Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201925

Area Forecast Discussion
National Weather Service Grand Rapids MI
324 PM EDT Sat May 20 2017

Synopsis/Short Term/Long Term/Marine

Issued at 324 PM EDT Sat May 20 2017

Low pressure over the Midwest will move northeast tonight. Showers
and isolated thunderstorms are possible through Sunday ahead of a
occluded front that will move across Lower Michigan Sunday
afternoon. Another low will generate showers and perhaps a few
storms mid week too. Highs through the middle of next week will
remain below normal.


.SHORT TERM...(This evening through Monday night)
Issued at 324 PM EDT Sat May 20 2017

Forecast concerns deal with pcpn trends through the period.

Latest regional radar shows a band of showers and isolated storms
moving north across the cwa. This rain is along and ahead of a warm
front. South of the front, dewpoints are in the 50s/60s while north
of the front dewpoints are in the 30s. After a period of steady rain
late this afternoon, pcpn will become more scattered tonight. There
is a little elevated instability tonight so we can`t rule out a
storm, but dynamics aren`t that strong. Precipitable water values
increase to over an inch and a half, so a few of the heavier showers
may produce some ponding on roads.

Once the occluded front moves through we`ll dry out as high pressure
builds in from the west. The next chance of rain looks to be Monday
night as another cold front moves across the state.

High temperatures will remain a bit below normal through the period.

.LONG TERM...(Tuesday through Saturday)
Issued at 324 PM EDT Sat May 20 2017

Forecast has not changed much in the long term from last night`s
thinking. We are still expecting an upper trough to dig into the
Great Lakes at the beginning of the period on Tuesday, affecting our
weather with rain and clouds into Thursday. The best chances for
rain looks to be Tuesday into Tuesday night when we also have
introduced a small chance for thunderstorms. CAPE values on the GFS
are in the 500-1000 J/KG range Tuesday which is plenty enough to
mention thunder. Rain chances decrease into Thursday and especially
on Friday with the upper low pulling away to the east. Friday and
Saturday look dry with surface and upper ridging located nearby.

Bottom line...showery from Tuesday into Thursday, then drying out
for Friday and Saturday. Temperatures will be cooler than normal mid
week with highs in the 60s, warming back towards normal by Saturday.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 212 PM EDT Sat May 20 2017

A warm front will gradually lift north into Southern Lower
Michigan tonight with periods of rain along with some embedded
thunderstorms. Aviation conditions will steadily deteriorate this
afternoon and evening with TAF sites trending from VFR to MVFR to
IFR this evening and tonight. By midnight expecting fairly
widespread IFR conditions with ceilings below 1000ft. Best chances
for thunderstorms will come tonight with a second batch of
showers/storms that is forecast to move in towards midnight.
Another round of showers and storms may move through with the
occluded front between 1000am and 200pm on Sunday. Winds will
gradually veer from east and southeast this evening to southwest
behind the occluded front on Sunday.


Issued at 324 PM EDT Sat May 20 2017

Waves will be well below advisory criteria tonight. Wind speeds will
be close though, especially at the 5 mile mark. Opted not to issue a
small craft advisory as most winds will remain below 25 knots.


Issued at 1005 AM EDT Sat May 20 2017

Rivers levels have returned to near normal. Expecting rain totals
ranging from a half inch in Central Lower Michigan to around an inch
in Southern Lower Michigan today through Sunday with locally higher
totals possible. Additional precipitation is possible Monday night
through Friday. Some river rises are likely with this rain, but we
are not expecting flooding at this time. Smaller streams and rivers
may need to be watched for above bankfull rises. Main stem rivers
should be able to handle this rainfall fine unless guidance trends




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