National Weather Service Raw Text Product

784 
FXUS63 KGRR 201752
AFDGRR

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

LATEST UPDATE...
Aviation

.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 18Z TAFS through 18Z Friday afternoon)
Issued at 151 PM EDT Thu Sep 20 2018

VFR conditions will prevail although some brief IFR is possible in
thunderstorms from late afternoon through early evening. Strong
gusty winds and hail are also possible with these thunderstorms,
which should end by late evening.

Low level wind shear can be expected later tonight. Strong west to
southwest winds with gusts over 30 knots will reach the ground
during Friday morning with the strong winds continuing through the
afternoon.

&&

.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...Ostuno
HYDROLOGY...63
MARINE...Meade