Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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501
FXUS63 KGRR 241158
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
658 AM EST Sat Feb 24 2018

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 328 AM EST Sat Feb 24 2018

Our next rain event moves through the area tonight with a few
isolated thunderstorms, the heaviest rainfall misses us to the
south and northwest. Enough cold air follows this system so that
we may see some flurries or sprinkles during the mid morning hours
of Sunday then skies should clear by mid afternoon. Mostly sunny
and warmer than normal conditions are expected Monday into
Tuesday. A weak cold front may bring showers later Tuesday into
Wednesday. Then a system from the southern plains moves into the
area to bring more rain, later Wednesday into Thursday. This may
be enough to cause more flooding issues.

&&

.SHORT TERM...(Today through Monday)
Issued at 328 AM EST Sat Feb 24 2018

The main issue with this forecast is the rain and possible
thunderstorms tonight. However is has become very clear the
heaviest precipitation amounts will be northwest and southeast of
this area. The system does go negative tilt at upper levels over
us tonight but the air cold enough for snow is well to our north
and west and the deep tropical moisture feed misses us to the
southeast. The instability is marginal but we can not rule out an
isolated thunderstorm this evening.

The reason the heaviest precipitation misses southwest Michigan is
that the upper low that develops will be north of Lake Superior
so that keeps the precipitation with the upper low mostly north
and west of this area. As for why the heavy rain south of this
area, that is due to the upper jet core tracking across the
Detroit area Sunday morning. That puts southwest Michigan in the
left exit region of the upper jet after midnight, which is why we
get any precipitation at all from this but the right entrance
region stays south of I-80 so that will be where the heaviest rain
and significant thunderstorms will be.

More than likely there will be a brief period of freezing rain
north and east of Grand Rapids tonight as the showers move in but
due to the light amount of rainfall expected and that temperatures
have been for the most part above freezing plus even in areas that
get freezing rain temperatures will be just near 32 so any ice
accumulation will be very limited.

Colder air follows this system so we may see a brief period of
flurries or sprinkles mid morning Sunday then the dry air surges
in and skies should clear. High pressure rules then into Monday.

.LONG TERM...(Monday night through Friday)
Issued at 328 AM EST Sat Feb 24 2018

Rain chances will increase Tuesday night when a weak cold front
moves south across the state. Little in the way of upper support is
noted with the front so rain will be light and may mix with wet snow
by Wednesday morning north of I-96.

The best chance of precipitation will be Wednesday night through
Thursday night. The ecmwf and gfs are at odds on the strength of the
system. The gfs is 20mb deeper with the low than the ecmwf and
consequently develops much more rain. Latest gfs paints 1-2 inches
of rain over the cwa with the higher amounts over the east central
cwa. We`ll watch this closely especially in light of the ongoing
flooding across the cwa. The ecmwf only created around a quarter
inch. The discrepancies can be traced to the different upper
dynamics. The gfs shows a much more robust low than the ecmwf which
shows a weaker trough farther north.

Once the system goes by, colder air will filter in again and
lingering pcpn may change to light snow.

Warmer weather is again on the way with highs in the mid 50s Tuesday
and Wednesday. Otherwise, highs in the 40s can be expected.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 658 AM EST Sat Feb 24 2018

It would seem the low clouds will clear out of western sections
more so than eastern sections today. MKG has already gone to
BKN250 for a ceiling as of 11z. It would seem this clearing should
reach GRR and AZO and maybe BTL. That is due to winds becoming
easterly, since the clouds are clearing from west to east, if the
winds turns to the east to soon, which I think it will, that will
not allow the eastern taf sites to clear the low clouds like the
western taf sites.

In any event, the next system comes in tonight with showers and
isolated thunderstorms. Since the risk of a thunderstorm is very
low, I did not put that in any of the TAF sites. If it were to
happen, the I-69 taf sites would be the most likely to see that
happen. All areas will see IFR ceilings and MVFR visibilities with
the showers tonight. The showers should end before 12z at all but
the most eastern TAF sites.

Due to winds being 40 to 50 knots above 1500 ft AGL, we have low
level wind shear in all of the TAF forecasts tonight.

&&

.HYDROLOGY...
Issued at 328 PM EST Fri Feb 23 2018

Levels along the Grand, Thornapple, and Kalamazoo Rivers have
crested in upriver locations and will crest over the next couple
days downriver. Many river forecast points in these basins are
seeing river levels that challenge modern records over the last 5 to
50 years. Moderate to major flooding is ongoing in the most flood-
prone locations. In Kalamazoo this is a record high river level, but
flood impacts compared to Sep 2008 are reportedly not quite so bad
due to recent flood mitigation projects. Still, many roads are
closed and properties are taking on water. In Kent County, the crest
on Saturday is expected to climb just a little under the levels of
April 2013, but flood impacts will nonetheless be substantial near
the river especially in Comstock Park.

A tenth of an inch of rain fell Friday morning, and a quarter to
locally half inch of rain is expected Saturday night. This may not
increase the flood impacts much if at all, but it could delay the
recession of river levels.

Please see the latest flood statements or river hydrographs for
details. Hydrographs are available by going to weather.gov/grr and
clicking on the "Rivers and Lakes" link above the map of flood
warnings.

&&

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

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...04
AVIATION...WDM
HYDROLOGY...CAS



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