200
FXUS63 KGRR 211741
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
141 PM EDT Sat Sep 21 2024

.KEY MESSAGES...

- Dense fog early this morning, then sunny and hot today

- Showers and scattered storms Sunday

- Starting Monday, a week of near to above-normal temperatures,
  and a chance of showers/storms on Tuesday

&&

.UPDATE...
Issued at 924 AM EDT Sat Sep 21 2024

Fog is dissipating or lifting into low stratus, which will also be
mixing out in the next hour or two. The Dense Fog advisory has
been cancelled.

&&

.DISCUSSION...
Issued at 332 AM EDT Sat Sep 21 2024

- Dense fog early this morning, then sunny and hot today

Surface obs show widespread dense fog with vsbys aob 1/4sm early
this morning. A Dense Fog Advisory is in effect until 10am; fog
will slowly burn off this morning, leading to a sunny day. H8
temps climb to 18c this afternoon which will lead to surface
temps in the mid 80s...roughly a dozen degrees above normal.
Humidity this afternoon will be somewhat mitigated by mixing down
some drier air from aloft, which will send dewpoints into the 50s.

- Showers and scattered storms Sunday

An upper trough will move across southern Canada today and Sunday
and push a cold front through the region. Most of the dynamics
with this system will remain well north of the cwa. However, there
will be a strong push of moisture with it...precipitable water
values jump to around 2 inches Sunday thanks to a 40 kt LLJ.
Instability will be lacking though; MUCAPE is a few hundred joules
at best. Mid level lapse rates are only around 5c/km, which is not
impressive. Thus, most of the precipitation will be showers and
perhaps isolated storms. QPF could range from a half inch to over
an inch, which would be welcome to an area that hasn`t seen much
rain lately.

- Starting Monday, a week of near to above-normal temperatures, and
a chance of showers/storms on Tuesday

Temperature-wise next week, while there remains a fair amount of
spread among the ensembles in the day-to-day temperatures (depending
on a complex evolution of the upper-level wave pattern), the median
of the solutions are pointing to temperatures closer to normal early
in the week, then warming up a little later in the week. This is a
break from the abnormally long stretch of 80-degree weather that
occurred in mid-September. Normal highs/lows for this time of year
are on their way to about 70/50 this time of year.

Daily rain chances have been a challenge to predict for the upcoming
week. Ensemble systems and their constituent members have provided a
wide range of plausible rain outcomes: ranging between a mostly dry
week, to a somewhat showery week, to a one-or-two episode heavy rain
threat. Adding to that are run-to-run inconsistencies among each of
the ensembles. This can be forgiven since the interaction and
evolution of the wave patterns between the northern-stream polar jet
and the southern-stream subtropical jet is set to be rather chaotic.

After Sunday, the best chance of rain still appears to be Tuesday as
the southern-stream low out of the Desert Southwest becomes
incorporated into the northern stream as it moves over the Great
Lakes. However, the CMC ensemble is much less enthusiastic about
Tuesday, and there are still a portion of members among the GFS and
ECMWF ensemble that stay dry too. Outlier solutions on the rainy
side indicate the potential for over an inch of rain in spots on
Tue. The chance of rain is relatively higher in southern locations.
Later in the week, models have been tending to develop cut-off lows
between the northern and southern jets over or near the Great Lakes
region. Where a cut-off low might form will dictate whether late
week is dry or has a rain episode. In the latest run of the
ensembles, there is a cluster of solutions that offers a chance of a
soaking rain on Friday, but this cluster is in the minority.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 141 PM EDT Sat Sep 21 2024

Skies are clear this afternoon as morning fog has dissipated. VFR
continues into tonight with mid to high clouds moving in after 04z
ahead of our next system. Westerly winds around 10 knots today
become light tonight and increase from the south Saturday morning.

The main aviation concern, widespread rain showers approach MKG
after 12z, and may reach GRR/BTL/AZO through 18z Sat.However,
with low-level dry air it is uncertain the extent these showers
will bring impacts until after more moisture arrives near 18z.
Therefore except at MKG where SHRA to be prevailing have added
PROB30 groups for SHRA at BTL/AZO/GRR. At MKG, signs exist that
heavier showers could cause MVFR or lower conditions after 15z.
Have elected to add this as a PROB30 with this package as while
the chance exists, aforementioned dry air may prevent these
conditions from prevailing throguh 18z.

&&

.MARINE...
Issued at 332 AM EDT Sat Sep 21 2024

Low wave heights expected today, but they`re likely to increase
later Sunday into Monday as north winds develop behind a cold
front. Two to four foot waves will be common during this time
period but conditions will probably be just under Small Craft
Advisory criteria.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

UPDATE...Ostuno
DISCUSSION...04/CAS
AVIATION...Thomas
MARINE...04