Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201931
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
331 PM EDT Fri Sep 20 2019

LATEST UPDATE...
Synopsis/Discussion/Marine

.SYNOPSIS...
Issued at 329 PM EDT Fri Sep 20 2019

* Warm and humid conditions expected on Saturday

* Biggest concern as we head into the weekend continues to be the
  threat for heavy rainfall Saturday night through Sunday night.

* Drier and cooler for the first part of the work week

&&

.DISCUSSION...(This evening through next Friday)
Issued at 329 PM EDT Fri Sep 20 2019

We will likely to see dry conditions across the area for the
remainder of today and tonight behind the passage of an an upper
wave and as heights recover. We are looking at a mild and muggy
September night, as both temperatures and dewpoints should remain
above 60 degrees for most of West Michigan.

Chances for showers and thunderstorms will then increase through the
day tomorrow as better moisture begins to advect into the area as
southwest flow begins to ramp up. Best chance for activity will be
along a wave of deeper theta-e advection in the afternoon and
early evening. Can`t rule out a few stronger storms in the Lansing
and Jackson areas by late afternoon and evening, as some forecast
model soundings (RUC and NAM) show the cap eroding and develop
1000-1500 J/kg of SB CAPE (ML CAPE will be lower). Not all models
are eroding the cap however, so will need to keep an eye on this
over the next few model runs. CAPE profiles are more tall and
skinny, but there is an inverted V look to soundings which hints
at some risk for strong downburst winds. Shear profiles are weak
given pretty uniform direction and speed, and storms should be of
the pulse variety and not very organized. Temperatures in the
afternoon should reach into the upper 70s to mid 80s, with
dewpoints in the 60s making it feel more like the low to upper
80s. It will be a breezy day, with a solid 15 to 20 knots of wind
in the mixed layer keeping sustained speeds about that high and
supporting gusts to around 30 mph.

The upper ridge that has been parked over the Eastern U.S. for much
of the week will be displaced by the high-amplitude trough pushing in
from the west. Ahead of this trough, tropical moisture will feed
up into the Plains and Great Lakes within deep south to southwest
flow between the advancing trough and a subtropical ridge that
will hold over the Southeast U.S. Models have been consistent the
last few days with a plume of precipitable water just over 2
inches setting up over West Michigan Saturday afternoon through
Sunday evening. While rain and thunderstorms may occur anytime
during this timeframe, the heavier rounds currently looked to be
timed from very late Saturday night through early Sunday morning
(initial surface trough pushed through the area as upper wave
lifts through Ontario) and again Sunday evening into Sunday night
(main surface trough and ahead of upper trough axis). Potential
for rain totals to exceed two inches, locally 3 inches, looks best
near and west of U.S. 131 where potential is higher to be
affected by both rounds. Flooding will be the main concern,
especially on roads and in low-lying areas.

Some showers will linger into Monday before surface high pressure
builds and brings dry weather Monday night and Tuesday.
Temperatures will become more fall-like behind the front for early
next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 119 PM EDT Fri Sep 20 2019

VFR conditions are likely through the TAF period, however a quick
shower or thunderstorm that could briefly drop conditions to MVFR
cannot be ruled out over the next 24 hours, particularly after
12Z Saturday morning. We will mainly be dealing with high and
mid-level clouds for the remainder of today and tonight. Lower
clouds will begin to fill in Saturday afternoon, but look to
remain VFR until Saturday night when low-level moisture transport
into the area will really ramp up. Southwest winds will also
increase for Saturday afternoon, especially after 18Z when gusts
could reach 30 knots.


&&

.MARINE...
Issued at 329 PM EDT Fri Sep 20 2019

South to southwest winds will increase in advance of a
slow-moving cold front Saturday afternoon through Sunday evening.
Small Craft Advisories are now in effect through this period,
initially for water north of Holland, but the other nearshore
waters will need to be added for Sunday. In addition, beach
conditions will be dangerous for swimmers due to large waves and
strong currents. Currents will be especially strong on the south
side of piers. Large waves in the 4 to 7 feet will also wash over
the piers, making them dangerous to venture out onto. The extended
period of large waves will also create favorable conditions for
beach erosion, which will be amplified by heavy rains.

Moderate wave heights in the 3 to 4 foot range look to continue
behind the front Sunday night into monday as winds turn northwest.
Calmer conditions are expected Monday night and Tuesday as high
pressure builds into the area.

&&

.HYDROLOGY...
Issued at 119 PM EDT Fri Sep 20 2019

Several rounds of heavy rain will be moving through Lower Michigan
this weekend. At this point it looks like most spots will end up
with a total of 1-2" of rain, with some spots near the lakeshore
possibly topping 3" by the time the rain ends Monday morning. This
will cause some pretty good rises on most of the rivers in the area,
but at this point no rivers are expected to exceed bankfull. The
rivers that might approach bankfull are most likely the White and
Pere Marquette rivers, where heavy rain last week has limited how
much rain the soil is able to absorb.

Of greater concern is the potential for localized short-duration
flooding of some small streams as well as ponding on roadways at low-
spots and in poor drainage areas. The heaviest rain is likely to
fall in the communities along the Lake Michigan shoreline, so the
row of counties along the Lake are at the highest risk for the
localized flooding. The heaviest rain looks like it will fall Sunday
evening and overnight, and be moving out of the area by Monday
morning. Water on roads at night is much harder to see, so drivers
should slow down and pay extra attention.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Saturday afternoon through Sunday
     evening for MIZ037-043-050-056.

     Lakeshore Flood Advisory from noon Saturday to 8 PM EDT Sunday
     for MIZ037-043-050-056.

LM...Small Craft Advisory from noon Saturday to 8 PM EDT Sunday for
     LMZ846>849.

&&

$$

SYNOPSIS...HLO
DISCUSSION...HLO
AVIATION...HLO
MARINE...HLO
HYDRO...AD


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