Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171156

Area Forecast Discussion
National Weather Service Grand Rapids MI
756 AM EDT Thu Aug 17 2017


Issued at 336 AM EDT Thu Aug 17 2017

Expect humid conditions today with occasional showers and
thunderstorms as a low pressure system tracks through Wisconsin.
Friday will be breezy, cooler, and less humid with some lingering
clouds and light showers. A warming trend is forecast over the
weekend into early next week.


.SHORT TERM...(Today through Saturday)
Issued at 336 AM EDT Thu Aug 17 2017

The primary time frame of concern for severe weather today is
roughly Noon to 6 PM and the main area of concern is east of
highway 131, particularly the LAN and JXN areas. This is where
favorable instability/shear ingredients could come together,
although it is highly contingent on at least a couple hours of sfc
heating in the wake of morning warm advection showers.

The current band of showers crossing the area represents the
leading edge/gradient of the higher PWAT air. After this passes
through, another more solid area of showers over IL, associated
with a 35-40 kt LLJ, should impact the area between roughly 8 AM
and Noon.

It is in the wake of the second area of rain where a relatively
brief window will exist to get some heating and sfc based
instability before the increasing southwest low level flow off Lk
MI sweeps eastward. This convection, on the leading edge of the
mid level dry slot and convergent lake shadow could become quickly
severe with sfc dew pts in the lower 70s in place by that time
(and a tongue of H8 dew pts near 17C).

Deep layer shear of 35 to 40 kts could lead to an organized
band/cluster of storms developing east of Highway 131 by 18Z,
with even some support of a tornado if/where sfc wind remains
backed enough as the 35-40kt southwest low level jet is still
overhead. Low LCL heights may also favor a quick spin up. The main
threat of svr wx (and locally heavy rain) looks to shift east of
our area by 22Z.

The best risk of showers on friday is north of I-96 where deeper
moisture is progged in the commahead of the system and H8 thermal
troughing is most pronounced. Still, mainly just light qpf
amounts expected.

Another shortwave traverses the area late Friday night and
Saturday morning with about 500-1000 J/KG of MU Cape advertised by
the GFS. Currently the guidance indicates best probability of
shower/isolated tstms with this feature will be south of I-96,
but only worth chc pops at this time.

.LONG TERM...(Saturday night through Wednesday)
Issued at 336 AM EDT Thu Aug 17 2017

The long term period will be book ended by dry weather with pcpn in
the middle. High pressure over the Great Lakes Saturday night will
drift east by early next week. The weekend looks dry with seasonal
temperatures.  The zonal flow aloft will begin to buckle early next
week as a short wave moves toward the Great Lakes. Chances for rain
will begin to increase Monday afternoon first as a warm front moves
north through the cwa and then as a cold front follows on Tuesday.
Tuesday looks like the best chance of showers/storms. Shear values
are progd to ramp up to around 40kts Tuesday afternoon so we`ll need
to keep an eye on the severe potential.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 755 AM EDT Thu Aug 17 2017

Showers and isolated tstms this morning will lead to numerous MVFR
cigs. Showers becoming less numerous this afternoon and VFR should
be more predominate, although breezy southwest winds and any
tstms that redevelop (mainly east of GRR) could produce brief IFR
or lower vsbys in heavy rain as well as some strong gusty winds.

VFR this evening within the dry slot, then conditions will trend
downward to MVFR overnight as the stratocu clouds and isolated
showers on the back side of the low wrap back into the area from
northwest to southeast.


Issued at 336 AM EDT Thu Aug 17 2017

No change to the existing marine headlines for Holland north
today for the south-southwest flow. However will issue a new
Small Craft Advisory and Beach Hazard Statement for areas south
of Holland starting tonight at 6 PM when the flow becomes more
westerly. The advisories will run through Friday.


Issued at 330 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.


MI...Beach Hazards Statement from 6 PM EDT this evening through
     Friday evening for MIZ064-071.

     Beach Hazards Statement from noon EDT today through Friday
     evening for MIZ037-043-050-056.

LM...Small Craft Advisory from noon today to 9 PM EDT Friday for

     Small Craft Advisory from 6 PM this evening to 9 PM EDT Friday
     for LMZ844-845.



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