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Area forecast discussion 
National Weather Service Grand Rapids Michigan
732 PM EDT Wednesday Aug 20 2014

Latest update...

issued at 341 PM EDT Wednesday Aug 20 2014

An active warm front will push toward southwest lower Michigan
Thursday and Thursday night triggering scattered showers and
thunderstorms. The storms on Thursday night could produce heavy rain
and a risk of flooding. A low threat for damaging winds will also

Very warm and humid weather can be expected over the weekend into
early next week with highs possibly reaching 90. A pop up shower or
storm will be possible each day but most of the time it will be dry.


Short term...(this evening through Friday night)
issued at 341 PM EDT Wednesday Aug 20 2014

The main focus is on the potential for heavy rain/flooding and
severe weather Thursday night as an active warm front across Iowa/Illinois
shifts northeast.

Several jet cores/speed maxima embedded within building upper ridge
over the central/northern plains will impact the area over the next 48
hours. Departure of the upper low currently over eastern Upper Michigan allows
an influx of warm/unstable air from the southwest with extremely
high precipitable water air of around 2.2 inches.

This pattern very much supports the chance of flash flooding where
storms develop and repeat in a northwest to southeast fashion. The
European model (ecmwf) has been quite consistent in placing highest quantitative precipitation forecast Thursday
night over SW Michigan... and am heavily favoring its solution at this
time. Still some uncertainty however regarding low level jet
placement/behavior which will dictate where best quantitative precipitation forecast sets up.

Guidance also suggests the possibility of isolated severe storms
with damaging winds in a northwest to southeast corridor from roughly Minneapolis
to Milwaukee/Chicago to Kalamazoo. 0-6km deep layer shear is prognosticated
to be 35 to 45 kts with mu convective available potential energy of 1500-2500 j/kg. Mesoscale convective system activity
tracking southeast along this corridor may contain bowing line segments
which could produce an episode or two of severe storms south of I-96
Thursday night. Elevated nature of the convection however could be a
limiting factor.

Thursday nights convection is expected to exit early Friday morning
leaving only scattered activity at best in its wake. The cooling
effects of the nocturnal convection will probably keep the surface warm
front suppressed south of the state for Friday with some stratus
persisting through at least early afternoon.

Long term...(saturday through wednesday)
issued at 341 PM EDT Wednesday Aug 20 2014

The long term portion of the forecast looks rather warm and unsettled
through the period. Precipitation chances will not be high most of the
period...however small chances will remain present through much of the
period with a couple of better opportunities.

Warm front is expected to be situated to our SW at the beginning of
the period on Sat morning. This warm front will make a good push
north by sun as the upper ridge builds over the area in response to
the upper low taking shape across The Rockies. A chance of showers/
storms will exist as this front moves through. It does look like the
best chance of precipitation with the front will remain west of the area where
the best low level jet is focused.

Once the front moves through...we can expect temperatures approaching 90
then for the end of the weekend and into early next week with h850
temperatures around 20c. Humidity levels will increase also. Models are
trying to kick out some light diurnal precipitation each day with the great
deal of instability that is expected to be present. We should be
rather capped sun and Monday based on forecast soundings. Can not rule
something popping up along a boundary such as a lake breeze or such.

A better chance of showers/storms will come late in the period. The
Rockies low will gradually lift to the NE...and flatten the upper
ridge in the process. The lowering heights/cooling middle levels will
combine with a front allowed to drop down to bring better chances of


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 732 PM EDT Wednesday Aug 20 2014

Beside the fog issue tonight another forecasting issue has
developed... that is the risk of convection overnight.
As it turns out the entrance region of the polar jet speed maximum
crosses lower Michigan around 06z. That lift increases the
moisture in the 900 mb to 700 mb layer which in turn creates
elevated instability. The good news is the air above 600 mb is
very dry yet so any cells that would try to develop would likely
entrain to much dry air. Still the possibility exists for a few
showers and maybe even a thunderstorm in the 06z-09z time frame at
most of the taf sites. Actually the more western taf sites would
be have a higher risk of seeing the convection as the axis of the jet
core does not get east of Lansing. The hrrr 20z run does show
convection developing in that time frame over southwest lower
Michigan. I put vcsh for this for now but we will have to watch
to see if anything develops.

Otherwise with light winds fog may redevelop overnight however if
the aforementioned convection develops there may be enough clouds
around to keep the fog from becoming dense.

The warm front convection should being to effect our taf sites
after 16z so I have thunderstorms in the vicinity for that.


issued at 341 PM EDT Wednesday Aug 20 2014

Main concern is areas of dense fog over the next several days as
very warm and humid air mass moves over the cooler lake waters. Winds
and waves are not expected to exceed small craft criteria at this


issued at 341 PM EDT Wednesday Aug 20 2014

Excessive rainfall is possible Thursday night with precipitable waters of 2.2
inches and an approaching warm front under a building upper ridge.
This pattern supports the possibility of flooding/flash flooding
across parts of the upper Midwest/southern Great Lakes region including SW lower
Michigan. Depending on how the storms set up and persist... areal flooding
is possible Thursday night followed by the potential for river
flooding in the days following. Some uncertainty still exists
however in low level jet behavior and placement of best quantitative precipitation forecast.


GRR watches/warnings/advisories...


short term...Meade
long term...njj

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