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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1254 am EDT Thursday Jul 28 2016



Aviation...

Weak frontal boundary will sag south of the terminals overnight with
light northeast flow expanding through the forecast area. This flow
will persist Thursday and increase with better boundary layer mixing
and a tightening pressure gradient as low pressure ripples through
the Northern Ohio valley along this boundary. While diurnal cumulus
will be common, shras/tsras will focus south of terminals for the
most part. Chance seems to small to include as prob30 group attm.
That said, lower VFR cigs in the 7-10kft range will be more common
from midday into early evening.

For dtw...northeast flow in wake of passing front will persist
throughout the forecast, increasing to 8-10 knots for a time during
best mixing Thursday afternoon. Ceilings should remain above 5kft and
any shra activity will be spotty with low confidence in timing and
placement

//dtw threshold probabilities...

* low for thunderstorms impacting kdtw Thursday afternoon.

* Low for ceilings at 5000 feet Thursday afternoon.

&&

Previous discussion...
issued at 402 PM EDT Wed Jul 27 2016

Discussion...

Forecast reasoning provided in the updated forecast discussion
earlier today remains in good agreement with the sels mesoscale discussion issued at
1747 UTC. The combination of low level equivalent potential
temperature content and introduction of midlevel cyclonic flow has
allowed convection to initiate and develop along the northern
instability gradient. Showers and thunderstorms should continue to
fill in with time along the sagging frontal boundary. A strong to
isolated severe thunderstorm threat exists primarily with any new or
fresh convective updrafts. The main potential impacts from
thunderstorm activity will be to 40 mph and brief heavy
rainfall with slow storm movement of west to east at 20 mph. Given the
overachievement to convection coverage thus far, did bump up pops
into the likely or numerous range across the Saginaw Valley and
northern thumb. A pronounced anticyclonic curvature to the fair
weather cumulus streets south of M 46 suggests a continued strong
ridge influence. Expecting dry weather for southeastern Michigan
south of the I 69 corridor. Shower and thunderstorm activity should
push out of the area by 2 to 3z.

Thursday. A relatively high amount of uncertainty has been
surrounding this period with little inter-model continuity
pertaining to the evolution of midlevel moisture and cyclonic flow
that is now over Wisconsin. The European model (ecmwf) model which had been very
progressive in bringing the potential wave into southeastern
Michigan for the daytime Thursday has backed off considerably with
this solution. This has always been sort of a tricky period
forecasting with a total lack of any clean or pristine potential
vorticity advection in the background of very weak 1000-500mb
geopotential height falls. Therefore, it has been difficult to go
above a chance pop.

The weather for Thursday will be governed by a synoptic cold front
or equivalent potential temperature boundary that will be in the
process of slowly settling southward through lower Michigan. Models
agree fairly well the boundary will not totally clear the area until
Thursday night. Increased high temperatures for the Detroit Metro
slightly more into the upper 80s, but these values could still be a
degree or two too cool. With easterly flow in place to the north of
the boundary there is a reverse Lake Shadow look to the thermal and
moisture fields. The end result is a better Theta E ridge structure
developing over western lower Michigan due to increased low level
convergence. The 27.12z European model (ecmwf) is in agreement with the best
precipitation potential occurring to the west of the cwa.
Significantly rolled probability of precipitation back with low to mid chance pops, dry in
The Thumb. It is appearing there will be a diurnal flare up to the
chances as well, and may need total maturation of boundary layer,
after 21z to get activity into semich. MLCAPES in the southern and
western sections of the cwa are expected to be limited in the 250 to
750 j/kg range. There is a very low potential for strong
thunderstorm development.

Friday. There is again a muddled solution with surface high pressure
trying to maintain control. The problem is that overall flow pattern
aloft is a blocked one and is actually cyclonic. Combine this will
some very persistent, weak, ragged potential vorticity of low
confidence, a continued non zero chance for showers and
thunderstorms exists on Friday. The problem is, cannot justify
blanketing a pop for the setup. Decided to go dry and will allow
future shifts to reevaluate. With lower dewpoints, temperatures
Friday will definitely feel less humid with highs in the low 80s.

Broad upper troughing over the upper Midwest this weekend will give
way to rising heights next week. This will result in a near normal
temperatures this weekend and a warming trend next week. As for
precipitation chances this weekend, medium-range guidance and
ensembles continue to differ amongst each other and in a run-to-run
sense on whether any consolidation of vorticity can take place. If
it does, it could result in a meaningful precipitation event (e.G.,
12z gem). If the pattern remains ill-defined, it will likely result
in no more than a few showers for areas south of M-59. At this point
will continue to play the blender approach with low chance pops over
the south this weekend.

Marine...

Light wind in advance of a cold front will turn moderate
northwesterly upon its passage late tonight into Thursday.
Accompanying frontal passage will be a chance of thunderstorms
across the waters. Elevated waves will exist in the nearshore
waters, but are not expected to meet Small Craft Advisory criteria
at this time. Moderate winds will turn northeasterly as high
pressure builds in Friday and Saturday. Small craft advisories will
be necessitated during this time due to persistent onshore flow.

&&

DTX watches/warnings/advisories...
Michigan...Beach hazards statement from 11 am EDT this morning through this
evening for miz048-049-055-063.

Lake Huron...Small Craft Advisory from 11 am this morning to 4 am EDT Friday for
lhz421-422-441>443.

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

$$

Aviation.....Dg
discussion...cb/dt
marine.......jvc



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