905
FXUS63 KGRR 121934
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
334 PM EDT Sun May 12 2024

.KEY MESSAGES...

- Small Window for Aurora This Evening Before Clouds Move In

- Shower and Thunderstorm Chances Tonight through Monday Night

- Rain Ends Tuesday

- Additional Chances of Rain Late Week|

&&

.DISCUSSION...
Issued at 334 PM EDT Sun May 12 2024


- Small Window for Aurora This Evening Before Clouds Move In

For those looking to view the Northern Lights tonight we remain in
an active period as a X class flare (the highest level, although
there are varying degrees of X class flares) came off the sun at
midday. kP index values and magnetometers from across the mid
latitudes show limited activity as this time. With additional
flares inbound though, aurora remains a possibility tonight. The
issue is like Friday evening, clouds are pushing in from the
northwest after dark. There may be a window from roughly 1000pm to
midnight or so where clouds have not yet obscured the sky. That
window only becomes relevant though if we increase aurora activity
from current relatively low levels. Stay tuned...or look skyward.

- Shower and Thunderstorm Chances Tonight through Monday Night

The main focus of the next day or two is the chances for showers
and thunderstorms tonight, Monday and Monday night. Two main upper
shortwaves of note during this time frame, one is passing well to
our north across Ontario. The other lifts into the Plains from the
Central Rockies tonight. A cold front associated with the Ontario
wave will push into the area from the north tonight. As the
Ontario low pulls away to the east the boundary stalls over the
area on Monday as the plains system approaches. It is not until
Monday night into Tuesday before the boundary sags south of the
area.

Assessing thunderstorm chances each period beginning with
tonight...showers and storms will rapidly increase this evening
over Wisconsin and Northern Lake Michigan where a Slight Risk has
been issued by the Storm Prediction Center (SPC). Storms will try
to advect in across Central Lower Michigan this evening after
800pm and continue roughly through about 300am. The 3km NAM tries
to push MUCAPE values into the area that are over 1000 j/kg this
evening. That may be a bit high. HRRR MUCAPE is much lower, on the
order of a few hundred joules. We are in a marginal risk from the
SPC, but feel severe weather is a low risk in our forecast area
tonight. Storms will be very much diurnally driven and with
limited CAPE to work with in the first place it will be tough to
produce strong storms. Cannot rule out some small hail if a
stronger storm can survive into our area given steep mid level
lapse rates. Updraft helicity swaths tell the tale tonight with
best chances at severe in Wisconsin and weakening storms as they
move our direction.

The HREF has a good flare up of convective activity in our area
Monday afternoon, especially near and south of Interstate 96. We
are likely looking at around 1,000 j/kg of MUCAPE tomorrow with
potentially a bit more if we can develop breaks in the cloud cover
located near the stalled front. Deep layer shear is only 20-30
knots where storms will be located so organization and severe
threat remains on the low side.

The front remains in place for Monday night slowly sagging south
with time. Once we get beyond the evening hours, where some
embedded thunderstorms remain, we transition into more of a
synoptic scale rain. The Plains upper low and its surface
reflection will be moving into the Ohio Valley Monday night.

Regarding rainfall...the HREF probability matched mean is showing
all areas seeing 0.25 to 0.75. Pockets of 1.00 to 1.50 inches will
be possible in heavier swaths. We are not expecting any
significant hydro concerns with this rainfall.

- Rain Ends Tuesday

Rain chances will end from north to south as the day progresses on
Tuesday. This will be driven by a mid-level trough driving the
frontal boundary responsible for our precipitation south of the
state, with northerly flow behind it. This northerly flow keeps West
Michigan in the 60s for highs Tuesday.

Surface ridging and a return to southerly flow Wednesday causes
highs to return to the 70s. The surface high pressure system paired
with upper-level ridging and the exiting of low-level moisture means
dry conditions are favored Wednesday and much of Thursday. There may
even be decent sunshine given the lack of moisture.

- Additional Chances of Rain Late Week

Precipitation returns to the forecast late Thursday into Friday as a
mid-level wave approaches the state. Confidence in precipitation is
higher than 24 hours ago with the Euro/EPS trending towards the
GFS/GEFS suggesting troughing arriving by Friday. Ensemble forecasts
suggest that a surface low develops in association with this trough
but the position and strength of this feature varies. Given the
favorable synoptic setup for rain but still spread in where the best
rainfall axis will be, feel the NBM broad 40-55 percent PoPs are
reasonable. As the amplitude and tilt of the trough and associated
location and strength of the surface low refine in the coming days,
POPs may increase across the region, particularly where the
synoptic setup provides the best setup for rainfall.

Signals for additional precipitation exist into the weekend, but
considerable spread remains in the mid-level pattern in whether
troughing persists into the weekend or a period of ridging takes
over. Given the weaker signal for precipitation under the less
certain synoptic conditions, will leave the broad slight chance to
low end chance (25-35 percent) chances of precipitation for Saturday
in the outgoing forecast.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 108 PM EDT Sun May 12 2024

Two aviation related weather concerns exist within this TAF
window. First is the arrival of showers tonight as a weak frontal
boundary slides into lower Michigan. Given expected coverage of
showers have maintained the previous VCSH at GRR/LAN and
prevailing SHRA at MKG. A stray shower could clip the I94 corridor
but confidence is to low to include in any form for BTL/JXN/AZO.
There be isolated thunder within these showers but confidence in
direct impacts to any terminal is to low to include VCTS. Will
defer adding thunder to the TAFs to future updates. The other
concern is LLWS. A 40-50 knot wind maximum around 2kft brings the
potential from late this evening to 12z Monday of non-convective
LLWS for all terminals.

&&

.MARINE...
Issued at 334 PM EDT Sun May 12 2024

South flow has increased ahead of an approaching cold front and we
have therefore issued a Small Craft Advisory (SCA) for areas near
Big and Little Sable Points. High resolution models show 20+
knots of wind in the profile this evening. Webcams at Manistee
indicate a developing wave field with waves splashing up onto the
south pier. The WaveWatch3 is showing waves of 3-5 feet this
evening before subsiding overnight. South of the points we should
be ok with less wind in the profile. In addition thunderstorms
moving in from Wisconsin tonight will affect the Points as well
as winds/waves will be locally higher near any storms.

Winds and waves should be below SCA criteria for Monday and most
of Monday night. Winds and waves look to increase however on the
backside of a low pressure system producing stronger northerly
flow on Lake Michigan beginning late Monday night. SCA`s look
likely late Monday night into Tuesday at least for winds, if not
waves as well.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 2 AM EDT Monday for LMZ848-849.

&&

$$

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