442
FXUS63 KGRR 080653
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
253 AM EDT Sat Jun 8 2024

.KEY MESSAGES...

- Showers Today, Breezy Sunday

- Risk for some Thunderstorms Thursday Evening

&&

.DISCUSSION...
Issued at 251 AM EDT Sat Jun 8 2024

- Showers Today, Breezy Sunday

Presently, shortwave ridging is keeping the area dry and clear
causing temps to fall into the 50s. However, rain showers are
beginning to develop across the northern Mississippi River Valley
thanks to a 700mb shortwave. These will continue to develop and
spread southeastward into West Michigan starting across Central
Lower this morning and expanding south through the day. Given LIs
staying positive today into tonight and little to no MUCAPE have
removed thunder from the forecast. HREF LPMM progs suggest 0.25"-
0.5" is possible north of I96 while less is expected south. Showers
wind down by 2am as the mid-level wave exits the area.

Sunday will be generally dry. A weak shortwave crosses through the
area in the afternoon causing increased clouds especially north of
I96. An isolated shower cannot be ruled out, however any coverage
would be to low to justify any PoPs in the forecast at present. A 20-
35 knot LLJ moves overhead Sunday, and when combined with mixing
heights climbing to over 5kft gusts of 20-30 mph are possible. Highs
today and Sunday will generally be from the upper 60s to mid 70s.

- Risk for some Thunderstorms Thursday Evening

Models are advertising a cold front dropping down from the northwest
during this time.  Zonal flow out ahead of this feature draws in an
unstable air mass into the CWA during the afternoon/evening.  A low
level jet is shown to strengthen over MI as this front moves in.
Also, a mid level jet streak of 50 to 70 knots moves in which will
lead to stronger deep layer shear. However the limiting parameter
for organized convection is the instability with ensemble surface
based CAPE values shown to remain well under 1000 J/kg.  If the
instability trends upward with time, we could end up with stronger
storms, which will need to be monitored.  For now, we will feature
chance POPs in the forecast for Thursday evening, which is supported
by the latest ensemble qpf trends.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 135 AM EDT Sat Jun 8 2024

An expanding area of rain will be spreading into the TAF sites
from the west this afternoon. The atmosphere will be relatively
dry in the low levels at the onset of the rain so VFR conditions
are predicted even as this precipitation begins. However as the
low levels gradually saturate in the afternoon and evening, MVFR
conditions and even some IFR are expected to expand, especially
near and southeast of KGRR. Once the lower conditions develop,
they are likely to persist through 06z Sunday, even as the
rain/drizzle ends.

&&

.MARINE...
Issued at 251 AM EDT Sat Jun 8 2024

Weaker gradient winds and diminished winds aloft will keep winds and
waves below levels hazardous to small craft today. The next day to
watch for potential conditions hazardous to small craft would be
later Sunday into Monday. A high pressure system across Central
Canada drifts south increasing gradient winds. Uncertainty as to the
extent winds increase for any sustained period of time exists, as
well as how well we can mix gusts down. This will be monitored for
the next 24 hours for the need for a Small Craft Advisory or a Beach
Hazards Statement if confidence increases.

&&

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

&&

$$

DISCUSSION...MJS/Thomas
AVIATION...MJS
MARINE...Thomas