Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 141727

Area Forecast Discussion
National Weather Service Grand Rapids MI
127 PM EDT Mon Aug 14 2017


Issued at 329 AM EDT Mon Aug 14 2017

Chances for a few showers and storms will occur today through
Tuesday as a weak low pressure system approaches and moves through
the region. Tuesday night looks to be dry as the front pushes south
of the state. Chances for showers and storms increase mid week,
especially Wednesday night into Thursday as a stronger low works
through the area. High temperatures will remain seasonable for this
time of year around 80 into the low 80s the next few days.


Issued at 1100 AM EDT Mon Aug 14 2017

No major changes made to the forecast, although some tweaks were
made to expand shower/storm chances a bit further south.

Warm air advection is ongoing over the area this morning. The mid
levels of the atmosphere are somewhat unstable already with
Altocu Castellanus present overhead.

We are still expecting spotty shower/storm activity to develop
this afternoon, maxed out over the NE corner of the area. Combined
with the mid-level instability, will be some lower level
instability from the heating of the day and additional moisture
moving in. It seems we will see somewhat of an interior trough
develop this afternoon with the light wind regime, and lake
breezes from Lakes Huron and Michigan moving inland.

Instability could increase to around 1000 j/kg of CAPE inland this
afternoon. CAPE profiles are rather thin which will result in
relatively weaker updraft strength. Deep layer shear is actually
decent around 35 knots, so we can not rule out a stronger storm.


.SHORT TERM...(Today through Wednesday)
Issued at 329 AM EDT Mon Aug 14 2017

Chances for precipitation in the short term have increased just a
bit as compared to 24 hours ago.

The first chance for precipitation looks to possibly be even before
daybreak this morning as a few showers develop from Grand Rapids to
the north and east. This precipitation is associated with a weak
warm air advection regime. 850mb moisture transport is meager but
enough to possibly kick off a shower or two. Have a 20 pct chance in
the forecast.

Today, instability increases to the point where both showers and
thunderstorms are possible. We are situated in an area where a weak
warm front is located across the area ahead of a low moving our
direction from the Northern Plains. 850mb LI`s via the NAM dip to -
3C, while CAPE values increase to around 1000 j/kg. Not expecting
anything widespread but some scattered precipitation in certainly
possible. The NAMnest corroborates this thinking with the best
chances over the northeast CWA.

Tonight, the main upper shortwave from upstream digs southeast into
our area with most likely the best chances for precipitation in the
short term. Much of the evening may be dry with showers and storms
developing from the west overnight. Envision scattered showers and
storms moving in off of Lake Michigan after midnight.

Tuesday, the low and associated weak cold front progress south
through the forecast area. Best chances for a few showers and storms
will be over Southern Lower Michigan on Tuesday towards Lansing,
Battle Creek and Jackson.

Tuesday night looks dry as the front sags south of the area and
stalls. On Wednesday the front begins to come back north as a warm
front, but I wonder if a good portion of the day doesn`t remain dry.
Forecast CAPE values are meager on Wednesday and our pops may be a
bit ambitious given that fact.

.LONG TERM...(Wednesday night through Sunday)
Issued at 329 AM EDT Mon Aug 14 2017

It will be very warm and humid Wednesday night into Thursday and the
warm front will bring scattered showers and thunderstorms Wednesday

The sfc low/cold front will bring more showers and thunderstorms
Thursday. Some strong to potentially severe convection may develop
given lift from the approaching front and ample low level moisture
in place in conjunction with potential for rather strong instability
to develop. However this will be largely contingent on extent of
cloud cover. More robust convective potential will also be mitigated
by lack of stronger deep layer shear, and stronger upper level
forcing should stay north of our fcst area.

A slightly cooler and less humid airmass will move in behind that
system Thursday night through Friday when fair weather will return.
Several upper level disturbances will bring potential for a few
showers and storms for the weekend. High temperatures through most
of the long range fcst period will range from the upper 70`s to
lower 80`s with min temps generally ranging from the middle 50`s to
middle 60`s.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 117 PM EDT Mon Aug 14 2017

Visible satellite shows cumulus field continuing to develop this
afternoon. Looking out the window...cumulus have some vertical
extent. Water vapor imagery shows a plume of higher moisture over
northern Illinois heading in our direction. An area of showers had
developed in Central Michigan over the past half hour...moving
northeast. A few returns were also noted along the western shore
of Lake Michigan.

Main issue for the 18Z TAFs is precipitation wording this
afternoon. Agree with widely scattered coverage as depicted by
short range models. Opting for VCTS wording...but reserve the
right to make a last minute adjustment. KGRR and KLAN appear to be
the prime candidates for stronger precipitation wording for this

After a bit of a lull this evening...rain chances increase again
after 06Z stronger wave moves through the region.
Rain should end from west to east between 12Z and 15Z Tuesday.

Model soundings indicate low cloud potential overnight...targeting
areas that receive rain this afternoon. Definitely something to
keep any eye on this afternoon and evening...but not enough
confidence to insert into this set of TAFs.


Issued at 329 AM EDT Mon Aug 14 2017

A fairly slack gradient is expected through the short term period.
Winds in BUFKIT profiles over the lake show generally 10 knots or
less through daybreak on Wednesday. The lighter wind regime early
this week should keep waves at or below 2 feet into mid week.

The next time frame of concern for winds and waves looks to be
Wednesday night into Thursday as southerly winds increase.


Issued at 1144 AM EDT Mon Aug 14 2017

Isolated to scattered showers may result in a brief dampening of
the ground in localized spots this afternoon. Better chances for
rain will come in overnight tonight and early Tuesday morning.
Measurable rainfall of around one-tenth of an inch may fall. Some
locations could receive a couple of tenths of beneficial rainfall.

A more active pattern is expected to setup for the midweek period
as a result of a strengthening wave that passes over the Great
Lakes Region. A deepening trough may result in showery activity
toward next weekend. It is possible for 5-7 day totals to approach
an inch or more in spots. Given recent dry weather, growing
moisture deficits, and near/below normal river levels, flood
concerns remain low.




LONG TERM...Laurens
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