Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 241645
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1245 PM EDT Sun Sep 24 2017

LATEST UPDATE...
Hydro

.SYNOPSIS...
Issued at 324 AM EDT Sun Sep 24 2017

Our unusably late season heat wave will last into Tuesday before
a cold front finally comes through early Wednesday to bring in
cooler air. A second cold front late in the week will bring
seasonably chilly air into the area. It is not out of the question
we could have showers and even a thunderstorm early Wednesday
with that cold front but at this point widespread precipitation
is not expected.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 324 AM EDT Sun Sep 24 2017

Today will be day four of our unprecedented late season heat
wave. Monday will likely be day five for this heat wave (3 or
more consecutive days with highs of 90 or more). Tuesday is on
the fence for day six since heights and thickness fall as the
front gets closer. Still it would not surprise me that most areas
inland will still have highs near 90. I would expect daily record
highs will again be set today and Monday at Grand Rapids, Lansing
and likely Muskegon.

There are two things working together that are helping the upper
high to remain in place into Monday. First and foremost is the
deep western trough. Second is the upper level outflow from Maria.
Finally through, an upstream shortwave over the eastern Pacific
gets close enough to the western CONUS trough to push it north
and east over the top of our upper ridge feature. It is for that
reason (the through is forced over the upper ridge) that keeps the
better dynamics north of Southwest Michigan, limiting the chances
for convection with the front.

The lack of rain and subsidence under the upper ridge is helping
to lower surface dew points some. Today those lower surface dew
points will make the heat stress a touch less. The maximum heat
index values will mostly be at or under 95 degrees instead of near
100 degree as they have been at many of our observation sites the
past 3 days.

The subsidence under the upper high will also make day time
heating convection very unlikely the next two days. Even on
Tuesday model sounding are only marginal at best for afternoon
convection.

.LONG TERM...(Tuesday night through Saturday)
Issued at 324 AM EDT Sun Sep 24 2017

After a period of record, to near record high temperatures early in
the week, a sharp change in temperatures is still expected in the
Wed/Thu time frame as a cold front pushes through. H8 temps plunge
from around 16C Tuesday night to 6C by Wednesday night.  Wednesday
will still be mild with highs in the 70s, but Thursday should only
see 60s for highs.  60s, or even some highs in the upper 50s, should
continue into next weekend.

Rain chances however appear to remain low through the week.  We may
see widely scattered or isolated showers and storms with the front
Tuesday night.  Then as the pattern shifts to a more northwest flow
we may see more widely scattered showers Friday and Friday night.
The ECMWF has an upper low moving into the Great Lakes in this time
frame, the GFS and it`s ensembles show high pressure building in
from the west.  More likely is that a quicker moving short wave
comes through Friday and perhaps some lake effect showers Friday
night with H8 temps lowering to around 4C.

All in all, a significant shift towards Fall for the latter half of
the week, with nearly a 40 degree temperature drop from this weekend
to next weekend.  However, a well needed soaking rain does not
appear in the offering.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 717 AM EDT Sun Sep 24 2017

Any patchy fog should be gone by 15z, otherwise clear skies and
for the most part a light south southeast winds into Monday.

&&

.MARINE...
Issued at 324 AM EDT Sun Sep 24 2017

Surface high pressure overhead will continue to mean light winds.
This will keep wave highs favorable for beach activities as will
the bright sunshine an unseasonably warm lake surface
temperatures. The frontal system early Wednesday may have enough
wind to result in a Small Craft Advisory.

&&

.HYDROLOGY...
Issued at 1245 PM EDT Sun Sep 24 2017

Rivers are running around normal to below normal for the time of
year. Meanwhile, the US Drought Monitor is indicating dry conditions
across Southern Lower Michigan. Near-record warmth is expected today
and Monday. The next chance for rain is Wednesday, but it`s not a
good chance. No river issues are expected through the week.

&&

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

&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...JK
AVIATION...WDM
HYDROLOGY...63
MARINE...WDM



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