Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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689
FXUS63 KGRR 290748
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
348 AM EDT Sat Apr 29 2017

LATEST UPDATE...
Marine

.SYNOPSIS...
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 06Z TAFS through 06Z Saturday night)
Issued at 125 AM EDT Sat Apr 29 2017

We`re heading into a wet period in which mvfr and lower cigs/vsybs
are expected. A large area of showers/storms is devleoping along
the I-44 corridor over OK/MO and latest radar trends show that
expanding rapidly northward. We`ll see showers develop later this
morning as a warm front moves north. This frontal boundary will
remain nearby for the next 36 hours and be a focal point for
continued showers/storms.


&&

.MARINE...
Issued at 347 AM EDT Sat Apr 29 2017

I issued the Small Craft Advisory for tonight through Monday
morning.

&&

.HYDROLOGY...
Issued at 333 PM EDT Fri Apr 28 2017

In general, a fairly widespread 2" to possibly 4" of rain is
expected this weekend. This will certainly lead to river rises and
some areas may reach flood stage by early next week. At this point
no major flooding is anticipated. Forecasts will continue to be
adjusted with the projected rainfall and once the rain gets into the
basins the accuracy of the forecasts will increase. It is important
not to focus on the exact crest values necessarily since they may
change with the daily updates as the rain gets closer to the region
and eventually begins occurring overhead.

The greatest impacts at this point look somewhat similar to earlier
this month for the Sycamore Creek and Looking Glass River. Rises
above flood stage look likely. On the Grand, the river may hold just
below flood stage. If rainfall ends up being more 3"-5" across this
basin then higher crests would be achieved and flooding would likely
materialize.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 2 PM EDT this afternoon 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...WDM
SHORT TERM...WDM
LONG TERM...04
AVIATION...04
HYDROLOGY...Hoving
MARINE...WDM



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