Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171713
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
113 PM EDT Thu Aug 17 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
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.

&&

.UPDATE...
Issued at 1135 AM EDT Thu Aug 17 2017

CAPE values remain low late this morning, generally under 1k.
There are pockets of clearing over Lake Michigan and NW Indiana
behind the current wave of pcpn crossing the CWA, leading to the
potential to become more unstable this afternoon.

A warm front currently lies from roughly LDM to MOP as of 15Z. A
vigorous surface low will be crossing Northern WI and into the
U.P. today and dragging a cold front through SW MI early to mid
evening.

0-6km bulk shear values remains 30-35 kts through the day, with
close to 20 kts with the first 1km. CAPE values will likely remain
near 1k over the Western and Central CWA as this area will be
slow to recover from the morning showers. However we have the
potential to see CAPE increase toward 2K over the Eastern CWA,
toward U.S. 127, for a brief time this afternoon.

The window for severe storms appears short, mainly from roughly
3 PM until 6 or 7 PM. And agree with SPC that the main risk is
over the Eastern CWA. However we need to monitor the potential for
rapid development of these storms, which could quickly become
rotating storms, especially north of I-96, near the warm front.

&&

.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 18Z TAFS through 18Z Friday afternoon)
Issued at 113 PM EDT Thu Aug 17 2017

Radar showing latest wave of showers and thunderstorms over
southeastern Michigan...with widely scattered showers from
northwest Lower Michigan across Lake Michigan into Wisconsin.
Short range model guidance suggests that the best chances for
widespread showers and thunderstorms will at KJXN and KLAN from
19Z to 22Z. Will keep a vicinity wording at the remaining TAF
sites as chances are non zero.

Ceilings are ranging from below 1 kft AGL at KMKG to around 1.5
kft AGL at KAZO/KLAN/KBTL to at or above 5 kft AGL at KGRR and
KJXN. Upstream ceilings in the 1.5 to 2.5 kft AGL range are
widespread...with below 1 kft AGL over northwest Lower Michigan.
Will keep a MVFR ceiling area wide this afternoon...with the
exception of KMKG...where IFR ceilings will likely continue.

Should see some temporary improvement in ceilings to possibly VFR
conditions this evening as evident over Wisconsin over the past
few hours...but expect a return to MVFR later tonight. The rain
chances will increase as the lower ceilings return...which was
depicted well in the current forecast.

&&

.MARINE...
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.


&&

.HYDROLOGY...
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.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from 6 PM EDT this evening through
     Friday evening for MIZ064-071.

     Beach Hazards Statement through Friday evening for MIZ037-043-
     050-056.

LM...Small Craft Advisory until 9 PM EDT Friday for LMZ846>849.

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

&&

$$

UPDATE...JK
SYNOPSIS...Meade
SHORT TERM...Meade
LONG TERM...04
AVIATION...MWS
HYDROLOGY...Hoving
MARINE...Meade



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