Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171757
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
157 PM EDT Sat Jun 17 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 250 AM EDT Sat Jun 17 2017

Several rounds of thunderstorms will likely impact the region
today with a risk for severe weather mainly closer to the MI/IN
border tonight. Some gusty winds and hail will be possible.
Conditions dry out Sunday afternoon before some scattered showers
and perhaps some thunderstorms develop Monday.

&&

.UPDATE...
Issued at 1104 AM EDT Sat Jun 17 2017

Radar is fairly quiet at this time over SW Lower MI, except for a
weak broken line of showers from near Grand Haven to Saginaw Bay.
Quite a bit of convective inhibition exists across the cwa, but
will likely weaken by the afternoon. An axis of higher
precipitable water exists mainly where the current weak showers
are occuring. Models continue to want to develop convection across
the central cwa this afternoon. Thinking if it does develop it may
be a tad farther northern closer to where the boundary between
clouds and sunshine exists. This is also an area that shows steep
mid level lapse rates. Will keep the mention of decent pops in the
grids this afternoon.

&&

.SHORT TERM...(Today through Monday)
Issued at 250 AM EDT Sat Jun 17 2017

The main challenge is determining timing and placement of convection
today and tonight, and whether strong to severe storms will occur.
There may be several waves of thunderstorms this morning and
afternoon for areas near and north of I-96, with a risk for severe
weather primarily near and south of I-96 tonight but especially
toward the MI/IN border.

The forecast for today and tonight is somewhat tricky, with much
of our severe threat tonight hinging on what occurs into the
afternoon. It now seems plausible that there will be multiple
waves of thunderstorms from this morning into the afternoon for
areas near and north of I-96, with the threat area shifting toward
I-94 late afternoon. The H850 LLJ strengthens to near 35 kts this
afternoon over the region, in the midst of a very moist
atmosphere supporting PW values of 2.00". Surface based
instability will be sufficient today (1000+ J/kg) but the
atmosphere may not recover this evening near and north of I-96 to
support what would otherwise be more vigorous convection tonight.

High res CAMs from the HRRR/ESRL HRRR to the NAM 3km and NCEP WRF
ARW/NMM support multiple rounds of storms today across the region
but not a persistent rain for all areas. With such a moisture-rich
atmosphere, these storms will be efficient rain producers. Locally
heavy rain is expected. It seems that any wind gusts and hail
produced from these storms through the afternoon would be sub-
severe. The strongest mid level jet core (50 kts) does not arrive
until after 00z and the CAPE might not be large enough in the hail
growth zone to generate severe hail. Still, some isolated instances
of damaging winds and large hail can`t be ruled out into the
afternoon.

By the time the best dynamics arrive tonight (in the 00z-06z time
frame), our severe weather risk may be waning due to loss of daytime
instability. The ESRL HRRR and NAM 3km suggest that a line of storms
will develop from Chicago/southern Lake Michigan down into Missouri
around 00z and then move eastward. It seems that areas near and
south of I-94 stand the best chance at getting damaging winds and
large hail from this activity. Even so, there may be convection on
the nose of the H850 WAA that could generate hail north of this
region.

Convection may persist into early Sunday morning before drying
occurs during the day. Cyclonic flow over Lower Michigan combined
with falling H850 temps and some marginal sfc based instability may
help generate at least scattered showers and perhaps a few
thunderstorms Monday afternoon away from Lake Michigan.

.LONG TERM...(Monday night through Friday)
Issued at 250 AM EDT Sat Jun 17 2017

We continue to expect a fairly unsettled weather pattern for much of
the long term. Rain chances will exist almost everyday, but we
should not see any days that are a complete washout. Temperatures
will start out cooler than average, and moderate to near to slightly
above average.

The best chances for showers/storms on Tue will come during the
afternoon/evening hours. This will be the result of an upper trough
remaining overhead, and another impulse moving through from NW to
SE. The impulse of note does still have a chance of coming through
during peak heating, which could produce some stronger storms.

The best chance of a dry day still looks to come on Wed. The upper
trough overhead earlier in the week will slip far enough east to
allow ridging to build in.

Rain chances will increase then once again for Thu and Fri. The heat
makes a push toward the area on Thu as the sfc ridge moves east and
upper heights build overhead. Storms will become possible at the
leading edge of the heat moving in with a warm front. Storms will
continue to be possible through Friday when a cold front associated
with a fairly strong sfc low across Wrn Ontario pushes through the
area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 157 PM EDT Sat Jun 17 2017

Have decided to keep thunder out of the TAFs until after
midnight. Currently most of the shower/tstm activity is passing
north of the TAF sites and once the MCV currently over wrn lwr MI
is east of the area, subsidence in it`s wake should limit
convective development.

Coverage of showers and storms is expected to increase later this
evening and continue overnight as the low level jet increases and
the cold front/upr shortwave trough approaches from the west. IFR
or lower conditions may develop if heavier rains set up over the
terminals.

Also, A band of MVFR cigs will likely accompany the cold frontal
passage late tonight/early Sunday morning, impacting any one TAF
site for 3-4 hours. South-southwest sfc winds will be 10-20 knots
today and tonight, shifting westerly Sunday morning.

&&

.MARINE...
Issued at 250 AM EDT Sat Jun 17 2017

Waves will build on Sunday with 2 to 4 footers possible and steady
west winds. Until then, southwest winds may generate 1 to 3 foot
waves today. No small craft advisories are anticipated this
weekend.


&&

.HYDROLOGY...
Issued at 1202 PM EDT Sat Jun 17 2017

Showers and thunderstorms moved through West Michigan prior to
sunrise...with a swath of one to three inch amounts across
northern portions of Ottawa and Kent counties. The Rogue River at
Rockford rose almost 2 feet in a few hours this morning due to the
runoff from these storms...but has already started to fall.

Satellite shows mesoscale vortex over Chicago...with plenty of
moisture available ahead of this feature. Expect showers and
thunderstorms to develop this afternoon through tonight...and
maintain a southwest to northeast orientation along the axis of
highest precipitable water values. Heavy rainfall will definitely
be possible given the high moisture availability and increased
precipitation efficiency.

Could see quick rises on some of the smaller rivers across the
region through Sunday morning...but above bankfull rises are not
anticipated at this time.

&&

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

$$

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



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