Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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531
FXUS63 KGRR 190528
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
128 AM EDT Wed Sep 19 2018

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 300 PM EDT Tue Sep 18 2018

A few showers and thunderstorms are forecast tonight through
Wednesday afternoon. A warm front will lift northward across our
area Wednesday night and bring a much better chance for widespread
thunderstorms late Wednesday night into early Thursday. A cold
front moving in from the west will bring more thunderstorms
Thursday night. Some severe storms with damaging winds and hail
and heavy rain are possible early Thursday and then again Thursday
night. A much cooler and drier airmass will move in behind that
system for Friday through the weekend.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 300 PM EDT Tue Sep 18 2018

Primary short term fcst concerns involve determining potential for
convection through the short term fcst period.

A few showers and perhaps an isolated storm will continue to
develop late this afternoon into this evening along and south of the
weakening cold front that is moving in from the north and is located
near the I-96 corridor attm. Scattered showers and thunderstorms
will develop Wednesday as elevated instability begins to increase
to the north of the warm front.

A much better chance for widespread rain and convection will come
late Wednesday night into Thursday morning as the warm front
lifts to the north and elevated instability ramps up significantly
north of that boundary.

Convection will also develop on the nose of a 30-35 kt llj off to
our west Wednesday night and then roll east along the boundary
into our area. The strongest convection will contain heavy rain
and potentially hail and gusty winds. A few marginally severe
storms are possible overnight.

On Thursday after early morning convection clears out it will
become very warm and humid as the warm front moves north of our
fcst area. Increasing instability out ahead of the approaching
cold front in conjunction with strong deep layer shear and lift
from the approaching front with ample low level moisture in place
will set the stage for convective development Thursday night.

Strongest convection will bring potential for damaging wind gusts
and large hail and heavy rain. There is also potential for an
isolated tornado given ample deep layer shear and favorable speed
and directional wind shear along with low lcl heights. However
the severe wx threat is mitigated by unfavorable cold frontal
timing as well as lack of pva or favorable upper level dynamics.

.LONG TERM...(Friday through Tuesday)
Issued at 300 PM EDT Tue Sep 18 2018

A few showers and storms will linger Friday morning until fropa.
After fropa occurs we expect some instability showers to develop
in the afternoon and evening before high pressure builds in to
bring fair wx Saturday. Temps will average pretty close to normal
for this time of year late in the weekend into early next weekend
as the upper pattern becomes very zonal before ridging begins to
amplify over the eastern CONUS again by Tuesday/Wednesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 128 AM EDT Wed Sep 19 2018

Mid and high clouds are in the process of clearing out over the
terminals. This clearing looks to be short lived, especially for
most of the terminals (except KMKG). This is because we expect low
clouds to develop and spread in from the E/NE. These clouds are
expected to range from lower MVFR to higher IFR. The clouds and a
light breeze will likely keep fog limited. The clouds will lift
after sunrise into a VFR cumulus deck by afternoon. We can not
rule out an isolated shower/storm this afternoon, however most
areas should stay dry through the daylight hours.

We are expecting showers and storms to develop toward 04z as a
warm front takes shape over the region. It looks like that the
terminals that will be impacted by these will be the nrn locations
(KGRR and KMKG). Further south the chc looks too low to mention at
this time.

&&

.MARINE...
Issued at 300 PM EDT Tue Sep 18 2018

Wave heights of around 1 to 3 feet are forecast tonight and
Wednesday and winds will become offshore tonight through
Wednesday. Strong southerly winds will develop Thursday by which
time a small craft advisory will be required through Friday.
Winds could potentially reach gale force Thursday night and
Friday.


&&

.HYDROLOGY...
Issued at 300 PM EDT Tue Sep 18 2018

River levels remain steady following mostly dry weather over the
past week and a half. Coverage of showers and thunderstorms through
Wednesday evening will be isolated to scattered. Although individual
storms may contain heavy rain, flooding is not anticipated through
Wednesday evening.

A higher coverage of rain and thunderstorms is expected late
Wednesday night into Thursday morning. This round looks to bring
between one-half inch and one inch of rain to areas near and north
of a line from Holland to Grand Rapids to Mt Pleasant. Local totals
will be higher in thunderstorms, which will be capable of producing
rainfall rates in excess of one inch per hour. There looks to be
relative lull in activity Thursday afternoon before another round of
showers and thunderstorms moves through the area Thursday night into
early Friday morning. This round will also have the potential
produce rainfall rates in excess of one inch per hour.

The quick-hitting nature of these rounds of heavy rain will be more
likely to cause localized flooding on roads, in low-lying areas, and
along small creeks, than flooding of streams and rivers. Drier
weather is expected Friday afternoon through Sunday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...Laurens
AVIATION...NJJ
HYDROLOGY...HLO
MARINE...Laurens



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