871
FXUS63 KGRR 271411
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1011 AM EDT Sat Apr 27 2024

.KEY MESSAGES...

- Slight Risk for Storms This Evening and Tonight

- Scattered Showers and Storms into Sunday Evening

- Chances for rain almost every forecast period Sun Night through Fri

- Continued warm right through the upcoming work week

&&

.UPDATE...
Issued at 1010 AM EDT Sat Apr 27 2024

No change in expectations through early evening. Rain showers
have exited the CWA so have removed morning PoPs from the
forecast. Satellite shows an area of clearer skies crossing the
state, which should fill back in over the course of the afternoon
thanks to cloud development to our west. Looking at the 12z APX
and DTX soundings, we have a well capped environment at present,
which will destabilize over the coming hours as daytime heating
and low- level moisture affect low level stability. However, given
the lack of a frontal boundary or trigger, expect at worst
isolated showers or storms this afternoon and for most if not all
remain dry through the afternoon.

The latest SPC D1 outlook has shifted the slight risk further
south into lower Michigan, north of a South Haven to Ithaca line
and a marginal risk southeast, given the expected frontal trigger
for evening storms will be further south and east than was
expected yesterday. Storms look to initiate in the 7-9pm range off
of this front and push southeast through early Sunday. Primary
threats will be large hail and damaging winds with these storms.
Other than the shift in the D1 Slight Risk current expectations
remain on track.

&&

.DISCUSSION...
Issued at 355 AM EDT Sat Apr 27 2024

- Slight Risk for Storms This Evening and Tonight

Michigan sits in the warm sector of a surface low currently centered
over northwest Iowa / southeast Minnesota. As that low traverses
northeast into Lake Superior today, scattered showers and storms
will move through the area. A patch of these warm sector showers
is currently traveling northeast from IL/IN into southern MI this
morning. Elevated instability will increase through the morning,
so some rumbles of thunder are possible.

Better chances for strong to severe storms exist later this evening
and overnight as a cold front moves in from the northwest.
Surface heating builds instability throughout the day with HREF
MUCAPE up to 1000 J/kg and bulk shear near 35 to 40 knots. Low-
level convergence will focus storm initiation near the surface
boundary near and north of a line from Muskegon to Clare. SPC
maintains a Slight Risk for that area today, with Marginal to the
southeast. Main timing for the strong to severe storms will be
from 8pm to about 2am with wind and hail as the primary storm
threats.

- Scattered Showers and Storms into Sunday Evening

Active weather will continue into Sunday, though the risk for severe
weather is low. Another upper low will eject out of the Southern
Plains with its associated surface low moving into the region
Sunday. We remain in the warm sector of this surface low with
HREF MUCAPE up to 500-1000 J/kg by the afternoon. So expect
continued showery conditions with some weak storms. HREF
probability-matched mean total QPF by Sunday evening pushes 1 to
1.75 inches for areas north of a line from Muskegon to Saginaw.

- Chances for rain almost every forecast period Sun Night through Fri

The saying about "April Showers" will be very applicable to our
forecast over the next 7 days despite crossing into the month of
May. Chances for precipitation will be in the forecast each 12 hour
forecast period through Friday with the exception of Tuesday.

The system affecting our weather this weekend will exit the area to
the east on Monday into Monday night. Chances for showers and
thunderstorms will exist Sunday night through Monday night. Sunday
night a warm front will be slowly lifting northward. Monday we will
be positioned in the warm sector of the weakening system and Monday
night the cold front goes east. Looking at ECMWF and GFS ensemble
member plots of CAPE it is clear to see that deep moisture remains
resulting in CAPE in every ensemble member. Showers and storms are
in the forecast each of these 3 forecast periods. Lack of upper
support in terms of a significant shortwave will likely limit and
severe potential, but strength of the low level jet (30-45 knots)
suggest we will need to monitor. As of now, the SPC has us in
general thunder for Sunday night through Monday night.

One dry forecast period on Tuesday with ridging moving through the
Great Lakes will be short lived as rain chances reappear Tuesday
night. Tuesday night a cold front moves in associated with a
shortwave aloft off to our northwest. Thursday a warm front will try
to push in from the south with a low remaining in the area then
towards Friday.

- Continued warm right through the upcoming work week

GFS and ECMWF MOS guidance shows highs in Grand Rapids in the 70s
Monday through Friday. 850mb temperatures are supportive of the warm
weather theme as well with +8C to +14C in place much of the time.
Normal highs are in the low 60s this time of year so we will be at
least 10 degrees above seasonal normals. Bottom line to the forecast
is warm and showery through the period.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 755 AM EDT Sat Apr 27 2024

A warm front will surge north of the TAF sites in the next hour or
two. Strong southwest winds will develop in its wake and a broad
area of MVFR ceilings will develop lasting from around 14Z to 20Z.
We are expecting mainly dry weather in this time frame. The winds
will be rather strong at 15-25 knots with gusts of 30-40 knots
possible. Additional storms will develop this evening north of
the TAF sites and gradually try to work in from the north
overnight. Overall an unsettled aviation weather pattern with
stronger winds and storms in the forecast.

&&

.MARINE...
Issued at 355 AM EDT Sat Apr 27 2024

Hazardous conditions will continue through tonight as southwest
winds quickly build this morning. Southwest winds to 20-25 knots
are expected with gusts pushing 30 knots. Areas of fog may
develop tonight through Monday as warm moist air with higher dew
points in the mid to upper 50s moves into the region. This warm
moist air over cold water will result in fog, though strong
surface wind may help lift inversion heights and disperse fog.

&&

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

&&

$$

UPDATE...Thomas
DISCUSSION...Duke/Thielke
AVIATION...Duke
MARINE...Thielke