069
FXUS63 KGRR 050653
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
253 AM 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

&&

.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 /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 154 AM EDT Fri Jul 5 2024

Based on observations and guidance, have given MKG IFR/LIFR
visibility restrictions overnight for this update. Surface
dewpoint temperatures are in the mid/upper 60s which is at or just
above reported water temperatures that arrived the morning of
July 4. Satellite nighttime microphysics RGB product indicates probable
fog/low clouds by the immediate lakeshore where a band of slightly
colder water resides (due to prior upwelling). The GOES satellite
probability of IFR product similarly latches onto this area.

Outside of MKG and farther inland, expect patches of MVFR and
even localized IFR restrictions areawide given the juicy airmass
and the potential of radiation fog development. Have also begun to
introduce VCTS this afternoon, predominantly for our eastern
terminals.

&&

.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.

&&

$$

DISCUSSION...TJT/Thomas
AVIATION...TJT
MARINE...Thomas