Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
650 am EDT Wednesday Sep 17 2014
High pressure will sustain a dry and stable environment in modest
westerly flow today. This will limit the coverage of VFR diurnal
cumulus development this afternoon. A clear sky and light wind
through the first half of tonight. Cold front will drop south
across Southeast Michigan late tonight. This will prompt the southward
expansion in stratocu from north to south by middle morning on
//Dtw threshold threats...
Previous discussion...issued 357 am EDT Wednesday Sep 17 2014
Short term... today and tonight
Surface high pressure tucked beneath low amplitude middle level
northwest flow will sustain a dry and stable environment throughout
Southeast Michigan today. This will yield plenty of sun with just a
limited amount of diurnal cumulus development during peak heating.
General moderation of the resident airmass under full sun will apply
roughly a 2 to 3 degree increase through the 850-925 mb layer. This
will support highs largely in the upper 60s.
A modest southward expansion in height falls will commence tonight
as an elongated shortwave shears across southwest Canada. While
both the main corridor of attendant forcing and deeper moisture
content remains forecast to hold well to the north...this process
will work a cold front southward and through the region tonight.
Clear sky to dominate the early portion of the night with the
frontal timing centered after 06z. Substantial low level moistening
with the onset of cold air advection and aided by the emerging
Post-frontal northeast flow off Lake Huron will then direct an
expanding stratus deck from north to south into the region through
the early-middle morning hours. Lows in the 40s tonight.
A stable anticyclone is forecasted to expand into southeastern
Michigan on Thursday as the center of the surface high slides
southeastward along the northern shores of Lake Superior and Lake
Huron. Model soundings are painting a picture of shallow cold air
advecting into County Warning Area for the morning hours capped by a very stable
profile in the 4 to 17.5 kft layer. Solutions show the backdoor near
surface front coasting into the Michigan landmass prior to the start
of the period then becoming muddled or washed out during diurnal
heating processes. Regardless a northern flow trajectory is expected
to result over all of the area by the afternoon. Despite the shallow
cold air advection the only wind magnitude of note will be along the Lake Huron
shoreline and thumb where gusty winds in excess of 10 knots will
likely persist throughout the day. A shallow steep lapse rate pocket
at the top of the mixed layer along with moisture fluxing off of Lake
Huron is expected to result in overcast to mostly cloudy skies
during the morning across the northern half to 2/3rds of the
County Warning Area...scattering out during the late afternoon under an onslaught of
midlevel subsidence. Overall wind profile and lack of a pressure
gradient gives one pause in how much the shallow airmass will advect
into the far southern County Warning Area and lead to cloud cover. For this
reason...expecting more sunshine south and warmer daytime
temperatures as a result. A gradient in temperatures is
expected...readings ranging from the middle 50s in the northern
thumb to the middle 60s downriver and Lenawee County.
A subtle ridge axis rolling through the central part of North
America along with a very dry and stable midlevel column will
continue very quiet weather Thursday night. Favorable radiating
cooling conditions will develop Thursday night and make Friday
morning temperatures very interesting for those areas that
experience cool middle 50s highs from the day before. Pattern
recognition supports going quite cool for Thursday morning
temperatures up north where a widespread middle to upper 30s will be
The weekend period remains uncertain with intra and inter model
continuity lacking. The forecast for this weekend hinges on the
arrival time of a very Stout upper level jet core that is expected
to dig right into the upper Midwest. Precipitation chances will ramp
up ahead of this with the overall configuration of the jet leading
to a duration of outstanding warm air advection as well as deep
column moisture. The problem is the 17.00z GFS and European model (ecmwf) are in
disagreement with quantitative precipitation forecast by as much as 24 hours. The potential for a
shortwave ridging episode also suggests that a ton of future
precipitation timing refinements will be needed.
Calm winds will back to moderate southwesterly flow ahead of an
approaching front today. The passage of the front tonight will
result in moderate to fresh northeast flow forcing cooler air to
spread across area waters. With winds approaching 25 knots and
significant wave heights forecasted to be at/greater than 4
feet...issued small craft advisories for all nearshore areas of Lake
Huron including Saginaw Bay. The advisories will be in effect from
this evening into the late morning hours on Thursday. Winds will
ease Thursday night into Friday before southerly winds begin to ramp
up ahead of the next system late Friday.
Small Craft Advisory...nearshore waters from Port Austin to Port
Huron including Saginaw Bay...from 10 PM Wednesday to 10 am
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).