-- Discussion containing changed information from previous version are highlighted. --
668
FXUS63 KGRR 071402
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1002 AM EDT Tue May 7 2024

.KEY MESSAGES...

- Afternoon-evening thunderstorms, possibly severe

- Rain on Thursday and Saturday

&&

.UPDATE...
-- Changed Discussion --
Issued at 957 AM EDT Tue May 7 2024

The going forecast is on track and no changes are planned.
Regional radar shows a band of slowly weakening convection
stretching from Wisconsin south to the IL/IN border. As this
convection moves east, it should continue to weaken as it moves
into stable air over the cwa. However, this convection is also the
leading edge of the warm front and once it moves by, we`ll likely
destabilize fairly quickly. Incoming short wave noted on water
vapor imagery will help to generate stronger storms with some
severe potential too, especially across the southern cwa. All
modes of severe will be possible due to strong shear and decent
helicity.
-- End Changed Discussion --


&&

.DISCUSSION...
Issued at 400 AM EDT Tue May 7 2024

- Afternoon-evening thunderstorms, possibly severe

On the southeast periphery of a maturing low over the Dakotas, a 500
mb trough and vorticity maximum will move from the central Plains to
the western Great Lakes today. Upper level height falls and an
approaching left exit region of a 250 mb jet streak will broadly
support ascent and cooling in the midlevels. In the lower levels,
the nose of a southerly 850 mb jet will slide through northern
Indiana and southern Michigan while a surface warm front is
expected to progress northward to roughly a Muskegon-Jackson line
during the afternoon. A frontal triple point and possibly a
deepening surface pressure field is expected in the vicinity of
Chicago/Milwaukee by late afternoon.

Ongoing line of thunderstorms marching from the Plains to
Mississippi river valley overnight is expected to arrive in
southwest Lower Michigan around 11 AM to Noon and progress
northeastward toward Clare by 3-4 PM while weakening as it moves
into a more stable environment. Severe weather is not likely with
this first round, though lightning may still be a hazard for
southwest locations. In its wake, rapid moistening of the low levels
(through some combination of rainfall and advection) along with
cooling mid-upper levels (with the arrival of the 500 mb trough) is
likely to support development of scattered thunderstorms (possibly
severe) between 4-10 PM, even if there is considerable cloud cover.
HREF mean surface-based CAPE in southwest Michigan reaches 1000 J/kg
by 5 PM, south of a Muskegon-Jackson line, though even if
convective inhibition of the surface remains high, there may be
a less-inhibited effective unstable layer above the surface on
both sides of the warm front.

Most of the CAMs show similar character of convection this evening,
with a mode of scattered to numerous cells of small to moderate
width evolving into clusters across southern Michigan, and perhaps
other quasi-linear thunderstorms farther north crossing Lake
Michigan. These storms should be strongly sheared, given an expected
0-3 km wind difference of 30 to 40 knots. The magnitude of 0-1 km
streamwise vorticity available for the given storm motion is
questionable, however. There may be cells which briefly encounter a
more favorable environment to attain supercell characteristics,
perhaps in vicinity of the warm front, or with less predictable
environmental modification from preceding cells or interactions
between cells. Can`t really rule out any severe hazard in this
environment, though the most likely limiting factors may be if
potential instability isn`t realized or if updraft width remains
modest.

- Rain on Thursday and Saturday

A couple shortwave troughs will result in chances for showers
peaking Thursday and Saturday. Confidence is above average with good
model agreement and ensemble support. One trend that has been noted
since yesterday is a further south shift in the track of the low on
Thursday which could mean the rain stays south of central Lower
Michigan. This is not yet completely reflected in the current grids.

An area of mid-level frontogenesis north of a low pressure center
tracking across the Ohio Valley will result in rain moving in late
Wednesday night and continuing through Thursday afternoon. Northerly
flow on the back side of the low will advect in a cool Canadian
airmass. We will have to watch for frost potential if skies clear
out Thursday night.

Shortwave ridging should continue the fair weather for Friday into
Friday night but clouds will be moving in after midnight as the next
shortwave trough approaches, with rain moving back in for Saturday.
The trough cits off into a 500 mb low across our area by Saturday
and there could be some diurnal enhancement of the showers Saturday
afternoon. Given the steep lapse rates, some thunder is possible and
even some hail with any deeper updrafts.

The low lifts out gradually early next week but there is low
confidence on how quick the showers end, with model differences and
ensemble spread increasing after Sunday.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 747 AM EDT Tue May 7 2024

BKN stratus deck with ceilings 2300 to 3000 feet AGL this morning
will be followed by a line of showers and thunderstorms arriving
around 18Z. Then scattered thunderstorms later this afternoon that
could bring local IFR conditions along with hail and strong gusty
winds.

Southeast winds this morning around 10 knots will go south this
afternoon and then southwest this evening.

&&

.MARINE...
Issued at 400 AM EDT Tue May 7 2024

East-southeasterly winds today become southerly this evening and
southwesterly tonight. Hazards to boaters will come from
thunderstorms crossing over and/or developing over Lake Michigan
between 11 AM and 9 PM. These may produce lightning, hail, or
stronger winds.

&&

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

&&

$$

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