678
FXUS63 KGRR 200553
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
153 AM EDT Thu Jun 20 2024

.KEY MESSAGES...

- Temps not as Hot Thursday north

- Very warm and windy Saturday

- Thunderstorms Friday through Sunday, especially Saturday night

- Dry and somewhat more comfortable Sunday through Tuesday

&&

.UPDATE...
Issued at 1013 PM EDT Wed Jun 19 2024

Scattered convection continues overnight. Isolated storms have
moved northward towards the I 94 corridor. While the hot and
humid air mass over SW Michigan provided enough instability for
storms earlier this evening, they were stifled due to the
divergent pattern and weak shear. There is a boundary that extends
from Big Rapids to the southwest that has a persistent line of
showers. There were some storms along this boundary over mid Lake
Michigan. However, not expecting for these showers to intensify,
especially since there is weak shear, it is nighttime and the -10C
is at 20K feet.

&&

.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.

- 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 /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 153 AM EDT Thu Jun 20 2024

VFR prevailing much of the time over the next 24 hours although
brief periods of MVFR/IFR are possible if and when any of the
widely scattered showers and storms impact a terminal. The most
likely timeframe for thunderstorms is later today between roughly
20Z and 02Z, and mainly near and south of a GRR to LAN line. MKG
should be excluded from the thunder threat later today due to a
stable onshore/lake breeze. Scattered showers and storms weakening
and ending after sunset then the main concern will become areas
of fog and stratus developing overnight, especially where rain
occurred.


&&

.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 8 PM EDT Friday for MIZ056-057-064>067-
     071>074.
MARINE...None.

&&

$$

UPDATE...Ceru
DISCUSSION...MJS/TJT
AVIATION...Meade
MARINE...MJS