Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 160730

Area Forecast Discussion
National Weather Service Grand Rapids MI
330 AM EDT Wed Aug 16 2017

Synopsis/Short Term/Long Term/Marine

Issued at 330 AM EDT Wed Aug 16 2017

High pressure will provide pleasant weather today. Low pressure
tracking northeast through Wisconsin on Thursday will send very
humid air into the region that will help trigger numerous showers
and thunderstorms. Some of the storms could be strong and produce
torrential rainfall. Cooler and breezy weather will follow on
Friday then temperatures will moderate back to near to above normal
over the weekend into early next week.


.SHORT TERM...(Today through Friday)
Issued at 330 AM EDT Wed Aug 16 2017

Not much change regarding the Thursday system. Will continue to
run with high pops as fairly impressive H5 shortwave trough for
this time of year interacts with High PWAT air around 2 inches
and sfc dew pts in the lower 70s. It appears we may have one round
of rain with the warm front/low level jet very late tonight into
Thursday morning, then perhaps a bit of a lull before the
shortwave and sfc cold front/occlusion brings another round from
mid afternoon into the evening.

The Thursday afternoon/evening storms will have the potential of
producing excessive rainfall rates given very moist air mass,
deep/warm cloud layer and low LCLs. Severe wx threat is contingent
upon how much heating/instability can develop, but progged deep
layer values of 30-35 kt are sufficient for organized convection
and capes could exceed 2000 J/KG if any cloud thinning occurs.
The svr wx/hvy rain threat should end by midnight Thursday night
with the frontal passage and arrival of drier/more stable air
mass from the west.

Considerable clouds and lingering showers expected Friday morning,
followed by decreasing clouds later in the day as the sfc low
lifts out.

.LONG TERM...(Friday night through Tuesday)
Issued at 330 AM EDT Wed Aug 16 2017

A trough will move across the Great Lakes Saturday bringing a chance
of showers and thunderstorms. Zonal flow aloft will push systems
quickly through the cwa. Another cold front will move through
Tuesday resulting in another chance of showers/storms. In between,
we`ll see typical summer weather with dewpoints well into the 60s,
which means humid weather. Highs will be in the lower to mid 80s. A
good amount of sunshine too can be expected outside of the


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 136 AM EDT Wed Aug 16 2017

There may be some mvfr fog from 09z-12-13z along the I-94 taf
sites due to the rain that fell last night. Otherwise vfr is
expected. Showers and thunderstorms may develop late in the
period, but most likely will hold off until after 06z.


Issued at 330 AM EDT Wed Aug 16 2017

Confidence is fairly high that we will need small craft advisories
and beach hazard statements Thursday night into Friday. Winds are
expected to increase significantly out of the west-northwest as
the sfc low tracks across ern upr MI and into Ontario.


Issued at 1137 AM EDT Tue Aug 15 2017

Observations reflect 24-hour rainfall amounts of 0.25-0.50 and
locally around 1.00 inch between I-96 and I-94, stretching from
Lake Michigan, east to the Detroit area. A couple of the rivers in
this swath (e.g. Jackson) show rises, but are expected to remain
well within banks. Another area of beneficial rain fell from
Ludington to Harrison and locations to the northeast.

The midweek system is beginning to look a bit more impressive with
each update in guidance. Precipitable water values are around and
above 2 inches, especially along and south of I-96, late
Wednesday night into Thursday afternoon. It is looking more
plausible that at least locally heavy rain could result as the
best lift moves in along the warm front Wednesday night into
Thursday morning. Another round of rain could develop with the
passage of the cold front during the day Thursday. Timing could
play a role in determining thunderstorm intensity, which could
affect overall rainfall amounts. Given the anomalous moisture
values and overall potential, this system may need monitored as
the forecast evolves through Wednesday.




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