Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
712 am EDT Tuesday Oct 21 2014
A cold/moist northerly flow will sustain an extensive MVFR stratus
canopy across Southeast Michigan today. This pattern may support a
few light showers/areas of drizzle...particularly this morning.
Northern winds will see a period of gustiness as daytime mixing
peaks this afternoon...gust potential in the 20 knot range. There
remains enough evidence in recent model guidance to maintain a
more pessimistic outlook in the clearing potential tonight.
Ceiling heights in the 2500 to 3500ft range will be favored during
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet through this evening.
Previous discussion...issued 351 am EDT Tuesday Oct 21 2014
Short term...today and tonight
549 dm closed middle level circulation currently exiting Southern Lake
Huron will effectively reinforce an already high amplitude middle level
trough now compassing much of the eastern Continental U.S.. trailing sheared
out short wave energy pivoting through the periphery of this system
will maintain a higher degree of DCVA locally today...particularly
early on. This agitated flow will combine with an easing cold air
advective component in deep northerly flow to sustain an extensive
canopy of stratus today. Upstream observations compared to recent
hi res model guidance continue to suggest the potential for some
pockets of light showers to emerge...positioning of both the better
ascent and a slight additive component from Lake Huron leaving The
Thumb corridor with the greatest potential. 850 mb temperatures
remain forecast to bottom out at roughly -2c by this afternoon.
Given the expected cloud cover...this points to seeing very little
recovery in temperatures for today. Highs upper 40s to lower 50s.
Both the degree and location of clearing remains the biggest
forecast challenge for tonight. 925 mb thermal troughing will be
slow to exit given the expansive nature of the background
reinforcing northerly gradient working through the departing Lower
Middle level height field. Latest sounding analysis and low level relative humidity
fields suggest that drier air will attempt to make some inroads late
this evening from the northwest/west...which may promote some cloud
breaks /favoring the Tri-Cities and Highway 23 corridor/. However
the low level flow will tend to veer slightly with time...a favored
trajectory off Lake Huron in support of the maintenance and/or
renewed development of stratus overnight. This setup favors keeping
with a more pessimistic outlook on clearing until near term trends
suggest otherwise. This in turn will cap the nocturnal cooling
potential. Lows will range from middle 30 in the Tri-Cities to lower
40s in portions of metropolitan Detroit.
Long term...Wednesday through Monday
Upper low now spinning over the eastern Great Lakes will push off to
the East Coast by Wednesday morning. This will leave Southeast
Michigan in cool northerly flow off Lake Huron...with just enough
moisture expected under the inversion early to keep some stratus
around. Models are then fairly aggressive with drier air moving into
the area during the afternoon as ridging at the surface and aloft
Cascades down into the central Great Lakes. Model forecast soundings
support clearing skies through the afternoon as we mix into this
drier air aloft. The high should remain over the area into
Thursday...keeping dry and quiet weather over Southeast Michigan.
Temperatures will run on the cool side of normal as h850 temperatures are
slow to moderate...only back up to 7-8c by Thursday afternoon. This
should hold highs down in the low/middle 50s. Clear skies and high
pressure should allow for a good radiational cooling set-up
Wednesday night...with min temperatures falling into the low 30s for
locations outside of the Detroit heat island.
Next item to watch will be the upper wave now along the West Coast.
This feature is forecast by medium-range models to track across the
western and central Great Lakes late Thursday night through
Friday...potentially scooping up additional energy from the shearing
wave lifting out of Chihuahua. At this time...GFS/Euro do not show
much moisture surging up into the area ahead of this feature. Given
the preceding dry airmass and only weak isentropic ascent...will
keep the forecast dry but reflect an increase in high and middle level
clouds. Another cold front will push across Michigan with this
system...with current timing prognosticated for late Friday night into
Upper ridging then looks to slide over the area again for the
weekend...bringing quiet weather.
Winds will increase from the northeast today behind the passage of a
cold front. Wind gusts are expected to range between 25 and 30 knots
today over Lake Huron. The strong northerly winds will also allow
wave heights to build as high as 8 feet over the southern half of
the lake. Winds and large waves will gradually decrease overnight as
low pressure over the eastern Great Lakes tracks east and a ridge of
high pressure builds into the western Great Lakes. This high will
remain overhead during the middle-week period...allowing for calmer
Further to the south...slightly lower wind speeds are expected over
Lake St Clair and far Western Lake Erie today...with gusts expected
to top out below 25 knots.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 6 am Wednesday.
Small Craft Advisory...nearshore waters from Harbor Beach to Port
Huron...until 6 am Wednesday.
Small Craft Advisory...inner Saginaw Bay...until 10 PM Tuesday.
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).