Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201756

Area Forecast Discussion
National Weather Service Grand Rapids MI
1256 PM EST Tue Feb 20 2018


Issued at 316 AM EST Tue Feb 20 2018

A area of low pressure will track slowly through the Great Lakes
Region through tonight. This system will lead to heavy rain for
much of the area along with a risk for flooding. The wind will
shift behind the departing storm later tonight...drawing in a
colder airmass that will last into Thursday.


Issued at 1133 AM EST Tue Feb 20 2018

The morning wave of rain passed through the heart of the CWA.
Rainfall amounts of 2 to 2.5 inches are common over the SW portion
of the CWA as of 10 AM. Flood impact reports have been received
across much of the region.

This afternoon`s rain will be focused over the southern CWA with
around an additional half inch expected. Meanwhile the evening
wave, that was over IA and MO as of 16Z, is pegged for the heart
of the CWA again. This then slides SE through the CWA overnight
with the front.

So have adjusted the axis of additional heavy rain a bit farther
NW, rather than just over the far SE. Now expect the axis to be
from roughly Van Buren to Clinton Counties where an additional 1
to 2 inches is expected. This could bring totals to 4 to 5 inches
in some locales before it winds down early Wednesday morning.
North of a Grand Haven to Midland line generally less than an
additional inch is expected through tonight.

Decided to expire the flood watch over the far NW CWA...Mason and
Lake Counties. The area will stay in the 40s today and snow melt
should not be excessive. Also additional heavy rain is not
expected there, generally another quarter to half inch is
expected. Area rivers are only expected to reach advisory levels
with little impact in this area.

UPDATE Issued at 711 AM EST Tue Feb 20 2018

Since it looks very likely that the rounds of heavy rain will
continue into this evening and were are starting to hear of some
impacts...we issued a flood warning for most of the region and
dropped the watch.

Water levels were rising across much of the watch area with some
nuisance flooding developing. After looking at radar and
observations...rainfall rates have approached a half inch per
hour recently...especially upstream around Chicago and more
recently around Michigan City. Nearly all available model
guidance shows multiple rounds of additional heavy rain into
tonight with at least another one to two inches of rain depending
on location. The blossoming area of heavy rain well upstream in
OK and KS is shown to spread into MI mid to late this afternoon.


.SHORT TERM...(Today through Thursday)
Issued at 316 AM EST Tue Feb 20 2018

With the axis of very high pwat values remaining over the CWA
through the day...along with deep unidirectional southwest flow
and a right entrance region of an upper jet...the risk for very
heavy rain will persist. Will keep the flood watch going into
tonight. We already have seen roughly 0.5 to 1.0 inches of rain
with plenty more upstream. Elevated instability stays in place so
a risk for thunder will continue. Some fog will continue as
well...especially up north where the snow pack persisted.

The band of precipitation is forecasted to shift southeast through
the CWA tonight. With colder air wrapping in from the
northwest...a brief period of mixed precipitation will be
possible. A few slick spots possible as temperatures fall below

A large area of high pressure then builds in from the west for
Wednesday into Thursday. Models are in reasonable agreement on
this it looks like we will dry out somewhat.

.LONG TERM...(Thursday night through Monday)
Issued at 316 AM EST Tue Feb 20 2018

Forecast concerns during the long term revolve around rain potential
Saturday night and Sunday.

Both the gfs and ecmwf agree that the upper flow will remain
southwesterly which points toward above normal temperatures. Thus,
much of the precipitation that develops will be rain, which probably
isn`t a good thing considering the ongoing rain event and how rivers
are moving toward flood stage.

Both models show low pressure moving through the Great Lakes
Thursday night and Friday morning. There is little in the way of
upper support and the gfs and ecmwf paint the bulk of the
precipitation south of the cwa in Indiana and Ohio. The issue
Thursday night is the potential for some light freezing rain given
cold ground temperatures and lows in the lower 30s. This could be
the type of situation that flies under the pun...and then
surprises in the early morning hours.

The next event Saturday night and Sunday has some upper support with
it and so will likely produce more precipitation. The ecmwf and gfs
are somewhat similar in track and take low pressure from the
southern Plains to Wisconsin. However the gfs is much
stronger...984mb vs 1001mb...and produces quite a bit more
precipitation in the form of mixed rain and snow changing to rain.
The reason for the difference is the stronger upper wave on the gfs.
It`s a bit difficult to pin down qpf at this point, but confidence
is fairly high we`ll see precipitation.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1257 PM EST Tue Feb 20 2018

Widespread rain will bring MVFR to IFR conditions today and
tonight. There could be some isolated tstms as well, but did not
include in TAFs due to low probability. The rain will end from
northwest to southeast late tonight... at around 06Z at MKG and
closer to 12Z at JXN. The rain could end as a brief period of
freezing rain and/or snow, but felt the icing threat is too low
and too short lived to include FZRA in the TAFs. On Wednesday
conditions are expected to improve to VFR by 18Z. Southwest winds
around 10 kts will shift northwesterly with the sfc frontal
passage tonight.


Issued at 314 PM EST Mon Feb 19 2018

A notable flood event is forecast to materialize over the next 48
hours. A combination of heavy rainfall, melting snow and frozen soil
is expected to lead to efficient runoff over the next couple days.
Flood conditions are expected to continue along streams and rivers
through the course of the week with some sites forecast to reach
moderate category and potentially higher.

Guidance continues to be rather persistent in painting a wide swath
of 1 to 3 inches in total rainfall across much of Lower Michigan.
The greatest potential to reach or exceed 3 inches will exist over
the southern one-third of the state. A few thunderstorms could be
embedded in the overall shield of rainfall, locally enhancing totals
tonight through early Wednesday.

All sites are affected by some form of a flood product,
incorporating information on river rises through the next several
days. While most sites rise to minor to moderate flood levels, few
sites are forecast to achieve benchmark levels. It is important to
note that any record forecast will be highly dependent on whether
forecast rainfall verifies and whether the extent of the modeled
snowpack truly includes the expected water equivalent. If rainfall
occurs as expected and rivers continues to rise, the rate of rise
will almost certainly slow with higher levels, as water continues to
spread out.

Residents and local officials should keep up-to-date with latest
forecasts and remain vigilant with regard to river levels over the
next several days.




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