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Area forecast discussion 
National Weather Service Grand Rapids Michigan
325 am EDT Friday Aug 29 2014

Latest update...
synopsis/short term/long term/marine

issued at 325 am EDT Friday Aug 29 2014

Showers and a few thunderstorms are possible today as a warm front
moves through the area. More widespread thunderstorm activity is
expected Saturday ahead of the cold front...which comes through
early Sunday morning. Behind the cold front on Sunday expect
partial clearing by afternoon. Then a strong cold front will bring
the risk of addi tonal storms Monday.


Short term...(today through sunday)
issued at 325 am EDT Friday Aug 29 2014

We are tracking showers and thunderstorms today through Saturday
night. While there could be some showers and isolated thunderstorms
around would seem our best potential for widespread
convection is Saturday as the upper wave moves through.

The big picture finds a opening up southern stream closed upper
low moving east as it shears out and moves over Michigan during
the day time Saturday. The models as nearly always weak these
feature to fast so I would expect strong dynamics Saturday then
any of the models currently would suggest. In any event as this
upper wave heads toward Michigan it displaces a shortwave ridge
currently in place over Michigan. That results in a surge of middle
level moisture that should lead to some shower and thunderstorm
activity over the western and norther sections of the County Warning Area today.
Then the passage of the main system Saturday will bring more
widespread convection to the area.

Looking at the 1000/850 moisture transport vectors from the
GFS...NAM and European model (ecmwf) for today the story they tell is similar to
what they showed yesterday and the day before that. For the most
part the best moisture transport will be north and west of the GRR
County Warning Area today. The low level jet axis is west of Michigan early this
morning and is mostly over northern lower and Upper Michigan this
afternoon into tonight. Also the instability is rather limited and
what there is of it is mostly over western sections of lower
Michigan into this afternoon. The shortwave ridge ahead of the
upper wave has brought to warm of air above 700 mb so that
vertical totals are marginal at best for thunderstorms today. What
bring the showers to the area today is a surge of middle level
moisture as the precipitable water surges from under a seven tents
of an inch early this morning to above one and three quarter
inches over most of the County Warning Area by noon. The focus of this will be on
the warm front near the axis of the low level jet. That means most
of the convection will be north and west of the GRR County Warning Area today.

Saturday is when the main upper wave moves through the area and
there is workable low level jet in the area and some upper level
jet support so we should have widespread showers and a few non
severe thunderstorms (cape is limited and deep layer shear is weak

All of this moves out Saturday night and Sunday fines US with
short waver riding. So I expect clearing to occur during the day
Sunday as dry air moves in from the northwest.

Long term...(sunday night through thursday) issued at 325 am
EDT Friday Aug 29 2014

The main weather in the long term will be focused on the convection
chances early on in the period with a system that will be moving
through the area. The area will see a break in the weather for Sun
night in the wake of the departing system from Sat/Sat night...and
well out ahead of the system poised to move through the area on Monday.

The break will be a short one as the storm chances will be increasing
on Monday morning and lasting into Monday evening. Short wave energy
expected to come onshore across Oregon on Sat will traverse the
country over the Holiday weekend and will be lifting into Ontario by
Monday afternoon. The short wave remains mainly north of the area.
However the associated low level jet and strong moisture transport does move
right into the area Monday. We envision a residual line of showers/
storms coming into the area from the west on Monday...and strengthening
as it encounters an increasingly unstable air mass with diurnal
heating that takes place.

We can not rule some of the storms becoming severe Monday. Deep layer
shear will be on the increase with the upper jet nearby. Li/S are
expected to drop down around -6c with convective available potential energy approaching over 1500
j/kg. The low level jet overhead will increase the 0-3 km shear which will
help the cause. The limitations of possible severe weather would be
limited instability if not much heating can take place...and not
ideal upper jet dynamics overhead.

Most of the rest of the week should be rather quiet and seasonable
for the area. Some residual chances of precipitation will hold on Tuesday until the
secondary short wave moves through. After that...we get into a
nearly zonal flow with lower Michigan just downstream of some


Aviation...(for the 06z tafs through 06z Friday night)
issued at 145 am EDT Friday Aug 29 2014

Two potential periods of precipitation are the main items of
interest for aviation interests over the next 24 hours. Middle/high
clouds are streaming in out ahead of the showers and storms that
have been ongoing across WI and Illinois last evening and early this
morning. We will see the threat for showers increase mainly north
of kgrr by 12z as a wave moves into that area. This will keep the
threat in through about 16z or so.

The next threat will then begin after 00z Sat as another wave of
low pressure will approach the area. Showers and a few storms will
become possible...although the much better threat will come after
06z Sat. Conditions should be mainly VFR except under some heavier
showers further north.


issued at 325 am EDT Friday Aug 29 2014

We will be hoisting a Small Craft Advisory for the northern marine zones
for today and lasting through Sat. We will also hoist a Small Craft
Advisory for the remainder of the zones for tonight and Sat. The
gradient will be increasing ahead of the incoming system out of the
south. This will last into Sat evening before it diminishes a bit.


issued at 328 PM EDT Thursday Aug 28 2014

The heavy rain threat appears to be limited to the Muskegon and
pere Marquette basins as an inch of quantitative precipitation forecast can be expected there by
Sunday morning...which would result in rises by early next week.
Much less rain is expected in the grand and Kalamazoo river


GRR watches/warnings/advisories...


short term...wdm
long term...njj

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