Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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339
FXUS63 KDTX 221914
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
314 PM EDT Sat Jul 22 2017

.DISCUSSION...

Humid/moist airmass down low with low level convergence leading to
areas of low clouds and drizzle up across north half of the CWA.
Meanwhile, toward the south, warm and dry mid levels in place, with
RAP13 analysis indicating 500 MB temps around -5 C and 700 MB Temps
around 11 C. Still, MLCAPES pushing at or just above 1000 J/kg this
afternoon potentially yielding a thunderstorm capable of producing
heavy rain before weak wave of low pressure slides east by Sunset,
allowing dew pts to slip under 70 degrees with little if any
shower/thunderstorm activity expected this evening/tonight. With mid
clouds exiting, enough surface/near surface moisture to support
areas of fog with calm winds.

Upper level wave/low over southern Manitoba this afternoon swinging
southeast through Lake Superior/U.P. tonight and through the Straits
Tomorrow. One surface low is currently over West Central
Illinois/Northern Missouri and will be tracking eastward, sliding
south of the Michigan border early tomorrow morning, with more of an
inverted trough extending northwest through lower Michigan before
low coming out of Western Great Lakes takes over and drops southeast
into Eastern Great Lakes Sunday night. With moisture quality
concerns and mid level dry slot in place, especially over far
southern Lower Michigan expected to limit coverage of activity. 12z
NAM attempts to advect back dew pts around 70 degrees, yielding
CAPES in the 1500-2000 J/kg range, which would be sufficient for
isolated severe storms with 0-6 KM bulk shear in excess of 30 knots.
However, moisture/850-700 MB Theta-E axis is very narrow, and mid
level lapse rates are still nothing special, around 6.5 C/KM from
700-500 MB. Mid level lapse rates then drop off by early evening as
actual cold front swings through, and it looks like we will be hard
pressed to see more than scattered coverage of activity, especially
if we hold onto a lot of cloud cover through the day and maxes come
up short of the projected highs in the mid 80s.

Surge of low level cold advection, helped out by northeast push off
Lake Huron Sunday Night, setting us up for cooler Monday with highs
in the 70s.

High pressure will keep dry and pleasant conditions in place on
Tuesday with highs near 80. An upper level low tracking east through
southern Canada will bring the next chance for active weather as a
cold front pushes through the region during the midweek period.
Southwesterly flow ahead of the cold front will bring warmer, more
humid conditions across the region on Wednesday with highs in the mid
to upper 80s. Model guidance has trended slightly slower for the
frontal passage keeping the chance for thunderstorms in the forecast
for Wednesday night and Thursday. High pressure then builds back into
the region late next week as an amplifying upper level trough over
the Northeast brings a return to dry, pleasant conditions and near
average temperatures.

&&

.MARINE...

Near stationary low pressure across southern and central Lower
Michigan tonight and Sunday combined with strengthening high
pressure over James Bay will support an increasing easterly gradient
across Lake Huron this afternoon into Sunday morning. During the day
Sunday, winds will veer toward the southeast while the stronger
winds become focused across the northern third of Lake Huron. This
will result from deepening surface low pressure across Lower
Michigan. Winds gusts across far northern Lake Huron may actually
top 20 knots on Sunday. This low pressure system will also provide a
chance for thunderstorms on Sunday to the entire region. The low
will depart to the eastern Great Lakes on Monday. Northeasterly
winds are expected to develop on the back side of this low Sunday
night into Monday. Winds may become gusty at times across Lake
Huron, especially Saginaw Bay where hazardous small craft conditions
may develop.

&&

.PREV DISCUSSION...
Issued at 1242 PM EDT Sat Jul 22 2017

AVIATION...

The region of showers which tracked across the terminals this
morning will continue exiting to the east of Se Mi early this
afternoon. While there is a region of mid level subsidence behind
this feature, very high low level moisture and daytime heating will
support some degree of scattered showers this afternoon. Some late
morning clearing over portions of srn Mi will continue to rapidly
fill in with diurnal cu given the abundance of low level moisture.
The expectation is that bases will gradually rise to MVFR with
persistent daytime heating. There is however some concern that lower
based clouds may hold on through the evening from time to time. Sfc
troughing will hold across Se Mi through the night. Given the ample
low level moisture and some flux off the lakes under light easterly
flow, there is a good potential for fog and/or low stratus
development tonight. For this reason, some IFR conditions have been
added to the terminals.

For DTW...Recent satellite data suggests a better mixing potential
across metro Detroit this afternoon. Thus, cloud bases should
undergo a steady rising trend heading into the evening.

//DTW THRESHOLD PROBABILITIES...

* Low in thunderstorms Sunday afternoon.

* High in ceilings at or below 5000 feet this afternoon through
  Sunday morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...SF/JD
MARINE.......SC
AVIATION.....SC


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at www.weather.gov/detroit.



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