660
FXUS63 KGRR 221603
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1203 PM EDT Sat Jun 22 2024

.KEY MESSAGES...

-Potential severe weather and flooding today/this evening

-Dry Monday, strong storms possible Tuesday/Tuesday night

-Dry Wednesday/Thursday, storminess returns next weekend

&&

.UPDATE...
Issued at 1137 AM EDT Sat Jun 22 2024

Based on the latest wave model forecasts, we expanded the small
craft advisory and beach hazards statement one county/zone further
north.

One area of heavy rain was tracking through the Pentwater
to Ludington region. It appears that fairly widespread amounts of
around an inch may accompany this initial batch. Additional heavy
rain is likely this afternoon and evening with the storms in
southern WI tracking east northeast. Off of SPC`s meso analysis
page, PWAT values were around 2 inches in this region with really
impressive 925-850 mb moisture transport going on. 12z soundings
from GRB and APX support an efficient warm rain event along with
warm topped convection with the MPL well under the trop. Rainfall
rates could easily top an inch per hour. Thus we will maintain the
flood watch.

As for the severe potential, low level convergence was maximized
closer to the warm front over the northern zones with divergence
flow further south. Instability was building throughout the area
with ML CAPE values up to 500 J/kg and bulk effective shear values
over 30 knots. An area of stronger 0-1 km shear vectors was noted
west of Pentwater and moving northeast. The flow remains
relatively divergent for much of the CWA for a few more hours so
we may struggle to develop widespread convection. However by mid
afternoon and more so late afternoon we could start to see an
increase in the low level convergence which would support newer
storm development. We will also need to monitor the trends of the
storms in southern WI as when the try to track into the CWA, they
could generate enough low level lift to persist. Given the
favorable instability and shear at that time, we could end up with
an organized cluster of storms rolling through. Until then, it
appears the northern parts of the region is the favored location
for strongest storms given stronger 0-1 km bulk shear values
through mid afternoon.



&&

.DISCUSSION...
Issued at 332 AM EDT Sat Jun 22 2024

-Potential severe weather and flooding today/this evening

A warm front has lifted to the northern cwa early this morning
where it`s expected to become quasi stationary today. Low pressure
in Nebraska will move ENE toward Wisconsin by this afternoon. A
strong southwest flow ahead of the low is helping to bring in
moisture-rich air and precipitable water values in excess of 2
inches is expected today.

There will be some destabilization ahead of the low across the
northern and central cwa today. Thunderstorms will likely spread
east from Wisconsin. CAPE values are not particularly impressive.
However, shear values increase to 45 knots this afternoon. An
impressive LLJ is progd to move toward the northwest CWA this
afternoon. A weak short wave aloft will aid in convective
generation. It`s possible that as the shear increases we could see
some low topped supercells develop. Impressive helicity values
around 400 m2/s2 move over the northern rows this afternoon near
the frontal boundary. It`s not out of the question a tornado or
two could spin up along the boundary.

As the low crosses northern Lower around 00z, the trailing cold
front will begin to move south. Showers/storms will form along the
cold front and move across the rest of the cwa during the evening
and overnight. Areas south of I-96 will likely see a bit more
destabilization today due to a bit more sunshine. And so any
storms that form along the cold front will be able to tap into
that instability. Strong wind gusts and torrential rainfall will
be the main threats. Precipitation will be ending by Sunday
morning.

Today will be the last day of the Heat Advisory. Heat indices will
top out in the mid 90s again today. A cold front will move across
the state tonight leading to highs in the 75 to 80 degree range
Sunday.

-Dry Monday; strong storms possible Tuesday/Tuesday night

Monday will feature sunshine and comfortable temps/humidity with
surface ridging in control before a warm front returns on Tuesday
and sends back the heat and humidity for a day. GFS has SB Capes
exceeding 3000 J/KG Tuesday afternoon once the warm sector arrives,
setting the stage for the possibility of some stronger storms by
evening as a cold front and H5 shortwave trough approach from the
northwest.

-Dry Wednesday/Thursday; storminess returns next weekend

The cold front exits early Wednesday and we dry out by afternoon as
cooler/drier air arrives from the north. High pressure settles in
for Thursday with dry weather still prevailing, but then models
diverge which leads to low confidence timing the return of our next
chance of showers and storms. The more progressive GFS has rain
chances returning already on Friday whereas ECMWF holds off until
Saturday.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 745 AM EDT Sat Jun 22 2024

Showers and storms impacting MKG and GRR through Noon with ocnl
MVFR/IFR vsbys then the rain lifts primarily north of these two
terminals this afternoon before returning again this evening.

Otherwise VFR weather prevailing all day for the terminals south
and east of GRR.  The potential for showers and storms does not
really ramp up across this area until after about 02Z tonight.
Winds increasing to 12-22 kts out of the southwest by Noon today.

After the passage of a cold front late tonight the storm threat
ends then lower cigs in the MVFR or IFR category will press
southward after 06Z.

&&

.MARINE...
Issued at 332 AM EDT Sat Jun 22 2024

Wind and waves will increase behind a cold front tonight
necessitating a Small Craft Advisory and Beach Hazards statement
south of Grand Haven. Wind and waves will decrease late tonight.
Additionally, thunderstorms will pose a threat to boaters and
beachgoers today and tonight.

&&

.HYDROLOGY...
Issued at 332 AM EDT Sat Jun 22 2024

Very juicy air with precipitable water values north of 2 inches
are expected across the cwa today. A stationary frontal boundary
over the northern cwa will focus showers and thunderstorms today.
Any of these storms will be capable of producing torrential
rainfall. Training echos will be an issue today and local
probability matched mean values hint at the potential for 2 to 4
inches of rain along US-10 in an area that saw locally heavy
rainfall from storms yesterday. As a result, we have issued a
Flood Watch for the northern 3 rows of counties from noon today
through 6 am Sunday. Local flooding of poor drainage areas is
possible.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through late tonight for MIZ037>040-043>046-050>052.
     Beach Hazards Statement from 8 PM EDT this evening through
     Sunday morning for MIZ050-056-064-071.
     Heat Advisory until 8 PM EDT this evening for MIZ056-057-064>067-
     071>074.
MARINE...Small Craft Advisory from 8 PM this evening to noon EDT Sunday
     for LMZ844>847.

&&

$$

UPDATE...MJS
DISCUSSION...Meade/04
AVIATION...Meade
MARINE...04
HYDROLOGY...04