Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211732
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
132 PM EDT SUN AUG 21 2016

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 323 AM EDT Sun Aug 21 2016

Showers and a few thunderstorms are expected today into early this
evening as a fall like storm system departs the area. This will
bring the coolest aid Southwest Michigan has seen since early
June. Skies should clear tonight allowing temperatures to fall
into the mid to upper 40s over northern areas to the mid 50s for
most of the rest of the area. It will be mostly clear Monday into
Tuesday with a slow warming trend. The next frontal system will
bring showers and thunderstorms back to the area later Wednesday
into Thursday.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 323 AM EDT Sun Aug 21 2016

The main issue today is the convection expected into early this
evening. Beyond that given that it should clear tonight, one has
to wonder of we will not see lows in the 40s over northern area?
Little question Monday and Tuesday will be mostly clear with a
slow warming trend.

Today we have the upper level cold pool moving through the area.
When this area moved through Minnesota on Sunday there were
scattered thunderstorms. As this moves over southwest Michigan
this afternoon all the models show significant instability with
the NAM showing the greatest instability. Both the NAM and GFS by
early afternoon show most unstable cape in the range of 500 to
1000 j/kg inland of US-131. This area of instability drops
southeast from NW lower during the mid morning hours to the
Detroit area by early this evening. On both the GFS and NAM the
equilibrium level is near the trop (around 300 mb or about 30,000
ft). Note too the SPC has all of lower Michigan in general thunder
today too. This lead me to add thunder to the forecast today.

I want to point out the NCAR ensembles for 1 hour precipitation
amount of 0.01 inches or more are near 100% early this afternoon
between I-94 and I-96. One other aspect to this is one has to
consider the upper level jet core will be south of Michigan today,
which is why we are in the upper level cold pool, meaning we are
on the cold side of the polar jet. During the fall and winter
months this is key to lake effect snow showers. Since it is August
we get rain showers instead of snow showers. Remember too the lake
surface temperature is in the mid 70s. So, like in the fall the
air coming over Lake Michigan at the surface is colder than the
lake. That creates instability convection as long as the low
levels are moist enough, which they will be today. Given all of
this I increased the pop to likely across the area and added
scattered thunderstorms.

Tonight the polar jet lifts out of the area ending the threat of
showers and allowing skies to clear. In fact upper level ridging
builds into the area for Monday and Tuesday. Surface high pressure
moves overhead Monday afternoon and to the east of the area Monday
night. This will lead to a slow warming trend but keep the skies
mostly clear. Light winds and nearly clear skies at night should
allow lows to fall into the 40s to near 50 north and lower to mid
50s elsewhere.

.LONG TERM...(Tuesday night through Saturday)
Issued at 323 AM EDT Sun Aug 21 2016

High pressure will be moving away from the central Great Lakes
Tuesday and a south flow of warmer air will be in place. Highs
around 80 are expected for a few days during the middle of the week
until a cold front moves through by the weekend.

Chances for showers and storms will increase Wednesday with highest
chances Wednesday night as the cold front moves across the cwa. Back
to dry wx Friday and Saturday with highs in the 70s and lower
humidity. Chances for severe storms Wednesday-Thursday don`t look
great at this point as shear values are progd to be less than 25
knots.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 132 PM EDT Sun Aug 21 2016

Scattered showers and areas of MVFR conditions will continue until
late afternoon. There could be an isolated thunderstorm, but have
left this out of the TAFs due to the isolated nature.

Expect both the showers and MVFR will become isolated by about
21z and gradually diminish into the evening.

West winds of 10-15 knots will continue through the afternoon, but
drop quickly by early evening.

&&

.MARINE...
Issued at 323 AM EDT Sun Aug 21 2016

I plan on no changes to our marine headlines. Just about all of
our models both Hi Res and otherwise show the core of strongest
winds through early this morning then winds in the 20 to 25 knot
range continue into early afternoon before decreasing
significantly by evening. So beach conditions folks wanting to
sail in our near shore waters should be significantly improved
Monday and Tuesday.


&&

.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...Beach Hazards Statement until 1 AM EDT Monday for MIZ037-043-050-
     056-064-071.

LM...Small Craft Advisory until 1 AM EDT Monday for LMZ844>849.

&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...99
AVIATION...JK
HYDROLOGY...Hoving
MARINE...WDM


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