Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1149 PM EST Wednesday Dec 11 2013
The low level flow over Lake Michigan will continue to back to the west
this morning as high pressure slides into the Ohio Valley. This is
allowing lake strato cumulus and a few flurries to funnel back toward
the terminals. The dry air over Southeast Michigan has been eroding the leading
edge a bit. Given observational trends...these clouds should
overspread the terminals between 06z and 08z. Additional moisture
will move in from the west after 12z...which should allow more
widespread strato cumulus to infiltrate Southeast Michigan...with some
flurries/light snow possible at fnt and mbs as the flow backs to
For dtw...the clouds have been struggling to push toward metropolitan with
given the dry air. Upstream observations still support some periodic
broken ceilings this morning.
//Dtw threshold threats...
* medium confidence in ceilings below 5000 feet this morning and
Previous discussion...issued 951 PM EST Wednesday Dec 11 2013
There has been a long enough period of clear skies to all some
decoupling...causing winds to fall below 10 miles per hour. So while this
should inhibit the need for a Wind Chill Advisory...the light
winds will still create wind chills of 5 to 10 below through the
The 00z DTX sounding showed a shallow inversion near 3500 feet above ground level.
Ongoing middle level subsidence tonight will deepen this inversion.
This should support some advancement of the strato cumulus field now over
western lower Michigan into Southeast Michigan overnight as the low level flow continues
to back to the west. So the current forecast of increasing clouds
looks reasonable. It will be rather easy to generate some flurries
out of the strato cumulus as well. So in short...the only update to the
forecast will be to add some flurries overnight.
Previous discussion...issued 351 PM EST Wednesday Dec 11 2013
Short term... afternoon through tonight
A fast-moving wave of low pressure will usher an Arctic cold front
the rest of the way through Southeast Michigan during the late afternoon.
Light system snow will continue to fade as the middle level wave shears
eastward which will leave a narrow band of snow showers left to move
through with the surface front. This band perked up nicely between
the I-69 and M-59 corridors during the early afternoon as the
boundary became a hybrid lake effect convergent band and shallow
frontal circulation. The key to the response was the lake moisture
injected into an already unstable layer below 700 mb within the
frontal convergence zone. This allowed the vertical motion field
within the otherwise shallow frontal circulation to produce a burst
of healthy dendrites and a quick accumulation up to an inch. The
band will remain similarly or slightly less capable as it moves the
rest of the way through the region by 5 PM.
Cold temperatures and wind chill readings will be the main concern
for the tonight period. The passage of the front will open the door
on an Arctic air mass that has confined afternoon temperatures to
the single digits below zero over northern Wisconsin. The lake will
act as a buffer on this air mass thanks to westerly wind but not
enough to prevent min temperatures from reaching the lower single digits in
our area. Decreasing clouds during the evening and light but fresh
snow cover will also contribute to falling temperatures during the
night and will lead to some decoupling of the surface wind. The
expectation is then for a 10 to 15 miles per hour westerly wind to actually
help hold temperatures in the single digits but remain light enough
for wind chill near but warmer than the -15f advisory threshold
through sunrise Thursday.
Thermal trough axis (850 mb temperatures around -20 c/-26 c at 700 mb) over
the central Great Lakes to start the day will pull eastward as
surface high over the Midwest tracks along/south of the Ohio River
during the day. The airmass still will likely be sufficiently cold
to generate scattered light snow showers across north half of the
County Warning Area as southwest warm advection sets in over Lake Michigan with
saturation and low level convergence noted 925-850 mb. There is even
solid 700 mb fgen/saturation noted which could allow for the
activity to be even more widespread. No complaints with inherited
maxes...upper teens which is right in line with latest guidance.
We will be stuck in baroclinic zone Thursday night into
Friday...with back door cold front sliding down from the northern
Great Lakes...resulting in low level wind shift during Friday. With
low level convergence and moisture flux coming off Lake Huron...snow
showers are possible...but wind shear from 925-850 mb resulting in
shallow cloud depths will likely prevent activity from being as
robust to today's Arctic front.
The focus for Friday night into Saturday centers around three
distant shortwaves/upper waves...which look to be interacting with
each other...but significant phasing between the waves does not
appear likely...as 12z Euro indicates more of an inverted surface
trough extending northeast. Still...the lead upper wave just off
Baja California California will deliver enough deformation/low level fgen
forcing/moisture advection to support snow Friday night into
Saturday...highest amounts toward the Ohio border...where 2.5+ g/kg
of specific humidity is noted at 850 mb. The other factor to
consider is the long fetched easterly flow at 925 mb off Lake Erie
contributing to the fgen/snowfall totals...and could see 3-6 inch
snowfall for locations along/south of Eight Mile...with 1 to 3
inches elsewhere. The 12z Euro and GFS look to be in pretty good
agreement from upper air/dynamic tropopause perspective. This is a
welcome trend...as the 00z Euro was outlier with the second upper
wave hanging back across far southern Arizona/northern Mexico
Saturday morning...causing the center of the lead Baja California upper wave to
track through Kentucky...which would be too far south for
appreciable synoptic snow. Obviously...we got a tight baroclinic
zone/gradient and initially strong upper level confluent
flow...coupled with the systems still out in the
Pacific..confidence is average at best.
Have included a chance of snow showers for the area for Saturday
night through Sunday to account for lingering showers from the
departing low pressure system. High pressure returns on Sunday to
the southern County Warning Area while a clipper system tracks across the northern
Great Lakes. This clipper could bring another chance for some snow
to the Saginaw Valley and thumb regions. Otherwise...very cold
Arctic air filters in for Sunday night with lows in the single
digits and wind chills dropping to below zero with some locations
seeing wind chills down to -10 degrees. The cold air will stick
around through Monday before slightly warmer air returns /highs in
the middle 20s and lows in the upper teens/ through middle week. A series
of disturbances will impact the region through middle week...each
bringing a chance of snow to the area.
Will allow the rest of the gale warnings to expire...as westerly
gradient has eased just enough to keep gusts topping out around 30
knots. High pressure over the Midwest (1041 mb) will slide southeast
and weaken slightly...but another surface cold front dropping south
through the northern Great Lakes will maintain westerly wind gusts
around 30 knots for much the open waters of Lake Huron tomorrow. The
front will slowly slide south Thursday night into Friday...allowing
winds to turn to the northeast. Northeast winds of 15 to 25 knots
are expected across all of Lake Huron by Friday night...with
funneling into Saginaw Bay leading to large waves.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 7 PM Thursday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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