000
FXUS63 KGRR 210506
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
106 AM EDT Mon Aug 21 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 312 PM EDT Sun Aug 20 2017

High pressure will continue to bring fair weather this evening. It
will become quite warm and humid Monday and an approaching low
pressure system will bring rain and thunderstorms Monday night
into Tuesday. A much cooler airmass will move in behind that
system for mid to late week.

&&

.UPDATE...
Issued at 1129 PM EDT Sun Aug 20 2017

High Res models (NAMnest and experimental HRRR) continue to show
a mainly dry night for Southwest Lower Michigan. An upper
shortwave is forecast to move through the area tonight however and
do not feel comfortable going with a completely dry forecast.
Showers and thunderstorms have recently developed in Northeast
Illinois and some of this activity may work into at least southern
portions of the forecast area overnight. Have a 20-30% chance for
showers and thunderstorms in the forecast area wide. The low level
jet does stay mainly south of our area, so not expecting anything
widespread. Some light fog may develop late in a light wind regime
as temps drop to near the dew point.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 312 PM EDT Sun Aug 20 2017

Fair wx will continue late this afternoon through this evening
with high pressure still in control of the wx pattern. A cold
front approaching from the nw will bring a very small chance of
showers to our far northern fcst area toward daybreak Monday.
Showers in association with a warm front to the w/sw of our area
should stay sw of our fcst area overnight.

Dry wx will continue through most of Monday morning and afternoon
though a few showers and isolated thunderstorms may develop in an
increasingly warm and humid and unstable airmass out ahead of the
cold front.

A consensus of 12Z shorter range guidance time height rh progs and
latest numerical guidance all suggest that rather extensive cloud
cover will linger through Monday afternoon resulting in unfavorable
viewing conditions for the eclipse in our area. However completely
overcast conditions are not expected so the eclipse may briefly be
viewable from time to time from portions of our fcst area.

The best chance for rain and thunderstorms will come late Monday
evening into early Tuesday morning along and ahead of the
approaching cold front. Severe wx is not anticipated given
unfavorable frontal timing with decreasing instability overnight.
However locally heavy rain is possible late Monday evening into
early Tuesday morning when 1000-850 mb moisture transport will be
strongest in conjunction with precipitable water values
approaching two inches at that time.

Rain and perhaps a few thunderstorms will linger Tuesday morning
and early afternoon prior to fropa. However instability prior to
fropa Tuesday will be very weak given extensive clouds/showers in
place Tuesday morning.

Cold frontal timing and a consensus of latest 12Z guidance all
suggests that stronger convective redevelopment later Tuesday
afternoon will occur well to the southeast of our fcst area. This
notion is supported by the spc day 3 convective outlook. Brisk west
to northwest winds will usher in a drier airmass late Tuesday
through Tuesday night when skies will gradually clear.

.LONG TERM...(Wednesday through Sunday)
Issued at 312 PM EDT Sun Aug 20 2017

The overriding message of this coming weeks weather will be our slow
progression into a more "Fall Like" weather pattern. As I pointed
out yesterday, the frequency of days with highs at or above 80 falls
below 50 percent, at GRR, by mid week. Our afternoon highs from
Wednesday through Friday will surely show this to be true.

As it turns out the last time we had the QBO turn from positive to
negative (2014) during the spring into early summer, like it has
this year, we saw a similar pattern where below normal temperatures
becoming more frequent that warmer than normal temperatures. You may
remember how cold of a winter 2014/2015 was. In any event, this
tendency toward colder temperatures occurs due to a more meridional
upper air pattern,  verses zonal being favored when the QBO is
positive. In fact we are now seeing this. When the QBO is negative
there tends to be more blocking in the mid to high latitudes than
when it is positive. That is why we get more meridional flow.  Of
course this could also mean an extend period of warmer than normal
temperatures if the upper air pattern blocks us close to the upper
ridge position. My point here is we should whatever patten we get
into to be more persistent that last summer or fall. At this point
we have an upper troughs over eastern North America and that seems
to be favored as we go into the fall. In fact the CFSv2 and the 45
day ECMWF from the last such run do show this through September.

Getting back to our forecast for Wednesday through Sunday, we see
one such upper trough digging into the eastern CONUS, which puts
southwest Michigan under the upper trough. As was pointed out by our
previous shift, this means unstable air due to cold air at mid
levels relative to warmer air near the surface. The sounding over
our area on Wednesday are not so dry at mid levels that convection
is out of the question, so do not be surprise do see showers added
to the Wednesday forecast before to long.

Another upper wave digs into the eastern trough in the Friday time
frame so afternoon showers would again be possible.  Over the
weekend a system current over western Asia, moves across the Pacific
and tries to merge with the Gulf of Alaska upper low. This sends
another Pacific upper wave our way. This will likely result in
another major  precipitation event for Southwest Michigan but I
believe this is beyond our current forecast period, more in the
Tue/Wed time frame. One other aspect that I am nearly sure of is do
not expect any significantly warm weather in Southwest Michigan in
the next week or so.

Bottom line below normal precipitation and temperatures are favored
over wetter and warmer than normal Wednesday through Sunday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 103 AM EDT Mon Aug 21 2017

An area of showers and thunderstorms is moving across northern
IL/IN. A few showers may move near the southern TAF sites
overnight and we added a vcsh comment to the early portion of the
TAF. From the afternoon through the evening, isolated
showers/storms are possible as low pressure approaches Lower
Michigan. More numerous showers/storms are expected after 06z.

&&

.MARINE...
Issued at 312 PM EDT Sun Aug 20 2017

Winds and waves will remain relatively light through Monday.
Thunderstorms will pose a hazard to mariners Monday night into
early Tuesday and winds and wave heights will increase Tuesday
through Wednesday.


&&

.HYDROLOGY...
Issued at 312 PM EDT Sun Aug 20 2017

Area rivers are around normal levels for the time of the year and
well below bankfull. Though we are going to get localized heavy rain
of over an inch late Monday night into early Tuesday, most areas
will not see even minor river flooding. Some water covered roads are
possible.

&&

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

$$

UPDATE...Duke
SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...WDM
AVIATION...04
HYDROLOGY...63
MARINE...Laurens