Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
701 PM EDT sun Sep 21 2014
Post frontal cold air advection should sustain gusty north-northwest winds
through the rest of the evening /gusts over 20 knots at times/.
Gradual cooling in the boundary layer late tonight into Monday morning
will lead to a slow drop in the winds as the night wears on.
Residual low level moisture within a deepening inversion will
sustain MVFR stratus through the night. The moisture depth will
become much more shallow during the morning as very dry air advects
atop the inversion. This may allow some breaks in the
overcast...especially considering the slight downslope from the northwest
flow. A clearing trend is expected by early Monday afternoon as the dry
air mixes into the cloud layer.
For dtw...residual light rain should end roughly between 01 and 02z.
//Dtw threshold threats...
* high confidence in ceilings below 5kft tonight...medium confidence
Previous discussion...issued 336 PM EDT sun Sep 21 2014
Upper level low/circulation pivoting through the central Great Lakes
this afternoon...supporting plenty of showers (categorical) working
their way through Southeast Michigan. Surface cold front now
starting to surge southward through Southern Lower Michigan...and
should mostly be out of the County Warning Area around 00z...with just a low/slight
chance of some leftover light showers across the far eastern
section of the County Warning Area for a couple hours Post 00z. The added
convergence along the front is enhancing the showers
presently...with even a few lightening strikes noted earlier
toward Saginaw Bay. With only about 500 j/kg of surface based cape
to work with...expecting thunder activity to remain isolated. The
12z NAM is Adamant low level moisture will remain trapped
underneath the developing subsidence inversion behind the upper
level trough axis overnight...with clouds locked in the 2000-3000
foot level. The 12z GFS/Euro are drier...but still enough evidence
in the 925-850 mb layer to go with more pessimistic sky forecast
tonight...as clouds are still locked in north of the Great Lakes
in Ontario. Would refer more northerly flow (vs northwest) off
Lake Huron as 850 mb temperatures drop to zero or slightly colder to be
100 percent confident in the clouds...but none-the-less...have
average to above confidence in mostly cloudy skies prevailing
through the night...which should keep mins on the plus side of 40
degrees...about 20 degrees colder compared to this mornings lows.
Long term...Monday through Sunday
Sprawling high pressure system will build into the area Monday and
bring clearing skies. The Canadian origin of this high will result
in cool condtions with highs in the lower 60s for the most part. By
Monday night...return southwest flow is already setting up around
this high as the center becomes established over the Ohio Valley.
This will hold low temperatures close to seasonal averages in the
middle 40s. Temperatures will continue to moderate into Tuesday as
high pressure moves east and combines with another high pressure
center dropping southeast through Quebec. Temperatures are expected
to climb to around 70 on Tuesday under mostly sunny skies and fall
into the upper 40s/lower 50s Tuesday night as the quiet conditions
prevail from the strong high pressure that evolves over upstate New
York and the New England states.
For the extended...sprawling high pressure will extend from the
western Atlantic Ocean to the Ohio Valley and Great Lakes region. Southeast
Michigan will remain under the influence of this high for several days as
the upper level pattern becomes stagnant under an Omega blocking
high pattern. Temperatures will be on the increase through the week
with above normal values expected.
Scattered showers and isolated thunderstorms will end as low
pressure shifts quickly east overnight. Northerly flow in the wake
of this system will remain gusty into tonight...before slowly easing
late tonight into Monday. Even as the flow subsides...wave action
will be slower to fall so small craft advisories will remain in
effect over portions of the Lake Huron nearshore zones into Monday
afternoon. Favorable marine conditions then set up for the remainder
of the week as high pressure becomes established over much of the
northeastern United States.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 10 am Monday.
Small Craft Advisory...inner Saginaw Bay...until 8 am Monday.
Small Craft Advisory...nearshore waters from Harbor Beach to Port
Huron...until 4 PM Monday.
Lake St Clair...
Small Craft Advisory...until 8 PM Sunday.
Michigan waters of Lake Erie...
Small Craft Advisory...until 8 PM Sunday.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).