Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 220147

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


Issued at 815 PM EDT Fri Jul 21 2017

An area of low pressure will track eastward along the quasi-
stationary front just south of Michigan tonight into Saturday.
It will keep most of the strong storms closer to I-80 than I-94.
There will be some periods of light rain tonight over areas near
and south of Grand Rapids with the risk of thunderstorm around
midnight west of Battle Cleek, near and south of I-94. A few
instability thunderstorms are possible Sunday and the truly cooler
air moves into the area.

Canadian high pressure will move through the area early next week
bringing slightly cooler than normal temperature and considerable


Issued at 946 PM EDT Fri Jul 21 2017

The thunderstorm complex continue, for the most part, tracking
southeast. Watching the individual cells in the storm complex
over WI, IA, IN and IN most of the cells are tracking southeast
too. However over the past 1/2 hour one cell just south of the
city of CHI is tracking northward. Likely this is some sort of
boundary interaction. On the other hand the RAP and HRRR low level
jet shows a surge northward just ahead of the MCV assoicated with
the complex of storms near CHI moves across Southern Lake
Michigan and Northern Indiana over the next few hours. It is more
than possible a few storms could make it into Van Buren and
Kalamazoo Counties in the 11 pm to 2 am time frame. Otherwise near
and north of I-96 it would seem to me there little if any threat
for thunderstorms tonight.

The way the surface wave tracks across the CWA Saturday it would
seem the instability will stay south of I-94 most of the day. I
see very little chance of showers so I lowered the POP over most
of the area through the afternoon.

UPDATE Issued at 815 PM EDT Fri Jul 21 2017

It seems to this forecast that the threat for any serious severe
storms is once again been trumped by the frontal boundary being
closer to I-80 than I-96. While there is a real surface wave on
that front, and that will allow some storms to track east to east
southeast just north of the warm front, that warm front does not
really push northward into our area overnight. That tells me the
significant convection will also stay south of our area.

Going along with this idea of you look at the SPC-HRRR model
browser and look at the trends of models heavy rainfall between
17z runs and the 22z run for any hour between 01z and 09z, the
heavy rainfall is father south with each run. So the trend suggest
the strongest convection will miss our CWA to the south.

The best chance for any thunderstorms would be west of Battle
Creek along and south of I-94 between 11 pm and 2 am as the best
support moves through this area.

As a result I trended back on the precipitation (POP) threat and
put the significant QPF significantly farther south. I may have
to do cut back on the QPF even farther later this evening.


.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 00Z TAFS through 00Z Saturday evening)
Issued at 815 PM EDT Fri Jul 21 2017

For the most part the convective threat is south of our TAF sites
tonight. Still I could see, being north of the surface wave
tracking along the front to our south, there will be an area of
low clouds and light fog Saturday morning. This should clear by
mid afternoon but that will not be long enough to allow enough
instability for afternoon thunderstorms. My bottom line to this
is VFR most of the night (threat of thunderstorms best at AZO
after 05z). The other TAF sites are to far east or north to be
impacted by this event.


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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