Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 220151
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
951 PM EDT THU JUL 21 2016

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 334 PM EDT Thu Jul 21 2016

A weak cold front will move slowly south tonight triggering
additional thunderstorms. Locally heavy rain is possible from
these storms. The cold front will continue to slip southward into
Saturday so lingering thunderstorms are possible south of I-94
Friday morning. Otherwise hot and humid conditions are expected
Friday and Saturday as this is a really weak cold front. A much
stronger cold front will move across the area Sunday. that will
trigger additional storms. Temperatures will be close to normal
early next week behind the cold front that comes through Sunday.

&&

.UPDATE...
Issued at 950 PM EDT Thu Jul 21 2016

MCS impacting Srn WI/Nrn IL has stolen the show tonight. Hi-Res/
CAM guidance has really been struggling in this pattern as 24
hours ago it looked like that Wisconsin MCS would be sliding south
through Michigan later tonight. Short range guidance also did not
do well with the line of storms that came through the area earlier
today.

Showers and a few tstms may still redevelop/expand eastward
tonight over srn lwr MI due to H8 theta-e advection from the west.
However svr wx looks unlikely given ongoing active MCS just to our
west.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 334 PM EDT Thu Jul 21 2016

The main issue tonight will be the threat of additional
thunderstorms. The heat advisory will be continued for Friday. It
may be needed Saturday but I am not sure enough of that to extend
it at this time.

The current area of thunderstorms should be totally out of our CWA
by 4 pm. Then the question becomes can storms redevelop? There is
a cold front dropping south into our forecast area tonight and it
will only slowly move south through the CWA, not clearing the
southern CWA till Friday afternoon. So the low level jet may help
redevelop storms this evening that will sink south overnight.

Once those storms clear the area by mid morning Friday then I
expect hot and humid conditions to prevail through Saturday night.
the next cold front may trigger more storms Saturday night.

Assuming the morning convection does clear the area Friday, I
would expect highs in the mid 90s. Combine that with dew points
near 70 and heat index values should reach around 100. This may
happen Saturday but my confidence in that is too low to extend
the headline at this time.

.LONG TERM...(Sunday through Thursday)
Issued at 334 PM EDT Thu Jul 21 2016

Main weather in the long term comes in the first period or two,
Sunday and Sunday night. A shortwave trough will be traversing the
northern Great Lakes and Ontario which will swing a cold front
through the area. The ECMWF is indicating a 30-35 knot LLJ over the
CWA Sunday afternoon and evening. CAPE values via the ECMWF are on
the order of 2000-3000 J/KG. Deep layer shear is 30-40 knots across
Central Lower, so a severe threat will need to be watched going
forward.

Still warm and humid heading into Sunday, with some relief coming in
the wake of the cold front. Highs Monday through Wednesday will be
more typical in the lower to middle 80s in most areas. Dry weather
is forecast Monday through Wednesday with a high shifting east
through the area. Another shortwave in zonal flow will bring
additional chances for showers and storms in the Wednesday night
and Thursday time frame.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 803 PM EDT Thu Jul 21 2016

Fcst confidence is low over the next 24 hours in regards to
coverage/timing of convection as well as the risk for late night
and early morning stratus/fog.

The line of storms that went through earlier has stabilized the
air mass temporarily, but the latest RAP guidance shows MU Capes
near 3500 J/KG and strong moisture advection at 850 MB returning
between now and 06z. This could cause showers and tstms to
redevelop, however the tstm complex currently in srn WI may
block/disrupt that incoming moisture/instability.

For now will only include a mention of showers in the TAFs
tonight and amend if the tstm potential increases later on. RAP
guidance also indicates a decent potential for some IFR stratus
from 06z-12z, but am skeptical of this too with all the mid and high
level clouds around, so for now only went with a light/MVFR fog
scenario.

Believe the threat of thunder is too low on Friday to have CBs in
the TAFs given westerly flow and subsidence in place, so went VFR
most of the day once the fog lifts.


&&

.MARINE...
Issued at 334 PM EDT Thu Jul 21 2016

I will continue the beach hazard and Small Craft Advisories into
this evening then let them expire this evening. The core of the
strongest winds will come through between now and 8 pm this
evening. Beyond that I see no big issues for our Near Shore.


&&

.HYDROLOGY...
Issued at 334 PM EDT Thu Jul 21 2016

Rainfall is not expected to be heavy enough to result in large
scale flooding of rivers or streams in the coming week. Even so,
locally heavy rain from thunderstorms tonight may cause some
localized flooding in areas with poor drainage and flashy streams.

&&

.GRR Watches/Warnings/Advisories...
MI...Heat Advisory until 7 PM EDT Friday for MIZ044>046-050>052-
     056>059-064>067-071>074.

LM...None.
&&

$$

UPDATE...Meade
SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...Duke
AVIATION...Meade
HYDROLOGY...WDM
MARINE...WDM



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