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Area forecast discussion 
National Weather Service Grand Rapids Michigan
1250 PM EDT Thursday Sep 3 2015

Latest update...
mesoscale discussion

Mesoscale discussion...
issued at 1250 PM EDT Thursday Sep 3 2015

Storm Prediction Center updated day 1 places the southern GRR zones in the marginal
risk for this afternoon which seems reasonable as surface temperatures are
expected to be in the middle to upper 80s yielding convective available potential energy near 2000.
Deep layer shear is only 20 to 25 knots so storm Mode is expected
to be pulse type with the main threat isolated downbursts.

We will also have to watch for any training or backbuilding
storms as two inch per hour rain rates were observed with some of
the deeper updrafts earlier today.


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 1110 am EDT Thursday Sep 3 2015

No immediate concerns to be aware of today regarding river

Heavy rainfall associated with thunderstorm activity will quickly
pass through the area this morning and through early afternoon.
Ponding may occur on roadways and low lying areas in and around
the heavier rainfall. Much of the heavier activity will vacate
the region by middle afternoon.

The next 7 days remains active and includes multiple chances for
rain. 7 day area average rainfall of one to two inches is possible
with locally higher amounts in the vicinity of thunderstorms.
Recent dry weather and low river levels...coupled with the
piecewise fashion of the rainfall...should allow for the ground to
soak up much of the onset of moisture. Runoff thereafter is
likely to be handled well by they have plenty of room
to take on additional water.


GRR watches/warnings/advisories...


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

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