Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1145 PM EST Sat Dec 27 2014
Modest westerly winds and MVFR ceilings have settled in behind a
departing cold front during the overnight hours. High pressure will
bring a slug of drier air during the late morning and early
afternoon hours on Sunday...clearing out the stratus deck and
leaving high clouds over the taf sites.
For dtw...despite ceilings briefly rising to VFR upstream observations
indicate the MVFR stratus deck should quickly move over the Airport
by taf issuance.
//Dtw threshold probabilities...
* high for ceiling below 5000 feet through early morning Sunday.
Previous discussion...issued 352 PM EST Sat Dec 27 2014
Regional radar mosaic has a fineline feature just east of Benton
Harbor and Grand Rapids at 20z. This feature coincides with modeled
positioning of the 950mb extension of surface cold front that will
slide through southeastern Michigan this evening. All indicatations
point to the cold front sliding through the Tri Cities around
23z...Flint at 00z...and through the far southeastern County Warning Area by 03-04z.
Precipitation ahead of this feature has struggled...due to the loss
in synoptic scale support for ascent with the middle to upper level
ridge overhead...along with a reasonably stable midlevel profile.
Stood firm with chance probability of precipitation ahead of the cold front as moisture
streamlining ahead of the cold front could generate some shower
One exception to a broadbrush pop approach is the far southeast from
Detroit southward to the Ohio border. Models show a textbook
positive d(mag)/dt response to the upper level jet branch along with
an organized curvature change. This is expected to lead to a
significant increase in right entrance region dynamics that will
draw deeper moisture and frontogenesis northward. This forcing is
expected to lift far enough northward to bring a short duration of
showers...less than .10 inch of rain south of Detroit after 00z.
Cold advection is then expected to be in overdrive with 850mb
temperatures dropping to -7c by Sunday 12z. Subsidence in the
postfrontal environment will lead to little to no potential for
precipation late...while clouds will hold with residual lower
tropospheric moisture holding through the end of the forecast
Long term... Sunday through next Saturday
Today's cold front will be the first in a two stage downward trend
in temperatures finishing the weekend and starting next week. Steady
but weak cold advection during Sunday balanced by partial sunshine
will result in afternoon maximum temperatures in the lower to middle 30s...which
is still at or slightly above normal. A secondary front/trough will
then settle over lower Michigan during Monday and bring in a truer
polar air mass that will take temperatures below normal for the first half
of next week. Maximum temperatures in the upper 20s to lower 30s Monday will be
followed by 20s areawide Tuesday...and with no more than a few
flurries either day.
The swing toward below normal temperatures in the Great Lakes will
be promoted by maturation of split flow in the large scale upper air
pattern over North America. The main component for the Great Lakes
will be the upper low over central Canada that will stay progressive
in the northern stream. Trailing northwest flow aloft will then
support a slow southward intrusion of polar air during the first
half of the week while a broad trough develops in the southern
stream over the central rockies. The main component of the trough in
the southern stream is shown digging into the Pacific northwest today...
close enough to the upper air network for an improved Sample in the
12z model runs. This brings added confidence to a cut off low
solution over Southern California by middle week keeping the Great
Lakes subjected to weak and moisture-starved northern stream short
waves that will bring a steady supply of cold air to the region.
Limiting factors for precipitation start out mainly as unfavorable
conditions for lake effect activation Sunday. Post-frontal westerly
flow will be light with modest instability over the water...but lake
effect will be limited primarily by shallow convective depth below
875 mb/3000 feet. By the time moisture and convective depth deepens
with the polar front...the wind is turned to the northwest...which
is a Dry Lake effect trajectory for our area under all but the most
extreme scenarios. The polar front itself will need that lake boost
as it will only carry specific humidity averaging less than 1 g/kg
in the 850-700 mb layer...good enough for solid cloud coverage but
too low for meaningful precipitation through Tuesday.
Polar high pressure will be settled over the central part of the
country during the second half of the week. Westerly low level flow
from this high into the Great Lakes will maintain the below normal
temperature pattern Wednesday through Friday. Conditions will be
otherwise dry and benign until possibly next weekend when model
solutions indicate the southern stream closed low will phase with
the northern stream and lift east of The Rockies and into the Great
Lakes. The latest global solutions still exhibit considerable spread
in timing...track...and intensity. Given the cut-off character of
the southern stream system during middle week...slower and farther east
solutions are preferred...which give the Great Lakes a better chance
of snow than rain by Saturday.
The surface low exiting The Straits during the afternoon is
deepening on schedule with model depictions...and wind gusts Post
front over Southern Lake Michigan are already near 30 knots. This
supports upgrading to a Gale Warning for about a 6 hour period
tonight for frequent gusts by the time the Post-frontal northwest
flow arrives over Northern Lake Huron. Small craft advisories remain
in effect for Saginaw Bay and the nearshore waters to Harbor Beach
for a combination of 30 knot gusts and high waves. Much colder air
will surge over the Great Lakes and remain in place...but high
pressure will bring lighter wind during the first half of the week.
Lake Huron...Gale Warning until 4 am EST Sunday for lhz361-362.
Small Craft Advisory until 4 am EST Sunday for lhz421-422-441.
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).