Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 110948

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
548 AM EDT Fri Aug 11 2017


Early morning showers with isolated thunder is working east out of
the area. Should have several VFR hours through the early afternoon
before the showers and storms begin to pop up in advance of an
approaching cold front. Timing the cold front puts it through MBS
around 18Z and DTW closer to 22Z. With a bit higher confidence in
convection this afternoon, opted for tempo groups for some sites to
account for the storms. As storms clear out tonight, will have to
watch for falling CIGS. Opted for MVFR to start and can assess after
we see where the rain falls and how fast it clears out.

For DTW...Still highlighting 21-00Z for the best chance of afternoon
convection although some isolated cells could start initiating as
early as 18-19Z in the vicinity.


* Low for ceilings at or below 5000 feet early this morning. Medium
  after 15Z this afternoon and again after 06Z tonight.

* Low for thunderstorms after 18Z Friday.


Issued at 308 AM EDT Fri Aug 11 2017


Sfc low pressure will track into ern Upper Mi this morning,
associated with an upper trough slowly churning across the northern
Great Lakes. There is a mid level moisture plume now extending
across wrn and cntl Lower Mi, expected to pass through Se Mi in the
09Z to 14Z time frame. Upper level divergence within the exit region
of an upper jet diving across the mid Mississippi Valley will
overspread Se Mi this morning. This combined with the potential for
some nocturnal cloud top cooling early this morning suggests the
upstream convection will hold together to some degree as it works
east this morning.

Lingering elevated convection will diminish by late morning giving
way for afternoon convective development rooted in the boundary
layer. The focus for showers/thunderstorms later today will be
convergence along/ahead of a sfc cold front set to pass through Se
Mi this afternoon. The degree of daytime destabilization will affect
the coverage and intensity of afternoon/evening thunderstorms.
Considering the potential for some residual mid clouds, 0-1km ML
cape will likely peak around 1500 j/kg. Deep layer shear is expected
to range from 30 to 35 knots. Despite the overall marginal
cape/shear parameters, relatively cold mid level temps will
contribute to steep mid level lapse rates. This and a push of mid
level dry air from the west will support good downdraft cape,
suggesting a potential for some strong to severe storms. If there is
some over achievement on daytime heating and/or boundary layer
moisture advection this afternoon, severe storm chances will be much
higher. Convective mode is likely to be a combination of linear and
super cellular (0-1km helicity actually looks quite good along the
eastern third of the forecast area later today).

The depth of mid level dry air will increase in the post frontal
environment, ending the risk of convection from west to east during
the late afternoon and early evening. The upper trough and
associated sfc low will slowly exit east of the UP tonight into
Saturday morning. Low level moisture wrapping around the sfc low
will filter into Se Mi overnight within the low level thermal
trough. This moisture will lift into a fairly extensive strato cu
field on Saturday. Convective cloud depths will be fairly shallow,
but my be deep enough to support some light showers across the thumb
region. A secondary sfc trough axis forecast to push down across
Lake Huron may also contribute to a few afternoon showers in the
thumb. Otherwise the cloud cover and relatively cool low level
airmass will keep daytime highs confined mainly to the 70s.

High pressure will expand into Lower Mi during the later half of the
weekend amidst mid level confluent flow. With some sunshine and a
little moderation in the low level temps, highs on Sunday will push
into the upper 70s/near 80 over much of the area. Model solutions
suggest some weak mid level short wave features will traverse the
region on Monday. These will likely only result in some mid/high
level clouds as the low levels will be quite dry. Building mid level
heights are forecast across the ern US during the course of the work
week, suggesting a modest warming trend across the southern Great
Lakes region.


A weak low pressure system will track slowly across northern MI
today and tonight. This system will pull a cold front through the
eastern lakes this evening likely resulting in showers and
thunderstorms. Winds will be southerly ahead of the system today
before switching to the northwest behind the cold front later this
evening and overnight. Wind gusts today will stay below 20 knots,
but more unstable flow on Saturday should cause gusts to increase
closer to 25 knots across the Huron basin.


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



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