Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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218
FXUS63 KDTX 301105
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
705 AM EDT SAT JUL 30 2016

.AVIATION...

Better coverage of showers and thunderstorms today across
southeast Michigan will warrant vicinity showers for much of the
day. Longest and most persistent showers expected across MBS
during the morning while a prob30 group for thunderstorms is in
the forecast late this afternoon for farther south locations.
Outside of this activity, looking at ceilings at or mostly above
5000 feet. Scattered shower activity is expected to persist
tonight (did not include in tafs due to coverage concerns), but if
clouds break up, fog will become likely. confidence is shaky with
this scenario, but will include light (mvfr) fog development for
now.

For DTW...Diurnal heating should lead to additional showers and
possible thunderstorms today. Timing remains difficult to determine.
Continued light northeast winds during the day, becoming light and
variable late tonight.

//DTW Threshold Probabilities...

* Low in thunderstorms and ceilings aob 5000 feet.

&&

.PREV DISCUSSION...
Issued at 405 AM EDT Sat Jul 30 2016

DISCUSSION...

Persistent mid level deformation remains in place over lower
Michigan early this morning. Left over from yesterday, it is now
getting a boost from an upper wave sliding eastward from the south
end of Lake Michigan. Early morning observational data indicates
expansion of elevated convection over lower Michigan occurring
within the mid level theta-e ridge analyzed roughly along the I-69
corridor at 00Z. There is loose model agreement that indicates the
moisture axis will shift northward during the remainder of the
morning as the upper wave moves from the south end of Lake Michigan
eastward along the Ohio border. This should favor the Tri Cities and
Thumb region for likely/numerous POP coverage during the short term
but plan to broaden higher POPs southward toward the M-59 corridor
as well to account for potential of enhanced forcing near the mid
level dry slot interface. Convective trends in this scenario remain
difficult to pin down as clusters of showers/storms will not be well-
behaved within the larger scale pattern. The general west to east
orientation of the moisture axis and west to east cell motion will
keep locally heavy rainfall as the primary hazard through the
morning.

The transition to surface based convection will take place as
diurnal heating peaks during the late afternoon. This will shift the
higher concentration of convection toward the Ohio border closer to
the surface low. The surface low and fronts will remain south of the
border but the tendency for inverted troughiness extending northward
into lower Michigan will help with low level convergence. The
general consensus of mesoscale models (HRRR, NMM, ARW, 4KM NAM)
indicates surface based CAPE building into the 1500-2000 J/kg range
as far north as the M-59 corridor but mostly I-96/696 southward.
Given the weak wind profile, expect convective mode similar to
yesterday which turned out to be pulse-type, slow-moving multicell
clusters prone to heavy rainfall. Measures of deep moisture remain
high as PW is projected around 1.7 inches with 700 mb dewpoint
around 3C. The WPC day 1 slight risk area for excessive rainfall
seems reasonably placed over the south 1/2-1/3 of SE Michigan and
within the SPC day 1 general thunder outlook. The difficulty in
pinning down convective trends precludes longer fused headline
issuance at this point but short fused issuances may be needed
again during late afternoon into early evening.

The upper wave moving into the northern plains during the early
morning will be the culprit in delaying the exit of this unsettled
pattern during Sunday. The collective output among the 00Z models
suggest this feature will slide over about the same region as
today`s wave and both feed off and reinforce the low to mid level
moisture axis. The ECMWF remains more progressive than the NAM, GFS,
and GEM and would confine chance POPs to the morning while the other
models are slower and more intense with the occlusion. Definitely
trended the Sunday forecast more pessimistic in this package to
build in the trend for upcoming model cycles.

The short wave ridge trailing the Sunday wave will support surface
high pressure sliding into the Great Lakes during Monday which will
then last through about mid week. The surface high is projected to
produce warm temperatures for the start of August but with lower
humidity. There are signals for a round of MCS type activity nearby
to our west during the second half of the week and possibly the next
front due in lower Michigan by next Friday.

MARINE...

Northeast winds holding around 20 knots over central and southern
Lake Huron through today will maintain elevated wave heights in the
nearshore waters. Small Craft Advisories have been reissued through
this evening. Northeast winds will decrease to around 10 knots over
the remainder of the weekend into early next week, allowing for
significant wave less than 4 foot range.

HYDROLOGY...

A better coverage of showers (scattered-numerous) and a few
thunderstorms are expected today over southeast Michigan, but still
not widespread. None-the-less, the stronger showers and
thunderstorms will be capable of producing heavy rainfall in excess
of half an inch within an hour, with isolated cells producing 1 inch
or greater not out of the question due to the slow movement.
Localized urban flooding will be possible. Activity will persist
tonight, but in a weakening state with the loss of daytime heating.
Another round of scattered activity expected on Sunday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Beach Hazards Statement through this evening FOR MIZ048-049-055-063.

Lake Huron...Small Craft Advisory until 10 PM EDT this evening FOR LHZ421-422-
     441>443.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....SF/BT
DISCUSSION...BT
MARINE.......SF
HYDROLOGY....SF

You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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