Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 191026
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
626 AM EDT FRI AUG 19 2016


.AVIATION...

VFR conditions have been holding across southeastern Michigan
perhaps because the true warm advection has remained to the west of
the terminals. Deeper warm air and moisture advection will increase
during the course of the day, particularly by late this afternoon
and evening. Widespread shower and thunderstorm development is not
anticipated this afternoon because substantial midlevel high
pressure is expected to become centered over the area. The better
support for a thunderstorm very late this afternoon and everly
evening is over at KMBS where thunderstorm activity may push
southward into the Saginaw Bay region.

For DTW...Very low confidence in thunderstorms directly impacting
the terminal today. What is knows is that thunderstorm activity will
remain isolated in coverage. The best time window will be between 20
and 24 UTC. Winds look to be light today, under 10 knots and
predominantly out of the south-southwest.

//DTW Threshold Probabilities...

* Low confidence in ceilings aob 5000 feet.

* Low confidence in thunderstorms today and tonight.

&&

.PREV DISCUSSION...
Issued at 408 AM EDT Fri AUG 19 2016

DISCUSSION...

The main upper level jet axis will settle southward into U.S. for
the end of the week and weekend, influencing the weather over
southeastern Michigan.

The process is already underway as a deep midlevel trough
and height fall region now diving directly southward out of Canada,
along the Rocky Mtn chain is causing a downstream response with
southwesterly return flow becoming increasingly more organized over
both the mid and upper Mississippi River Valleys. The big feature
that needs to work out of this developing southwesterly flow is a
massive midlevel high pressure circulation pushing out of IA in the
anticyclonic trajectory branch. The westerly flow residing on the
northern periphery of this anticyclonic gyre will spread across
northern Lake Michigan and northern Lower Michigan first allowing
deeper moisture and theta e wash to spread across those areas today.
It is this warm air or theta e advection that will support a chance
of showers and thunderstorms. Regional mosaic shows thunderstorm
activity now over portions of the U.P. in and near the Garden
Peninsula in this forcing. Expecting this activity to generally push
eastward during the morning hours with little consequence for
southeastern Michigan. As the morning progresses, low to midlevel
theta e advection eventually spread into much or all of the forecast
area. This will begin the low chance for a shower and thunderstorm.
However, model data strongly suggests that persistent anticyclonic
flow aloft and ridging will hold firm with the center of the
anticyclone eventually setting up directly over the cwa by this
evening. This will make it extremely difficult to witness any
significant coverage to any convective activity. Thus, just a low
chance for this afternoon. Hi-res explicit convection models
support some fresh convective initiation over portions of far
northeastern Lower Michigan late this afternoon with some
activity either pushing southward or additional development down
into the Saginaw Bay region. No widespread severe weather is
anticipated, particularly for the southern 2/3rds of the cwa with
0-6km bulk shear values of 20 knots or less. The one area where an
isolated strong to sub severe thunderstorm will be possible is in
vicinity of Saginaw Bay. Model soundings suggest that MLCAPES
could approach 2000 J/kg this afternoon. The latest Swody1 product
has the area in general thunder designation.

Saturday...Southeastern Michigan will continue to feel the effects
of the preceding high pressure system as models support
southeasterly system relative winds right through the morning hours.
This is expected to temper any convective development until the late
afternoon hours when flow veers to the southwest and becomes more
cyclonic. A fairly significant, but typical, late summer upper level
trough will pivot across the western Great Lakes late in the day.
There are some fairly notable differences noted in the positioning
of the surface low by 00Z Sunday, however there is good confidence
that some lead shortwave energy and main PV anomaly within the
trough will get kicked to the NE across Lower Michigan between 21-
00z. This will highlight the start of the most robust 1000-500mb
height falls over Michigan and should support a secondary warm front
or occlusion/triple point at least over a portion of lower Michigan.
However, there is enough gradient to the height falls that provides
some uncertainty as to where exactly the strongest forcing will push
across. It does remain possible the strongest forcing could occur
just north or west of the forecast area. Shear again remains
relatively weak early in they day at 20 knots, but an impulse pushes
the 0-6km bulk shear to 30 to 35 knot range by the evening hours.
The best instability appears to occur midday, out of phase with the
increasing shear. However, model soundings still support 1000 to
1250 of MLCAPE during the evening which is good enough to support an
isolated strong to severe thunderstorm risk. Given the uncertainty
to the placement of the dynamical forcing it remains possible that
future outlooks could upgrade the risk. The latest swody2 has a
marginal designation for severe weather across all of southeastern
Michigan. Best timeframe for impactful thunderstorm activity would be
late afternoon and evening.

The main cold front will push across lower Michigan between 06-12z
Saturday Night. One will notice this cold front as it has been quite
awhile since we had one to compare to this. Temperatures in the 70s
on Sunday will be some 10 degrees cooler than Saturday, and some 5
to 7 degrees cooler than averages. Humidity will try to lower, but
60s values will hold on Sunday before lowering more substantially
Sunday night. A steep lapse rate pocket between 3000 and 10000 ft
agl looks really good for instability showers potential, especially
post 20Z Sunday afternoon.

MARINE...

A weak frontal boundary will sag south into Lake Huron late today
and stall over the north portion of the lake tonight, bringing a
chance of showers and thunderstorms.

Low pressure will then track into the Great Lakes on Saturday,
bringing a better chance of showers and thunderstorms. South to
southwest winds will increase to some degree in advance of this
system on Saturday. A strong cold front will then sweep through the
area in its wake, bringing gusty west-northwest flow late Saturday
night into Sunday, with winds expected to top out around 30 knots
over the open waters of Lake Huron as cooler air settle in.

&&

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

$$

AVIATION.....CB
DISCUSSION...CB
MARINE.......CB


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