991
FXUS63 KGRR 090733
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
333 AM EDT Tue Jul 9 2024

.KEY MESSAGES...

- Heavy Rain, Flooding Possible Tonight into Wednesday

- Summer Like Precipitation Pattern Thursday through Monday

- Heat Builds During the Friday through Monday Time Frame

&&

.DISCUSSION...
Issued at 332 AM EDT Tue Jul 9 2024

- Heavy Rain, Flooding Possible Tonight into Wednesday

Dry conditions this morning give way to a chance of scattered
thunderstorms this afternoon mainly south of I96, as we destabilize
due to the influx of tropical moisture. 5000-1000 J/kg of CAPE with
ample moisture support locally heavy rainfall and lightning as the
main concerns if any storms develop.

More widespread rainfall arrives tonight across West Michigan as the
remnants of Beryl move into the Ohio Valley towards Detroit. A
favorable setup for high rainfall rates is expected as near
climatological max PWATs combine with a very deep warm cloud layer.
Guidance has trended northwest with the deformation axis and
heaviest rainfall compared to this time yesterday. Current thinking
supports a wide swath of 2-4 inches of rainfall south of a Mount
Pleasant to Grand Haven line with HREF LPMM guidance suggesting
locally higher amounts of 4-6+ inches of rain in the heaviest band.
Rainfall then tapers off to around 0.5-1 inches near US10. Some
uncertainty exists as to the exact placement of the heaviest band of
rainfall but there is strong support among both global and
convective allowing guidance for its development. Will be issuing a
flood watch near and south of the Mount Pleasant to Grand Haven line
for possible areal and creek/stream flooding. More on impacts will
be contained in the Hydro section.

The strong low-level jet with this system will also support gusts of
25-35 mph across the area. The widespread showers continue into the
evening before tapering off Wednesday Night as the remnants of
Beryl pull away from West Michigan.

- Summer Like Precipitation Pattern Thursday through Monday

Once we get beyond the tropical rainfall in the shorter term portion
of the forecast the remainder looks very mid summer like with
periodic chances for showers and storms. Overall we have low chances
for a few showers and storms through the period as there are not any
major features to latch on to. Aloft, there is some weak troughing
that persists between Thursday and Saturday. There is not a signal
within the ensemble members for good precipitation chances in the
Thursday through Saturday time frame. There may be some isolated to
scattered showers and storms that form from time to time, but our
forecast has dry periods with an occasional 20-30 pct chance mixed
in. Our completely dry periods as of now are both Thursday night and
Friday night with some diurnal activity in the afternoons.

The main chance for precipitation in the longer term looks to be
Sunday into Monday with a shortwave sliding through northwest flow
in a ridge rider scenario. A surface boundary looks to be in the
area with dew points in the GFS anyway that are near and above 80F
off to our west and southwest. So, any thunderstorm activity will
likely be working with extreme instability. In fact the GFS has
5000+ j/kg MUCAPE in place Sunday and Monday afternoons. Too early
to nail anything down that far out, but any storms working with that
much instability will be a focus of ours obviously. 40-50+ knots of
deep layer shear in the area make the concern even greater for
severe weather.

- Heat Builds During the Friday through Monday Time Frame

Heat will build into the Great Lakes region late in the work week
and through the weekend. Saturday night through Monday will feature
850mb temperatures that are largely in the +18C to +20C range. The
values are very mid summer like and will produce high temperatures
in the upper 80s to around 90 and not allow for much of a drop at
night. Sunday night and Monday night we have lows around 70 degrees.
For those look for a deep summer like weekend with warm temperatures
and a fair amount of sunshine the upcoming weekend may fit the bill.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 157 AM EDT Tue Jul 9 2024

A fair amount going on during this TAF period. We are starting off
mainly VFR, but there is some rain hanging on at JXN at 06Z. That
rain should be over fairly quick, but some light fog (MVFR) will
develop across the south at AZO, BTL and JXN. The potential is
there for JXN to dip lower, but left that out for now. We will
be VFR by mid morning at all sites.

The next focus is the threat for showers and thunderstorms across
the south at AZO, BTL and JXN this afternoon in the 18Z to 00Z
time frame. These showers and storms will be focused along I-94.
Occasional MVFR and IFR is possible for short periods with showers
and storms. This evening after 00Z we shift gears into rain
spreading in with the tropical moisture associated with the
remnants of Beryl. Visibilities and ceilings sink after 00z with
MVFR spreading into most TAF sites (MKG is the exception) between
00Z and 06Z this evening. IFR conditions surge in just beyond this
TAF period between 06Z and 12Z on Wednesday.

&&

.MARINE...
Issued at 332 AM EDT Tue Jul 9 2024

Weak gradient winds keep winds and waves below small craft
advisory/beach hazard statement levels today. Models trends support
increasing northeasterly winds of 20-25 knots gusting to 30 knots
early Wednesday morning into the evening hours south of Whitehall
and possible elevated waves as flow becomes more northerly Wednesday
afternoon. Will let day shift make the final call on a Small Craft
Advisory as high resolution model guidance continues to become
available. Winds then decrease below small craft criteria Thursday
into Friday as the low pulls away.

&&

.HYDROLOGY...
Issued at 332 AM EDT Tue Jul 9 2024

The remnants of what was Hurricane Beryl trek north east into the
Ohio Valley placing Southern Michigan in the favored region for the
heaviest rainfall. Historically, scenarios like this where a
tropical system transitions to extratropical are favorable for heavy
rain and flooding. A wide swath of 2-4 inches is forecast across the
Kalamazoo, St. Joseph, and Grand River basins with locally higher
amounts of 4-6+ inches in the heaviest bands.

Areal flooding wise, these amounts suggest flooding in urban areas,
poor drainage areas, and creeks and streams is likely in the general
region south of a Grand Haven to Mount Pleasant line. The greatest
impacts in this regard will be wherever the heaviest band of higher
totals sets up. An areal flood watch has been issued for this risk
from late this evening through Wednesday evening.

As for river flooding, yesterday`s day shift coordinated with the
NCRFC based on expected QPF and antecedent conditions. Current
forecasts suggests that minor flooding is likely across at least some
of our more flood prone forecast points like Sycamore Creek, the
Portage River, or Kalamazoo river. Major flooding is unlikely at
this time, though depending on where and how much the heaviest
rainfall is can`t rule out a point or two making a run for moderate
flooding. As the area of heaviest rainfall becomes clearer over the
next 24 hours will monitor river and contingency forecasts.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from this evening through Wednesday evening for
     MIZ046-051-052-056>059-064>067-071>074.
MARINE...None.

&&

$$

DISCUSSION...Thomas/Duke
AVIATION...Duke
MARINE...Thomas
HYDROLOGY...Thomas