Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
306 PM EST Friday Nov 27 2015
Short term...through tonight
Upper wave will continue to push across the northern Great Lakes
this evening before tracking east into Ontario/Quebec overnight. At
the surface...associated surface trough axis and trailing cold front
pushed across the area earlier today...with the surface front just
clearing the far southeast corner of the state within the past hour.
Strong cold air advection has brought falling temperatures on the
backside of the front...with the northern half of the forecast area
already in the 30s. Drier air has also been pushing in as evidenced
by falling dewpoints and decreasing coverage of precipitation on
radar. Wet-bulb effects and then just overall cold air will continue
to bring a chance for a brief changeover to snow on the backside of
the precipitation shield...with little or no accumulation expected.
Although the surface front has already cleared the area...elevated
portions of the frontal slope still remain over Southeast Michigan
as of 3pm (per model data...and banded fgen signatures on radar).
The elevated frontal slope (especially at 925mb) will be slower to
exit as it has become parallel to upper flow...it becomes
increasingly blocked by high strong high pressure extending through
the middle-Atlantic coast...and small surface waves ripple along it
tonight. Strong fgen (mostly lower level) and right entrance region
forcing will allow precipitation to regenerate along this boundary
this evening and through much of tonight...all while the upper front
slowly slides southward. Best chances for rain showers after sunset
will be along/south of M-59. Precipitation should mostly be out of
hereby 09z (4am).
Cloud forecast is a little bit challenging with drier air working in
from the north. Higher-res models strongly support at least a broken
stratus deck staying intact as low-level moisture remains trapped
under an inversion between 2000-3000 feet...aided by lake effect
processes as flow turns northeast off Lake Huron. The exception may
be over the northern portion of the Saginaw Valley. Confidence for
more clouds has increased today given satellite presentation over
the northern Great Lakes (lake effect)...even hours after frontal
passage. The clouds will try to offset cold air advection and keep
min temperatures in the upper 20s to low 30s. It appears enough wind and
the drier air will limit fog potential even over the wet ground. Did
not include fog in the forecast...but will monitor trends through
the evening and early part of the night.
Ongoing wave breaking over the West Coast will continue into
Saturday setting up a stagnant/blocky dipole pattern from British
Columbia to the Desert Southwest. Deep remnant subsidence layer will
translate east as this occurs, with strongly confluent northwest flow
downstream of this feature aiding the eastward expansion of strong
high pressure into the Great Lakes for Saturday and Sunday. Axis of
the 1032mb surface anticyclone will settle over the area during
Saturday and will aggressively erode any lingering cloud cover from
north to south. Resident airmass will support highs in the low to
middle 40s under emerging mostly sunny skies.
A sensitive low temperature forecast for satuday night as rather
minor adjustments in simulations of the northern stream have
resulted in nwp shifting the ridge slightly further north than
previous expectations for the overnight period. This will introduce
an easterly gradient that may be difficult to overcome in terms of
decoupling potential from Detroit south. Radiating conditions will
otherwise be ideal with an extremely dry column and clear skies.
Maintained aggressively cold forecast for low 20s/upper teens from I-
69 north, but sharpened the temperature gradient by increasing lows
across the south to 25 to 30 where cooling will be diminished. Mav
guidance remains the most preferable low temperature guidance.
Little to change expected on Sunday as upstream flow remains
stagnant and strong high pressure holds across the area. Once again
looking at highs of 40 to 45 under full sun.
Upper level ridge in place over the eastern Great Lakes will be
replaced by an approaching system moving up through the plains on
Monday. Monday is expected to stay dry until later Monday night
when when precipitation in the form of rain moves in. Current model runs
are generally tracking the center of the system up over
WI/mi...keeping Southeast Michigan in the warm sector for much of
the duration. Colder temperatures then quickly filter in behind the
departing system allowing for a chance of rain/snow mix Tuesday
night into Wednesday. While timing will be refined with each
run...precipitation looks to exit the area by later in the day on
Wednesday. High pressure then returns to the area for the end of
Small craft advisories for Outer Bay and the nearshore waters of
Southern Lake Huron remain on target. Moderate northwest winds will
be gusty through tonight and persistent onshore flow will easily
build waves above the 4 foot sig wave threshold this evening into
tonight. Building high pressure gradually diminish winds and waves
Saturday morning. Dominant high pressure will ensure light winds
through Monday. East wind will ramp up in advance of the next low
pressure system Monday night into Tuesday.
Aviation...issued 105 PM EST Friday Nov 27 2015
High confidence that cold front will pass through remaining
terminals (dtw...det...yip) right around 18z...as the front can
easily be seen on radar. Upstream observations and model data
continue to support several hours of IFR ceilings behind the frontal
passage before some drier low-level air lifts ceilings above 1000
feet (18-23z from north to south). MVFR ceilings are more likely
than not to hold through the night as northeast flow off the Great
Lakes maintains feed of low-level moisture and an inversion remains
intact between 2000-3000 feet. Mbs and fnt should remain dry through
this evening overnight as the front slides south. Upper portions of
the frontal boundary however are expected slow and then stall over
far southern Michigan and Northern Ohio tonight. Lift along this
boundary is expected to generate additional rounds of showers
tonight...especially near the Detroit-area terminals. These rounds
will be hard to time...and will require amendments or adjustments
to the forecast with future taf packages.
For dtw...observational evidence shows good support for a period of
IFR this afternoon. Confidence in timing an improvement to MVFR is
not high...but most likely timing looks around 23z. After
that...upper portion of the frontal boundary that slid through
around 18z will remain over Southeast Michigan through much of the
night. There is good confidence that lift along this boundary will
maintain showery pattern this evening and through much of
tonight...but rounds of showers will be very hard to time. Cold air
should hold off until late...making chances for snow to mix in low
//Dtw threshold probabilities...
* high in ceilings at or below 5kft through 10z tonight...medium through
Saturday morning and afternoon.
* High in precipitation falling as rain through 09z tonight.
Lake Huron...Small Craft Advisory until 4 am EST Saturday for lhz421-441>443.
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).