Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
358 am EDT Tuesday Sep 2 2014
Short term... today and tonight
A weak cold front will move through Southeast Michigan during the morning
and help focus scattered to numerous showers...with a rumble of
thunder...over about the south 1/2 of the area. The early morning
observation package and model analysis fields indicate a few things
at work in support of the rain pattern. The weak frontal zone will
serve as the focusing mechanism for lift generated by a sheared wave
within the entrance region of a 110 knots upper jet axis. Both of these
features will force a background of larger scale lift in the
southwest flow aloft over the Great Lakes. Model depictions of
frontogenesis are not surprisingly on the noisy side given the
overall lack of baroclinicity but the middle level Theta-E gradient
does respond with some contraction around the 700 mb level under the
larger scale forcing of the wave. Combine that with some late night
nocturnal destabilization and the result will be an expansion of
showers along and ahead of the surface front as it chugs through Southeast
Michigan. Model soundings also indicate just enough elevated
instability for a rumble of thunder before sunrise...and parcels
becoming surface based during the morning for a better chance...but
there is no concern for anything more than a brief heavy downpour.
A stray shower is possible immediately Post front during the early
afternoon but boundary layer moisture will be tailing off during the
rest of the day. Surface to 925 mb dry air advection is on the weak
side but upstream surface observations indicate some middle to upper
50s dewpoint air available to filter into lower Michigan by middle
afternoon. Increasingly neutral westerly flow aloft and the low
level dry air advection will Foster a decreasing cloud trend through
the evening before surface high pressure builds during the overnight
period. Model soundings indicate only a shallow amount of boundary
layer moisture available that looks more like a heavy dew than fog
as min temperatures settle in the middle 50s to lower 60s by sunrise Wednesday.
Strengthening upper level ridge (592-595 dam at 500 mb) over the
southeastern United States Wednesday/Thursday...with southeastern
Michigan lying on the outer fringe of this ridge...as 500 mb heights
rise to around 590 dam near the Ohio border by Thursday morning.
Meanwhile...strong 500 mb upper wave/low moving through the Pacific
northwest tonight will track northeast into Ontario Thursday night.
This will support a warm up as elevated warm air descends off The
Rockies and streams northeast toward the southern Great Lakes
region. 700 mb temperatures look to be warming into the lower teens on
Thursday...which will help cap activity. Assuming the warm front
cleanly lifts north of southern Lowe Michigan during the day...the
potential is there to make a run toward 90 degrees as 850 mb temperatures
of +20 c are advertised...per 00z Euro. Timing differences
(euro/GFS/canadian) on the frontal passage on Friday...and could see
a 10 degree north to south temperature gradient...but overall highs in the
80s should suffice. The surface cold front looks to be on the
southern fringe of the strong upper level westerlies...but
instability will be moderate to high...as lifted indice's prognosticated to be between
-5 to -10 c. Cooling middle levels (700 mb) eroding the cap...but middle
level lapse rates (700-500 mb) will also be diminishing.
Broad upper level trough over the weekend will offer up a
significant cool down...as 850 mb advertised to drop down in the middle
single numbers...which may just hold maxes at or below 70 degrees.
A few more showers and thunderstorms around this morning over Lake
Erie and Lake St Clair before high pressure takes control and builds
into the Great Lakes region on Wednesday. This high will provide
favorable boating conditions with light winds and warm temperatures
into Thursday. A cold front moving through the region on Friday will
bring another round of showers and thunderstorms...with much cooler
air to follow for the weekend.
Aviation...issued 1158 PM EDT Monday Sep 1 2014
Widely scattered showers will funnel into area overnight and
eventually focus over the southern County Warning Area late tonight into early
Tuesday as a secondary shortwave trough shifts through the area from
the southern stream. This activity will brush the I 94 terminals with
scattered...but somewhat persistent...activity around daybreak. To
the north...isolated rain showers will remain hit/miss into parts of the
overnight hours. Some patchy fog/stratus with IFR/MVFR conditions
will also be possible as low level moisture remains elevated. That
said...a general increase in middle/upper cloudiness with approaching
wave will limit this process to some extent. By middle/late morning
Tuesday...expect VFR conditions as drier air works into the area.
//Dtw threshold threats...
* low confidence that a thunderstorm will impact the terminal late
tonight into Tuesday morning.
* Low confidence in ceilings below 5000 feet late tonight into early
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).