Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Grand Rapids Michigan
650 PM EDT Thursday Aug 21 2014

Latest update...

issued at 330 PM EDT Thursday Aug 21 2014

Very warm and humid conditions will prevail for the next several
days. Widespread showers and thunderstorms are expected tonight but
only isolated showers and storms are expected from Friday through
the weekend.


issued at 539 PM EDT Thursday Aug 21 2014

Latest regional radar trends seem to be confirming the thoughts of
the southward trend in the expected precipitation for early this
evening...and likely overnight also. The potential severe and
flooding threats seem to be dropping also. Radar imagery indicates
a mesoscale convective vortex aloft over Muskegon and Ottawa counties as of 2130z with
some semblance of a mesolow at the surface over Barry County. As these
features shift east over the next couple of hours...a northwest flow
aloft and subsidence building in will likely limit our precipitation chances.

We are significantly lowering the rain chances for tonight as a
result of the latest trends. The axis of heaviest precipitation looks to
be setting up south of the state line. Even with a low level jet
strengthening a bit overnight...the boundary aloft will likely be
too far south for our County warning forecast area to see the worst of the effects.


Short term...(this evening through Saturday night)
issued at 330 PM EDT Thursday Aug 21 2014

Confidence is still too low to issue a Flood Watch for tonight.
Latest model guidance trends are to shift the axis of heaviest rain
tonight a bit farther south.

Previous guidance had been focusing the best quantitative precipitation forecast from Milwaukee to
Kalamazoo... but now it looks more like Chicago to South Bend. This
makes sense based on the low level jet forecasts which show a southwest
flow aimed toward SW lower Michigan this evening but then indicate it
veering westerly overnight. The severe weather threat has shifted a
tad farther south as well to the Chicago to South Bend corridor
where best instability and warm front will be.

However based on inconsistent model quantitative precipitation forecast and low level jet solutions for
tonight over the last few runs... still cant totally rule out
locally heavy rain/flooding over the SW County warning forecast area corner of the County warning forecast area
tonight. Also of concern is the presence of right entrance region
lift related to northwest to southeast oriented jetlet over Lake Huron.

In the short term... increased probability of precipitation considerably for the rest of
today as widespread rain from decaying WI mesoscale convective system moves east.

Long term...(sunday through thursday)
issued at 330 PM EDT Thursday Aug 21 2014

The long term portion of the forecast looks to be rather warm through a
good portion of the period and fairly unsettled with chances of precipitation
each day through the period.

The lowest threat of rain looks to be at the beginning of the period
on sun. This is when the upper ridge builds overhead as the upper
low takes shape across The Rockies. The core of the hot air should
be just west of the area...however it still looks very warm and
humid on sun. Forecast soundings show a fairly substantial cap between
5-10k feet that should keep most areas dry. We can not rule out an
isolated storm along some sort of boundary like a lake breeze with a
good deal of instability present.

The chance of showers and storms will then increase a bit on Monday and
remain possible through the end of the period on Thursday. The Rockies
low will lift NE...remaining north of Lake Superior. This will help
to flatten the ridge over the area...and allow the cold front to
sink south into the area by Tuesday. The warmest day looks to be on Monday
where 90 degrees will be possible. The front looks to stall out over
the southern portion of the area on Tuesday. The models are now showing
additional energy coming in from the west middle week helping to keep
the front active. This will keep the chance of showers/storms in for
the area through the end of the period. Temperatures will cool a bit with
the lower heights and more precipitation potentially around.


Aviation...(for the 00z tafs through 00z Friday evening)
issued at 649 PM EDT Thursday Aug 21 2014

It would seem the threat for significant rainfall has shifted well
south of the GRR taf sites tonight. Even so the frontal boundary
being south of Michigan and with a shallow layer of cold air (low
level east flow) to about 4000 feet above ground level (rap model soundings from
21z run) there will be overrunning of the warm moist air to the
south of the boundary over this shallow cold layer. That should
lead to a low layer of stratus and even fog. As a result in have
all taf sites becoming IFR in stratus and fog by 09z or so. It is
questionable if this will mix out Friday. The models suggest this
shallow boundary will mix out by early afternoon so I lift the
ceilings but do not be surprised if the low clouds hang in all day.


issued at 330 PM EDT Thursday Aug 21 2014

Minimal wind and wave action is expected the rest of this week
through the weekend. However areas of dense fog will pose a hazard
to mariners due to a very warm and humid airmass overrunning
relatively Cool Lake water.


issued at 330 PM EDT Thursday Aug 21 2014

Given the latest low level jet forecasts and guidance placement of best
quantitative precipitation forecast mainly south of Michigan... the flood threat seems to be lower than
previous thought. Will not issue a Flood Watch but some amounts in
excess of two inches cannot be completely ruled out SW of GRR.


GRR watches/warnings/advisories...


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

National Weather Service Glossary of Abbreviations