Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1209 PM EST Thursday Dec 12 2013
Northwest flow aloft and west flow at the surface will continue to
usher in strato-cumulus from Lake Michigan. A few passing snow showers
will be possible through 00z, but will seldom reduce visibilities to 5sm.
Several breaks in the clouds may allow brief periods of scattered coverage
today. A cold front enterring the area tonight will lead to lower ceilings
from ptk north, and a wind shift to the east daytime Friday. A broad
range of accumulating snow (1-5 inches) will likely occur Friday
For dtw...snow after this forecast period may accumulate to 3-5
inches. Look for updates to mention lower visibilities after 06z Sat.
//Dtw threshold threats...
* medium confidence in ceilings below 5000 feet through this
afternoon...low confidence tonight.
Previous discussion...issued 339 am EST Thursday Dec 12 2013
Short term...today and tonight
Next sheared middle level shortwave quickly diving across the upper MS
valley early this morning. Downstream low level wind field backing
in response...more depth to the westerly flow now promoting better
inland penetration of the lake moisture plume. Winds will
eventually settle to west-southwest in the wake of this wave...
as a defined convergence zone develops along the remnant lake
aggregate trough and a period of modest 850-925 mb warm air
advection emerges. This will continue to promote light snow
shower/flurry production within the expanding stratocu through the
day. Best potential for some minor accumulation will exist into
northern sections of the area...a more favored position for west-southwest
flow/convergence to direct a deeper lake plume.
Arctic air firmly entrenched across the region remains the main
story today and tonight. 950 mb temperatures solidly in the middle
negative teens will contain highs in the teens. Wind chills again
stuck in the single digits. The warm air advection will establish a
deep inversion layer heading into tonight. The persistent moisture
flux off Lake Michigan will ensure some degree of moisture remains
trapped below this inversion. Some open sky will be possible across
the south...a southwest trajectory favoring the intrusion of the
drier Arctic air residing across Indiana. The uncertainty in the
cloud coverage plays in targeting lows tonight...and will continue
to cap lows in the lower to middle teens.
Long term...Friday through Wednesday
Canadian high pressure will build eastward north of lakes Superior
and Lake Huron on Friday. Strong cold air advection on the southern
flank will support surface pressure rises which will push an Arctic
surface front down into central portions of Wisconsin/Michigan. This
is atypical due to the lake aggregate effect. The clincher will be a
localized ribbon of deep column subsidence which will be a
circumstance of the upper level polar jet configuration. This
subsidence will add a localized component of surface pressure rises that
will push the front southward as well. What will result is a well
developed baroclinic zone stalling out over the center of lower
Michigan...northern County Warning Area...for Friday night. Models have been hinting
at the potential for circulations to break out on this boundary in
addition to the background convergence this feature will Harbor.
Various models are supporting that precipitation will break out
along this frontal boundary before the southern moisture/winter
event pushes in from the south. Low level -div and deformation
appears supportive for early accumulating snow chances for the
northern thumb and Tri Cities . Snowfall accumulations of less than
an inch would not be surprising from late Friday afternoon through
6z Friday night. Have taken a very conservative approach with the
probability of precipitation as modeled quantitative precipitation forecast is very disorganized along this feature. In
addition...the area will be battling strong upper level confluence
during this timeframe.
The closed upper level low off the California coast near San Diego
will get kicked eastward today by an approaching upper level trough
in the Pacific. The upper level pv anomaly is forecasted to hold
together but dampen considerably by the time it reaches the Ohio
River valley...south of lower Michigan by midday Saturday.
Combination of left exit region dynamics with best warm air
advection/low level jet support will cause a large amount of
convection to break out across portions of Texas and the southern
Mississippi River valley on Friday. Latent heating...low to midlevel
pv...and strong warm air advection will cause the main kinematic
surge and main surface low feature to pass well south of the Great
Lakes...across Tennessee/Alabama/Carolinas on Saturday.
Meanwhile...a deep inverted surface trough feature and likely closed
secondary surface low will exist off the northern flank through
portions of Ohio. This distinct center will positioned by remnant
1000-500mb height falls off of the aforementioned potential
vorticity anomaly. The midlevel low associated with the pv anomaly
is expected to track across northern portions of Illinois late
Friday night and into lower Michigan on Saturday. It is the
combination of this inverted trough and midlevel low pressure system
that will bring the accumulating snow event to southeastern Michigan
to start the weekend. The models are in very good agreement for this
amount of lead time and brings increasing confidence to some details.
Initial low to midlevel Theta-E is forecasted to lift northward and
develop a compact gradient Friday night. From this Vantage
Point...the gradient is expected to lift into Northern
Ohio...eclipsing the Ohio/Michigan border...remaining south of Detroit.
North of this feature isentropic lift will exist and certainly
support light snow but will not have the benefit of the steeper
frontal surface. This is the likely reason for a rather sharp quantitative precipitation forecast
gradient and higher amounts over the southern two tiers of counties
prior to 12z Saturday. Confidence is high that blocking of the Theta
east advection will occur due to the strong surface ridging/anticyclone
that will have built southward on Friday. As Saturday
progresses...the midlevel low circulation will lift through Southern
Lower Michigan forcing higher magnitude and deeper isentropic ascent
over all of the County Warning Area. Snow intensity will increase during the
morning...and persist until northwesterly drier flow kicks in during
the afternoon. Forecaster confidence is high the midlevel low will
make it into lower Michigan for three reasons. 1...it will have some
remaining support from the parent pv anomaly. 2...it will become
increasingly supported by right entrance region dynamics of a jet
over portions of northern New York and Quebec. 3...there will be
some pulling northward by lake aggregate/thermal effects. Overall
feeling good about the larger signal of convergence provided by
inverted trough. The detriment of the system as a whole is that
overall moisture will deflect south which gives confidence in an
advisory event. We are on schedule to get some better sampling today
as the offshore California closed low washes ashore and a northern
stream trough structure hopefully passes over a more dense radiosonde observation
network. What has been confirmed repeatedly over the past 24 hours
is an inbound northern stream trough will remain well separated from
the passing southern stream energy...thus no phasing.
Given all the factors outlined above...expecting a widespread
advisory snow event for all of southeastern Michigan beginning
Saturday at midnight and persisting to approximately 00z Sunday.
These inverted trough type systems can turn into a longer duration
grinder. The cold system and the lack of true higher Theta-E content
in the low levels suggests higher snow ratios in the 12:1 to 13:1
range. Highest snowfall amounts...3 to 5 inches...are expected to
fall south of 8 mile. 2 to 4 inches will be possible between I 69
and M 59 as well as the eastern shoreline areas of The Thumb.
Easterly flow will bring a lake enhancement component there. For the
northwestern County Warning Area and the Tri Cities...1 to 3 inches is forecasted.
Wind is expected to be a non issue with this event.
Surface high pressure passing eastward through Canada will force a
cold front down through the northern Great Lakes tonight and into
portions of Wisconsin and Michigan on Friday. Westerly winds will
remain fresh today ahead of the frontal boundary...at 30 knots for
all areas. An abrupt wind shift to the northeast is anticipated
behind a backdoor front on Friday. 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.
heavy freezing spray warning...lhz361-lhz362...from 5 PM Thursday
to 3 am Friday.
Gale Warning...lhz361-lhz362...from 5 PM Thursday to 3 am Friday.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 3 am Friday.
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).