Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 241738
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1238 PM EST Fri Feb 24 2017

LATEST UPDATE...
Hydro

.SYNOPSIS...
Issued at 326 AM EST Fri Feb 24 2017

A strong area of low pressure will move through the region through
this evening. This system is bringing showers and storms to most of
the area. A wintry mix is falling north of M-20 this morning. This
system will bring the potential of severe weather later this
afternoon and early this evening to areas south of Lansing and east
of Kalamazoo.

Colder air will filter in over the area late tonight and on
Saturday. Light snow will develop at the same time, with some light
accumulations possible. A weak wave of low pressure will move
through on Sunday and bring another chance of some light snow, that
could mix in with a little bit of rain.

The weather will remain unsettled next week as multiple systems
could affect the area.

&&

.UPDATE...
Issued at 1109 AM EST Fri Feb 24 2017

Low pressure will track across the southern CWA this evening. This
will bring a warm front into the our area. The SPC outlook of
enhanced risk of severe storms lines up very well where this warm
front is expected to lift. This SE area of our CWA should see
CAPE values approach 1000 J/KG along with helicity values around
300 m2/s2. Concern for isolated tornadoes remains in this region.

Any storms going north of a Holland-St. Johns line could be
severe, but they will be entering the cooler and less unstable
air, so they should begin to weaken and become more elevated,
where hail becomes the main threat.

A conveyor belt of moisture is maintained over Western IN into SW
MI where a few showers exist late this morning. This should also
be the region of more development through the mid/late afternoon.
Convective allowing models suggest storms organizing after 21Z,
and then clearing the CWA by 04Z.

Temps are tough today with a large gradient across the CWA.
Expect highs around 40 north, to the lower 60s south. Nudged temps
upward across the central CWA. Most areas should see their max
temps in the early evening as the low comes through.

Have allowed the winter weather advisory to expire. An occasional
mix will still be possible across Highway 10, but impacts are no
longer expected.

&&

.SHORT TERM...(Today through Sunday)
Issued at 326 AM EST Fri Feb 24 2017

The main points of note in the short term are that we will maintain
the Winter Weather Advisory as is for the northern counties this
morning, and the best severe weather threat has shifted slightly a
bit SE compared to yesterday at this time.

Much of the advisory area is seeing light snow as of 08z this
morning. Temps are above freezing thus far, so no major impacts are
likely occurring. We expect that p-type will transition over to
sleet and freezing rain for a few hours this morning before changing
to rain by afternoon. Temps aloft are increasing ahead of the sfc
low to our SW, while temps and dew points at the sfc are dropping
due to the cooler and drier E/NE flow.

Elsewhere showers are ongoing, with some lightning moving across the
lake as of 08z. The storm chances will increase gradually through
today as the atmosphere becomes more unstable. All storms early this
morning will be of the elevated variety, with sfc based instability
arriving at the srn CWFA boundary around 12z this morning.

The severe threat will come after about 3 pm this afternoon and will
last into mid evening tonight. The convection allowing models
indicate there will be some isolated cells popping up toward mid
afternoon mainly in the warm sector which should end up being south
of a line from South Haven to Lansing. We then expect a more solid
line to form ahead of the cold front starting around 22-00z that
will push east. The warm sector should have over 1000 j/kg of low
level based cape available. This combined with the high deep layer
and 0-3km shear values will justify the severe threat. We continue
to expect damaging winds will be the main threat. A tornado is not
out of the question with the strong low level shear values.

The dry slot moves in then behind the front this evening, and then
the wrap around moisture and main wave will arrive over the western
portion of the area by 12z Sat. Temps will have dropped below
freezing, however the warm ground will take a bit to cool off enough
for accumulations to occur. We are thinking up to a couple of inches
will be possible, with the highest amounts being NW.

Ridging will build in quickly then for Sat night, diminishing most
of the snow/snow showers. Some light lake effect could hold on with
h850 temps in the minus teens C, but the ridging will squash the
vertical extent and capping the snow showers. We will see another
wave move through on Sun with some warming aloft. This will bring
another chance of light snow, possibly mixing with rain in the
afternoon as temps warm well into the 30s.

.LONG TERM...(Sunday night through Thursday)
Issued at 326 AM EST Fri Feb 24 2017

We`ll have several opportunities for pcpn during the extended
period, but no big storms are on the way.

A weak short wave passing over Lower Michigan Sunday night may
produce a period of light snow but not much more than that. We`ll
dry out for monday as weak ridging moves overhead. Both the gfs and
ecmwf show a system moving out of the southern Plains toward the
Great Lakes midweek. Mixed rain and snow are possible in the warm
advection pattern ahead of the low Tuesday. The gfs is a bit
stronger, especially aloft, and tracks the low over the cwa while
the ecmwf is much farther northwest of Wisconsin. We`ll keep high
pops for now and taper pops Wednesday into Thursday as the sfc low
moves east and the upper wave moves through.

Highs will be mostly in the 40s Monday through Wednesday before
cooler air flows in behind the system for Thursday and highs fall to
the mid 30s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 648 AM EST Fri Feb 24 2017

Plenty of impacts to account for in this set of fcsts, including
thunder, fog, and wind. IFR and lower conditions dominate the area
early this morning. IFR will likely remain in place for most of
the area most of the fcst period.

We will see another round of storms come through this morning via
the line that is over Lake Michigan to near Chicago as of 1145z.
This should exit the area by around 16z or so. There will be a
break, before additional showers and storms develop starting
around 19z or so and will continue until the front moves through
around 23z to 02-03z. The low will move between the I-96 and I-94
corridors, so conditions may be quite different with regard to
wind direction and cigs and vsbys.

Once the front moves through, drizzle will likely develop. Winds
will shift to the WNW to SW and become gusty overnight. Some snow
showers will move in at the Wrn terminals toward the end of the
fcst period.


&&

.MARINE...
Issued at 326 AM EST Fri Feb 24 2017

We have an ongoing Small Craft Advisory event this morning that we
will maintain the headlines for at this time. This is an offshore
flow type of event, so the winds and waves are worse away from the
shoreline. The peak of the event is just about here, and will then
taper off fairly quickly by this afternoon.

We are looking at a max SCA or minimum Gale event on Saturday behind
the cold front. We will hoist a Gale Watch for this event on
Saturday.


&&

.HYDROLOGY...
Issued at 1237 PM EST Fri Feb 24 2017

Widespread rainfall overnight will produce rises on area rivers
over the next few days. Rainfall was generally one quarter to one
half inch...with amounts in excess of one inch across northern
portions of Lower Michigan. Current forecasts keep all rivers
below flood stage...but additional rainfall today could produce
stronger rises. Heaviest rain today should fall over southern
sections of Lower Michigan...which received less rainfall
overnight.

Colder temperatures return for the weekend...with a series of storm
systems bringing additional precipitation to the region this week.
While temperatures are not expected to be cold enough for ice
formation on rivers...the additional precipitation could cause
slower recession or additional rises on area rivers.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Gale Watch from Saturday morning through Saturday evening for
     LMZ844>849.

&&

$$

UPDATE...JK
SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...04
AVIATION...NJJ
HYDROLOGY...MWS
MARINE...NJJ


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