Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 201953

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
353 PM EDT Sat May 20 2017


MVC from earlier convection over the Plains is now lifting from
southwest Michigan into Central Michigan, and should track through
the Saginaw Valley during the very late afternoon and early evening.
Showers and thunderstorms associated with this feature will continue
to lift across the area, and should continue to exhibit a gradual
weakening trend they encounter higher stability over Michigan.
Overall, expecting high coverage of measurable rainfall.

Large upper low over the Central Plains will slowly lift northeast
through the upper Midwest late tonight and Sunday, and into Ontario
by Monday. This system will provide a good chance for precipitation
as it lifts to our west, with the main area of precipitation still
expected to push across the area late tonight into Sunday. This will
occur as the better moisture axis works through the area and a
shortwave trough axis pivots through the Western and Central Great
Lakes late tonight and Sunday. There looks to be a break in the
forcing during the late morning to early afternoon, with some drier
air also filling into the area, before the cold front sweeps across
the area later in the day. Extent and strength of any second round
of convection will depend, per usual in Michigan, on how much we can
destabilize before the frontal passage and perhaps even on how much
dry air becomes entrained into the boundary layer. NAM/GFS show
activity becoming surface-based after some daytime heating, with
about 500-1200 J/kg of SBCAPE able to build from roughly Flint east
by the time the cold front works across late in the day. (This is
assuming we reach forecast highs.) Skinny CAPE profiles and not too
impressive wind field should limit the threat for severe storms.
Temperatures will be a little tricky given muddled start to day, but
southerly flow ushering in warmer air ahead of the front and mild
start to the day should translate to max temps in the 70s.

Another strong shortwave looks to pivot through Central/Northern
Michigan as it rotates around the upper low over Ontario on Monday.
Forcing and moisture from this feature looks to remain primarily
north of the area however, and expect that is where precipitation
will remain. More noteworthy item for Monday will be the development
of breezy conditions once again as a westerly gradient tightens
between surface low pressure over Ontario and high pressure
strengthening over the Plains and Ohio Valley. Temperatures will
cool in the post-frontal airmass on Monday, reaching into the 60s,
which is just slightly below seasonal normals.

Mid-level troughing in association with an cut-off low will bring
rain chances for much of the extended period. A surface low pushing
east from IL/WI into lower Michigan will start off by bringing rain
chances Tuesday into Wednesday. The cut-off low is then expected to
be stationed over or near the Ohio Valley, and will spin-up a series
of upper-level disturbances which will bring additional chances for
pop-up showers Wednesday into Thursday. While a rumble of thunder
may be possible embedded in a shower or two, stability parameters
remain not all that impressive, thus general thunder chances have
been left out of the forecast. As the cut-off low moves east into
New England, upper-level ridging is then expected to settle across
the North Central Plains and Great Lakes through Friday and
Saturday, which will act to diminish rain chances.

A warming trend is also looking more probable for Memorial Day
weekend. Expect daytime highs peaking in the 60s during the middle
of the week, with a warm-up into the 70s for both Friday and



High pressure over the eastern Great Lakes and low pressure over the
Midwest will combine to support continued moderate easterly wind
early tonight. Small craft advisories remain in effect through the
evening before a weakening and veering wind trend develops
overnight. This will shift stronger wind over north sections of Lake
Huron through Sunday as a weak cold front moves through the region.
Southwest flow will then strengthen during Monday as low pressure
stalls and deepens north of Lake Superior. This will likely produce
wind exceeding small craft advisory levels on Saginaw Bay before
diminishing Monday night. The middle of the week looks unsettled in
terms of rain as another low pressure system develops over Missouri
Tuesday and moves into Lower Michigan Wednesday.



A weakening pattern of showers with isolated thunderstorms will move
through SE Michigan during late afternoon. Locally heavy downpours
with this activity will be capable of a quick 0.25 inch of rainfall.
This will be followed by a break in coverage through most of the
evening before another round of showers and storms develops toward
midnight and lasts through mid Sunday morning. There will be a
better chance of thunderstorms and heavy rainfall during this time.
Additional amounts of 0.25 to 0.75 will be possible producing totals
up to 1 inch in spots. The break between the late afternoon and
overnight activity will minimize potential for flooding, although
some ponding on area roads and in prone areas will be possible.


Issued at 1258 PM EDT Sat May 20 2017


Showers/storms over Indiana are holding up well against the usual
tendency for weakening that occurs with convection of nocturnal
origin. This is thanks to MCV organization on the larger scale and
which will continue to support at least a pattern of showers into SE
Michigan terminals during the afternoon. The most recent short term
guidance indicates a weakening trend on intensity while mostly
maintaining coverage, and suggests holding off a little longer on
mention of thunder. Plan to continue monitoring trends and add with
updates as observational evidence firms up into Ohio/SW Lower Mi.
Moderate easterly flow will otherwise maintain VFR through the
evening before lowering to MVFR/IFR overnight as the warm front and
triple point move into the region. The low pressure system will also
bring in renewed coverage of nocturnal showers/storms overnight
through Sunday morning.


* Moderate for ceiling aob 5000 feet this afternoon and evening...
  high overnight.

* Low for thunderstorms during late afternoon and overnight.


MI...Lakeshore Flood Advisory until 8 PM EDT this evening for MIZ076-083.

Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ421-422-

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for LEZ444.




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