Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1159 PM EST Wednesday Dec 17 2014
Extensive stratus deck will hold firm through the taf period given
sufficient moisture trapped beneath a strong inversion. Recent
observational trends support a renewed period of MVFR ceiling.
These trends do suggest that ceiling heights may fall below 2000 feet
at times through the morning period. Little variability in this
condition is expected well into Thursday per recent model sounding
data. Modest west to west-northwest winds through the period.
For dtw...upstream trends indicate conditions will return to MVFR
during the early morning period. MVFR stratus will hold through
Thursday. There is the potential for a period of ceilings below 2000 feet
//Dtw threshold threats...
* high confidence in ceiling below 5000 feet through the taf period.
Previous discussion...issued 326 PM EST Wednesday Dec 17 2014
Short term...through tonight
Strong upper wave embedded within the longwave trough over the Great
Lakes and New England will move across lower Michigan this afternoon
and early evening. This will drag an area of middle-level deformation
down across northern and central Michigan as it tracks east through
the area...and also pull a surface trough now extending back from
the main low center over southern Ontario/Quebec down into central
Currently...widespread snow showers are developing over northern
lower Michigan within the area of deformation...deeper moisture to
saturate the dendritic growth zone...and ahead of the trough axis.
Activity is also getting a boost from moisture flux off Lake
Michigan. Area of better forcing and deeper moisture looks to just
clip the Saginaw Valley and thumb from late this afternoon through
about 9 PM...with lift weakening and steeper lapse rates from the
upper cold pool shifting east with the upper low and surface trough
tonight. This should prevent activity from seeping further
south...and keep the remainder of the night fairly quiet outside of
some potential for light lake effect bands to brush The Thumb. Snow
amounts over the Saginaw Valley and northern thumb should be fairly
light given only 1 g/kg of specific humidity and weakening
forcing...with amounts expected to top out around 0.5 inches.
Further south this Evening...Lake effect processes from off Lake
Michigan will continue to bring the chance for flurries or even a
light snow shower this evening before drier air arrives and the exit
of the upper cold pool allows lapse rates to become more stable.
Best potential will be before 8 PM...as upper cold pool briefly
steepens lapse rates.
Clouds are expected to remain locked into place tonight under an
inversion. This should prevent temperatures from falling too far
tonight...with mins in the middle 20s expected.
Long term...Thursday through Tuesday
The end of the week will remain relatively quiet as high pressure
builds in over the region behind the exiting surface trough.
Northerly flow will slacken as the gradient relaxes and drier air
filters in from the north. The one issue early on in the extended
is the chance for some lingering snow showers over The Thumb area
with the northerly flow...especially Thursday morning. Latest model
runs showed weakening of the gradient and with drier air moving in
have decreased probability of precipitation while leaving in a mention of flurries through
Thursday night. Continued cloudy conditions will be seen through the
day on Thursday before skies are expected to slightly improve
Friday. However...given this time of year would not be surprised if
low clouds remained which has been the case lately.
All signs still point to a quiet weekend as ascent from
deamplifying wave is still shown by nwp solutions to pass well to
our south. Continued dry and a bit warmer Monday ahead of a clipper
type system organizing over the northern plains. Remainder of the
forecast through Christmas evening will hinge upon very strong Pacific northwest
jet energy carving out a deep longwave trough over the central
Continental U.S.. indications are that the clipper will slow in response to
this jet energy and eventually be absorbed into a stronger surface
low lifting into the Great Lakes from the deep south. 12z models
indicate the cyclone becoming quite deep by the time it reaches our
area. Details in terms of precipitation type will depend on the low
track...with low confidence at this point...but current indications
are that impactful weather will become more likely toward Christmas
Gusty winds on Wednesday will eventually slacken as a low pressure
system and associated surface trough push to the east overnight.
Snow showers are also possible through the evening as the trough
swings through. As westerly/northwesterly flow is maintained through
the overnight hours...the Small Craft Advisory for outer Saginaw Bay
along with Lake Huron from Port Austin to Harbor Beach will be
continued. High pressure then builds in on Thursday which will
bring lighter northerly winds to the marine areas.
Lake Huron...Small Craft Advisory until 4 am EST Thursday for lhz421-441.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).