Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 191757
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
157 PM EDT FRI AUG 19 2016

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 330 AM EDT Fri Aug 19 2016

We are entering an active weather period Today across the Great
Lakes region, which will stretch into Sunday. Today we look for
scattered showers and thunderstorms to develop in a warm and moist
southerly flow of air. Tonight into Saturday low pressure will form
off to our southwest and gradually move into the northern Great
Lakes. The low will strengthen Saturday night and into Sunday
driving a cold front through our area Saturday evening. Showers
and thunderstorms are likely Saturday afternoon and evening along
and ahead of the cold front. Some of these storms may become
strong to severe. Strong westerly winds behind the low on Sunday
will bring cooler and showery weather to the area. Highs will push
into the 80s today, hold near 80 on Saturday beneath cloud cover
and hold in the 70s on Sunday.

&&

.UPDATE...
Issued at 1119 AM EDT Fri Aug 19 2016

The showers and storms will continue to ease into the CWA today.
We are removed from the best moisture and jet dynamics that
remain to our northwest, so in general the pcpn should slowly
weaken, but still make it across the CWA. Included a timing
element in the grids over the eastern CWA, where scattered pcpn
shouldn`t reach U.S. 127 until mid afternoon.

Satellite trends would indicate we will struggle to get much sun
today. This will lead to lesser instability, and also result in
temps only climbing slowly. Tweaked max temps slightly lower.
Despite temps struggling today, dew points will inch up with the
increased moisture, so it will remain very humid, and even become
increasingly uncomfortable into the afternoon and evening.

&&

.SHORT TERM...(Today through Sunday)
Issued at 330 AM EDT Fri Aug 19 2016

Shower and thunderstorm chances remain the focus right through the
short term. Ascertaining the threat of severe weather Saturday
afternoon and evening is also a focus.

Chances for showers and storms Today and Tonight look rather
scattered in nature with not much of a low level jet to speak of. In
general the low level jet through 12z Saturday is on the order of 20
knots or less. Our chances for showers and storms will likely be
tied to upstream convective vorticity maximums that move our
direction from Wisconsin. One such vort can be seen spinning across
far southwest Wisconsin at 0730z. Essentially have chance pops (30-
50 pct) both today and tonight. Hard not to think our chances will
be somewhat diurnally focused with CAPE values peaking this
afternoon in the 2000-3000 j/kg range. Given weaker wind fields any
threat for severe will come towards max heating in the strongest
cells.

Main time frame of concern will come Saturday afternoon and evening
with strong wind fields are forecast to come together late in the
day. The extent of convection tonight into Saturday morning will
likely determine our chances for severe later Saturday afternoon
and evening. Depth of the low is still in question as the NAM and
Canadian are stronger and slower. Bottom line is a broken line of
storms will work through the area between 18z and 03z. Severe
weather cannot be ruled out and agree with the marginal threat for
now.

.LONG TERM...(Sunday night through Thursday)
Issued at 330 AM EDT Fri Aug 19 2016

High pressure will produce fair wx with seasonably cool temperatures
Sunday night through Monday.  Temperatures will fall into the 40`s
Sunday night over our northern fcst area and high temps Monday will
only reach the lower to middle 70`s.

Fair weather will continue Tuesday and Wednesday and temperatures
will undergo a slow moderating trend in a south to southwest flow
waa pattern on the back side of the departing ridge. The next system
approaching from the west will bring the next chance for showers and
thunderstorms late Wednesday through Thursday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 157 PM EDT Fri Aug 19 2016

The threat of tstms this afternoon has diminished across sw lwr
MI due to extensive cloud cover/lack of instability, so have
removed CBs from the TAFs. Confidence is low as to whether or not
storms will develop tonight, so for now went with VCSH after
midnight. The chance for tstms will ramp up after 15z Saturday
and confidence is higher in this regard. Some fog/status could
develop later tonight and linger into early Saturday, especially
if we clear out this evening. A period of IFR or lower conditions
cannot be ruled out if this occurs, particularly 09z-13z.

&&

.MARINE...
Issued at 330 AM EDT Fri Aug 19 2016

Southerly winds today may push waves into the 2-4 foot range at
times, but overall expecting sub advisory conditions. The weekend
looks to be rough on Lake Michigan and that may start as early as
Saturday afternoon. Southerly flow begins to strengthen in earnest
then and winds by evening could be pushing into the 20-30 knot
range. Waves may reach Small Craft Adv criteria by evening. The main
time frame of concern however is Saturday night into Sunday. Models
have yet to come into full agreement on the depth and position of a
strong low moving through the western Great Lakes. We are at least
looking at Small Craft Adv criteria these two periods as well as
Beach Hazard Statement worthy conditions. If the deeper NAM/Canadian
solutions verify we could be talking a rather rare August gale
event. Bottom line, later in the weekend will be very atypical for
summer time in the Great Lakes. Waves of 4 to 7 feet look likely
later in the weekend with 6 to 10 footers in the realm of
possibilities.


&&

.HYDROLOGY...
Issued at 100 PM EDT Fri Aug 19 2016

Most rivers have crested or will very soon. The rivers within the Grand,
Kalamazoo, and St. Joseph river basins are flowing at or above the 90th
percentile for this time of year compared to historical readings. Any
added rainfall will continue to keep the rivers elevated. 24 hour rainfall
ending Saturday morning is not expected to cause additional rises. Locally
heavy rains Saturday afternoon and evening may need to be watched for
excessive runoff into elevated rivers given saturated soils in place.
In general, basin average amounts are expected to be 0.50"-1.00" through
Saturday night. This may, at the least, slow some of the receding flows
but could cause new rises into the weekend.

&&

.GRR Watches/Warnings/Advisories...
MI...None.
LM...None.
&&

$$

UPDATE...JK
SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...Laurens
AVIATION...Meade
HYDROLOGY...Hoving
MARINE...Duke



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