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801
FXUS63 KGRR 260400
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1200 AM EDT Wed Jul 26 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 303 PM EDT Tue Jul 25 2017
An area of high pressure will continue to push east of the region
into Wednesday. South to southwest winds around this system will
draw up an unseasonably moist airmass into Michigan. A cold front
will drop southeastward through the region later Wednesday into
Thursday morning. This will result in some showers and perhaps a
few thunderstorms. The pattern then dries out for the end of the
week.

&&

.UPDATE...
Issued at 1053 PM EDT Tue Jul 25 2017

Some showers and an isolated thunderstorm or two have advected off
the Wisconsin side of the lake and have crossed the mid lake line.
Both the RAP model and the objective analysis on the SPC
Mesoanalysis page show CAPE values on the order of 500 j/kg nosing
into West Central Lower Michigan at the present time. The RAP
model shows this instability weakening through the night to almost
nil by morning. Have 20 to 30 pct pops in the far northwest
through 200am. After 200am expecting isolated showers to dwindle
with the loss of instability. Otherwise, forecast was on track.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 303 PM EDT Tue Jul 25 2017

The south to southwesterly flow tonight draws up an increasingly
moist airmass. The warm air advection may lead to a few showers
especially towards Ludington. Some of the high res models are
showing that the showers could actually build further southeast
than Ludington later tonight...perhaps into Grand Rapids toward
daybreak. The atmosphere is shown to remain stable tonight...thus
no thunderstorms will be in the forecast.

Looking at Wednesday into Thursday. Overall the trend supports
less in the way of rainfall...but that could still change.
A slow moving surface frontal zone slips southeastward through the
CWA. The lift is forecasted to be weaker and less instability
present. However....PWAT values are shown to climb up over 1.75
inches. The latest GFS does show decent upper level divergence at
the jet level over MI. Thus we still need to monitor the trends on
this approaching system. Will have somewhat lower POPs for the
passage of this front.

Models are in decent agreement in showing the atmosphere drying
out as high pressure settles in from the north for Thursday night.

.LONG TERM...(Friday through Wednesday)
Issued at 303 PM EDT Tue Jul 25 2017

The overall trend is for little real change in our current weather
pattern. Temperatures should, for the most part be near normal and
precipitation will continue to be sporadic.

The upper air pattern for July likely will continue into at least
early August. That pattern is general northwest flow aloft, which is
the result of an overall large scale ridge over the western CONUS
into western Canada and weak trough over the eastern Canada and the
CONUS.

Over this coming weekend a shortwave upper ridge will be located
just west of Lower Michigan and it will crest over this area Sunday.
It will be accompanied by a large Canadian origin surface high. That
will mean dry air which means cool nights and warm days. Expect
temperatures to be near normal during the afternoon (80 to 85) but
cooler than normal at night, mostly lows in the 50s with 40s
possible in the normally cooler locations.

By early next week the next upstream Pacific origin shortwave will
crest the longwave ridge (near 110W) and start to dig southeast into
the eastern CONUS.  That will bring a cold front through this area
early next week. Normally that would have thunderstorms with it but
since this is Canadian air displacing Canadian air and there will
not be much inflow from the Gulf to recharge it, I would think we
will see more of the same, a dry cold front and temperatures remain
about the same, lows from the mid 50s to lower 60s and highs from
around 80 to the mid 80s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1145 PM EDT Tue Jul 25 2017

A few showers and isolated thunderstorms northwest of KMKG attm
should diminish as they move southeast but may affect KMKG during
the next couple of hours. Otherwise vfr conditions should
continue at all the terminals overnight through Wednesday.

An approaching cold front will bring potential for scattered
showers and isolated storms to develop Wednesday afternoon but
potential is too low to warrant mention in the terminal forecasts
until Wednesday evening at KMKG. Scattered showers and isolated
storms will likely not affect the rest of the terminals until late
Wednesday night into early Thursday.

&&

.MARINE...
Issued at 303 PM EDT Tue Jul 25 2017

The pressure gradient increases tonight into Wednesday. This will
support building waves as the southerly flow increases. It looks
like a close call for marine headlines later Wednesday into
Wednesday night for the northern zones. At this time the potential
is too low to go with headlines. Perhaps a greater risk for
hazardous waves exists for Thursday as a northerly flow and cold
air advection develops.


&&

.HYDROLOGY...
Issued at 1055 AM EDT Tue Jul 25 2017

With near- to drier-than-normal conditions lately and a dry pattern
setting up for the next week, scattered showers and thunderstorms
Wed and Thu will be a welcomed sight to those looking for rain.
This does not look like a guarantee for everyone to get appreciable
rainfall amounts. But with 1.5 to 2.0 PW values nosing in and
relatively slow storm cell movement, it`s possible for some
locations to receive over an inch or two of rain in a couple hours.
Excessive rainfall to the point of flooding does not appear likely
due to a questionable storm coverage extent over the area.

&&

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

$$

UPDATE...Duke
SYNOPSIS...MJS
SHORT TERM...MJS
LONG TERM...WDM
AVIATION...Laurens
HYDROLOGY...CAS
MARINE...MJS