-- Discussion containing changed information from previous version are highlighted. --
859
FXUS63 KGRR 081055
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
655 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/...
-- Changed Discussion --
Issued at 638 AM EDT Sat Jun 8 2024

Showers in western WI are tracking eastward toward the TAF sites.
The lower levels of the atmosphere will be relatively dry as the
showers arrive this afternoon so VFR conditions will prevail
initially. However as the low levels saturate, MVFR and some IFR
look likely, especially southeast of KGRR. At this time, there
does not appear to be enough instability generate an isolated
storm with this area of precipitation but that is something which
will need to be monitored. As the system responsible for the
showers and lower conditions moves away from the region later
tonight, drier air will filter in and that will result in
conditions returning to VFR for most locations by 12z Sun.
-- End Changed Discussion --


&&

.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