Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 151538

Area Forecast Discussion
National Weather Service Grand Rapids MI
1138 AM EDT Tue Aug 15 2017


Issued at 330 AM EDT Tue Aug 15 2017

A weak cold front will produce scattered showers this morning
before moving south of the area this afternoon. High pressure
provides dry weather tonight and Wednesday, then humid conditions
with showers and thunderstorms are expected on Thursday as a low
pressure system tracks across Wisconsin.


Issued at 1007 AM EDT Tue Aug 15 2017

Precipitation has exited our southeastern forecast area and have
updated the forecast to reflect this. Also bumped high temperatures
up a bit since we anticipate diminishing clouds and mostly
unimpeded diurnal heating this afternoon.


.SHORT TERM...(Today through Thursday)
Issued at 330 AM EDT Tue Aug 15 2017

Shortwave passes through early this morning taking most of the
ongoing shower activity with it. However lingering sfc convergence
and a dew point boundary may result in some new isolated diurnal
convection developing from mid morning through mid afternoon for
the I-94 corridor east of AZO. Some thunder would be possible if
this occurs since latest RAP guidance has SB Capes 500-1000 J/KG
developing near JXN by noon although dry air and subsidence
eventually ends that threat by 21Z or so.

Model trends have been for a slower solution with the mid week
system. Current expectation is for arrival of warm advection
showers and storms from the southwest very late Wednesday night,
then fairly widespread showers and tstms on Thursday with the
approach of the shortwave trough and the sfc cold front. PWATs
near 2 inches and sfc dew pts around 70 support the possibility of
some locally heavy rainfall, although the svr threat is
questionable due to the likelihood of considerable cloud cover.
Still, if any pockets of heating can occur, capes could reach
over 1500 J/Kg and deep layer shear of 30-35 knots would support
the possibility of some stronger/organized convection developing.

.LONG TERM...(Thursday night through Monday)
Issued at 330 AM EDT Tue Aug 15 2017

Showers and thunderstorms will linger into Thursday evening before
ending once the cold front moves through. Some strong to potentially
severe convection is possible into Thursday evening given favorable
cold frontal timing and potential for moderate instability to
develop along and just ahead of the front.

Severe potential is mitigated by several factors including stronger
upper level forcing staying north of our area. A consensus of latest
medium range guidance suggests sfc based cape values will stay near
or below 2000 j/kg. Stronger instability will likely develop south
of our area. So the overall severe wx threat Thursday evening
looks rather marginal.

Fair wx will return Friday and a cooler and drier airmass will
advect in behind the front.  In fact high temperatures Friday will
only reach the 70`s.

An upper level disturbance will move in from the west and bring a
few showers Saturday.  Fair wx will return Sunday and Monday as
another high pressure ridge builds in.  Temperatures by then will
undergo a gradual moderating trend to slightly above normal for this
time of year. Above normal temperatures are then anticipated through
the rest of next week.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 750 AM EDT Tue Aug 15 2017

A hodgepodge of flight conditions exists early this morning,
ranging from LIFR in fog/stratus to VFR/CLR. Conditions will trend
toward predominately VFR at all sites by mid morning and rain
showers will mostly end by 13Z. Can`t rule out an afternoon
shower or tstm redeveloping at BTL/JXN but that threat is very
low. Clearing skies tonight will lead to the possibility of
radiation fog, mainly after 06z. Will go MVFR now but some
IFR/LIFR cant be ruled out especially after 09z.


Issued at 330 AM EDT Tue Aug 15 2017

A brief period of stronger northerly winds this afternoon into
the early evening should promote choppy 2-3 ft seas, particularly
near big and little sable points and south of South Haven.
Otherwise relatively tranquil conditions on Lk MI. Looking farther
ahead, a period of hazardous winds and waves are possible on Lake
Michigan Thursday night into Friday.


Issued at 1137 AM EDT Tue Aug 15 2017

Observations reflect 24-hour rainfall amounts of 0.25-0.50 and
locally around 1.00 inch between I-96 and I-94, stretching from
Lake Michigan, east to the Detroit area. A couple of the rivers in
this swath (e.g. Jackson) show rises, but are expected to remain
well within banks. Another area of beneficial rain fell from
Ludington to Harrison and locations to the northeast.

The midweek system is beginning to look a bit more impressive with
each update in guidance. Precipitable water values are around and
above 2 inches, especially along and south of I-96, late
Wednesday night into Thursday afternoon. It is looking more
plausible that at least locally heavy rain could result as the
best lift moves in along the warm front Wednesday night into
Thursday morning. Another round of rain could develop with the
passage of the cold front during the day Thursday. Timing could
play a role in determining thunderstorm intensity, which could
affect overall rainfall amounts. Given the anomolous moisture
values and overall potential, this system may need monitored as
the forecast evolves through Wednesday.




LONG TERM...Laurens
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