Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201617
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1217 PM EDT Thu Sep 20 2018

LATEST UPDATE...
Hydro

.SYNOPSIS...
Issued at 320 AM EDT Thu Sep 20 2018

Showers and thunderstorms will occur today as a warm front lifts
north into Lower Michigan. Coverage of rain will decrease tonight
as the warm front lifts well north of the area, but a cold front
approaching from the west will bring another chance of showers
toward daybreak Friday. Breezy conditions will follow on Friday
behind the cold front, then dry but chilly weather is expected
over the weekend with Canadian high pressure in control.

&&

.DISCUSSION...(Today through next Wednesday)
Issued at 320 AM EDT Thu Sep 20 2018

Showers and thunderstorms associated with the elevated warm front
will become more numerous over the next few hours as low level
jet through the central plains bends/leans east in our direction.
These storms will be rooted around 850 mb, with the main threat
being heavy rainfall through mid morning as PWATs over 1.75 inches
arrive.

Probability-matched mean QPF guidance from the CAMs indicate
heaviest rain falling generally north of a Holland to Lansing
line this morning, where widespread 1 to 2 inch amounts appear
likely. Isolated higher amounts are possible, which may produce
localized nuisance flooding.

As morning convection slowly fades, surface based instability
begins to build from southwest to northeast this afternoon with
the arrival of the sfc warm front. SB Capes on the order of
1500-2500 J/KG are progged near and south of the front, which
should be located roughly along the I-96 corridor by early
evening.

Progged deep layer shear around 35 kts would be sufficient for
organized storms later this afternoon, and the presence of the
sfc warm front in the area creates concern for spin ups. The
latest idea presented by the CAMs in terms of late day diurnal sfc
based convection is for scattered storms to develop south and
east of GRR around 4 PM along an outflow/MCV leftover by morning
convection.

HREF guidance does show elevated values of 2-5 KM updraft
helicity southeast of GRR around 00Z, and the JXN HRRR fcst
sounding has a sharply curved hodograph between 0-1 KM. Thus any
late day sfc based convection will need to be watched very
closely for rotation.

Much of tonight should be dry and quite warm/muggy once the
diurnal evening convection diminishes and the warm front lifts
off to the north. The band of showers with the cold front after
09Z does not look very impressive in the CAMs but could contain
some gusty winds as it comes through. Turning mainly dry but
breezy on Friday as the mid level dry slot arrives. Wind gusts to
35 MPH possible once deeper mixing develops.

Sfc ridge and dry air over the weekend will promote pleasant
weather and cold nights with lows in the 40s. We could even see
some patchy frost near/north of U.S 10. Our next chance of rain
arrives Monday night/Tuesday as moisture streaming north from the
Gulf interacts with a cold front approaching from the west. This
has the potential to produce soaking rains which would be followed
by a significant cool down for the mid to late part of next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 751 AM EDT Thu Sep 20 2018

Showers and thunderstorms along a MKG-GRR-LAN line this morning
producing ocnl IFR/LIFR conditions in heavy rain through 15-16Z.
Conditions improving this afternoon as convection diminishes and
lifts north. New sctd tstms may develop south and east of GRR
after 19Z and linger through about 00Z. VFR/dry weather expected
tonight as the warm front lifts well north but turning breezy
overnight with southerly wind gusts over 25 kts possible after
06Z.

&&

.MARINE...
Issued at 320 AM EDT Thu Sep 20 2018

Will convert the Gale Watch to a Gale Warning, but only in the
northernmost zone north of Pentwater. Elsewhere will have Small
Craft Advisories. Initially tonight the hazard is the strong
southerly flow ahead of the cold front, then on Friday it will be
the strong north-northwest flow behind the front. Waves up to 10
feet are expected.

&&

.HYDROLOGY...
Issued at 1217 PM EDT Thu Sep 20 2018

River levels are around to a little above normal for the time of
year. Levels are also steady or slowly falling. Rain and
thunderstorms this morning dropped over an inch of rain across
Ottawa, southern Kent, Ionia, and southern Clinton Counties. This
includes much of the Grand River basin. This should result in river
level rises which remain mostly below flood stage along the Grand
River.

More storms are expected tonight into Friday morning. Rainfall rates
in excess of one inch per hour are possible. This could lead to
additional localized flooding on roads, in low-lying areas, and
along small creeks. Drier weather is expected Friday afternoon
through Sunday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Gale Warning from 11 PM this evening to 8 PM EDT Friday for
     LMZ849.

     Small Craft Advisory from 8 PM this evening to 8 AM EDT Saturday
     for LMZ844>848.

&&

$$

SYNOPSIS...Meade
DISCUSSION...Meade
AVIATION...Meade
HYDROLOGY...63
MARINE...Meade


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