Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 202329

Area Forecast Discussion
National Weather Service Grand Rapids MI
729 PM EDT Thu Jul 20 2017


Issued at 315 PM EDT Thu Jul 20 2017

A weak high pressure ridge will bring fair but continued very warm
and humid weather tonight through Friday morning. Several rounds
of thunderstorms are expected from late Friday through Saturday
night. Some strong to severe storms will be possible with heavy
rainfall and localized flooding possible as well.


.SHORT TERM...(This evening through Saturday night)
Issued at 315 PM EDT Thu Jul 20 2017

Main short term fcst concerns continue to involve assessment of
convective/svr wx/hvy rain potential through Saturday night.

Fair but warm and humid wx will continue tonight through most of
the day Friday with a weak high pressure ridge in place.
Thunderstorms through then should stay just south of our fcst
area in closer proximity to the west to east oriented boundary that
will continue to sink a little further south to near the MI/IN state
line by this evening. Patchy fog will develop during the early
morning hours Friday as winds go calm with the humid airmass in

It will become hot and quite humid Friday afternoon with high
temps reaching the upper 80`s to lower 90`s with dew point values
well into the 60`s.

We expect a strong llj and the stationary front to focus
development of a large MCS over the upper midwest Friday
afternoon and early evening. Mid to upper level thickness tools
for MCS steering/propogation and orientation of the stationary front
suggest that the MCS will move southeast along the instability
gradient late Friday through Friday night.

This MCS has potential to bring a corridor of damaging wind gusts
as it potentially evolves into one or several bowing segments given
strong instability and favorable 0-3km shear. Some of the latest
guidance solutions including the 12Z ECMWF and 12Z NAM continue to
bring the MCS into our fcst area. Other guidance solutions such as
operational GFS trends indicate that the MCS may for the most part
miss us just to the west/southwest. All things considered there is
enough potential to continue to carry likely to low categorical pops
for convection late Friday night at this time.

There continues to be potential for very heavy rainfall late
Friday night through Saturday as precipitable water values reach
2-2.25 inches by then along with very strong 1000-850 mb moisture
transport. Dew point values by then should reach the lower to
perhaps middle 70`s.

I strongly considered a flood watch headline for portions of our
fcst area for Friday night through Saturday. However ultimately I
felt it was a little too early for this given uncertainty regarding
convective evolution that far out in time. Thx for coord on the no
headline decision IWX.

Although the atmosphere may potentially be worked over somewhat by
showers and storms into early Saturday there is potential for
instability to quickly redevelop later Saturday. Deep layer shear
remains favorable at 35-40 kts.

The SPC day 3 outlook having all of our fcst area in the slgt for
severe wx looks excellent. Convective redevelopment will continue
through Saturday night as the main low pressure wave approaches
from the west and moves through Saturday night.

.LONG TERM...(Sunday through Thursday)
Issued at 315 PM EDT Thu Jul 20 2017

The main challenge in the long term deals with the potential for any
thunderstorms.   The greatest risk looks to be Sunday and again on
Thursday.  Models are in reasonable agreement on this scenario.

A low pressure system is forecasted to track eastward through Lower
Michigan on Sunday.  This system will drag a cold front through the
region.  With the atmosphere shown to be unstable...thunderstorms
will be as possibility.  Deep layer shear is forecasted to still be
elevated...thus organized storms may occur.

The wind turns northeast behind the front.  This will push the warm
and humid airmass south of the border.  A large area of high
pressure will then move in from Canada for Monday into Tuesday as a
mid level ridge sharpens up overhead.
Subsidence will act to keep the cloud cover to a minimum.  The
airmass may turn cooler than normal.

Yet another mid level wave heads on direction for the middle part of
the week.  Will feature a risk for storms then.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 729 PM EDT Thu Jul 20 2017

At this point the major issue I see tonight is fog developing.
Winds become light and variable as a surface high is over this
area. Dew points are near 70 currently and our forecast lows are
in mid 60s. The NAM shows the classic fog signature in the model
sounding from around 09z till 12z. The NAM visibility forecast
shows less than a 1/2 mile over most of western Lower Michigan
west near and west of US-131. As a result I put fog in most of the
forecasts, the dense at MKG and AZO.

The fog will quickly mix out then due to 5% RH from 700 mb to 300
mb over the area, I see few if any clouds during the day. Any
convection would be early Saturday morning.


Issued at 315 PM EDT Thu Jul 20 2017

Wave heights will continue to gradually subside this afternoon and
tonight as winds become light tonight. Some patchy fog may develop
overnight into Friday morning with a humid airmass in place.
Thunderstorms will pose a hazard to mariners late Friday through
Friday night and again Saturday through Saturday night with
locally higher winds and waves in and near storms.


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




SHORT TERM...Laurens
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