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000
FXUS63 KGRR 271902
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
302 PM EDT Sat Apr 27 2024

.KEY MESSAGES...

- Scattered Severe Storms Possible Tonight

- Additional Showers and Storms Expected Sunday into Monday

- Pleasant weather on Tuesday

- Periodic shower chances and mainly above normal temps mid-late
  week

&&

.DISCUSSION...
Issued at 301 PM EDT Sat Apr 27 2024

- Scattered Severe Storms Possible Tonight

A look at satellite shows that while plenty of diurnal cu
demonstrate the unstable boundary layer present, there is no
convective development thanks to a lack of forcing. Expect the
afternoon and first part of the evening to remain dry, though an
isolated shower or storm is not off the table.

Looking at tonight, a boundary sinking across Lake Michigan will
provide a focus for storm development after 7-8pm. Given MLCAPE
values of 800-800 J/kg and MUCAPE values of 800-1250 J/kg in 35-45
knot deep layer shear, multicell organization will be supportive of
scattered wind gusts to 60 mph and hail to around 1 inch in
diameter, mainly across the current SPC slight risk area from South
Haven to Ithaca. A marginal risk is in place to the southeast where
an isolated severe storm tonight cannot be ruled out. The primary
threat will be through 2am. A second area of showers and storms is
expected to move in along a warm front overnight bringing rain
across the southern CWA.

- Additional Showers and Storms Expected Sunday

Showers and storms continue into Sunday as the warm frontal boundary
stalls across Central Lower Michigan aided by weak ripples in the
500 mb flow and forcing from a jet streak to the north, keeping the
best coverage of showers and storms near and north of I96. Severe
weather does not look likely at this time given that showers and
clouds in the vicinity of the warm front look to promote less in the
way of destabilization (MUCAPE over 500 J/kg mainly south of the
warm front and main convection), and large scale forcing is on the
weaker side. However, if instability can build further north and to
a greater magnitude than currently forecast, organized convection
cannot be ruled out. We will need to monitor this into Sunday.

Another concern Sunday will be for areas of locally heavy rainfall
as mean cloud layer winds will be somewhat parallel to the warm
frontal boundary meaning training of storms is possible. HREF
probability matched mean values suggest that rain from Sunday
Morning to Monday morning could exceed the 1 inch mark in spots if
convection can train repeatedly over an area.

Widespread rainfall spreads over the whole CWA Sunday night into
Monday morning as better forcing arrives in the form of an
approaching cold frontal boundary and vorticity maximum embedded in
the parent trough as it ejects north. Thunderstorm chances continue
into Monday as instability (MUCAPE around 1000 J/kg) develops ahead
of a front that passes through the CWA Monday into Monday night.
Given decent shear, we will have to watch the extent to which
instability builds ahead of the front for organized convection
potential. However the extent of instability development remains
uncertain at present. Rain chances wind down Monday night as the
front passes through the area bringing notably drier air with it.

- Pleasant weather on Tuesday

The upper trough and surface fronts responsible for the
relatively unsettled/showery/stormy weather Sunday and Monday will
shift northeast of the region by Tuesday. This will set the stage
for a very pleasant Tuesday. With surface high pressure
ridging overhead, expect partly-mostly sunny skies, light winds, and
temperatures slightly above normal /highs in mid 60s to near 70/.

Though the rest of the week won`t be a washout, Tuesday looks to be
the only day without at least some chance for rain.

- Periodic shower chances and mainly above normal temps mid-late week

Models/ensembles suggest that upper trough over the NW quadrant of
the CONUS will eject a couple of short waves ENEwd across the
Upper MS Valley and Great Lakes region mid-late week. WAA ahead of
one progressive short wave and associated surface front will support
a low chance (20-30%) of showers Wednesday. The better chance (50-
60%) for showers and storms arrives around Thursday as a more potent
short wave and surface low /somewhat reminiscent of this weekend/
advances into the region. Current projections suggest surface low
will be east of Lower MI by late Friday and Saturday, allowing
cooler and more seasonable air to overspread the Great Lakes.

All in all, temperatures are expected to average somewhat above
normal this week, with highs in the upper 60s to middle 70s Wed-Fri,
before falling back in the low-mid 60s Sat. Normal highs for late
April are in the middle 60s.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 126 PM EDT Sat Apr 27 2024

Gusty SW winds in excess of 30 kts continues this afternoon over
SW Lower MI in ongoing warm air advection regime. This is also
bringing higher dew points into the region, and has resulted in
BKN Stratocu areawide. Cloud bases have been sub-3k ft at times
/MVFR/ but expect bases to lift into VFR range with a bit more
surface warming shortly. In spite of increasing warmth/moisture
and instability /MLCAPES above 1K J/kg/ the lack of a trigger and
a capped atmosphere will preclude convection this afternoon.

Attention will then turn to SW-NE oriented front forecast to move
into northern Lower MI this evening and then push to central
Lower MI by daybreak Sunday. This will support scattered to
numerous showers/thunderstorms mid evening onward. Persistent
warming/moistening from the SW, coupled with the front, as well as
additional convection moving the region from IL/IN late tonight
will support MVFR/IFR cigs/vsbys after midnight.

&&

.MARINE...
Issued at 301 PM EDT Sat Apr 27 2024

Going small craft advisory looks to be in solid shape as frequent
gusts of 25-30 knots have been observed today. Winds calm below
small craft criteria overnight with waves falling early Sunday
morning. Depending on how much winds calm overnight, areas of fog
may develop over the lake as warm moist air moves over the still
cooler lake waters and continue into Monday. Conditions hazardous to
small craft are possible Monday as waves and winds increase ahead of
a cold front.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 AM EDT Sunday for LMZ844>849.

&&

$$

DISCUSSION...Smith/Thomas
AVIATION...Smith
MARINE...Thomas