Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1245 PM EST Monday Dec 9 2013
Moist low-level flow will continue this afternoon into this evening
in the wake of a low pressure system. Drier air is beginning to work
into the area from the west behind another cold front. This will
allow lake effect showers to gradually taper off from south to north
through the afternoon and evening. Flurries are still expected from
the Detroit area southward...with occasional snow showers continuing
from ptk northward. Best chances for IFR visibility restrictions
will occur (ptk/fnt/mbs) before 20z before snow intensity begins to
wane. Ceilings will also gradually increase through the afternoon
and evening as winds increase and slightly drier air works in behind
the front. West-southwest winds will gust up to 28 knots until
subsiding this evening. Ceilings are expected to rise to VFR in and
around the Detroit terminals overnight as drier air noses in from
Another cold front will push across the area on Tuesday. This will
once again bring gusty winds from the southwest. Gusts to 30 knots
can be expected through the afternoon and evening.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet through this
evening...medium confidence tonight and Tuesday.
Previous discussion...issued 342 am EST Monday Dec 9 2013
Short term...today and tonight
Rapid northeastward ejection of the sheared pv filament and
attendant surface reflection will bring an abrupt end to the
isentropic ascent/warm air advection supported light precipitation this
morning...the advective process quickly overcome by a veering low
level wind field and pursuant increasing cold air advection.
Pockets of very light snow and/or freezing drizzle will exist
through the morning commute as this process unfolds...marginal
depth at the top of the moist layer for maintaining ice
nucleation allowing for some intermittent fluctuation in phase
at times. In any case..both coverage and intensity will be too
limited to contribute any additional accumulation or the
possibility of a light icing issue.
Temperatures noted this morning likely to be the warmest over the
next week...the narrow 'warmer' nose left behind leaving readings
generally in the upper 20s/lower 30s. Low level thermal profile
will see a definitive downward turn heading into the afternoon and
evening...the 925 mb layer dropping 2-3 degrees from the morning
high. This will cap highs within that upper 20s/lower 30s range.
Westerly gradient will tighten with time as the exiting surface low
undergoes some deepening. This in combination with some added depth
to the mixed layer given the cold air advection will contribute to a gusty latter
half of the day /25 to 30 miles per hour range/.
While the moisture depth will see some reduction as the considerably
drier inbound airmass works in from above...the moisture profile does
retain a higher degree of saturation with respect to ice at the
right layer /-10 to -13c/. The upstream lake moisture flux
contribution will be hindered by a lack of depth as well...but the
deepening westerly flow will at least help modulate the low level
environment downstream. This will ensure a solid stratus canopy
remains in place...and will provide at least a low chance for snow
shower production as lapse rates steepen this afternoon/evening.
This environment will lend support for a continued light snow
shower/flurry mention into tonight. The passage of a trailing shear
axis emanating off the first clipper system rotating through the
northern Great Lakes will slide through overnight. This will
reinvigorate the cold air advective process...but introduce a higher
degree of dry air advection into moist layer residing below the
inversion. This drying always provides a tough call on the degree
of clearing within a progressively colder and weakly cyclonic
westerly flow. The setup points toward leaning more pessimistic
until cloud trends suggest otherwise. This may cap the nocturnal
cooling potential by a degree or two. However the strength of this
airmass will still support lows well into the teens...coldest
locales making a run toward the single digits with the fresh snow
cover now in place.
Long term...Tuesday through Saturday
Southeastern Michigan will remain influenced by lower geopotential
heights with full connection to the Arctic through the middle of the
week. Cyclonic flow within the upper level troughing will guide
multiple dry clipper type disturbances through the Great Lakes.
The first shortwave will affect southeastern Michigan Tuesday
afternoon. This energy will reside on the exit region of an upper
level jetlet that will be set to work through the base of the
trough. Integrity of the exit region dynamics will largely be lost
while entering the trough axis as some larger scale broadening will
also be occurring with the much stronger and more influential East
Coast jet energy. Therefore...the weak thermal field that will be
attendant to the shortwave is expected to become muddled. The flare
up in afternoon light snow shower activity will likely come down to
what Lake Michigan enhancement can survive the trip across lower
Michigan. Preceding southwesterly flow should confine better support
for measuring to the northern portions of the County Warning Area as southeastern
sections witness a dry trajectory from south of the Lake Michigan
bowl. What begins as a good low level convergence signal along the
Downwind Lake Michigan shoreline is shown to be severely compromised
as it advects eastward through the County Warning Area between 21-00z. Cooling in
the 900-800mb layer for the afternoon is shown to largely destroy
the subsidence inversion and allow a good jump in mixing depths.
This will support the development of breezy conditions with
wind gusts potentially pushing 30 miles per hour for all areas. Windchills make
up the story here with values remaining in the single digits above
The entrance region dynamics of aforementioned jetlet will push into
the central Great Lakes region on Wednesday harboring the second
shortwave to impact the area. The upper level jet dynamics are
expected to be a little better organized with this particular
feature supporting modestly better ascent forcing. The issue for the
Wednesday system is that moisture will be severely lacking. Ingest
of the entrance region dynamics with little curvature to the flow is
shown to provide for better frontogenetical/deformation support.
This ribbon of fgen is expected to slide across the southern half of
the County Warning Area between 12-18z Wednesday. Too early to get precise with
location...but could see some Lake Michigan convergent banding.
Trailing this shortwave...very cold air will advect into the central
Great Lakes as 850mb temperatures are forecasted to fall below -20c.
The continuing story is the cold weather...wind chills struggling to
climb out of the single digits above zero Wednesday...dropping to
-10c for northern areas Wednesday night.
A reinforcing cold front will push across the Great Lakes this
afternoon. Increasing gradient flow in the wake of the front will
bring 35 knot gales to the northern two thirds of Lake Huron from
this afternoon through late tonight. With the fresh Arctic blast
dropping temperatures in the teens...a freezing spray warning has
been issued to accompany the Gale Warning. Another weak cold front
is expected to push across the central Great Lakes on Tuesday. Winds
will remain elevated but at this time appear to remain below gale
force. The coldest air of the week is then expected to push into the
region Wednesday and Wednesday night. This system has the potential
to generate both gale force winds and additional heavy freezing
heavy freezing spray warning...lhz361-lhz362-lhz363-lhz421-lhz441-
lhz462...from 4 PM Monday to 4 am Tuesday.
Gale Warning...lhz361-lhz362-lhz363-lhz421-lhz441-lhz462...from 4
PM Monday to 4 am Tuesday.
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).