Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 200908
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
508 AM EDT Thu Apr 20 2017

LATEST UPDATE...
Mesoscale Discussion

.MESOSCALE DISCUSSION...
Issued at 508 AM EDT Thu Apr 20 2017

Narrow line of storms with heavy rain and gusty winds moving
through the forecast area at this time. Wind gusts along the line
are generally 30 to 40 mph although there was a brief period of
much stronger gusts above 60 mph from Big Sable Point to Manistee
when the line moved onshore. We do not expect any more severe
gusts although radar velocity data does show evidence of ducting
waves in the vicinity of the deeper updrafts, so the 45 knot
velocities seen at about 800 feet AGL just west of GRR at this
time could possibly reach the ground.

Although heavy rain is occurring along the line of storms, it will
not be persistent enough to cause flash flooding.

&&

.SYNOPSIS...
Issued at 330 AM EDT Thu Apr 20 2017

Low pressure will bring a couple waves of shower and thunderstorm
activity through the area today. The first round will develop
along a warm front before dawn this morning. The second wave will
come with the trailing cold front this afternoon. A few of this
afternoon`s storms could be strong over far southeastern sections
of the forecast area.

Once the front clears Lower Michigan, much cooler air will push
into the region with highs in the 50s Friday and Saturday. Quieter
weather will ensue through the weekend and into the beginning of
next week. Active weather should hold off until late Tuesday when
another upper trough begins to dig into the Great Lakes Region.

&&

.SHORT TERM...(Today through Saturday)
Issued at 330 AM EDT Thu Apr 20 2017

The main issue in the short term deals with the system that is
approaching the area today. Showers and thunderstorms will come in
two rounds with the second wave having a the better potential for
severe weather over southeastern sections of the forecast area.

Showers and storms are currently developing as a result of warm
advection that is pushing into the area just ahead of the main
system. Much of the activity remains elevated with brief bouts of
locally heavy rainfall and a few strokes of lightning. Isolated
areas of small hail and gusty winds are possible, especially north
of Muskegon, where to bulk of the energy resides this morning.
This threat will be limited and activity is expected to remain
sub-severe.

As the surface low approaches with the trailing cold front this
afternoon, the core of the jet rounds the base of the upper
trough. Bulk shear of over 30kts and increasing instability could
result in organized thunderstorms along the cold front through
the early and mid-afternoon. High res guidance places much of the
convection near and southwest of the Grand Rapids area from 2pm
and beyond. The greatest threats will primarily be large hail and
damaging wind. An isolated tornado cannot be ruled out, but the
greatest risk for anything of that nature would likely be over
far southeastern sections of the CWA...south of Lansing/near
Jackson.

The front clears the area late in the afternoon with a few
isolated showers or drizzle left behind. Otherwise, cooler less
dramatic weather moves in through Friday with some sunshine
possible late in the day.

.LONG TERM...(Saturday night through Wednesday)
Issued at 330 AM EDT Thu Apr 20 2017

A large portion of the long term will be inactive with systems
passing well north and south of of the Great Lakes.

The upper level pattern becomes zonal through the day Sunday and
Monday. If current guidance holds, we will see a rather pleasant
end to the weekend, which will carry over into the beginning of
next week. Temperatures will return to near normal with highs in
the lower 60s both Sunday and Monday with partly to mostly sunny
skies.

An upper level trough begins to develop over the Upper Midwest as
a surface low tracks northeast through Minnesota early Tuesday. A
cold front extending southwest of the surface low will push
through Tuesday night. This could bring another round showers and
thunderstorms with the front pushing east by Wednesday morning.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 155 AM EDT Thu Apr 20 2017

An area of showers and thunderstorms continues to come together
over Lake Michigan at TAF issuance time. Development will work
eastward into SW Lower Michigan over the next few hours and will
be the first round of thunderstorms that is expected. High res
guidance has much of the first wave through by 10z. Aside from IFR
conditions resulting from TS, lowering cigs will be the main cause
for IFR conditions tonight.

Another round develops as the system approaches today. A cold
front will be pressing in from the west. Ahead of the front, winds
will gust out of the south and southwest in the 20-25kt range.
Thunderstorms will fire along and ahead of the front through early
afternoon. Storms could lead to IFR vsbys/cigs until the front
passes. Once the front clears, winds switch out of the west
through the evening. Rain chances will decrease from west to east
through the afternoon/evening.


&&

.MARINE...
Issued at 330 AM EDT Thu Apr 20 2017

An area of low pressure will approach the Great Lakes today with a
couple rounds of shower and thunderstorm activity expected. The
first round is ongoing and a will continue to push through prior
to the dawn hours this morning. Locally heavy rain, lightning and
isolated areas of small hail are possible with the first round.

The second round will come with a cold front later this morning
and could also include lightning and heavy rainfall. Ahead of the
front, southerly winds will continue to increase, resulting in
hazardous boating conditions. Once the front passes, winds will
turn out of the west Thursday night, increasing to around 30
knots. Waves could reach 6 feet in places through Thursday night.
Winds will eventually turn out of the northwest through Friday
morning with high wind and waves expected to continue.


&&

.HYDROLOGY...
Issued at 1118 AM EDT Wed Apr 19 2017

River levels have returned closer to normal in many locations but
still remain a little high for the time of year. Rivers are above
bankfull and falling at Ionia, Maple Rapids, and Eagle, while
Evart is above bankfull and rising. Rain totals Thursday in excess
of an inch are possible upstream of Evart, which may boost the
crest over the comping days. Croton may reach bankfull by Friday
also. More spotty rain totals are expected in the Grand and
Kalamazoo basins, which should have a limited effect on main stem
rivers. Smaller river basins that receive locally heavy rain
(including Eagle) would be susceptible to rise again.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 6 PM EDT Friday for LMZ844>849.

&&

$$

MESOSCALE...Ostuno
SYNOPSIS...JAM
SHORT TERM...JAM
LONG TERM...JAM
AVIATION...JAM
HYDROLOGY...CAS
MARINE...JAM



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