Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171927
AFDGRR

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

LATEST UPDATE...
Synopsis/Short Term/Long Term/Marine/Hydro

.SYNOPSIS...
Issued at 327 PM EDT Sun Sep 17 2017

Scattered showers and a few thunderstorms will continue into
tonight, mainly east of U.S. 131, but they will end by daybreak
Monday.  After a dry Monday there will be another chance of showers
late Monday night into Tuesday morning.  It appears like a mainly
dry week thereafter, with the next chance of showers coming Saturday
night into Sunday.  It will continue to be mild, with temperatures
running about 10 degrees above normal, with daily highs mainly 80 to
85.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 327 PM EDT Sun Sep 17 2017

A cold front will continue to move across SW MI this afternoon.
There will be post frontal showers/storms into tonight.  Another low
chance of showers moves in Monday night and linger into Tuesday
morning.

The front was already into the heart of the CWA this afternoon with
little fan fare.  However a somewhat sheared short wave will not
move cross the area until mid evening, so the pcpn chance will
continue into tonight east of U.S. 131.  By midnight only expect a
few showers to remain over the SE CWA, and these will exit prior to
daybreak Monday.  CAPE values support the continued threat of
thunder too.

Removed POPs for Monday afternoon and evening as the over-running
event seems to move in slower now as the front is progged to drift
farther south.  This chance of showers now appears to move in late
Monday night into Tuesday morning.  Its in this time period that the
front drifts a bit north, into Northern Indiana.  And along with
this, the models show a weak short wave coming through, with
somewhat various timing.

We can not rule out a random shower/storm Tuesday afternoon into
Tuesday night with the front nearby to our south.  But it seems like
we lack upper level support, so will anticipate that most areas stay
dry in this time period.

Slightly cooler for Monday and Tuesday behind today`s front.  Highs
mainly mid and upper 70s, which is still slightly above normal.

.LONG TERM...(Wednesday through Sunday)
Issued at 327 PM EDT Sun Sep 17 2017

The bottom line to this forecast is warm and dry through next
weekend. In fact it may actually get hot (highs near 90 degrees) Fri
into Sunday. The threat for measurable rain is not zero but it is
not all to far from zero either. Given we are in D1 drought
(abnormally dry) and that little of any rain is forecast, we will
have to watch for continued development of a drought in this area.
As a point in fact, as of today GRR is about 5.25 inches below
normal on the 60 day rainfall total (one of the drought indicators).
We have to go back to 1996 to find a drier period in mid to late
September.

What is going on is there is a an unusually strong and deep upper
low in the Gulf of Alaska.  With that there is a 150 knot jet core
diving into the back side of that system as I write this. This will
help to significantly deepen the upper trough over the eastern north
Pacific, western Canada and the western CONUS.  Meanwhile there is
an upper high over Alaska and currently flat ridge over the eastern
CONUS.  As the western system deepens early this week, the upper
ridge over the eastern CONUS builds stronger. By late in the week
and likely through the weekend we will have a closed upper high over
most of the eastern CONUS.This will track any precipitation events
well north into central Canada over the top of that blocking ridge.
It will also result in nearly clear skies and allow the maximum
warming for our daily high temperatures.

So what that means is we will likely be approaching record highs by
late this week into the weekend. Record highs at GRR are in the
lower 90s most of this week into early next week. Today we got to 87
so far and on both the ECMWF and GFS the 1000/850 thickness and 850
temps are warmer from Wed into Sun than they have been at any point
in the past 7 days, so GRR has gotten to 87 today, it would seem 90
is very much not out of the question later this week.  The
persistent dry weather will only make getting to 90 easier.  Our
forecast highs are very conservative at this point.

The only real chance of rain is Wednesday with the warm front but
given the upper dynamics I would think any rain with the warm front
will be well north of here.  Still have do have low chance pops for
that just in case.

Bottom line... SUMMER LIKE temperature and developing drought seem
likely this coming week into at least early next week.

&&

.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 327 PM EDT Sun Sep 17 2017

Rather quiet through the short term regarding winds and waves.
Isolated thunderstorms will be possible late Monday night into
Tuesday morning.

&&

.HYDROLOGY...
Issued at 327 PM EDT Sun Sep 17 2017

A quick hit of rainfall is possible this afternoon and evening along
a cold front as it pushes west to east. We could see up to a couple
of tenths of an inch, especially in or near thunderstorms. Another
round of precipitation is expected to move through Monday night into
Tuesday, dropping another couple tenths of an inch.

Overall, given recent dry weather, these amounts are not expected to
have much of an impact. Abnormally dry areas may only see little
relief before conditions trend toward the dry side beyond Tuesday.
Limited chances exist for precipitation through the week with
temperatures that will be above normal. There continue to be no
hydro concerns at this time.

&&

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

$$

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



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