Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 291734

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


Issued at 328 AM EDT Sat Apr 29 2017

The threat of heavy rainfall is our primary story for this
weekend. We have a storm coming northeast out of the southern
Plains that runs into a large Canadian Polar High over Ontario.
That blocks front front from getting to far north and results an
extended period of showers and thunderstorms from late this
afternoon into Monday morning. Rainfalls of over 2 inches are more
than possible over a large part of our area by Monday morning. It
will be cooler than normal through the weekend thanks to that
blocking Canadian high. The cold front comes through early Monday
but we remain in the systems cold air and wrap around showers into
Tuesday afternoon.


.SHORT TERM...(Today through Monday)
Issued at 328 AM EDT Sat Apr 29 2017

The primary issue will be the heavy rain threat from late this
afternoon into around mid morning Monday. I continued the areal
flood watch from 2 pm today till 8 am Monday as nothing as changed
from what I can see in the models (we have been watching this for
over a week now). The heaviest rain is expected during two time
periods, tonight and late Sunday into the early morning hours of
Monday. The axis of heaviest rainfall seems to be near US-131
south to north give or take 50 miles.

This is a classic case of a blocking Canadian high stalling the
warm front near or just south of I-94 tonight into Sunday. That
result in the moisture feed being pinned in largely the same
location. We have all the classic signatures for heavy rain, the
model sounds are saturated from near the surface to 200 mb tonight
into Sunday night. The cape is narrow but deep so we do not get
very strong thunderstorms, just heavy rainfall. The precipitable
water (between 1.5 inches and 1.8 inches on Sunday) is shown by
the Western Region Ensemble Situational Awareness table to be 3
standard deviations from normal with a return period at this time
of year at less than 1 day in 30 years. We also have a 40 to 60
knot low level jet crossing the warm front tonight then again
later Sunday into early Monday ahead of the cold front (occluded
front). There also remains excellent model continuity and now the
high res models like the HRRRX go out far enough and also support
the idea of heavy rain tonight into Monday Morning. So bottom
line....we will see periods of showers and thunderstorms this
evening into early Monday morning.

We get into the cold air wrap around showers Monday but there will
likely be a dry slot so we may get 6 hours of dry weather once the
front comes through before the showers move back in from the upper
cold pool instability.

.LONG TERM...(Monday night through Friday)
Issued at 328 AM EDT Sat Apr 29 2017

The weather during the long term will be front-loaded; rain is
expected Monday night and then diminishing Tuesday with dry weather
late Tuesday through Friday.

The deep upper low responsible for producing heavy rain across the
cwa this weekend will be over Michigan late Monday. Shower seem
likely Monday night as the low moves through and several short wave
rotate through the flow.

Skies will begin to clear Tuesday afternoon as the low moves
northeast into Canada. Another trough will approach Wednesday night,
but that is really supporting a low over the Tennessee Valley. The
ecmwf is a bit farther north than the gfs and would give us some
light rain over the eastern cwa Thursday afternoon, but the gfs wold
keep us dry. Given that both models seem to be transition the upper
pattern to a cutoff low over the deep south, we`ll lean toward the
drier gfs solution.

Chilly temps are expected through the period. Highs in the 50s are
expected Tuesday-Thursday before climbing to around 60 Friday.


.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.


Issued at 347 AM EDT Sat Apr 29 2017

I issued the Small Craft Advisory for tonight through Monday


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.


MI...Flood Watch through Monday morning for MIZ043>046-050>052-

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



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