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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
323 am EDT Wednesday Jul 30 2014

Short term... today and tonight

Patchy fog will be the first concern in the forecast as the rain
yesterday and last night left wet ground followed by light wind and
a decrease in clouds during the early morning. The greatest
potential for locally dense fog will be in the Tri Cities down to
Flint where radiational cooling conditions will be better and where
most of the rain fell. This will be monitored for trends in duration
and areal coverage for headline potential before sunrise.

The focus of the forecast for the rest of the day will be on the
short wave energy rounding the upper low over the south end of Lake
Michigan. Coverage of showers supported by this feature has
decreased considerably since midnight with the loss of diurnal
instability. However...the wave remains superimposed over the weak
northwest-southeast temperature and moisture gradient being
maintained by the long wave pattern. Low to middle level Theta-E
advection ahead of the short wave track has a chance to produce
showers throughout the morning as weak moisture transport generates
some elevated instability. Daytime heating will then add surface
based instability to the mix with some enhanced convergence within
the surface pressure reflection of the wave. Thunderstorm potential
will be similar to yesterday but with slightly higher surface
dewpoint and perhaps a degree or two increase in maximum temperature. A 75/53
parcel then modifies surface based cape to about 1300 j/kg in NAM
soundings. All that being said...expect chance probability of precipitation will cover this
scenario which will affect the area generally south of the M59
corridor but limited by an early eastward exit of the wave. The 00z
model package has a reliable handle on the situation based on
analysis fields...and forecasts show good temporal evolution through
the Ohio border region placing the bulk of afternoon convection to
our south and east before peak surface heating.

Middle level subsidence trailing the Ohio border wave will keep things
dry in the Tri Cities down near Flint during the day...but the next
upper level feature will arrive soon into tonight. This system is
shown clearly in early morning water vapor imagery over northern
Ontario and looks plenty capable of supporting showers tonight as it
moves through lower Michigan. Model solutions indicate the
circulation weakening from a vorticity perspective but not enough to
offset the shot of cold air aloft it will bring down over the
region. The wave will also be able to draw some low to middle level
moisture in from the west to help generate modest elevated
instability to promote scattered shower development through sunrise


Long term...Thursday through Tuesday

Upper low continues to churn near the Ontario/Quebec border...with
several notable shortwaves visible on water vapor imagery pivoting
around the main system. The larger of the waves (one over northern
Michigan and the other over Hudson bay) will rotate across the area
today and tonight...clearing the area to the east by 12z Thursday. The
center of the upper low appears to be located near the southern end
of St James Bay at the moment. Models are forecasting this will drop
through eastern Ontario Thursday and Thursday night...pushing a
surface trough across northern Michigan. The passage of the trough
should help focus showers and maybe some thunderstorms once again
across Southeast Michigan...with the best coverage during the
afternoon and evening as diurnal heating peaks. There is some
uncertainty with development and coverage of thunderstorms through
at least the first half of the day. GFS/NAM forecast soundings have
varied over the past few runs between a weak middle-level cap holding
and no cap with a skinny cape profile. They do advertise a trend for
the atmosphere to become uncapped by late evening however...with
cooler air aloft sliding down as the upper low draws closer. Will
continue to mention thunderstorms after 15z and through the
overnight period.

Shortwave energy diving down from Saskatchewan and Manitoba will
then allow a weak upper trough to form over the Midwest and western
Great Lakes early Friday. This trough will then slowly track across
the Great Lakes Friday and Saturday. Surface low associated with
this feature is expected to work across the southern Ohio
Valley...however some middle-level moisture is expected to increase
over Michigan Friday into Friday night as the trough approaches and
southwest flow increases slightly...with some moisture then
lingering into Saturday. This will once again allow showers and
thunderstorms to form again...especially each afternoon/evening as
we remain under the upper cold pool and diurnal heating helps to
increase lapse rates.

Shortwave ridge is expected to briefly build into the area for
Sunday bringing a respite from daily shower/thunderstorm chances.
Another upper trough is then advertised by medium range models to
drop across Ontario on Monday...pushing another surface trough
across the western and central Great Lakes and bringing showers and
thunderstorms back.

Temperatures through the extended period are expected to moderate a
bit as upper troughing over the area exhibits a general weakening
trend. Maximum temperatures Thursday through early next week should run in the
upper 70s to near 80...still below normal for this time of year.



Winds will remain fairly light through the end of the week...with
the most notable period of winds occuring from the southwest
thurdsay and Thursday night as a weak trough moves across the
western and central Great Lakes. Winds are still expected to remain
near or below 10 knots however. Otherwise...expect the area to
remain under a cool pattern with showers and thunderstorms
developing each day...mostly during the afternoon and evening.


Aviation...issued 1205 am EDT Wednesday Jul 30 2014


With showers coming to an end...attention will turn to fog potential
for the early morning hours. With clouds diminishing and ample
moisture at the surface due to rainfall...radiational cooling could
produce some fog by morning. The clearing will be faster and earlier
across the north which will put mbs at the biggest risk to see some
fog...less confident for fnt and ptk with clouds lingering longer.
Will include a period of MVFR visibility for mbs. Showers will again
develop across the region Wednesday afternoon as a trough sweeps
across the region.

For dtw...improving conditions through the night as persistent
clouds and showers scatter out. VFR conditions into Wednesday
afternoon until afternoon showers possibly impact the site.

//Dtw threshold threats...

* low confidence in ceiling below 5000 feet Wednesday morning
through afternoon.


DTX watches/warnings/advisories...
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.



long term....hlo

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