Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 210731

Area Forecast Discussion
National Weather Service Grand Rapids MI
331 AM EDT Fri Jul 21 2017

Synopsis/Short Term/Long Term/Marine

Issued at 330 AM EDT Fri Jul 21 2017

Low pressure and a warm front will bring showers and thunderstorms
tonight into Saturday with a risk of locally heavy rainfall and
perhaps even a damaging wind threat. Rain chances will decrease
Sunday and Sunday night, with a stretch of quiet and less humid
weather expected for the first half of next week.


.SHORT TERM...(Today through Sunday)
Issued at 330 AM EDT Fri Jul 21 2017

Main focus continues to be potential for heavy rain/flooding and
svr wx late tonight into Saturday morning. Currently the main
risk looks to be near and south of I-96.

Active warm front and main corridor of increased MCS potential is
expected to shift into the area later tonight into Saturday
morning as upr low/shortwave moves east across srn Saskatchewan
and Manitoba.

Pool of very high precipitable water air of 2.00 to 2.25
will push into sw lwr Mi with the help of a 30-50 kt swly low
level jet. This supports high PoPs for showers and tstms late
tonight/Sat morning, however the svr wx/hvy rain potential is
somewhat low confidence due to uncertainties on behavior of the
low level jet and MCS track/propagation.

Late night MCS placement/propagation should favor the instability
gradient, which several guidance sources show to be just
west/southwest of our area at 06Z tonight. This gradient is
progged to nudge farther northeast in sw lwr MI after 06Z, however
if considerable convection is developing/tracking through WI this
afternoon/evening, the advancement of the higher instability
toward sw lwr MI may be delayed/suppressed. Main timeframe of
concern for a damaging wind and/or flooding risk seems to be 08Z
to 14Z... near and south of I-96.

Considerable clouds and lingering showers Saturday as the sfc low
tracks through will probably hold down high temps. However
remaining warm and very humid, followed by a fog risk Saturday
night. Thunder risk on Sunday looks limited due to westerly low
level flow.

In the near term, patchy dense fog will impact the morning commute
today, followed by hot/humid conditions with highs around 90.
Sounding are too dry to support convection today, although we may
have some thicker debris clouds and light showers approaching from
the west toward 00Z.

.LONG TERM...(Sunday night through Thursday)
Issued at 330 AM EDT Fri Jul 21 2017

We continue to expect that rain chances will gradually end Sunday
night into Monday morning. Upper trough and secondary sfc cold front
will pass through the area Sun evening. Some instability will remain
through early evening from diurnal heating and the thermal trough

We then expect a quiet and comfortable period from Monday through
Wednesday. Upper ridge situated west of the area over the Plains
will support a Canadian sfc ridge overhead. A dry and cool low level
NE flow will bring the comfortable conditions to the area. The sfc
ridge will gradually shift east and allow a srly flow to start
bringing in some slightly warmer air.

The chance for showers/storms will begin to increase Wed night and
continue through Thursday. An upper low will rotate along Southern
Canada, and eventually bring a cold front down into the area. Right
now, it appears that there will not be a great influx of moisture
from the Gulf. This will keep instability and moisture a bit
limited, but still enough for a few storms.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 137 AM EDT Fri Jul 21 2017

Our main concerns over the next 24 hours is fog potential through
daybreak this morning, then convective potential toward the end of
the period.

High pressure settling in is bringing light winds to the area.
These light winds combined with the moist low levels will bring a
good chance of fog. Some sites have been seeing some brief bouts
of fog already this morning. KMKG seems to be more primed to see
lower conditions, while inland areas may not go quite as low. IFR
and lower does look possible at all sites through about 12-13z.

Conditions will be quiet then for most of the rest of the period.
Mainly clear skies with a light, and sometimes variable wind with
the high nearby. Convection will develop tonight across WI and
will try to push in late in the period. It looks possible for KMKG
to see some showers/storms to move in toward 06z Sat. The rest of
the sites should see this after 06z Sat.


Issued at 330 AM EDT Fri Jul 21 2017

Convection late tonight into Saturday morning create periods of
rough conditions Lake Michigan. Will need to monitor for the
possibility of some higher winds and waves on Sunday with cold
advection/nw flow behind the sfc low.


Issued at 315 PM EDT Thu Jul 20 2017

The best opportunity for heavy rainfall comes Friday night into
Saturday. A very moist airmass moves in which, combined with
favorable jet structures at 300 mb and 850 mb, will lead to the risk
for efficient rain producing thunderstorms. Since these storms do
not currently exist, it is difficult to say exactly which areas will
get the heaviest rain. Much may hinge on whether any storms that
develop in Wisconsin Friday night tend to migrate southward toward
Chicago and southern Lake Michigan (like early Thursday morning) or
expand eastward into western Lower Michigan. Repeated thunderstorm
movement over the same areas is possible for several hours, which
could lead to some flooding of roads and rapid river rises
especially on small rivers and streams. River responses to the 72
hour ensemble QPF values over our river basins are not raising any
big red flags at this point, as it would take several inches of
rainfall to start flooding main stem rivers. That is uncertain at
this time.

PWAT values approaching or exceeding 2.00" is expected for most, if
not all, of the region Friday night and Saturday. With the sfc low
remaining west of the region through Saturday afternoon, the threat
for heavy rainfall continues well into the day should any convection




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