FXUS63 KGRR 200349
Area Forecast Discussion
National Weather Service Grand Rapids MI
1149 PM EDT Thu Jul 19 2018
Issued at 204 PM EDT Thu Jul 19 2018
Our warm and dry weather pattern is about to come to an end.
A developing but slow moving storm system from western Minnesota
this afternoon to Southwest Michigan by Friday evening. It will
take till Monday for this storm system to be out of our area. This
system will bring beneficial rainfall across the area between
Friday morning and Sunday evening. Expect locally heavy rainfall
and scattered thunderstorms with this system Friday into Saturday.
On Sunday the showers and thunderstorms will be more widely
scattered but they will still be around.
Most the next week will be dry but expect another wet storm system
toward the weekend. Temperature will warm to above normal by
.SHORT TERM...(This evening through Saturday night)
Issued at 330 PM EDT Thu Jul 19 2018
It is becoming increasingly clear we will see a widespread rain
event Friday into Saturday. Some locations may get over 2 inches
of rain while others, nearby may get less than a half inch. This
will be a very slow moving system so some areas may see a
prolonged period of heavy rain (hence the marginal risk for
excessive rainfall from WPC for Friday into Saturday morning).
The models have come into very good agreement on the track of
this system, which is southeast. This track will favor the
heaviest rainfall over western Lower Michigan and that is shown by
numerous models and their ensemble members. If this rainfall
event actually happens it will help mitigate the developing
I see three periods of precipitation with this event. First it the
precipitation associated with increase in deep moisture. That
will rotate through the western CWA mostly during the early to mid
morning hours. Then there will be a break in the activity while
the occluded front lift area moves through mid to late afternoon.
There is decent moisture transport for this and since there will
be a break in the precipitation that will allow some afternoon
heating. That in turn will increase the instability and threat of
severe storms, mostly this would be a wind threat.
The third band of precipitation is wrap around on the north side
of the system which will happen Saturday into Saturday night.
Seems most of that rain will be near and north of I-96.
.LONG TERM...(Sunday through Thursday)
Issued at 300 PM EDT Thu Jul 19 2018
The issue here is when does the system that will bring our
rainfall starting early tomorrow morning depart from the area.
It seems Southwest Michigan may not get out of the circulation of
the system over the weekend until midday Monday and there may yet
be enough moisture and instability around for afternoon
convection. The model sounding are not all that dry through 500 mb
at 18z and there is positive area in the soundings to near 200
mb. So do not count out the risk of convection yet Monday.
Beyond that there is a northern stream system that will keep most
of it's energy in Canada most of next week. It would not be out of
the question some the dynamic energy associated with that system
could bring some scatted storms each afternoon but at this point I
am thinking dry will prevail. Also expect a slow warm up.
.Week Two...Friday through Wednesday
In the big picture we are seeing (partly due to the MJO going
through phases 5 and 6) ridging on both coasts with a longwave
trough over the central or east central CONUS and Canada. This
will result in overall cooler weather pattern. That pattern will
continue into early August actually.
There is a significant threat for another rain event next weekend
as a system now in the Gulf of Alaska climbs over the western
ridge and dives into the eastern trough in the Friday to Saturday
.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1130 PM EDT Thu Jul 19 2018
VFR conditions will continue overnight. Scattered rain showers
will slowly move in from the west overnight and may cause brief
reductions to MVFR Friday morning due to lowering ceilings and
visbys in heavier showers.
A mix of VFR and MVFR conditions is then forecast Friday morning
through Friday afternoon with scattered showers in the vicinity of
all the terminals. A few thunderstorms may develop as well with
the relatively best chc for those occurring during the late
afternoon and early evening hours.
Conditions will slowly deteriorate to predominantly MVFR Friday
evening due to lowering cigs and showers and isolated storms.
Brief reductions into the IFR flight category are possible in any
thunderstorms that develop Friday.
Issued at 204 PM EDT Thu Jul 19 2018
At this point winds and waves should remain below small craft
criteria through the weekend. Winds will be strong out of the
southeast tomorrow but being southeast the strongest winds and
larger waves will be west of our near shore area. Saturday
afternoon into Sunday morning north winds behind the system will
be rather strong but at this point the strongest winds and largest
waves will be outside of our Near Shore area.
Issued at 1136 AM EDT Thu Jul 19 2018
River levels are around to a little below normal for the time of
year and steady or slowly falling. Dry weather is expected into
tonight. A slow-moving low pressure system will then bring a period
of unsettled and possibly wet weather to the region Friday into
Sunday. Total rainfall amounts with this system could accumulate up
to 1.5 inches over the area. Locally heavy rainfall will be possible
in the thunderstorms. Main stem rivers are not expected to flood,
but any of the smaller rivers or creeks that get a thunderstorm
or 2 may have quick hitting rises and minor flooding.