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Area forecast discussion
National Weather Service Grand Rapids Michigan
320 am EDT Sat Sep 21 2019

Latest update...
synopsis/discussion/marine/Hydro

Synopsis...
issued at 320 am EDT Sat Sep 21 2019

- warm and humid with scattered afternoon thunderstorms

- two part heavy rain event tonight into early Monday

- cooler much of the coming week

- the next cold front in the Wednesday time frame may bring a
period of showers and isolated thunderstorms




&&

Discussion...(today through next friday)
issued at 320 am EDT Sat Sep 21 2019

The primary concern is the two part heavy rain event tonight into
Monday morning. This is one of those near classic Maddox- synoptic
type events with a strong low level jet, high precipitable water
Plum that is aided by a water vapor Plum from the tropics. What
makes this event so wet is that we get the moisture Plum from the
remnants of Imelda with the lead shortwave tonight then from the
eastern Pacific Hurricane Lorena and the main amplifying upper
wave closes off at upper levels over Lake Superior Sunday night.
These moisture plums are very odious on the high level water
vapor image loops. The precipitable water is forecast to be near 2
inches, which is near the record amount on the Storm Prediction Center sounding chart
for this area at this time of year. The Standard anomaly charts
show this to have a return period of greater than 30 year (having
2 inches of precipitable water over southwest Michigan in late
september). What has my eye is the closing off upper low Sunday
night. For some time only the European model (ecmwf) showed a significant surface
wave on the cold front due to that strongly developing upper wave.
Now, finally both the NAM and GFS show it too. The closing of
upper low in conjunction with around 2 inches of precipitable
water to work with results in a 50 knot low level jet to help the
cause. At this point I believe the heaviest rain will be
assoicatedwith the second part of this event, Sunday afternoon
into early Monday as the wave on the front moves through this
area. It has been my experience that the models tend to under-
forecast the rainfall from this type event. We may need to
consider an aerial Flood Watch for this event today.

One other aspect to this is while there is deep moisture and and
strong dynamics forcing the event, the instability is marginal at
best so, while there may well be some thunderstorms, really this
will mostly just be a heavy rain event with the favored area being
from US-131 to the Lake Shore, south of Big Rapids. If that
surface wave really develops the area of heaviest rain could be
shifted northward some. I do believe the 48 hour storm total will
exceed 3 inches in some places.

For today there is a shortwave the moves through the area this
afternoon into early this evening. This brings in the really deep
moisture. It is also when the instability, weak as it is, will be
greatest so I do expect some scattered non severe thunderstorms
from mid afternoon into early this evening. The heavy rain event,
part one starts after midnight with the lead waves moves into the
western Great Lakes.

It will cool off behind this system for Monday through Friday.
However two things of significance there. There is a strongly
digging northern stream polar jet for the Wednesday system. This
will likely bring a short period of showers and thunderstorms with
it. Then the next system behind that really sharply digs and
western Continental U.S. Trough and builds and eastern Continental U.S. Ridge by the
weekend of the 28th. Expect another period of very warm weather by
then.

&&

Aviation...(for the 06z tafs through 06z Saturday night)
issued at 139 am EDT Sat Sep 21 2019

There is a narrow area of VFR cigs broken cloud over assoicated
with the mid level warm front pushing northward. The night time
microphysics satellite images show this nicely. These clouds may
lower to MVFR over the next few hours but by then they should be
north of our taf sites. Winds will remain light during the early
morning hours and with skies becoming nearly clear and higher dew
point air pushing into this area from the south I believe we will
see some fog toward sunrise. I have MVFR/IFR fog at nearly all of
our taf sites around 12z.

During the day time the upstream system gets close enough to
increase wind speeds. By mid afternoon most of the central and
western taf sites will see winds of 15 to 25 knots.

There is enough instability and mid level moisture increases
enough for scattered showers and isolated thunderstorms at all taf
sites in the 18z to 02z time frame. The coverage is not high
enough for ME to put a from group for this so I put vcsh for now.

The main rain area with locally heavy rainfall will not arrive
until after 06z so that part of the system does not impact the
tafs yet.



&&

Marine...
issued at 320 am EDT Sat Sep 21 2019

Due to the strong winds assoicated with this system we will
surely have Small Craft Advisory conditions over central and
northern sections of our near shore by this afternoon, and over
the entire near shore tonight into Sunday. Due to the high water
levels, the high waves will wash over piers and breakwaters plus
cause some flooding in areas near the Lake Shore. So I will
continue all of our current marine headlines.

&&

Hydrology...
issued at 119 PM EDT Fri Sep 20 2019

Several rounds of heavy rain will be moving through lower Michigan
this weekend. At this point it looks like most spots will end up
with a total of 1-2" of rain, with some spots near the Lakeshore
possibly topping 3" by the time the rain ends Monday morning. This
will cause some pretty good rises on most of the rivers in the area,
but at this point no rivers are expected to exceed bankfull. The
rivers that might approach bankfull are most likely the white and
pere Marquette rivers, where heavy rain last week has limited how
much rain the soil is able to absorb.

Of greater concern is the potential for localized short-duration
flooding of some small streams as well as ponding on roadways at low-
spots and in poor drainage areas. The heaviest rain is likely to
fall in the communities along the Lake Michigan shoreline, so The
Row of counties along the lake are at the highest risk for the
localized flooding. The heaviest rain looks like it will fall Sunday
evening and overnight, and be moving out of the area by Monday
morning. Water on roads at night is much harder to see, so drivers
should slow down and pay extra attention.

&&

GRR watches/warnings/advisories...
Michigan...Beach hazards statement from this evening through Sunday evening
for miz064-071.

Lakeshore flood advisory from 8 PM this evening to 8 PM EDT
Sunday for miz064-071.

Beach hazards statement from noon EDT today through Sunday
evening for miz037-043-050-056.

Lakeshore flood advisory from noon today to 8 PM EDT Sunday for
miz037-043-050-056.

Lm...Small Craft Advisory from noon today to 8 PM EDT Sunday for
lmz846>849.

Small Craft Advisory from 8 PM this evening to 8 PM EDT Sunday
for lmz844-845.

&&

$$

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