening.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet this afternoon. Low
confidence this evening.
Gale Warning...lhz361-lhz362...until 5 am Friday.
Gale Warning...lhz363...until 10 PM Thursday.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 5 am Friday.
Small Craft Advisory...nearshore waters from Harbor Beach to Port
Huron...until 4 PM Thursday.
Small Craft Advisory...inner Saginaw Bay...until 10 PM Thursday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).