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Area forecast discussion 
National Weather Service Grand Rapids Michigan
837 PM EDT Wednesday Sep 2 2015

Latest update...

issued at 328 PM EDT Wednesday Sep 2 2015

A weak cold front will move south across lower Michigan during the
next 36 hours and chance for showers and thunderstorms will increase
accordingly. Temperatures will be a couple of degrees cooler
Thursday and Friday due to more clouds and showers. Temperatures
will climb back to the upper 80s by the end of the weekend however.


issued at 836 PM EDT Wednesday Sep 2 2015

As suggested by the short term discussion the short wave over
Wisconsin will bring additional convection to our County Warning Area early
Thursday morning. There is considerable middle level instability to
work with tonight so even though convection will not be surface
based. The 20 to 25 knot low level jet on the south side of the
upper level circulation reaches our northwest coast (mason/Oceana
counties after midnight) then slowly slides south with the upper
wave...not making a lot of progress inland. I am thinking we will
see convection move onshore after midnight over our northern County Warning Area
that will slowly slide south (develop south) through the early
morning hours. I do not expect severe storms due to the lack of
significant deep layer shear but we could see brief heavy


Short term...(this evening through Friday night)
issued at 328 PM EDT Wednesday Sep 2 2015

Forecast concerns deal with convective potential during the period.

The short wave that was generating convection yesterday over
northeast Nebraska and the eastern Dakotas has migrated to northeast
Wisconsin where it is generating showers/storms...some of which have
been severe. This wave is forecasted to move over northern lower later
tonight and Thursday. Currently we're sitting on 4000 j/kg SBCAPE
but not much shear. Scattered storms have developed over the far
northeast County Warning Area early this afternoon but have been well below severe
levels. The stronger storms over Wisconsin have been in a more
favorable shear environment with 40kts noted across northern Wisconsin.

Shear values are forecasted to remain 15kts or less Thursday across the
County Warning any storms that develop likely won't be severe. Given the
increase in cloud cover expected...instability will probably be less
than today too. That said...more forcing in the way of the weak cold
front and the upper wave should lead to more precipitation coverage. We/ve
increased probability of precipitation to likely during the afternoon. Another thing to
watch for will be to see if any storms develop on the convergence of
the eastward moving lake breeze as it interacts with the NE winds
behind the front. This cold front isn/T that strong so convergence
may not be significant.

The upper wave will be slow to depart so chances for showers and
storms will linger into Friday...although chances will be lower than
on Thursday.

Long term...(saturday through wednesday)
issued at 328 PM EDT Wednesday Sep 2 2015

The overall synoptic pattern changes little on Saturday and Sunday
with very warm and humid conditions continuing under the upper
ridge. Isolated showers and storms still cant be ruled out
especially during peak heating due to the combination of instability
and potential for mesoscale low level convergence zones.

The ridge begins to gradually break down early next week and a surface
cold front is prognosticated to come through Monday and Monday night.
However this front looks weak with the good upper support still well
off to our northwest. Given continued weak deep layer shear believe
that the severe weather threat on Labor Day is very low. The front
should provide less humid conditions behind it however for Tuesday
as it hangs up just south and east of the state.

Perhaps a better risk of organized strong to severe storms will come
next Wednesday when deep layer shear ramps up and models show the
front lifting back to the north... with some suggestion of a wave
developing/deepening as it moves northeast from Iowa.


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 758 PM EDT Wednesday Sep 2 2015

It should be quiet at all of the taf sites early this evening but
expect thunderstorms in the western tafs between 06z and 12z (or
so). An upper level shortwave over Wisconsin late this afternoon
will Cross Lake Michigan tonight and likely set of a period of
showers and thunderstorms (shown by NSSL waf...hrrr CR... rap13...
nam12) move across the western taf sites in the 06z to 12z time
frame. All of the models show the storms weakening east of US-131
in the 12z to 15z time frame. Last night the storms did not weaken
as they moved toward lan due to a weak low level jet that kept
them together. Tonight the low level jet again comes into the
picture. This time it is around the system in Wisconsin and it
strengthens overnight as is typical(shown by NAM and rap model).
The jet core moving south along the West Shore of Lake Michigan
between 09z and 16z. This supports the idea of the convection not
getting inland to far. So... that is how I played the tafs. I the previous forecaster... have showers with tempo
thunderstorms at mkg...GRR...azo...btl. As for Thursday... more
afternoon convection.


issued at 328 PM EDT Wednesday Sep 2 2015

Outside of a few thunderstorms...marine conditions should be quiet
over the next 24 hours. A light offshore wind is likely to develop
later Thursday as a weak cold front drops south.


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...04
long term...Meade

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