Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211726

Area Forecast Discussion
National Weather Service Grand Rapids MI
126 PM EDT Fri Jul 21 2017


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 inches
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 despite upr low/trough in the vcnty.

In the near term, patchy dense fog will impact the morning commute
today, followed by hot/humid conditions with highs around 90.
Soundings 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 18Z TAFS through 18Z Saturday afternoon)
Issued at 118 PM EDT Fri Jul 21 2017

The latest trend in the forecast is to bring some showers in from
the west later this afternoon into the evening...then a round of
storms closer to or just after midnight. Still much uncertainty as
to how this event will unfold...but radar is showing the rain in
Wisconsin building east...which may very well make it into KMKG
before 00z. Will keep cloud bases VFR through 00z...but it looks
like as the tropical airmass moves in...the cloud bases will be
lowering...especially later tonight. Any thunderstorms could
support IFR or lower conditions...but I was not confident enough
to go that low.

Low level moisture is shown to be high for Saturday morning. Will
go with MVFR cloud cover for now.


Issued at 330 AM EDT Fri Jul 21 2017

Thunderstorms with gusty winds late tonight into Saturday morning
may create periods of rough conditions on Lake Michigan. High dew
points over the relatively colder waters may contribute to areas
of dense fog at times. Will need to monitor for the possibility of
some higher winds and waves on Sunday into Monday 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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