Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 230947
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
547 AM EDT Sun Jul 23 2017


.AVIATION...

Stratus heights are expected to lower around sunrise as we reach
what is typically a diurnal minimum for ceilings/vis. The exception
will be at MBS where a decaying line of showers will bring better
mixing. Showers are expected to have little to no impact otherwise.
Improvement in ceilings this morning will be slow with light winds
promoting poor mixing. Better improvement will be seen in the
afternoon as the gradient increases in response to an approaching
cold front. The front should generate showers and thunderstorms out
ahead of it, with scattered coverage justifying a TEMPO group in the
TAFS. The frontal passage will usher in drier air, ending chances
for rain/thunder and improving ceilings.

For DTW...Some uncertainty as to whether IFR ceilings just north of
the area will progress into DTW. Gradient over the past few hours
have supported 4-6 knots of wind, and will lean towards a more
optimistic forecast. Cold front working towards the area look to
generate a scattered coverage of showers and thunderstorms this
afternoon, with most likely window to see activity at DTW between
19Z and 22Z.

//DTW THRESHOLD PROBABILITIES...

* Low in thunderstorms Sunday afternoon.

* High in ceilings at or below 5000 feet through late afternoon.

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

&&

.PREV DISCUSSION...
Issued at 301 AM EDT Sun Jul 23 2017

DISCUSSION...

Thunderstorm chances remain the most notable forecast challenge for
today as ambiguity in surface parameters remain significant to the
eventual coverage/nature of convection. First, low level convergence
to force convection is somewhat nebulous within weak gradient around
broad inverted troughing which extends from weak low to the east on
northwest to main shortwave and associated low pressure working into
the northern Mississippi Valley/northern Great Lakes.

Lead convection over Lake Michigan/Wisconsin is forecast to weaken
or dissipate outright early this morning with additional showers or
storms then developing in weakly capped environment by midday. At
that point, forcing will be rather weak so do not expect initial
activity to be very well organized. To further complicate issues,
degree of instability by that time is highly questionable with the
rather extensive low stratus deck covering roughly the northeastern
2/3rd of the forecast area.

Better convergence and upper support from aforementioned shortwave
and associated cold front will provide a better chance at organized
storms from mid/late afternoon into early evening. As this wave
progresses into the Straits by early evening, upper flow will also
increase notably with 0-6km bulk shear AOA 30 knots. All of these
factors will increase the likelihood of strong to locally severe
storms late in the day, but as was noted previously, overall quality
of moisture/instability remains questionable and will most likely
lead to relatively scattered activity. The best coverage of storms
looks to occur over the northern forecast area where forcing and
upper support are best. This, however, is assuming the stratus now
covering area scatters out to some extent.

Surface high pressure will then build into area in the wake of this
cold front, providing somewhat cooler and notably less humid airmass
during the Monday/Tuesday time frame. Expect Monday to be especially
cool as northeast flow behind this system draws across Lake Huron
into the region. Afternoon temperatures will range from the lower
70s near the lake to mid/upper 70s further south/southwest. Highs
will moderate back in the upper 70s/lower 80s on Tuesday, but low
humidity levels will persist.

Increased south/southwest flow between exiting surface high and next
approaching Canadian shortwave and attendant trough/cold front will
lead to warmer and more humid conditions by Wednesday into Thursday.
This will also be the time frame for the next chance of showers and
storms (Wednesday night into Thursday) as the cold front progresses
through the region. A return to cooler/less humid weather can also
be expected after the passage of this front. While this transition
will begin late Thursday, it will be most noticed Friday/Saturday as
high temperatures edge back into the 70s/lower 80s. Lows in the 50s
to lower 60s will also bring a number of pleasant nights.

MARINE...

Low pressure tracking through the area will bring fairly light winds
to western Lake Erie and Lake St Clair today. Lake Huron will remain
along and north of the low track, promoting stronger east winds
today over the northern portion of the lake, with sustained speeds
even reaching about 20 knots today. A cold front will drop through
the area late today and tonight, triggering scattered showers and
thunderstorms across the area and allowing winds to increase from
the northeast over all of Lake Huron. Winds will then turn north
late tonight and Monday as the low tracks through the eastern Great
Lakes and high pressure builds into the Upper Midwest and then
finally into Michigan late. While winds will increase over all
marine areas, Lake Huron will once again become the focus for
concern as gusts top 20 knots and the long fetch over the water
allows wave heights to grow in excess of 4 feet over Saginaw Bay and
across the nearshore waters from Port Austin to Harbor Beach. Small
Craft Advisories are in effect late tonight through Monday. High
pressure settling in overhead will then allow for light winds and
improving wave conditions by Monday night.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from late tonight through Monday evening
     for MIZ049.

Lake Huron...Small Craft Advisory from 8 AM to 8 PM EDT Monday for LHZ421-422-
     441.

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

$$

AVIATION.....HLO
DISCUSSION...DG
MARINE.......HLO


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at www.weather.gov/detroit.



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