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Area forecast discussion 
National Weather Service Grand Rapids Michigan
932 am EDT Thursday Sep 3 2015

Latest update...
mesoscale discussion

Mesoscale discussion...
issued at 926 am EDT Thursday Sep 3 2015

A line of sub severe convection extending from Gladwin to Montana
pleasant to the Lake Michigan waters near Grand Haven will
continue to move east southeast over the next couple hours.
Despite a dry wedge at 500mb and cold pool formation...the line is
only producing winds of 10 to 25 miles per hour according to spotters and
mesonet observations. We will continue to monitor this line as it
moves into an area that should have more surface instability through
late morning and early afternoon.


issued at 304 am EDT Thursday Sep 3 2015

A weak cold front will push through the region today
leading to showers and thunderstorms with locally heavy rain.
The unsettled weather will persist Friday into Saturday as a warm
front lifts northward through the area. The unseasonably warm
airmass will persist through Labor Day.


Short term...(today through saturday)
issued at 304 am EDT Thursday Sep 3 2015

The wave dropping southeast from the Green Bay region will track
through lower Michigan this morning into the afternoon hours. Even
without this lift showers and thunderstorms have been developing
here in our forecast area. Thus with the added lift arriving we
will see expanding showers and thunderstorms for much of southwest
lower Michigan. Lake Michigan will add some moisture to this system as well.
I featured high probability of precipitation through the day to account for the passage of
this feature. At this point with limited shear any severe weather
should be local and brief. Heavy rain could become an
issue...especially if the cells train over the same region. Will
maintain these risks in the hazardous weather outlook.

Cyclonic flow aloft persists into Friday. The atmosphere is shown
to destabilize through the day. So it looks like showers and
thunderstorms will develop. I did keep Friday night mainly dry
thinking the convection will be diurnally driven.

I did add fog to tonights forecast. With cooling temperatures in a
moist airmass...thinking after midnight we should see fog

Long term...(saturday night through wednesday)
issued at 304 am EDT Thursday Sep 3 2015

After a hot and dry day Sunday (maybe 90 again?)...some unsettled
weather is expected for Monday through Wednesday with a weak front
that may get hung up across the Great Lakes. 00z guidance suggests a
weak cold front may try to move in Monday afternoon. The European model (ecmwf) is
slowing down this front in response to h500 ridging trying to hang
on with a deep upper low centering near lake winnepeg by 18z Monday.
This may wash out the front over our region...becoming stationary
for a time and then lifting back north as a warm front Tuesday.

Winds from h500 and below will be less than 30 knots until Tuesday
night into Wednesday when better upper level forcing arrives. With
weaker wind fields...cold pool dominated convection will once again
be the likely Mode for any storms Monday and Tuesday before the
primary cold front arrives by Wednesday. There could be more
rainfall than what the European model (ecmwf) and GFS are indicating during this
period. The GFS dew points are likely too low on Tuesday (mid 40s
for GRR off BUFKIT soundings) and as such the precipitation is likely
underdone. Until the main front arrives on Wednesday there is a risk
for some locally heavy rainfall with a weakly forced environment.
Cooler air follows for the second half of the week.


Aviation...(for the 12z tafs through 12z Friday morning)
issued at 731 am EDT Thursday Sep 3 2015

One complex of thunderstorms will track through the taf sites
leading to some impacts through 18z. After that we could see more
storms forming...but most likely for southern taf sites. There was
some IFR fog to start the day...that should diminish by 15z.

Tonight fog could lead to IFR or lower impacts...mainly after


issued at 304 am EDT Thursday Sep 3 2015

The pressure gradient remains relatively weak into Saturday. This
will act to keep the winds and waves low. With an offshore flow
developing tonight...that will also limit the wave heights. Fog
could develop tonight...which was added to the forecast.


issued at 1147 am EDT Wednesday Sep 2 2015

Confidence is increasing that our area will see at least scattered
thunderstorms Thursday... or perhaps a more expansive cluster. A
slow moving thunderstorm over the city of Mason produced rainfall
rates of over 1 inch per half hour Wednesday morning. While brief
heavy rainfall is not much of a concern in most locations... some
storms may persist locally and produce more significant rainfall
totals. This may cause some flooding issues in urban and poor
drainage areas.


GRR watches/warnings/advisories...


short term...mjs
long term...Hoving

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