Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 270023
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
823 PM EDT Wed Jul 26 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 324 PM EDT Wed Jul 26 2017

A frontal system will gradually sink south through the area through
Friday. This will bring sporadic chances of showers and storms to
the area. Severe weather is not likely, but a few of the storms
could briefly become strong with some heavier downpours possible.
The rain chances will become less likely Thursday through Friday,
but the chance will remain until Friday afternoon. Cooler weather
will follow by Friday.

A dry period of weather looks likely for this weekend into early
next week. Temperatures will start out cool and comfortable early
this weekend, and then become very warm by mid-week next week.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 324 PM EDT Wed Jul 26 2017

Our main concerns through most of the short term are on convective
trends and severe threat into Friday.

We had one more organized wave of sub-severe storms move through
along and N of M-20 earlier, and additional scattered storms
currently continue up there as of 19z. The main moisture transport
is shifting NE, so these seem to be associated with smaller short
waves training up there. Instability is limited (less than 1000 j/kg
of CAPE) by the earlier storms and the continued cloud cover, which
is keeping the current storms relatively weak.

We are expecting a more organized low level jet to develop to our SW
this evening, and stay south of the area through Thursday. This
should keep the better moisture inflow and instability locked up to
our south. That said, we still have a front that will be sinking
south through the area through tonight. This will have the potential
to pop off a shower or storm, but the potential for anything
widespread or significant is quite low.

The small chance of a shower or storm will remain on Thursday, as
the front will continue slowly south through the remainder of the
area. The better chance for rain will be south of I-96 where better
moisture will remain. The chance even down there will still be
scattered at best. The flow will become more NE through time behind
the front, which will gradually bring in drier air.

Even with the drier coming in Thu night into Fri, rain chances will
remain until Friday afternoon. We will see a short wave that is
currently across Saskatchewan be driven SE and will deepen as the
Wrn long wave ridge amplifies. This will move in overnight Thu night
and finally move east Fri afternoon. This would be a much bigger
concern if it were to move through during the heat of the day, or if
more moisture was available. That said, the dynamics alone justify a
chance of pcpn, even with it coming through overnight. We will then
dry up once it moves by.

.LONG TERM...(Saturday through Wednesday)
Issued at 324 PM EDT Wed Jul 26 2017

It would seem a rather tranquil weather pattern will prevail through
next week. High temperature should be near normal while low
temperatures will be slightly below normal.

Over the next week we will see the western upper level ridge build
while an eastern trough, deepens.  Southwest Michigan will be on the
downstream side of the upper ridge, which is typically a place where
little precipitation would be expected.  The eastern trough and
western ridge are not steep enough to bring down any really cold air
either. So even through we will get 2 cold fronts to come through
this area, one in the Monday time frame, then next one in the
Thursday time frame, I would expect little of any precipitation with
these systems. What these fronts will do is keep our afternoon
temperatures from rising above normal.

.Week Two Outlook

One issue we will have to be watching over the next few weeks the
potential for a developing drought over Southwest Michigan. Based on
the latest U.S. Drought Monitor, parts of southern Lower Michigan,
south of I-96 are already abnormally dry. Already, most of the
southern 1/3 of lower Michigan is now running between 1 and 2 inches
below normal on the mean 60 day rainfall. Typically drought becomes
and issue when the 60 day mean rainfall falls below 2 inches.
Sections of Lower Michigan. The 6 to 10 and 8 to 14 day CPC forecast
suggest below normal precipitation is suggested. I believe with the
persistent northwest upper level flow pattern  (upper ridge to our
west and trough to our east) most of the precipitation events will
be missing us to the south. This is also shown by the latest CFSv2
which shows most of Lower Michigan getting between 30 and 50% of our
normal rainfall for the next 10 days.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 823 PM EDT Wed Jul 26 2017

An area of rain was located over Central Lower Michigan at 00z,
stretching back into Southern Wisconsin. This area of rain should
slowly sag southward into the TAF sites tonight, but it will
likely be losing a bit of coverage/intensity with time. Expecting
MVFR conditions to gradually show up a the I-96 TAF sites tonight.

To the south along I-94, we will be watching the showers/storms
over Northern Illinois, which may impact AZO/BTL and JXN
overnight. Late tonight we will likely see MVFR visibilities
develop at all TAF sites due to some light fog.

The MVFR ceilings and visibilities should lift through the course
of Thursday morning. VFR weather is expected Thursday afternoon. A
cold front will be pushing through the area however during the
afternoon so a few showers and storms are possible.

&&

.MARINE...
Issued at 324 PM EDT Wed Jul 26 2017

South/Southwest winds are up a bit out ahead of the incoming wave,
but are staying just below Small Craft Advisory criteria. These
winds/waves will come down this evening, and should not be a problem
into Thursday.

There is a fairly strong wave dropping down Thu night and Fri that
could cause winds and waves to increase enough for possible
headlines. Those winds do look to be offshore in nature. This will
cause shoreline water temperatures to likely take a nose dive with
upwelling expected.


&&

.HYDROLOGY...
Issued at 324 PM EDT Wed Jul 26 2017

Thunderstorms rolling along the US-10 / M-20 corridors early this
afternoon have produced rainfall amounts between a half to one inch.
Given PW values around 1.75 inches in that area, any convection will
have efficient rainfall rates. Additional storms developing upstream
in Wisconsin will affect some of the same areas through this
evening. Much of the area has been dry over the last two weeks, so
this rain is beneficial. WPC indicates only a marginal risk of
excessive rainfall, and it is easy to agree with that. There is
chance of showers and storms in the southern portion of Lower
Michigan Thursday into Friday morning, but the questionable coverage
of convection will keep the risk of excessive rain very low.

&&

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

$$

SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...WDM
AVIATION...Duke
HYDROLOGY...CAS
MARINE...NJJ



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