Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 161848
AFDGRR

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

LATEST UPDATE...
Hydro

.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 212 PM EDT Wed Aug 16 2017

Multiple rounds of thunderstorms are possible Thursday. This may
lead to localized heavy rainfall where storms more frequently
occur. Early to mid morning showers and storms are expected along
a warm front and afternoon/evening showers and storms are possible
especially east of US 131. PWAT values will be quite high,
approaching 2.0 inches, along with surface dew points above 70.
There is some potential for a few training thunderstorms with MBE
velocities occasionally dropping below 10 kts. However, this does
not look like a classic flood setup outside of localized ponding
of roads. We will continue to monitor trends.


&&

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

$$

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


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