Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
112 PM EDT Tuesday Oct 13 2015
Drier air will mix into the lower levels early this afternoon as
winds increase slightly in speed and moisture associated with low
pressure pulls away from the area. This will allow ceilings to
slowly rise several hundred feet...but still remain bkn-ovc.
Prevailing ceilings will most likely remain MVFR...but there is
small potential to see heights rise to low-end VFR near the Detroit
area terminals. Winds will turn more northeasterly and decrease in
speed overnight. This will allow additional dry air to infiltrate
into the area and we could see some breaks in the stratus after 06z
as flow decreases across Lake Michigan. Winds will remain gusty
through early evening from the west-northwest before decreasing in
speed around 00z.
//Dtw threshold probabilities...
* medium confidence for ceilings below 5000 feet through 06z.
Previous discussion...issued 353 am EDT Tuesday Oct 13 2015
Short term...today and tonight
Cold and well-mixed west-northwest flow has arrived as promised early this
morning. Strong system relative isentropic descent along the 290k
surface has provided a pop of wind as high as 30 kts with the
initial arrival of the cold air but generally expecting a more
uniform 20 knots through sunrise. Steep lapse rates will further
steepen with the onset of heating this morning, yielding high
confidence in persistence of stratus and a modest increase in wind
gusts to 25 kts as the boundary layer deepens. Guidance continues to
offer a wide range of highs from the low 60s to close to 50. Given
that Southeast Michigan will remain socked in with thick stratus
through the daylight hours, felt it was a safe bet to adjust the
forecast toward the colder guidance. The bias-corrected 03z sref
offers a picture that very closely matches forecaster expectations.
Forecast highs of 52-54 along the glacial ridge with 55 to 56 in
favored downslope areas of Saginaw Valley and Wayne/Macomb counties.
In addition, a few brief light showers during peak heating will
possible especially within favored lake effect corridors.
The primary uncertainty heading into tonight will be the degree of
stratus erosion that results from the encroaching surface
anticyclone to the west. Boundary layer winds will veer to a
slightly drier northwest fetch as the high approaches and this
appears to be enough to trigger the guidance to erode simulated
cloud. Even so, forecast soundings remain quite moist and the
persistence of mixed conditions suggests continued potential for
some degree of stratus maintenance right into Wednesday morning. The
combination of sensitivity to boundary layer conditions and high
uncertainty in the difficult-to-forecast realm of stratus erosion
suggests the use of higher resolution ensemble information may be in
order. Leaned toward a blend of the high resolution guidance with a
significant influence from the 03z sref. This forecast allows for
partial clearing across the west and southwest where ridging will
build in most quickly. Correspondingly lowered temperatures within
this corridor into the lower 40s while maintaining middle to upper 40s
elsewhere. Still steered well clear of allowing lows to reach their
full potential, especially given skepticism that clouds will erode
much at all.
Long term...Wednesday through Monday
The feature of interest to begin the long term period will be the
extensive polar jet core digging through the Ohio River valley and
into the middle Atlantic. Models continue to advertise very benign
weather for Wednesday. A curious jet configuratoin with a melding of
twin jet axes over the northern plains and the far western Great
Lakes. It is tough to really pin down the upper level jet
dynamics...but models are overwhelming in the amount of deep column
dry air that will sweep through the entire region. This lends high
confidence for dry weather. A persistent trough axis will extend
back towards Lake Huron and Michigan from northeastern Canada. Have
to believe that a significant amount of moisture will remain trapped
under the stable midlevels. NAM 850-925 mb forecasts suggest that a good
amount of saturation will persist and the forecast will call for
stratus and stratocumulus to prove very stubborn. Maintained the
cloudy mention through the afternoon...before clouds dissipate with
the loss of daytime heating.
Thursday is an interesting forecast period as planetary lobe/deep
trough structure will swing across far Northern Lake Huron. This
deep midlevel trough will not only support a signifant amount of
shortwave energy that will crash directly through southeastern
Michigan...but a jet streak on the Anchor Point of the upper level
jet axis will support an organized frontogenetical response.
Confidence is high in the forcing...its just a matter of timing
which is important given how transient the jet streak will be and
the sensitivity to the tilting of the trough axis. There are some
indications that most of the widespread precipitation could develop
immediately to the south and east of the County Warning Area. The other important
narrative for the Thursday period will be the potential for a non
convective wind episode Thursday afternoon. There is alot going for
it...downward momentum transport in the wake of a cold front
Thursday afternoon...and high end thermodynamic profile supportive
of deep mixing. In fact...models soundings now show that mixing
heights could reach upwards of 6000 feet above ground level. Need some time here to
dial this in. The Theta-E ridge flashing across the area and warm
air advection will support highs in the 60s.
A Gale Warning remains in effect for Northern Lake Huron and small
craft advisories remain in effect for the nearshore waters of Lake
Huron...Lake St Clair...and the Michigan waters of Lake Erie. Deep
cold air advection today will continue to support gusty west to
northwest winds area wide. These winds are expected to get a boost
later today due to daytime heating. A modest westerly wind field is
forecasted for Wednesday as a weak surface ridge bubble works across
the region. The next cold front is expected to push rapidly through
the central Great Lakes Thursday afternoon. It appears that
conditions could be aligning to support a non-convective wind event
that will likely require marine headlines for all areas.
Lake Huron...Small Craft Advisory until 11 PM EDT this evening for lhz422.
Small Craft Advisory until 6 am EDT Wednesday for lhz421-441>443.
Gale Warning until 11 PM EDT this evening for lhz361-362.
Lake St Clair...Small Craft Advisory until 11 PM EDT this evening for lcz460.
Michigan waters of Lake Erie...Small Craft Advisory until 11 PM EDT this evening for lez444.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).