Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1140 PM EDT Monday Aug 31 2015
MVFR/IFR fog is settling in across Southeast Michigan with just
sporadic observations of 1/4 mile visibility so far. General forecast
reasoning remains unchanged although the 06z set of tafs will carry
an earlier onset time for all locations. Very favorable radiating
conditions may yet favor a period of fog below Airport minimums.
Trends will be monitored accordingly...but conditions will quickly
improve as heating increases 13-14z. VFR will then prevail through
Tuesday as high based cumulus develops.
For dtw...potential remains for visibility to fall to 1 mile or lower but
prefer to see supporting observational trends in the immediate
vicinity before adjusting the forecast downward. High based cumulus is
expected to develop between 4-5kft on Tuesday with ceiling development
by about 19z.
//Dtw threshold probabilities...
* low for visibility at or below 1/2 mile and/or ceiling at or below below 200 feet after 08z.
* Medium for ceiling at or below 5kft after 18z Tuesday
Previous discussion...issued 346 PM EDT Monday Aug 31 2015
Short term...through tonight
Upper ridge axis will remain over Michigan tonight...while surface
high pressure center remains stretched from the eastern Great Lakes
down through the Tennessee Valley. Large scale subsidence will keep
weather quiet over the area...despite weak upper low spinning nearby
over Indiana. Instability and most of the moisture from this feature
should remain south of the Michigan border tonight.
Main forecast challenge tonight will be potential for fog to
redevelop. There are a few things that hint at less fog than last
night...subsidence and some mixing today should allow for an overall
drying in the boundary layer...and no precipitation is expected
today/tonight. Will continue to highlight areas of fog for now...and
watch trends tonight as skies clear and winds become light.
Temperatures should remain mild as an increasingly warm airmass
builds into Southeast Michigan. Expecting to see min temperatures in the
60s...even close to 70 in urban metropolitan Detroit.
The warming trend will continue to gain traction over Southeast Michigan
Tuesday and start off the month of September well above normal.
Highs in the middle to upper 80s will be about 10 degrees above
climatology at all locations. The warm pattern remains expected to
last through the next several days as high temperatures show
potential to touch 90 each day during the second half of the week.
At the same time...heat index will likely remain below headline
criteria but still reach uncomfortable levels in the middle to upper
90s at times as surface dewpoint climbs to around 70f. Confidence
remains on the high side that the large scale pattern supporting the
warmth will produce highs at least middle to upper 80s well into the
Holiday weekend. Minor temperature modifications are possible
depending on just spotty shower or thunderstorm potential each day.
Satellite imagery today depicts a powerful long wave trough digging
into western Canada that is part of a blocky upper air pattern over
the North Pacific. This will be primarily responsible for
maintenance of the expansive and persistent large scale ridge over
the central and eastern United States during the next several days.
Local modification of the ridge structure will occur as smaller
scale waves move through the westerlies in Canada and also due to
the smaller scale middle level shear axis bisecting the ridge structure
from the Ohio Valley to western Gulf Coast. Circulations within this
feature could destabilize the thermal profile just enough for
isolated thunderstorm development Tuesday afternoon through
Wednesday. Predictability on this is quite low and subject to
availability of surface focusing mechanisms limited to lake breeze
boundaries that have little or no organization this time of year but
which favor locations from about Port Huron to along and south of
the M-59 corridor.
Thursday through Tuesday.
A chance of showers and thunderstorms are expected Thursday and
Friday as the surface high continues to shift east. A south to
southwest flow will result on the backside of the high allowing for
low level moisture to filter into the area. Above average
temperatures combined with the influx of moist air and an upper
level trough could provide enough forcing to trigger a few hit and
miss showers and thunderstorms across the County Warning Area. Probability of precipitation at this time are
running around 20 percent. Dryer conditions return Saturday and then
the pattern repeats for Sunday through Tuesday with low chances of
showers and thunderstorms back in the forecast.
A broad area of surface high pressure will produce favorable marine
wind and wave conditions through the week. Warm and humid air
associated with the high will support areas of fog at times. There
is also some isolated thunderstorm potential each day mostly over
land areas or just close to shorelines. There is a slightly better
chance of storms over the northern portion of Lake Huron Tuesday
night into Wednesday as a weakening cold front enters that area.
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).