Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 210009

Area Forecast Discussion
National Weather Service Grand Rapids MI
809 PM EDT WED JUL 20 2016


Issued at 256 PM EDT Wed Jul 20 2016

Hot and humid weather will move into the region over the next
couple of days. Afternoon heat index values will rise to around
100 both Thursday and Friday. A very warm a humid night on
Thursday will not allow for much relief to the hot conditions.
Within this stifling air will come periodic thunderstorms. The
best chance of storms will come Thursday evening, but widely
scattered storms will be possible Thursday and Friday. A weak cold
front drops through the area Friday evening which will give the
area a slight break from the very hot and humid weather.

The weekend into next week remains warm with daytime highs mainly
85 to 90, but the humidity should be less. Another period of
scattered storms should occur Saturday night through Sunday night.


.SHORT TERM...(This evening through Friday night)
Issued at 256 PM EDT Wed Jul 20 2016

Will issue a heat advisory starting Thursday afternoon and extend
it into Friday evening when front brings some relief. Storm
chances will be within the short term too, with the best chance
coming Thursday evening.

Temps will continue to warm as the upper ridge to our west builds
our way into Thursday. It does flatten out into Friday, but
little relief will occur until a surface front comes through
Friday evening. Many areas across southern lower, and away from
the lake will see low to mid 90s both days, along with dew points
in the 70s. This pushes heat index readings toward or exceeding
100. So will issue a heat advisory for both days. Also being a
factor, will be the very warm and humid night on Thursday, not
allowing people to recover from the Thursday heat, but jumping
right back into stifling heat Friday. We could struggle to reach
the 100 degree criteria in some spots if we see widely scattered
storms, along with debris clouds, but it will be very humid and
uncomfortable, and even dangerous if prolonged exposure occurs.

As for the storms...this is a ridge rider type set up. MCSs will
largely be triggered by nocturnal low level jets. Tonight`s jet
is progged to set up over WI and the U.P. Some of these storms
should slip over the ridge and into the GRR CWA tomorrow morning.
But they should weaken as they get farther removed from the LLJ.
Expect these to remain widely scattered into the area, so will
only have 30 POPs over much of the CWA. The jet weakens through
the day and the storms should too.

Another nocturnal LLJ develops Thursday night and noses into
Lower MI, and should bring our best chance of storms. Some of
these storms could be severe with the bulk shear values around
25-30 knots. At this point it appears the MCS develops over
central/northern lower and sags south through the night.

Some of these storms could survive over the southern CWA into
Friday. But with a weak front dropping in, the storms will push
south.  This front will also give us some relief from the heat.

.LONG TERM...(Saturday through Wednesday)
Issued at 256 PM EDT Wed Jul 20 2016

Heat and humidity will remain in place over the weekend, with the
upper ridge slowly dampening out. The flattening of the ridge will
be due to a shortwave trough moving east along the U.S./Canadian
border. Highs over the weekend will be around 90.

The shortwave will pivot a low pressure system through the Great
Lakes over the weekend with a warm front lifting into the area
Saturday night. A cold front will sweep through the area on Sunday.
The upper system will then move through the area on Monday. Chances
for showers and thunderstorms will exist from Saturday night through

High pressure will build in for Tuesday and Wednesday and have a dry
forecast in place those two days.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 809 PM EDT Wed Jul 20 2016

Conditions should continue to be predominately VFR tonight and
Thursday, although a period of light/MVFR fog or haze is possible
late tonight and Thursday morning as higher dew points of 70-75
move into the area. Light south winds tonight and early Thursday
will increase Thursday afternoon to 10 to 20 kts.

Was not confident enough to fcst showers or tstms at any of the
terminals on Thursday as the main threat appears to hold off until
after 00z Thursday evening. Some convection can`t be ruled out on
Thursday, but the probability/expected coverage is not high
enough at this point to include in the TAFs.


Issued at 256 PM EDT Wed Jul 20 2016

Choppy waters, but below SCA criteria expected through Thursday
evening before a front drops in causing winds to relax somewhat.


Issued at 332 PM EDT Wed Jul 20 2016

The main hydro concern through the end of the week will be localized
flooding due to heavy rainfall. Showers and storms are expected
mainly Thursday afternoon into early Friday morning. With low
confidence in coverage, the pattern will continue to support some
locations not seeing enough rain and some locations getting too much
too quickly.

Basin-average rainfall of a half inch to 1 inch is forecast, but
strong thunderstorms could produce localized rainfall totals well
over 1 inch. As is typical with summer convection, rainfall totals
will probably vary substantially over short distances.


.GRR Watches/Warnings/Advisories...
MI...Heat Advisory from 1 PM Thursday to 7 PM EDT Friday for



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