Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 210235

Area Forecast Discussion
National Weather Service Grand Rapids MI
1035 PM EDT Sat May 20 2017


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.


Issued at 1035 PM EDT Sat May 20 2017

Updated POPs based on current radar trends and hi res models.
Severe threat appears minimal across all but the far southern
forecast area where there will be some sfc based instability.

Main threat will be isolated downbursts in small bowing line
segments and even a remote chance for a weak brief tornado as
storm relative helicity values will be high in backed flow north
of sfc warm front. However, rotation in storms further south in
Indiana has not been noted recently so that threat appears minimal
as well.


.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 00Z TAFS through 00Z Sunday evening)
Issued at 758 PM EDT Sat May 20 2017

Ceilings will generally be in the 1500 to 2500 foot range across
the southern half of Lower Michigan early this evening then lower
to blo 1000 feet later this evening as showers and a few
thunderstorms move in from the south. The better chances for
thunder will be across far southern Lower Michigan.

East winds will continue to gust to 20 knots this evening. Winds
will go southwest on Sunday with some gusts over 20 knots.


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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