Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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676
FXUS63 KGRR 161748
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
148 PM EDT Wed Aug 16 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
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.

&&

.UPDATE...
Issued at 1100 AM EDT Wed Aug 16 2017

Added a slight chance POP to the far NE CWA for late today and
this evening. Lake breeze interaction may occur in this region off
of Lake Huron. With increase dew points/low level moisture and
instability through the day, some diurnal convection is possible
mainly after 4 PM and then dieing off by sunset.

&&

.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 shear 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
showers/storms.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 147 PM EDT Wed Aug 16 2017

Main impacts to aviation operations in this forecast period are
the thunderstorms and low clouds expected late tonight and Thu
morning.

Until tonight, diurnal cumulus based around 4-5k ft will be the
rule away from the lakeshore through around sunset. Winds will
generally be under 10 knots from the SE, with onshore wind at the
lakeshore.

We expect storms to start developing over the area after 06z
tonight. These look to be spotty in nature at this time. MVFR cigs
and vsbys are expected with any storms with some local IFR
possible. More widespread low clouds will start forming around or
after 08z as the warm front approaches. Storms will remain
possible until warm front pushes through from South to North Thu
morning.

Once the warm front pushes through, thunderstorm chances will
temporarily end, low clouds will blow out, and gusty SSW winds
will develop. Additional strong storms will be possible later
Thursday, after this valid fcst period.

&&

.MARINE...
Issued at 1100 AM EDT Wed Aug 16 2017

Rough conditions out on the lake may occur a bit sooner than
expected on Thursday as strong southerly flow proceeds the
surface low. Solid likelihood we will need a small craft advisory
by afternoon for areas north of Grand Haven. We will likely issue
this with today`s afternoon package. then as we get into Thursday
night, the entire nearshore area, including south of Grand Haven,
will likely need a SCA as brisk winds swing around to the west.


&&

.HYDROLOGY...
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.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

UPDATE...JK
SYNOPSIS...Meade
SHORT TERM...Meade
LONG TERM...04
AVIATION...NJJ
HYDROLOGY...JAM
MARINE...JK



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