Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
FXUS63 KDTX 221715
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
115 PM EDT FRI JUL 22 2016
Weak pressure pattern will provide light winds with a westerly
component through this evening while mixing up to about 6k ft may
bring a few higher winds gusts. Winds will become variable after
sunset and through most of Saturday morning before mixing brings
back a westerly component. Warming aloft will increase cap strength
allowing only a few late afternoon and evening cu 5-6k ft. A dew
point boundary will be settling across the northern taf sites this
afternoon and evening. There may be a enough convergence along this
to produce a narrow band of broken cu with a very slight chance of
breaking the cap and producing an isolated storm. Chances are too
slight and area too limited to mention in MBS and FNT tafs.
For DTW...winds will start off taf period variable or light
northwesterly but should shift more westerly with daytime mixing
along with a few higher winds gusts mixing down late afternoon and
evening. Winds will go light and variable tonight and become more
westerly component Saturday afternoon as boundary mixes. Just a few
to sct cu 5-6k ft possible today and expect clear skies tonight and
Saturday as increasing inversion aloft caps off cu development.
Some cirrus is possible through taf period from any convection
that may fire upstream.
//DTW Threshold Probabilities...
Issued at 1154 AM EDT Fri Jul 22 2016
Tweaked the sky cover grids to account for departing cirrus and
the next wave of thinner and more disperse cirrus that will spread
over the southern CWA during the next few hours. Temperatures are
pretty much on target so far, and don`t expect incoming thinner
cirrus to have much of an impact. Dewpoints are generally hovering
around 70 as we approach the noon hour and there is no support
for advection to bring in anything much higher. Localized higher
Tds are evident near the water, such as 74 in eastern Monroe
County, but generally expecting Tds to hover within a degree or
two of 70 save for isolated local effects. Moisture boundary
still on target to cross the Tri- Cities around 20z and Flint a
couple hours later, allowing the humidity to drop off prior to
sunset. Heat advisory still on target with upper 90s to low 100s
indices in most of the advised area - hottest in Metro Detroit.
Downward adjustment in dewpoints will likely keep the Thumb from
reaching critera, but there is no sense to cancelling during peak
heating when it will still be oppressively hot and humid. The
headline will run its course.
Issued at 315 AM EDT Fri Jul 22 2016
MCS over northern IL continues to propagate south and southwest away
from Lower MI. Despite most of the models not having a good
initialization of these feature, there is very good agreement with the
evolution of today over Lower MI. There is a surge of high theta-e
air that will take place mainly south of I-96 / I-69 corridor from
forecast time to 12z. Expect elevated showers and storms to
continue to develop early this morning and continue until about 12z
as they push south of the state and weaken as they move away from a
weakening nocturnal low level jet.
After this...not much to support additional storms to develop. There
is a dew point front that moves to about I-69 corridor by 00z this
evening. This front might have just enough convergence to kick off a
storm with 3000-4000 J/kg of ML CAPE. Model soundings indicate a cap
to overcome and with little forcing, that may be difficult. If a
storm does go, not only does it have that CAPE but also 35 kts of 0-
6 km bulk shear. Will continue the 20 to 30 pops for the south half
of the forecast area this afternoon and the SPC marginal risk looks
to be about right.
Continue to see warming at 850 mb and 925 mb today with temps
reaching 21C and up to 29C respectively. This should allow the
temps to get into the mid 90s across the forecast area. The caveat
is the clouds. Dense debris clouds were over the state early this
morning, along with any clouds that can develop with the showers and
storms along the theta-e surge this morning. Expect these all to
dissipate by 14z and that should be enough time to get some good
insolation. Dew points continue to rise early this morning and that
will continue into the morning with most locations getting into the
lower and mid 70s. That will allow heat indices to get into the mid
and upper 90s along and north of M-46 and above 100 south of that
line. The highest dew points will stay south of the previously
mentioned front over metro Detroit. There is certainly a decent shot
of getting to heat warning criteria with heat indices of 105 or
higher for three hours. But this is also the area that will likely
have the most clouds and still need dew points to get to 75 which
might be at the top of the expected range. Will hold off at this time
and let any morning updates issue if necessary.
The drier and stable air takes over for tonight, Saturday and most
of Saturday night. Not sure dew points will fall as low as the NAM
has, but somewhere around 60 to the lower 60s seems reasonable.
Still have the warm air at 925 and 850 mb for Saturday. This should
allow temperatures to warm nicely again especially with the dry air in
place. Mid 90s seem possible again for areas south and west of a
Midland to Flint to Detroit line. With those lower dew points, heat
indices only get into the mid 90s and may not need an advisory to
be extended into Saturday, especially since overnight lows will
fall into the 60s for most areas outside of metro Detroit.
There will be a temperature gradient to the northeast with north to
northeast winds off of Lake Huron for the Thumb region.
The next system moves in late Saturday night, Sunday and Sunday
night. This model run seems to be less stout with the wind field,
struggling to get to 30 knots of 0-6 km shear, and available
instability, maybe ML CAPEs of 1000 to 1500 J/kg. There are good jet
dynamics for late July, a couple of shortwaves within the trough
structure and a cold frontal passage Sunday night. All that leads
to that SPC Marginal risk of severe weather.
Dry weather for Monday through Wednesday with temperatures slowly
cooling back to average. Next chance of showers and storms will be
late next week.
A weak surface trough and cold front will drop southward through the
Central Great Lakes today and this evening. Thunderstorms will again
be possible in a continued warm and humid airmass as this occurs,
with greatest potential over Lake St Clair and Western Lake Erie.
Coverage today is expected to be less than yesterday, with only
scattered storms expected to develop. West to southwest winds today
will remain at or below 15 knots. High pressure will build in
overnight through Saturday night, bringing light winds and quieter
weather. Low pressure tracking across Ontario will push the next
cold front across the area on Sunday, bringing a good chance of
thunderstorms and an increase in southerly winds.
Shower and thunderstorm activity today will be much more scattered
in nature, and mainly occur south of the M-59 corridor. While
widespread heavy rainfall is not expected, locations that experience
a thunderstorm may see a quick accumulation of one-quarter of
an inch or more of rain.
The next chance of widespread thunderstorms will come Sunday and
Sunday night as a low pressure system and associated cold front
cross the region. Average rainfall looks to range between 0.25 and
0.5 inches, but isolated locations could see totals up to one inch.
MI...Heat Advisory until 10 PM EDT this evening FOR MIZ047>049-053>055-
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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