Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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593
FXUS63 KGRR 201145
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
645 AM EST Tue Feb 20 2018

LATEST UPDATE...
Aviation

.SYNOPSIS...
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.

&&

.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
freezing.

A large area of high pressure then builds in from the west for
Wednesday into Thursday. Models are in reasonable agreement on
this scenario...so 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 radar...no 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 12Z TAFS through 12Z Wednesday morning)
Issued at 643 AM EST Tue Feb 20 2018

IFR and lower conditions are forecasted for much of the period as
a storm system moves into the region. Wind shear is also likely to
occur. The wind will shift to the northwest later tonight as the
storm system goes by...which may support improving conditions by
12z Wed.

&&

.HYDROLOGY...
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.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Wednesday afternoon for MIZ037>040-043>046-
     050>052-056>059-064>067-071>074.

LM...None.
&&

$$

SYNOPSIS...MJS
SHORT TERM...MJS
LONG TERM...04
AVIATION...MJS
HYDROLOGY...JAM



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