Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211930

Area Forecast Discussion
National Weather Service Grand Rapids MI
330 PM EDT Fri Jul 21 2017

Synopsis/Short Term/Long Term/Hydro/Marine

Issued at 330 PM EDT Fri Jul 21 2017

Showers and thunderstorms with heavy rain and some gusty winds
will be on the increase overnight before slowly moving out on
Saturday. Mostly fair weather will follow for Sunday and early
next week as high pressure builds in.


.SHORT TERM...(This evening through Sunday night)
Issued at 330 PM EDT Fri Jul 21 2017

Very high precipitable water values will mean we will have to
monitor radar trends for excessive rainfall tonight. The model
guidance including the HRRR and NCAR models have trended further
south with the higher risk of storms tonight but the southern
forecast area may still get clipped by the organized storms which
will also bring the threat of strong winds.

We have adjusted POPs and QPF further south and lowered POPs on
Saturday based on the belief that convection tonight will impede
instability and moisture transport on Saturday.

.LONG TERM...(Monday through Friday)
Issued at 330 PM EDT Fri Jul 21 2017

The weather to start the period will be characterized by a mid level
ridge which will be building in from the Upper Plains.  This feature
will help to dry the airmass out and lead to a cooling trend with
the temperatures.  Will feature dry conditions for Monday and
Tuesday as the surface high tracks through.

Mid level height falls occur over the Great Lakes region as we go
into Wednesday and Wednesday night. This will be the result of an
upper low that will be tracking eastward from Ontario into Quebec.
Models are in reasonable agreement on the timing and track of this
system. Thus some showers look possible ahead of and along the cold
front associated with this system   With instability developing as
well... a risk for a few storms exists. Overall the forecasted
forcing and instability look will only go with a low
chance for measurable precipitation.

The atmosphere then dries out for the end of the week as a northwest
flow and subsidence develops behind the departing front.  The new
High Res Euro does stall the front out just south of the MI border
on Thursday and tracks a weak surface wave along it.  If this system
trends further north with time...some rain may occur for southern
parts of the CWA.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 118 PM EDT Fri Jul 21 2017

The latest trend in the forecast is to bring some showers in from
the west later this afternoon into the evening...then a round of
storms closer to or just after midnight. Still much uncertainty as
to how this event will unfold...but radar is showing the rain in
Wisconsin building east...which may very well make it into KMKG
before 00z. Will keep cloud bases VFR through 00z...but it looks
like as the tropical airmass moves in...the cloud bases will be
lowering...especially later tonight. Any thunderstorms could
support IFR or lower conditions...but I was not confident enough
to go that low.

Low level moisture is shown to be high for Saturday morning. Will
go with MVFR cloud cover for now.


Issued at 330 PM EDT Fri Jul 21 2017

Winds and waves will generally remain relatively light through the
weekend. The exception could be for some gusty winds and higher
waves tonight in strong thunderstorms south of Muskegon and
Saturday morning south of of South Haven. There could be some
dense fog at times through Saturday before drier air moves in.


Issued at 330 PM EDT Fri Jul 21 2017

The favored region for locally heavy rainfall through early Saturday
will be south of I-96 and particularly near the MI/IN border. Hi res
models have trended further south with thunderstorm activity
overnight into Saturday morning due to poor handling of earlier
convection. Therefore, the risk for heavy rainfall has shifted
accordingly. Localized flooding will still be possible where any
storms persist. This may be more of a concern near I-94 and toward
the MI/IN border. Rises of small rivers and streams will be possible
but it is not looking like the main stem rivers will have any big
issues unless convective trends change significantly.




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