Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171637
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1237 PM EDT Sun Sep 17 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 328 AM EDT Sun Sep 17 2017

A cold front will bring scattered showers and thunderstorms this
afternoon and evening. Weak high pressure will provide mostly fair
weather for Monday, but showers will return by Tuesday as low
pressure moves towards Lower Michigan.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 328 AM EDT Sun Sep 17 2017

Looks like the better chance for rain will be on Monday night and
Tuesday. The front today is washing out and the mesoscale models
show little action with it until afternoon across the eastern
zones as diurnal instability increases. Otherwise, slight and low
chance POPs will cover for today. Severe threat is low due to weak
shear and marginal instability.

The return flow begins late Monday night and isentropic ascent
will aid in showers developing and perhaps a rumble of thunder by
Tuesday morning.

.LONG TERM...(Tuesday night through Saturday)
Issued at 328 AM EDT Sun Sep 17 2017

If you like Fall weather, you`re out of luck this week. Summer like
weather will continue at least through next weekend with highs in
the lower 80s and lows in the lower 60s. Dewpoints will be in the
lower to mid 60s for much of the week too, so it`s going to feel
humid.

The reason for the above normal temperatures is a large dome of high
pressure that`s going to be camped out over the eastern portion of
the country. Under the ridge of high pressure, h8 temps in the 15-
18c range are progd through the period. The upper jet is carving out
a deep trough over the western US and then heading straight toward
Hudson Bay, which essentially creates the ridge over the Great
Lakes. The only question is whether we`ll see any rain. Most spots
will stay dry, which isn`t good since the southern cwa is abnormally
dry. The models show a couple of fairly weak short waves trying to
move through, but they`ll have difficulty with the ridge in place.
Isolated pcpn at best. The western cwa may have a little better
chance of seeing rain Wednesday night as a cold front moves toward
Lake Michigan. But that front is going to weaken and not make it
through the cwa. So, this is basically a pretty dry forecast.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1237 PM EDT Sun Sep 17 2017

For the most part expect VFR with winds mostly less than 15 knots
(and actually mostly under 10 knots) through Monday. There is a
front coming through this afternoon with instability ahead of it.
So it is possible AZO, BTL, LAN and JXN could see a shower or
thunderstorm this afternoon and for LAN and JXN that risk could
linger into early evening. There could be some IFR or MVFR
visibilities and ceilings between 06z and say 15z Monday for
those same TAF sites a shallow layer of cold air slips south over
the central and eastern CWA behind the initial cold front later
this evening.

The problem for those storms though is most of the good mid level
forcing is well north of Lower Michigan this afternoon and
evening. The latest run of the ESRL HRRR shows little if any
convection over our CWA this afternoon actually. Meanwhile the RAP
model has all the convection between I-69 and US-131 this
afternoon. I am favoring the ESRL HRRR as it typically is better
with short term deep convection. Model sounding surely support
convection as in the 18z to 00z time frame there is no cap and the
EL is near the tropopause and surface based capes are near 1500
j/kg. However even with that being so, there is a problem with
meaningful convection developing. There is little in the way of
meaningful low level jet and there is little in the way of upper
divergence over this area this afternoon. So I put VCTS in those
listed TAF sites.

Once the sun sets, the convective threat will end but as suggested
in my initial discussion paragraph, I would expect some low
ceiling/visibilities around sunrise Monday as a shallow layer of
cooler air moves south into the area. That will mix out by late
morning so all areas should be solid VFR by 15z-17z.


&&

.MARINE...
Issued at 328 AM EDT Sun Sep 17 2017

Winds and waves will be decreasing later today as a weak cold
front moves through. There will be a few showers and thunderstorms
along the front but no severe weather is expected.


&&

.HYDROLOGY...
Issued at 1142 AM EDT Sat Sep 16 2017

Dry weather continues with no concerns for area rivers.

A cold front will bring chances for showers and a few
thunderstorms Sunday afternoon/evening. Widespread precipitation
is not likely, as the front continues to weaken and the best
support for rainfall is well off to the north. Another weak system
brings additional chances for light precipitation Monday night.

Overall rainfall amounts through the week are forecast at one-half
an inch or less.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...Ostuno
SHORT TERM...Ostuno
LONG TERM...04
AVIATION...WDM
HYDROLOGY...JAM
MARINE...Ostuno



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