Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
552 am EST Thursday Dec 5 2013
With cold frontal passage ongoing....expect gusty west/southwest winds which
will maximize (near 25 knots) during best mixing middle morning into
the afternoon. Remaining MVFR ceilings will be replaced with VFR ceilings
with time...with broken lower VFR ceilings the rule into tonight within
unsettle cold westerly flow around departing low pressure. Pressure
gradient with relax by this evening...so wind speeds will come down
to 10 knots or less.
//Dtw threshold threats...
* medium confidence in ceilings at or below 5000 feet this morning.
Previous discussion...issued 351 am EST Thursday Dec 5 2013
Short term...today and tonight
A strong cold front will progress through the area around daybreak.
Temperatures will start the in the middle/upper 30s (saginaw valley) to
the middle/upper 40s (detroit south) and fall throughout the day as cold air advection
ensues in the wake of this frontal boundary. Any rain showers early
this morning will end as drier westerly flow develops. Winds will be
rather gusty at times (with gusts peaking at 25-30 mph).
Cold air will continue to deepen over the area tonight with minimum
temperatures falling into the upper teens north to middle 20s closer to
the Ohio state line. An area of light snow will spread into the Ohio
Valley during this time frame and may brush the far southern County Warning Area. If
nothing else...cloud cover associated with the responsible shortwave
will keep temperatures slightly milder tonight than they otherwise
High pressure will progressively build into the area for Friday and
Saturday as confluent upper flow transitions to more aggressive
height rises on Saturday in response to the deepening western U.S.
Trough. Colder air will ooze into Southeast Michigan during this time with
850mb prognosticated to be around -9c at 12z Friday and -14c by Saturday
morning. Cold advection and diurnal heating will aid boundary layer
growth up to around 900mb on both days supporting forecast highs in
the upper 20s/low 30s Friday & low 20s Saturday. The trickier aspect
of the forecast will be the extent of boundary layer cloud cover.
Planviews of relative humidity at 900mb reveal a likely and expected pattern of
potential cloud cover given cold westerly flow across the lakes.
However, forecast soundings from the NAM suggest a rather thin layer
of saturation which supports the more optimistic approach to the sky
forecast noted in the previous discussion. Depth of the cloud layer
will be Worth watching, however, as the column will be cold enough
to support dendrite formation at the top of the boundary layer by
Friday night. Given aforementioned issues, will leave mention of
flurries out at this time.
The only other issue of note is the light snow that will be ongoing
to our south on Friday as a shortwave lifts across the Great Lakes and
produces a weak baroclinic response along the front to our east.
Cross-sections across the baroclinic zone and upper jet suggest that
the nature of the response will be dominated by a frontal
circulation rooted in the right entrance of the strong jet streak
positioned overhead. The dominance of the frontal response during
this event is the likely culprit in the more broad quantitative precipitation forecast depicted by
the global models compared to the much sharper cutoffs shown by the
NAM/arw/nmm. These higher resolution depictions are therefore the
preferred guidance and have trimmed probability of precipitation accordingly.
Gale warnings remain in effect for the northern third of Lake Huron
as a strong westerly breeze is still forecast to gust to gale force
beginning with the passage of a cold front this morning. In
addition...a more southwesterly component to the wind across the
central axis of Lake Huron will result in a more favorable fetch and
therefore a southward expansion of the Gale Warning to include
Central Lake Huron. Outer Saginaw Bay and the nearshore waters of
The Thumb will experience a prolonged period of near gales...and
while they may still experience an occasional gale force
gust...gales are expected to be neither long lived nor particularly
widespread in these zones. Winds will remain below 30 knots for all
locations south of Harbor Beach. Westerly flow will continue gusting
to near 30 knots across the northern half of Lake Huron into the
weekend with a marginal gale possible again on Saturday as strong
high pressure builds into the region.
Gale Warning...lhz361-lhz362...until 7 am Friday.
Gale Warning...lhz363...from 10 am Thursday to 10 PM Thursday.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 5 am Friday.
Small Craft Advisory...nearshore waters from Harbor Beach to Port
Huron...until 7 PM Thursday.
Small Craft Advisory...inner Saginaw Bay...until 10 PM Thursday.
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).