Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 201421 CCA

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1020 AM EDT THU OCT 20 2016


Forecast remains on track this morning...just a few cosmetic
changes made to hone in on coverage and timing. Rain shield will
expand northwestward deeper into the CWA as the upstream trough
acquires a neutral tilt through this morning and afternoon.
Expectation is largely confirmed by ongoing additional development
along the northwest periphery of the existing rain shield in NW

Only other item of note is developing lake effect in Huron and
Sanilac county. NAM12 0-1km theta-e lapse rates of -4 to 5C are
supportive of moderate convective development ongoing through the
afternoon, likely migrating south and east with time as the trough
approaches. Convective depth is healthy, slightly overshooting the
700mb cap to about 10kft. Periodic heavy downpours will therefore
be possible mainly in Sanilac and possibly northern Saint Clair
County through the afternoon.


Issued at 705 AM EDT Thu OCT 20 2016


While convection to the south has disrupted moisture convergence
along elevated fgen banding over the area, expect some degree of
shower activity to persist. In fact, with additional upper level
support lifting into the area from the southwest, still anticipate a
re-expansion of rain from mid/late morning into the afternoon. Lower
VFR ceilings are still expected to edge down to MVFR, but overall,
have increased ceilings with this forecast given drier low level air
that has remained in place. Will end rain chances are 00z or so, but
lake effect/enhanced banding in conjunction with forcing along main
upper trough axis late this evening/overnight will have to be re-

For DTW...Expect occasional showers with MVFR cigs/vsbys at times as
shortwave trough lifts slowly through the area. NNE flow 10-12 knots
should back to NW with time late today into tonight. Additional will
be possible tonight as upper trough axis works through area and some
lake enhancement becomes possible as cold air deepens across the

//DTW Threshold Probabilities...

* High confidence in ceilings aob 5000 feet today, low confidence

Issued at 306 AM EDT Thu OCT 20 2016


Upper trough axis shifting from the Central Plains into the Great
Lakes and Tennessee Valley will send surface low pressure up across
the Ohio Valley and Eastern Great Lakes today and into the Eastern
Great Lakes by this evening.

Thermal gradient over Michigan will tighten as the low lifts into
the Ohio Valley, pushing the mid-level warm front into Michigan and
as a cold front over the Northern Great Lakes begins to slowly sink
into the area. Strong mid-level FGEN looks to become positioned over
far Southeast Michigan this morning, most robust along and south of
a line from Howell to Port Huron per latest model consensus. Right
entrance region forcing will act to enhance lift from mid-level
FGEN, supporting widespread rainfall that will extend up into much
of the area. Forecast soundings and observations of ceiling heights
still support a lot of dry air below 8000 feet. This dry air should
be quickly overcome in the next few hours as better forcing works up
into the area. Rain looks to last much of the day as the mid-level
front remains positioned over Southeast Michigan, though forcing
(and rainfall) will start to weaken after 4PM.

Question is how far north and west the rain shield will extend as
cooler and drier air from the cold front wraps into the backside of
the system. Dprog/Dt analysis shows a slight shift to the left with
the surface low track over the past 24 hours supporting a westward
expansion of high pops through the Flint area. Still some question
as to how moisture transport will respond to better instability
(convection) and forcing to our southeast and east and whether this
could deflect deeper moisture from reaching too far into Michigan.
Radar/lightning analysis shows thunderstorms lined up along lower
portions of the front stretching from the Ohio Valley back through
the Central Plains. Idea is that this will keep rain shield from
reaching too far into the Saginaw Valley and may keep rainfall
totals area-wide away from the higher end of guidance (NAM). Clouds,
rain, and cooler air seeping into the area in the lower-levels will
limit temperatures today into upper 50s to near 60. This is not too
far off current readings early this morning.

Cold front will drop into the area as low pressure pulls away
supporting a gradual reduction in precipitation chances overnight.
H850 temps will fall below 0C behind the trough axis by Friday
morning. Lake-H850 delta T values between 16-22C and long fetch from
north winds will generate lake effect rain showers over the thumb
with strong flow even bringing a low chance for these to reach into
the Detroit-area Friday into Friday evening. The threat for showers
may be enhanced by shortwave energy dropping down into the area from
Saskatchewan during peak heating, but will leave most of the
forecast area dry for now with just an expansive stratocu field with
expectation for very dry air in the low-levels away from the lake
(surface dewpoints in the 30s). These showers should shift back over
the lake Friday night as flow turns more to the northwest. Friday
will be a cool day as Michigan resides within the thermal trough,
with max temps only reaching the 50s. Lows look to drop back down
into the 30s Friday night, with frost threat mitigated by north-
northwest gradient which will strengthen overnight as a backdoor
cold front pushes down into the thumb. The exception could be over
the western fringe of the forecast area where gradient may be a
little lighter. Confidence at this point is not high enough to
include any mention of frost. The reinforcing shot of cool air will
keep readings cool on Saturday.

Medium range models are in good agreement with racing a clipper
across the Great Lakes late Sunday and Sunday night. Track right now
would only favor pops over the northern portion of the forecast area
Sunday night. Increased southerly flow ahead of the system would
boost max temps into the upper 50s to low 60s Sunday. This is close
to normal.


Northerly winds will strengthen during the course of the day as low
pressure lifts through the Ohio Valley into the eastern Great Lakes.
Moderate to fresh winds will continue out of the north to northwest
tonight through Saturday as colder air spreads over area waters. The
persistent northerly flow over such a long fetch of Lake Huron will
build waves to hazardous levels in the nearshore zones by evening
with a prolonged period of elevated winds and waves then continuing
into Saturday as the aforementioned low pressure system undergoes
significant deepening over the New England states heading into the

A Small Craft Advisory will remain in effect during this time frame
for the Lake Huron nearshore waters. The strongest wind gusts will
occur from late Friday into Saturday as much colder air in pulled
across the area into the developing low pressure. At this time, it
appears that wind gusts should remain below gale force, averaging 25
to 30 knots over the Lake Huron open waters.


Light to occasionally moderate rain will continue today as low
pressure lifts through the Ohio Valley on its way to the eastern
Great Lakes tonight. Given the fact that this rain will fall on and
off for much of the day, expected rainfall totals in the one quarter
to one half inch range to be common. The highest rainfall totals
will occur along and south of the Interstate 69 corridor with a drop
off to much lower amounts north-northwest into the Saginaw Valley
where some locations may receive little to no rainfall.


Lake Huron...Small Craft Advisory from 4 PM this afternoon to 10 PM EDT Saturday
     FOR LHZ442-443.

     Small Craft Advisory until 10 PM EDT Saturday FOR LHZ421-441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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