Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 291929
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
329 PM EDT Sat Apr 29 2017

LATEST UPDATE...
Synopsis/Short Term/Long Term/Marine

.SYNOPSIS...
Issued at 329 PM EDT Sat Apr 29 2017

Showers and a few thunderstorms are expected over the course of
the weekend into early next week, well ahead an upper low that
will track to our west late Sunday into Monday. Chances for rain
will exist until the system rotates north through the midweek
period. Some of the rain could be heavy at times, especially in
and around thunderstorms late tonight into Sunday. Overall
rainfall amounts may lead to flooding along area rivers and in
flood prone areas into next week.

There is likely to be a wide spread in temperatures Sunday with
highs south of the front pushing toward 70 degrees. Otherwise,
below normal temperatures are expected until late next week, when
we moderate back into the 60s area wide.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 329 PM EDT Sat Apr 29 2017

A good soaking rain is still on track to affect much of Lower
Michigan over the weekend. The heaviest rainfall is likely to fall
tonight into early Sunday and again Sunday night into Monday.

A warm front will continue to push north out of the Ohio Valley
tonight into Sunday. A mid-level wave will travel northeast along
the front in the overnight period. This feature will combine with
isentropic ascent and will be the focal point for ongoing
convection to track through the area. A few embedded thunderstorms
are possible with the high likelihood for thunder along and south
of I-96. Expect the first good wave of heavier rain to pass
through late this evening into early Sunday morning.

The upper level low continues to intensity through the day
Sunday. Plenty of support remains out ahead of the low thanks in
part to a coupled upper jet that will be in place over western
Lake Superior. Moisture transport remains strong out of the gulf
with PWATs persistently in the 1.2-1.8 inch range. A broad
surface low continues to be guided just west of Lake Michigan.
This will be our second good wave of moisture with the core of
the lift moving through Sunday evening into the early morning
hours Monday.

While the low continues to rotate northward through the day
Monday, a few rain showers will linger with a couple of weak
upper wave rotating around the out edge of the main circulation.
Precipitation will be light and scattered in the nature. Expect a
pretty damp, cool start to the work week.

.LONG TERM...(Tuesday through Saturday)
Issued at 329 PM EDT Sat Apr 29 2017

A well needed drying out period should be seen through much of the
long term. Temps will be cool, but we should see a slow moderation
during the latter half of the week.

The upper low is expected to be moving northeast across the Superior
region and toward Quebec on Tuesday.  As it departs, the showers
will continue to become more scattered.  Most areas probably won`t
begin to dry out until Tuesday afternoon or into the evening.  It
does appear we will partially clear Tuesday night, and with
diminishing winds, some areas of frost will be possible with temps
dropping into the 30s.  I have added this to the forecast, however
impacts are still expected to be low given the time of year.

The core of the coldest air behind the system slowly moves east,
leading to a slow but gradual warming trend by Wednesday night and
continuing into next weekend.  After highs only around 50; 10 to 15
degrees below normal on Tuesday, we will warm to closer to normal
temps by Saturday, in the 60s.

A well needed dry spell should be in the offering starting Tuesday
night.  A short wave is shown to come through Thursday night, while
low pressure is expected to be over the Gulf Coast states. The upper
wave should act to pull the surface low northeast into the OH/TN
valley.  So there could be some isolated light showers over the SE
CWA late Wednesday night through Thursday night. Otherwise much of
the Tuesday night through Saturday period appears dry.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 134 PM EDT Sat Apr 29 2017

Conditions were still VFR as of TAF time, with only some patchy
MVFR ceilings around. However conditions will lower through the
rest of the afternoon and into the evening as deeper moisture and
steadier rains arrive. This downward trend will occur over the
I-94 TAF sites first, to MVFR by about 20Z, and to IFR around 23Z.
The I-96 TAFs will see the downward trend about 3 to 5 hours
later. Once areas reach IFR, or even some LIFR, these conditions
are expected to linger through 18Z Sun and beyond.

A few embedded thunderstorms will also spread in from the south,
mainly after 08Z. Most of these should stay to the south and east
of GRR, so kept thunder out of the GRR and MKG TAFs for now.

ENE winds will increase this evening and occasionally gust around
25 knots overnight. The winds should begin to simmer down by late
Sunday morning.


&&

.MARINE...
Issued at 329 PM EDT Sat Apr 29 2017

The small craft advisory will continue as planned. Waves will
generally range from 2-4 feet in most instances with offshore flow
up to around 25 knots. Periodic episodes of waves in the 3-5 feet
range will occur, specially near the points (Ludington/Manistee).

The bigger concern will be in the later half of Monday when winds
shift out of the south and southwest. Higher waves are likely with
an onshore flow. Waves of 3-5+ will become more persistent through
Monday night. The advisory may need to be extended or reissued for
late Monday in later updates.

&&

.HYDROLOGY...
Issued at 122 PM EDT Sat Apr 29 2017

Flood advisories continue for the Grand River at Ionia and on the
Maple River at Maple Rapids. A flood watch continues for the same
four points. River statements have been issued for numerous
locations that might experience flooding towards the middle of the
coming week. However, forecast certainty for these sites remains
very low at due to high dependence on the character of precipitation
that occurs over the next 48 hours and how well the models handle
the subsequent river crests.

It is quite certain, however, that widespread rainfall affecting all
area river basins will fall between now and Monday morning.  The
latest round of river forecasts were based on 48 hour WPC QPF, which
is slightly greater and shifted farther north than WFO QPF. SREF
guidance shows mean values around 2", although there are a couple
outliers with 4+", making the median QPF a little bit below 2".
Interestingly, downscaled calibrated GEFS output shows only a 40-50%
chance of exceeding 2 inches in most spots. However, we favor the
ECE, whose ensemble members show strong consensus squarely in the 2-
3" total QPF range, in line with previous thinking. As noted
already, local maxima in precipitation are likely but there is
little predictability with exactly where they will occur. This of
course has potentially significant implications with what river
basins are most affected.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Monday morning for MIZ043>046-050>052-
     056>059-064>066-071-072.

LM...Small Craft Advisory from 8 PM this evening to 11 AM EDT Monday
     for LMZ844>849.

&&

$$

SYNOPSIS...JAM
SHORT TERM...JAM
LONG TERM...JK
AVIATION...JK
HYDROLOGY...TJT
MARINE...JAM



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