Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 170222
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1022 PM EDT Wed Aug 16 2017

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 334 PM EDT Wed Aug 16 2017

Showers and storms will arrive overnight and continue into Thursday.
Some storms could be strong to severe Thursday afternoon, perhaps
lingering into Thursday evening.  After a muggy highs in the upper
70s to lower 80s on Thursday, we will cool down to the low and mid
70s Friday and become less humid.  A few showers may continue into
Friday morning, north of I-96.

The weekend looks mainly dry, then another chance of showers and
storms should move back in Monday night through Tuesday night. Daily
highs for the weekend into next week should remain in the upper 70s
to mid 80s.

&&

.UPDATE...
Issued at 1022 PM EDT Wed Aug 16 2017

Based on the radar trends, and trends in the HRRR and ESRL HRRR I
have increased the pop to near 100% over the western CWA after
midnight.

The instability is not all that great (most unstable cape near 600
j/kg, but what is really happening is the deep moisture is coming
into our area. Precipitable water values rise from under 1.5" at
8 pm to over 2.0", which is within a tenth of an inches of the
all time record for the 17th of August. The 1000/850 moisture
transport increases dramatically between now and midnight over our
western CWA and more so our NW CWA. Both the HRRR and RAP models
show two low level jets tonight, one around midnight that is
mostly focused on NW lower Michigan then a second one, currently
helping the convection in MO, heads into our Southern CWA toward
Sunrise. So I see two periods of showers / isolated thunderstorms,
one around midnight then next around sunrise. Given the amounts
of water in the air, we will likely get some brief heavy down
pours, I can see isolated locations getting well over an inch
tonight then again around sunrise with the next band of
convection. Most areas through will likely see less than a quarter
inch between now and noon.

As for thunderstorms tomorrow afternoon, the primary low level jet
is aimed near SE MI. We do get another one that is aimed toward NW
Lower Michigan in the evening, that needs to be watched for late
day convection with the cold front.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 255 PM EDT Wed Aug 16 2017

Showers and storms will move in overnight and continue Thursday.
Severe storms are possible Thursday afternoon/early evening,
depending on the timing of several ingredients.

A quiet evening will become increasingly wet overnight as the upper
ridging that was overhead moves east, and a strong upper wave moves
into the Western Great Lakes by daybreak.  A warm advection surge of
moisture and lift will be over the heart of the CWA by 12Z.  Showers
and a few storms will be on the increase from 06Z, and covering much
of the CWA by 12Z.  No severe weather is expected with this wave of
pcpn.

However severe storms will be possible into Thursday.  But it will
depend on all factors coming together.  With the morning showers and
storms exiting we will need to destabilize again into the afternoon.
The models do indicate some clearing behind the morning wave,
allowing for instability to build into the afternoon. The most
likely region where this occurs should be for areas east of U.S.
131. Bulk shear values are most favorable into the afternoon for
organized storms with hail, around 35 knots, then it tails off to
around 25 knots by early evening. However a large portion of this
shear is in the lowest 1km into the evening.  A warm front will be
crossing the CWA, passably supplying more shear locally, and with
low LCLs we may see the risk of an isolated tornado.  Damaging winds
will also be possible with bowing line segments.

Precipitable water values spike to over two inches by Thursday
morning.  So we will also have to watch the heavy rain potential,
with very heavy localized rains likely with the WAA rains. Additional
heavy rains will be possible into the afternoon, but this will be
very more localized with the diurnal convection.

A breezy Friday with some warp around showers possible in the
morning.  Temps will be about 5 degrees cooler, along with falling
dew points.  More isolated showers and storms possible late Friday
night with another upper wave coming through.

.LONG TERM...(Saturday through Wednesday)
Issued at 255 PM EDT Wed Aug 16 2017

Two main periods of rain look possible in the long term, one on
Saturday, and then another centered around the Tuesday timeframe. We
are looking for temperatures to warm up quite a bit by Monday of
next week, before a pronounced cool down comes in for mid-week next
week.

A decent short wave will dive in quickly on Saturday, in the wake of
the upper system supporting the system for tomorrow into Friday.
There is decent agreement that this looks to affect mainly the srn
portion of the CWFA. We would not be surprised if this system digs a
little further south, and moves south of the area. We will roll with
chcs of showers/storms for Sat for the time being.

Dry and increasingly warmer weather is expected for Sunday and
Monday. We will see a flat upper ridge migrate over the area. Even
though the ridge will not be very strong, we will see a solid SW
flow bring in very warm air aloft from the Plains states. The
atmosphere will likely be capped through Monday. This is favorable
for eclipse viewing in the area Mon, aside from some small cumulus
development or high clouds that might approach the area.

The threat for rain will increase beginning Mon night and will
likely max out on Tue. The flat ridge will slip east, and we will
see an amplifying trough taking shape to our west. The stronger
nature of the wave combined with some Gulf moisture advecting in
looks to bring a good shot of showers/storms to the area. This
should move out by Wed, leaving a much cooler and drier air mass in
its wake.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 751 PM EDT Wed Aug 16 2017

There is some concern for convection to move through the TAF sites
overnight and during the day Thursday. As is typical for here,
seems most of the convection will struggle to reach our TAF sites.
The large area of convection ahead of the warm front lifts north
over Wisconsin this evening and we will mostly see virga from mid
clouds at least till around 06z. There is some chance a few
showers or Thunderstorms will come through the TAF sites at two
times. The first window (based on the HRRR and ESLR HRRR) is
between 05z and 09z (west to east) and from 09z to 14z (west to
east). The trend through is for the more widespread convection to
stay north of MKG so I only put VCTS at any of the TAF sites.

As the warm front comes through around sunrise I am expecting a
period of MVFR CIG but not much rainfall. As the cold front pushes
through the area Thursday afternoon, again it is not out of the
question there would be thunderstorms after all there is
instability and low level convergence ahead of the cold front.
However most of the models show a lot of mid-level subsidence and
the Low Level jet on the NAM is aimed at TVC in the 21z to 00z
time frame. More than likely any convection would be toward the
Detroit area in the evening.

However we will be watching just in case the storms do become
better organized for our TAF sites.


&&

.MARINE...
Issued at 255 PM EDT Wed Aug 16 2017

Will issue a small craft advisory for Thursday afternoon north of
Holland and extend it through Friday.  Southerly winds will pick up
Thursday, then become westerly by late Thursday night.  Most likely
we will need to add the areas south of Holland into Thursday night
as this wind shift occurs.


&&

.HYDROLOGY...
Issued at 330 PM EDT Wed Aug 16 2017

Multiple rounds of thunderstorms are possible Thursday. This may
lead to localized heavy rainfall where storms more frequently
occur. Early to mid morning showers and storms are expected along
a warm front and afternoon/evening showers and storms are possible
especially east of US 131. PWAT values will be quite high,
approaching 2.0 inches, along with surface dew points above 70.
There is some potential for a few training thunderstorms with MBE
velocities occasionally dropping below 10 kts. However, this does
not look like a classic flood setup outside of localized ponding
of roads. We will continue to monitor trends.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Thursday afternoon through Friday
     evening for MIZ037-043-050-056.

LM...Small Craft Advisory from noon Thursday to 9 PM EDT Friday for
     LMZ846>849.

&&

$$

UPDATE...WDM
SYNOPSIS...JK
SHORT TERM...JK
LONG TERM...NJJ
AVIATION...WDM
HYDROLOGY...Hoving
MARINE...JK


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