Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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334
FXUS63 KGRR 201745
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
145 PM EDT Sun May 20 2018

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 330 AM EDT Sun May 20 2018

A stalled frontal boundary south of Michigan will continue to
produce considerable clouds and periodic showers through Tuesday.
High pressure will provide dry weather for the midweek period before
a risk of showers and thunderstorms returns for the start of
Memorial Weekend.

Temperatures will remain on the cool side through Monday with
highs in the 60s, then begin to moderate Tuesday. Highs around 80
are expected from Wednesday through Saturday, with more humid
conditions developing by the holiday weekend.

&&

.UPDATE...
Issued at 1037 AM EDT Sun May 20 2018

Based on water vapor and radar presentation upstream over
Wisconsin along with recent HRRR/HRRRX guidance...will keep rain
chances into this afternoon mainly south of I-96. Well defined PV
maximum noted on mid-level water vapor imagery /CH09/ should help
keep precipitation intact as it exits southern WI and crosses
Lake MI.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 330 AM EDT Sun May 20 2018

Batch of showers and isolated tstms currently over southern
Wisconsin are expected to weaken/decrease in coverage as they
move east into slightly drier and more stable air mass over
Michigan. Still, some of these will survive and will need to carry
chc pops today, particularly west of Highway 131. Not much hope
for sunshine except perhaps north of M-46 and along the immediate
lakeshore this afternoon.

Surface ridging becomes more pronounced later today and tonight,
temporarily pushing the baroclinic zone a tad farther south and
resulting in a decreasing cloud pattern north of I-96. A patchy
frost threat could even develop in the far north around Leota late
tonight due to the clearing and drier air.

Clouds and scattered showers bulge back north again for Monday as
Nebraska shortwave moves northeast. This wave eventually gets
caught up in faster westerly mid level flow over WI/MI and finally
comes through the area on Tuesday.

Bottom line...our best chance of showers looks to be on Monday
and Monday night as the shortwave approaches but still a small
chance on Tuesday as it comes through - although mainly near and
east of Highway 131 ahead of a developing afternoon lake breeze
front.

No thunder expected today and tonight, and the risk on
Monday/Monday night is still quite low and mainly limited to areas
south of I-96. Instability stays mainly to our south.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM EDT Sun May 20 2018

High pressure over Lake Michigan at the start of the period will
produce dry weather until Thursday night. By then, the upper ridge
axis will be east of the cwa and developing low pressure over the
Upper Mississippi Valley will start to play a role in our weather.
Increasing southwesterly flow will interact with short waves
emanating out of the trough to the west to produce showers across
much of the Midwest into Michigan. Both the gfs and ecmwf track the
low from northern MN southeast across Lower Michigan over the
weekend. We`ll be on the warm side of the system, so we`re looking
at a muggy airmass with a scattered showers and thunderstorms.

Highs during the period will be near 80.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 141 PM EDT Sun May 20 2018

MVFR conditions will persist for parts of the region this
afternoon. Some drier air was dropping down from the north and
should support rising ceiling levels by late afternoon. An area of
rain moving in from the west is expected to weaken with time...but
it will likely cross several of the TAF sites through mid
afternoon.

A warm front will lift up from the south Monday morning. This will
bring increasing levels of moisture along with rain. Impacts are
expected to increase through the day. Some instability may move in
by the afternoon...but confidence on storms was not high enough to
put them in the forecast at this point.

&&

.MARINE...
Issued at 1037 AM EDT Sun May 20 2018

Current forecast looks on track with respect to a noon expiration
of the small craft advisory. Ludington buoy last reported a
waveheight of 3.6 feet with a decreasing trend along with the wind.


&&

.HYDROLOGY...
Issued at 1127 AM EDT Sun May 20 2018

Many rivers especially in southern Michigan are running much above
normal for this time of year, with lowland nuisance flooding still
occurring at several points. Most river levels are falling.
Showers on Monday have the potential to produce around a half inch
of rain on average in southern Michigan, though it wouldn`t be
surprising if some locations picked up closer to an inch.
Depending on the basins where any heavier rain may fall, this
could prolong the nuisance flooding. The middle part of the week
will be dry, then rain chances return toward the end of the week.

&&

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

$$

UPDATE...TJT
SYNOPSIS...Meade
SHORT TERM...Meade
LONG TERM...04
AVIATION...MJS
HYDROLOGY...CAS
MARINE...TJT



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