937
FXUS63 KGRR 051759
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
159 PM EDT Fri Jul 5 2024

.KEY MESSAGES...

- Strong Storms Possible This Afternoon

- Unsettled Weather At Times This Weekend

- Showers and thunderstorms most probable Monday Night/Tuesday

- Generally dry with seasonal temperatures starting Wednesday

&&

.UPDATE...
Issued at 1200 PM EDT Fri Jul 5 2024

A compact yet potent shortwave can be clearly seen in both model
data and water vapor imagery over Wisconsin in the mid levels.
This shortwave is pushing to the east and will drive into the area
this evening. Down at the surface weak low pressure is centered
out over Central Lake Michigan west of Big and Little Sable
Points. A warm front stretches to the east from near Fremont to
Mount Pleasant to Saginaw. A weak cold front stretches south down
the lake from the low and can be seen in visible satellite
imagery as a fine line in the what is likely a line of cumulus.
The front is approaching the Lake Michigan shoreline now in
Western Lower Michigan. Dew points in the warm sector of the
system (which is the bulk of our forecast area) are in the middle
60s. Surface based CAPE values have reached 1,000 j/kg. Combine
the low level moisture with what should be cooling mid levels
associated with the shortwave and we are looking at instability
values increasing with SBCAPE nudging further into the 1,000 to
2,000 j/kg range.

We expect a diurnal increase in both shower and thunderstorm
activity this afternoon as the upstream shortwave provides
synoptic scale lift. The convection will likely be favored near
both fronts (warm front lifting further into Central Lower and the
cold front sweeping east over much of the area). That said while
looking at the last few runs of the HREF, storms will be more
likely over Central Lower Michigan near the warm front.

There are chances for isolated severe thunderstorms this
afternoon and evening due to a combination of increasing
instability and deep layer shear values near and even above 40
knots due to strong mid and upper level jets in the area. Bottom
line we are not expecting widespread organized severe weather, but
isolated severe storms are certainly possible between 100pm and
800pm. Hail to around 1 inch in diameter and wind gusts to around
60 mph will be the main threats.

&&

.DISCUSSION...
Issued at 250 AM EDT Fri Jul 5 2024

- Strong Storms Possible This Afternoon and Evening

Ample moisture across the area will lead to patchy fog this morning,
dissipating shortly after daybreak. At the same time, a dying
cluster of showers and storms arrives mainly impacting our northwest
CWA just after daybreak.

The focus then turns to this afternoon as a robust 500mb trough
arrives from the west, driving an occluding surface low across the
CWA. Guidance is consistent in redeveloping one or more clusters of
convection along a cool front after about noon. At this point,
significant occlusion looks to hold off until this evening allowing
MLCAPE values to climb north of 500 J/kg and MUCAPE north of 1000
J/kg across the area, however uncertainty in the extent of
destabilization does exist across guidance. One thing to monitor
going into the daytime hours is the potential for locally better
instability if sunshine can break through, as with dewpoints in the
70s it wont take much for instability to overachieve. With 35+ knots
of deep layer shear, storm organization is likely. Greatest coverage
of storms is expected north of M46 where an upper-level vorticity
maxima and enhanced forcing from the triple point provide better
lift, though storms are possible CWA wide.

With no significant low-level jet and by extension weak low-level
shear, along with the moderate nature of instability, severe weather
will not be widespread. However, DCAPE values north of 800 J/kg and
low-level lapse rates exceeding 7C/km will support a damaging wind
threat and cold mid-level temperatures will also add a localized
hail threat in any storms that develop this afternoon. This is
covered by the marginal risk across the CWA. Coverage and intensity
of storms will diminish quickly after sunset as diurnal heating is
lost.

- Unsettled Weather At Times This Weekend

Going into Saturday, most of the area will be in weak cyclonic flow
as upper-level troughing lingers overhead to start the day. This
brings the chance of isolated diurnally driven showers near and east
of US127, especially north of I96. However, much of the area is
expected to remain dry. The weak cyclonic flow keeps temperatures
seasonally cool Saturday with highs in the low to mid 70s.

There is some uncertainty in the Sunday night and Monday timeframe
regarding precipitation chances, as there is a signal for a minor
leading PV max ahead of a meridionally phased and positively tilted
upper shortwave trough located farther upstream. At this time an
upward transition from 20 percent to 30-40 percent PoPs during this
time seems warranted.

- Showers and thunderstorms most probable Monday Night/Tuesday

There is reasonably good certainty with this part of the forecast
given the previously mentioned upper shortwave trough, whose
characteristics tends to be associated with good predictability.
Several consecutive runs of the ECE have shown a large majority of
members indicating precipitation during this time, which supports
the idea of good predictability. Given that this will be a
seasonably deep upper trough, we will need to prepared for the
possibility of severe weather and even localized flooding.

- Generally dry with seasonal temperatures starting Wednesday

Low precipitation chances (below 20%) seem justified at least
starting out given we will have confluent northwest flow aloft which
will help orient a surface high over the western Great Lakes.
Guidance PoPs remain low all the way into Friday. This latter part
might be a bit optimistic given that, as noted previously, we will
be beneath upper troughing and the surface high will have moved
east, yielding southeasterly return flow and a concomitant increase
in low level moisture.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 155 PM EDT Fri Jul 5 2024

Though current West Michigan TAFs do not explicitly contain
thunder, a small risk continues through the afternoon. The
greatest instability currently resides across central Lower MI,
and it`s beginning to appear this will remain the areas with
greatest coverage as a cold front now over Lake Michigan sweeps
across the state through early evening. Will amend TAFs if
development occurs and/or confidence grows.

Otherwise, generally VFR conditions this afternoon will give way
to MVFR conditions (1000-2500ft ceilings) overnight in
cool/cyclonic flow behind cold front. Some improvement is expected
is expected during the day Saturday, with areas of MVFR improving
to VFR afternoon - as clouds slowly diminish. South winds will
become west behind front this evening.

&&

.MARINE...
Issued at 250 AM EDT Fri Jul 5 2024

Light winds and a moist airmass will bring patchy fog over the
waters through mid-morning. Otherwise, with no notable wind in the
next few days, Small Craft Advisories and Beach Hazards Statements
are not expected. Winds under 10 knots this morning become
southwesterly at 10-15 knots later today before becoming
northwesterly late Friday as a cold front moves through. Scattered
showers and isolated storms will affect the nearshore waters north
of Grand Haven this morning.

&&

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

&&

$$

UPDATE...Duke
DISCUSSION...TJT/Thomas
AVIATION...Smith
MARINE...Thomas