228
FXUS63 KGRR 191931
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
331 PM EDT Wed Jun 19 2024

.KEY MESSAGES...

- Temps not as Hot Thursday north

- Risk for Strong/Svr Storms this evening

- Very warm and windy Saturday

- Thunderstorms Friday through Sunday, especially Saturday night

- Dry and somewhat more comfortable Sunday through Tuesday



&&

.DISCUSSION...
Issued at 303 PM EDT Wed Jun 19 2024

- Temps not as Hot starting Thursday north

Heat index values are well into the 90s at this time so we made
no changes to the headline through the remainder of today.
However, looking at Thursday, there has been a trend for a
southward shift of the axis of hottest air then. This supports
dropping the Heat Advisory for Thursday for the region north of a
line from Ottawa County to Ingham County. We already issued the
NPW reflecting this change.  If this southward trend continues,
further changes may be needed the headline area.


- Risk for Strong/Svr Storms this evening

DCAPE around the area is favorable for gusty winds with any
stronger storms. ML CAPE values are over 1000 J/kg so decent
instability is in place. Effective bulk shear values have dropped
off and are under 25 knots. So far the low level convergence has
been very weak, but there are some indications that it may
increase somewhat near and southeast of a Kalamazoo to Lansing
line. If convection can develop in that region, we will be
monitoring it for possible strong wind gusts and locally heavy
rain. It`s worth noting that the main PWAT axis is shifting east
of the CWA so that dry air advection may limit the overall
convective coverage. The latest HRRR only has a few isolated cells
over the next few hours, but it does suggest some outflows from
any cells that do develop.

- Very warm and windy Saturday

Highs Saturday could vary substantially from northwest to southeast,
ranging from around 80 near Ludington to mid/upper 90s near Jackson.
Winds from the southwest Saturday afternoon will be quite strong;
although the official forecasts indicate gusts in the 20-30 MPH
range, expect model trends to bump this closer to the 30-40 MPH
range based on some of the more recent guidance. Mixing should be
substantial. A broadening spread of dewpoint forecasts among the
ensemble members hints at the mixing process and also suggests
uncertainty about how much drying will occur. The effects of heat
additionally may be curtailed by the development of thunderstorms in
the afternoon/event (see following section).

- Thunderstorms Friday through Sunday, especially Saturday night

As noted in the previous discussion, we will have a surface boundary
providing a source of weak low level convergence. Very flat ridging
aloft will offer episodic opportunities for minor PV maxima to zip
across the region and allow storms to fire and then cluster into
MCSs. Timing details are uncertain but activity will generally be
guided by the diurnal convective instability cycle.

Timing for the main round of thunderstorms looks increasingly locked
into Saturday night. A deeply mixed boundary layer ahead of this
activity will elevate the chances for damaging winds as this
activity moves through. Additionally, the upper shortwave trough
looks sharper and farther south than before. Hence, QC forcing could
be more substantial than previously thought. These ingredients point
to growing severe potential for this timeframe.

- Dry and somewhat more comfortable Sunday through Tuesday

As noted before, confidence is good for a dry Monday given sharp
shortwave ridging aloft behind the aforementioned trough. PoPs
return Tuesday night into Wednesday along with unseasonably warm
temperatures. Predictability for temperatures should be fairly good,
but advertised PoPs probably can be better attributed to climatology
rather than forecast certainty.
&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 142 PM EDT Wed Jun 19 2024

Main changes were to scale back thunderstorm chances this
afternoon and evening. The best prospects for storms will be
around JXN. Should storms occur in this area, they would likely
be associated with outflow boundaries from other thunderstorms
farther to the southeast. While JXN barely merits a VCTS mention,
there is an even smaller, but non-zero, probability for other
storms developing farther west around BTL and AZO in a similar
outflow-related fashion.

Separate activity over northeast WI could spread southeast into
northwest Lower MI after 00Z tonight, but current expectations are
for this to remain north of the MKG and GRR terminals.

Did include a brief period of MVFR visibility restrictions at all
terminals overnight...although MKG could temporarily experience IFR
ceilings and JXN could similarly experience a brief period of IFR
visibilities.

&&

.MARINE...
Issued at 303 PM EDT Wed Jun 19 2024

Based on the lowering winds and waves, we cancelled the beach
hazards and small craft advisory a little early. Saturday it
looking like the next big wind/wave event.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Heat Advisory until 10 PM EDT this evening for MIZ043>046-
     050>052-058-059.
     Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
     Heat Advisory until 8 PM EDT Friday for MIZ056-057-064>067-
     071>074.
MARINE...None.

&&

$$

DISCUSSION...MJS/TJT
AVIATION...TJT
MARINE...MJS