-- Discussion containing changed information from previous version are highlighted. --
049
FXUS63 KGRR 250604
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
204 AM EDT Sat May 25 2024

.KEY MESSAGES...

- Risk for Thunderstorms Tonight

- Severe Storms Possible Sunday

- Windy Monday with Cooling Temps Early Next Week

&&

.UPDATE...
Issued at 833 PM EDT Fri May 24 2024

We`ve been watching convection over Wisconsin slowly decrease as
instability has begun to wane. It appears as though our window for
severe storms is closing. SPC mesoanalysis indicated 500 j/kg
MUCAPE will be present over the southwest cwa for the next few
hours. However, the strongest shear remains over central Lake MI
and really isn`t progd to move east. Thus we`ll see 20kts of shear
overnight...if the progs are correct. This isn`t to say we won`t
see any storms; regional radars continue to show convective
development across north central IL along the cold front and that
is moving ENE. We`ll continue to monitor. We trimmed PoPs a tad
to reflect current trends.

&&

.DISCUSSION...
Issued at 235 PM EDT Fri May 24 2024

- Risk for Thunderstorms Tonight

So far the instability has been lacking over the CWA with upwards
of 500 J/kg MU CAPE now reaching into Van Buren County. Values
increase considerably further south into northern IL and IN where
stronger convection was noted. Looking at the high res models,
the deeper instability never makes it into the CWA this evening
but some increase is noted for the southern zones, with values
making a run at 1000 J/kg. While deep layer shear does increase
over the next couple of hours, potentially topping 35 knots, it
fades during the evening hours, when the instability will be
increasing. Based on the above it appears a potential for stronger
storms exists mainly over southwest parts of the CWA. The
diminishing outflow moving onshore in the next hour may lead to
some places seeing gusts up to around 30 mph, even without rain.


- Severe Storms Possible Sunday

The primary concern in the long term portion is the Sunday afternoon
and evening timeframe. The overall synoptic setup will be driven by
a mid-level wave ahead of the parent trough passing into the region
and an occluding strong low pressure system (EPS model climatology
is at a minimum with the MSLP forecast for the time of year), and a
strong associated low-level jet with speeds north of 40 knots. The
strong southerly flow ahead of the low and parent trough will bring
dewpoints in the 50s to 60s into the region.

Confidence is high in convection developing with a warm front
lifting north followed by an evening cold front aided by ample upper-
level forcing. The important question is what is the extent of any
possible severe threat Sunday afternoon and evening.

The uncertainty how far north does the occlusion/triple-point get.
The farther north, the more surface based instability will be
available and the greater the proportion of the CWA at risk for
strong to severe storms. Uncertainties in low track and by extension
the position of this occlusion are still present, though 12z
guidance does is starting to favor a warm sector developing in the
current Marginal risk area near and south of a Muskegon to Grand
Rapids to St. Johns line with MLCAPE north of 1000 J/kg. However,
given the northward trend of the last 36 hours monitoring for
further northward shifts is needed.

Given the strong dynamics in play with this system, deep layer shear
is expected to be high across the area at 45+ knots. Assuming
surface-based instability can be realized, all severe hazards are
possible Sunday afternoon across at least some portions of West
Michigan. Close monitoring and refinement of this will be needed
over the next few days. In addition, PWATS near to north of 1.5
inches support a localized heavy rain threat. NBM probabilities
suggest at least a 20-30 percent chance of over an inch of rainfall
Sunday Morning to Monday morning with a better idea of possible
rainfall expected as we enter CAM range.

- Windy Monday with Cooling Temps Early Next Week

Behind the cold front the longwave troughing lingers over the area
through mid-week with occasional showers diurnally driven with the
mid-level cold pool aloft. With this, Memorial Day and Tuesday do
not appear to be a total washout but showers will be present at
times. We dry out by Wednesday into Thursday as ridging both aloft
and at the surface make their way into the area. Highs will
generally be in the 60s.

One thing to note however is it will be windy areawide on Monday as
cold-air advection paired with the still strong low-level jet. EPS
probs of gusts over 40 mph are 50-60 percent across the CWA east of
US131 where the strongest portion of the LLJ will be. So regardless
of showers expect a windy Memorial Day across West Michigan.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
-- Changed Discussion --
Issued at 204 AM EDT Sat May 25 2024

A cold front stretches north to south over portions of eastern
Wisconsin down into Illinois at 200am. A band of MVFR/IFR ceilings
is located over Eastern Wisconsin and Northeast Illinois near and
just ahead of the front. It appears that the shower and
thunderstorm threat is largely over and the main issue in the TAFs
will be a period of IFR/MVFR conditions as the front moves
through. The ceilings will progress through the TAF sites from
west to east between 400am and noon. We cannot rule out an
isolated/scattered shower ahead of the front, so we have VCSH
wording in the TAFs. VFR weather is expected after noon.
-- End Changed Discussion --

&&

.MARINE...
Issued at 235 PM EDT Fri May 24 2024

A low level jet moves in later this afternoon and evening and that
will likely keep some gusts going for a few hours but it
diminishes later tonight. As a result, any increase in the
gradient winds and associated waves will likely remain under
hazardous levels. High pressure moves over the area Saturday so
low waves and winds are expected.

&&

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

&&

$$

UPDATE...04
DISCUSSION...MJS/Thomas
AVIATION...Duke/Laurens
MARINE...MJS/Thielke