Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 222344
AFDDTX

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


.AVIATION...

Very moist level environment firmly entrenched across the southeast
Michigan airspace heading into tonight.  Evening cloud trends
suggest some longevity to the existing higher based cloud cover
associated with an exiting weak low pressure system.  North of this
cloud deck , extensive low stratus already in place under northeast
flow, with the southern extent impacting the MBS corridor.  Further
southward expansion is expected over the course of the night,
augmented by some weak moisture flux off the lakes under light
east/northeast flow. This will translate into the development of low
stratus and/or fog, favoring restrictions down into IFR.  Potential
does exist for one or more locations to drop into LIFR toward
daybreak, but confidence remains low.

For DTW...A combination of low stratus and fog expected to emerge
during the early-mid morning period as winds turn light
northeasterly.  Low potential for conditions to dip into LIFR,
particularly if dense fog develops.

//DTW THRESHOLD PROBABILITIES...

* Low in thunderstorms Sunday afternoon.

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

* Low for vsby at or below 1/4 mile Sunday morning.

&&

.PREV DISCUSSION...
Issued at 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.

&&

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

$$

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


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at www.weather.gov/detroit.



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