Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
459 am EST Friday Nov 27 2015
The cold front has arrived which is now resulting in widespread rain
across lower Michigan. Current observation early this morning show MVFR
conditions associated with this area of rain but guidance and model
soundings continue to advertise IFR conditions after sunrise as
deeper moisture and stronger forcing/convergence arrives. Sped up
the frontal passage by a couple hours as this faster solution
continues to dominate. Drier air behind the front will attempt to
break up the clouds later this afternoon...more so for mbs/fnt...but
the Detroit area taf sites look to be locked in clouds longer due to
additional waves of energy in the jet persisting across the south
into the overnight. Southwest winds will swing around to the north
and eventually northeast behind the front.
For dtw...MVFR conditions much of the day will turn to IFR for
several hours as the deeper moisture arrives this morning into the
early afternoon. Winds will remain elevated today as the veer from
the southwest through north behind the front...to northeast tonight.
//Dtw threshold probabilities...
* high in ceilings at or below 5kft today...low for tonight.
* Low in ceilings at or below 200 feet roughly 12z-18z today.
* High in precipitation falling as rain today and into tonight.
Previous discussion...issued 346 am EST Friday Nov 27 2015
Short term...today and tonight
A slow moving cold front will pass southeast through lower Michigan early
this afternoon. Increasing coverage of precipitation can be seen on the
regional radar mosaic as of 08z on the nose of the next ll jet surge
working northeast toward Southern Lake Michigan. Several waves will ripple
along the elongated frontal surface through the day while the main surface
low exits NE through Ontario this morning. With the loss of the main
surface feature...models trending weaker and more sheared with the fgen
band and pressure falls over the region during the frontal passage. Should not
affect the forecast too much though as the surface front is quite strong
in terms of convergence...with favorable fgen...isentropic ascent
and ll jet energy in place. Combine that with middle level waves
streaking through aloft and we should have a good coverage of rain
through the early part of the day as the front drops through. Models
have also continued to trend faster with the frontal passage...bringing drier
Post frontal airmass in faster. Trimmed probability of precipitation on the backside some to
Honor this but overall probability of precipitation and quantitative precipitation forecast are still in good shape. Timing
of the front looks to be around 12z for mbs/fnt...and closer to 16z
for dtw. Question does remain of how long does precipitation continue for
locations south of i94? Latest hires continue advertise precipitation
continuing over that region through the overnight due to another
shortwave lifting along the front with the northern edge of the
isentropic lift and moisture shield re-establishing over the area.
Will have to watch how aggressive the dry air moves in from the
Thermal trend today and tonight will be quite notable. 24 maximum temperatures
have already occurred as of midnight tonight with most locations up
around 60f. Temperatures have already begun to fall slightly into the
middle/upper 50s and the rain shield will only aide in that trend.
Locations in northern lower already behind the surface front have
dropped into the 30s and 40s. With a 12z frontal passage through
mbs...afternoon highs will only be in the 40s. The later frontal passage
through Detroit will allow them to hold onto the 50s through
lunchtime before temperatures start falling. Cooler temperatures brings into
question the possibility of mixed precipitation or snowfall. The front will
lay out more west to east through the afternoon as the surface low pulls off
to the NE and the middle/upper level flow become more zonal. This will
keep the coldest air aloft over the northern Great Lakes while
bringing more moderate temperatures...around 0c at 850mb...to southern Michigan.
The latest European model (ecmwf) actually keep positive temperatures while the GFS drops to
almost -2c. Regardless...moisture will be lacking greatly Post
frontal thus it appears any attempt at a ptype changeover would be
brief and would not have a chance to accumulate.
Brisk northerly winds behind the front and dry area of high pressure
building in south/central Canada will allow temperatures to fall rapidly
tonight. Should fall to around the freezing mark but will likely set
up a good gradient across Southeast Michigan as clouds and shower linger across
the south with clearing skies moving in the Saginaw Valley. NE flow
off Lake Huron should help mitigate some of the cooler temperatures but
Midland/Bay and Saginaw will probably see some temperatures in the upper
20s by morning.
Elevated portions of the frontal zone will be near the Ohio border
to start Saturday and will continue to move away from the area
during the morning. Expect this will just contribute to the overall
cloud pattern associated with deepening northeast flow over lower
Michigan. The lingering frontal clouds will be confined to the
Detroit metropolitan area southward while stratocu from Lake Huron will
affect the remainder of Southeast Michigan. The incoming air mass is shown
in model data to be very dry in the low levels where 850 mb relative humidity is
below 10 percent through the day. Boundary layer mixing during the
afternoon will then lead to dissipation of the stratocu leaving
sunshine filtered only by some high clouds. The sunshine might be
Worth an upward nudge of a degree or two toward the warmer end of
maximum temperature guidance...but that still means upper 30s to lower 40s
across the area which are really just slightly below normal for the
end of November.
The rest of the weekend will be dry with minimal cloud cover and not
much temperature trend. Good radiating conditions will result in a
cold start to Sunday morning. No problem with min temperatures ranging from
the teens rural to middle 20s metropolitan areas with a few more locations
making the lower 40s Sunday afternoon.
A blocking pattern currently in the upper level flow over the
Pacific coast will continue to evolve/mature during the weekend.
This will allow the closed low over California and the central
rockies to move slowly into the plains by Monday and push the short
wave ridge east of the Great Lakes. Slower model solutions are
preferred in this scenario which is worked into the forecast by
pushing an entry level chance for rain in Southeast Michigan into Monday
afternoon. An arm of middle level Theta-E advection will then organize
ahead of the low and spread greater coverage of showers into the
region Monday night into Tuesday. No concerns with precipitation
type until possibly Tuesday night as the upper low moves overhead
and helps lower 1000-850 mb thickness below 1300 dm. The chance of
rain/snow mix in our going forecast looks good based on GFS forecast
soundings which indicate a mildish boundary layer temperature
profile and freezing level holding slightly above 1000 feet.
As with the slower arrival of the upper low...a slower departure is
also a preferred model solution. The European model (ecmwf) looks best in that
department as it takes most of Wednesday to move the system out of
lower Michigan. Cooler high pressure then takes over on the large
scale for the end of next week.
Small craft advisories remain in effect today and tonight for outer
Saginaw Bay and the nearshore waters of Southern Lake Huron. This
will be for increased northerly wind behind a cold front that will
bring a large pattern of rain showers to the Great Lakes. Post
frontal wind gusts to 25 kts will help waves easily build above the
4 feet sig wave threshold through Friday night. Light air will build
in with high pressure on Saturday with light to moderate winds
persisting through Monday. East wind will ramp up in advance of the
next low pressure system Monday night into Tuesday.
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).