Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 181147
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
647 AM EST Sun Feb 18 2018

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 308 AM EST Sun Feb 18 2018

An area of high pressure centered over Missouri this morning will
track up the Ohio Valley today. This fair weather system will
provide the region with mainly dry weather and close to normal
temperatures. Later tonight the southerly flow strengthens around
the backside of this departing high pressure system. As a result
some precipitation will start moving in. A brief period of mixed
precipitation will be possible at the onset...but it is forecasted
to transition quickly to rain showers for most of Monday.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 308 AM EST Sun Feb 18 2018

The main challenge deals with the impacts from the potential heavy
rain. Overall the timing of the arrival is a little delayed but
the pattern is still supportive of swaths of heavy rain
developing.

Variability exists in the models with the overall amount of
precipitation forecasted which generates uncertainty in terms of
how much precipitation we will see. The high res euro shows
generally 1.5 to 2.5 inches for the CWA while the GFS adds at
least another inch on top of that. Overall they are similar on the
setup with the right entrance region of a very strong upper jet
strengthening over the area along with a very anomalous amount of
moisture from the Gulf arriving in the region. Several low level
jets move in during this period. Elevated instability is shown as
well. Enough support is there to highlight a risk for flooding
with an ESF which I will send out. Will keep the high POPs and
risk for thunder going Mon through Tue.

.LONG TERM...(Tuesday night through Saturday)
Issued at 308 AM EST Sun Feb 18 2018

Rain will continue Tuesday night as one final low pressure wave
moves northeast along the slow moving frontal boundary. 1000-850 mb
moisture transport still looks favorable especially in the evening
when precipitable water values will still be up around 1.25 to 1.5
inches.

So another half an inch to inch of rainfall is expected Tuesday
night. Some flooding issues are likely to occur due to the
cumulative rainfall from Monday through Tuesday night and melting
snow cover with the unseasonably mild temps Mon/Tue.

There is a chance of thunderstorms Tuesday evening as well mainly
over our southern fcst area along and just ahead of the front.
Precipitation will become mixed with then change over to some wet
snow flakes late Tuesday night as caa commences after fropa but
there will be very little to no snow accumulation.

Fair weather will return Wednesday through Thursday as a high
pressure ridge takes hold of the wx pattern. The next low pressure
system and upper level disturbance will bring a chance of mixed
light precipitation Friday. Another system moving northeast out of
the southern Plains states will bring potential for more mixed pcpn
Saturday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 643 AM EST Sun Feb 18 2018

Some uncertainty with this forecast. Initially MVFR clouds
covering most of the region. Models show them breaking up over the
next 2 hrs or so. Then with the developing southerly
flow...indications are that most of the lower clouds in Indiana
and Illinois will expand as they move into MI later this morning.
I did not feature IFR conditions...although there is a risk for
that this afternoon. Will need to monitor trends. I did add
vicinity showers for later tonight as the low pressure system
nears the region. Temperatures will be close to freezing...so
there is a small risk for freezing rain toward 12z Mon.

&&

.HYDROLOGY...
Issued at 329 PM EST Sat Feb 17 2018

Confidence is increasing in 1 to 2 inches of rain with narrower
swaths of 3 inches possible Monday through Wednesday morning. River
levels are running above normal for this time of year in the upper
Grand basin (greater Lansing area), with a couple already near
bankfull. Snow water content on an area-wide average is estimated
around a half inch.

The rain and snowmelt is expected to cause most rivers to rise out
of their banks by Tue in headwaters and later in the week in
downstream main stems. Familiar nuisance flooding would be expected.
Some rivers may reach minor flood stage which is a little less
typical and starts impacting roads and some structures. A comparable
flood in recent memory occurred in April 2017. Right now we do not
expect most rivers to surpass what was seen in that event, although
smaller basins that receive bursts of heavier rain can quickly rise
higher than expected. Ice on the rivers may break apart then jam up
downstream, resulting in unpredictable fluctuations in levels.

How rare is this rain event? Two-day precip totals over 2 inches
during February have only occurred a handful of times in 120-plus
years of records at Muskegon, Grand Rapids, and Lansing. Most
recently it occurred in 1997 at GR and Lan, and 2001 at Mkg. The
records for two-day Feb precip amounts are 2.41 inches at Mkg in
2001, 3.56 in 1997 at GR, and 4.40 in 1938 at Lan.

Note: Current river level forecasts published on AHPS do not account
for precipitation expectations beyond 48 hours.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...MJS
SHORT TERM...MJS
LONG TERM...Laurens
AVIATION...MJS
HYDROLOGY...CAS



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