Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 231710
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
110 PM EDT Sun Jul 23 2017

.AVIATION...

Satellite and mesoanalysis indicate some surface based instability
developing entering early afternoon while low pressure remains
diffuse over lower Michigan. In the absence of better surface
focusing in the pressure field, expect best storm development will
be more isolated and/or a little later while cumulus churns under the
850-700 mb inversion. A prefrontal trough is forecast to sharpen up
during mid to late afternoon as the upper level circulation
approaches, both of which should give a boost to coverage and
intensity of storms mid afternoon into early evening. Otherwise,
ceiling will continue to trend upward into VFR and remain well into
the night before the associated cold front moves through the region.
The front will pull a large area of borderline IFR/MVFR clouds over
SE Michigan late tonight that will persist into Monday afternoon.

For DTW... Mid to late afternoon timing is preferred for the
greatest coverage of thunderstorms in D21 and affecting DTW. Activity
will clear DTW around 23Z as a weak prefrontal trough slides to the
east. Timing on the cold front and arrival of MVFR ceiling around 10Z
looks good.

//DTW THRESHOLD PROBABILITIES...

* Moderate for thunderstorms during late afternoon.

* High for ceiling at or below 5000 feet during afternoon and again
  late tonight.

&&

.PREV DISCUSSION...
Issued at 1027 AM EDT Sun Jul 23 2017

UPDATE...

There has been a steady erosion of the low stratus this morning over
portions of the area. A slightly higher based strato cu field remains
fairly extensive across the northern half of the forecast area.
Satellite trends do at least suggest some diminishing trend to this
cloud cover into early afternoon. The 12Z DTX sounding showed a warm
layer between 900 and 700mb, warmer than model soundings would
suggest. Mid level height falls will however persist across Se Mi
through the afternoon as the upper low now churning over wrn Upper Mi
tracks to the straits region this afternoon. These height falls
combined with diurnal heating should be enough to erode this warm
layer and initiate late afternoon/evening convection. Some enhanced
sfc troughing over Se Mi this afternoon should also sustain some
added boundary layer convergence. Model solutions also indicate some
degree of mid level moisture pooling across Lower Mi this afternoon.
Therefore scattered convection later today continues to look
reasonable. Based on the latest hi res suite, the most probable time
period for convection will be between 19Z and 23Z.

Assuming afternoon temps into the low 80s with sfc dewpoints in the
upper 60s, rap soundings suggest somewhat marginal instability (0-1km
ML CAPE 1000-1500 J/KG). Deep layer shear is expected to range from
20 to 30 knots, also somewhat marginal. Therefore the chances for
late day severe wx appear rather low. Portions of metro Detroit down
through Monroe have however cleared out a little more quickly this
morning, giving them a jump start on daytime heating, supporting
afternoon highs into the mid 80s. Sfc dewpoints across the far south
may also be able to creep up toward 70 deg. These factors suggest a
chance for a little higher instability in the far south and thus a
little higher severe wx probabilities.

The current forecast looks reasonable. An update will be issued
simply to push the higher pops a little later into the afternoon.

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.....BT
UPDATE.......SC
DISCUSSION...DG
MARINE.......HLO


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