930
FXUS63 KGRR 250031
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
731 PM EST Sun Nov 24 2024

.KEY MESSAGES...

- Showers Tonight into Tuesday

- Snow Possible Wednesday Into Thanksgiving

- Cold With Lake Effect Late Week

&&

.DISCUSSION...
Issued at 303 PM EST Sun Nov 24 2024

- Showers Tonight into Tuesday

A surface low will trek to the northeast from the southwest
tonight into tomorrow. This low will intensify tomorrow morning as
an upper level wave deepens to support the mid level flow. Models
are in fair agreement on timing and movement. Total QPF is fairly
lite, with less than a quarter inch of expected. Best chance of
precipitation will be Monday afternoon into Monday evening with
increased mid level moisture and position of LLJ which will aid
showers.
 Dry slot due to the strong northwesterly flow will end
precipitation mid Tuesday evening with a more zonal flow into
Wednesday.

 Temperatures will remain above normal through tomorrow before
dropping back to below normal on Tuesday. The cold front swinging
through Monday night will bring temperatures around freezing. Tuesday
morning.


- Snow Possible Wednesday Into Thanksgiving

Some precipitation is possible as soon as Wednesday as a nearby
upper-level jet leads to weak isentropic ascent. The lack of
defining shortwaves or surface features will keep PoPs under 50
percent. Looking at thermal profiles this looks to start as a light
snow if it starts before daytime heating kicks in. Any snow
transitions to a rain/snow mix and possibly all rain before
reverting to snow as temps cool Wednesday Night.

Uncertainty still remains for the expected outcome of a surface low
tracking across the Ohio Valley Thursday morning. The driving
synoptic pattern will be the merging of a current upper-level low
off of the Pacific NW into the longwave pattern which then ejects
southward into the north-central CONUS. The associated low looks to
track across the Ohio Valley with a swath of deformation induced
snowfall on the north end. The concern is if this is far enough
north accumulating snow would be possible, especially south of I96,
on Thanksgiving day.

12z guidance continues to consolidate on a more broad upper-level
low and further south surface low track, which would result in light
snow mainly along the I94 corridor. There are still some ensemble
members, mainly in the EPS camp, that have a more robust upper-low
bringing more widespread snowfall to the area. Will keep the
outgoing forecast in line with the favored solution that keeps the
bulk of any snow out of the forecast area. However, given potential
holiday impacts we will continue to monitor for the possible
northerly low/stronger upper-low scenario.

- Cold With Lake Effect Late Week

No matter how the upper-level wave and associated synoptic snow
evolves for Thanksgiving, colder weather and a ramp up of the lake
effect snow machine are likely Friday into the weekend. Longwave
troughing carrying modified arctic air arrives over the area
Thursday into Friday with 850mb temps falling to near -10C. With
lake temps over +10C this generates robust over-lake instability.
Combined with overlying cold air advection causing inversion heights
to rise, periods of accumulating lake effect snow are likely late
this week into the weekend, particularly in the first couple of
rows of counties inland from the lakeshore. Overall flow looks to
prevail to be from the northwest favoring the southwest CWA for the
most substantial snowfall with a secondary maxima in the northwest
CWA. This could be a notable amount of snow with NBM and Long Range
Ensemble probabilities of 4+" of snow in the 50-70 percent range
across the SW CWA by Sunday evening. However given we are still
several days out there are plenty of details that need to be ironed
out over the next week. It will also truly begin to feel like winter
across West Michigan next weekend as highs below freezing become
likely.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 720 PM EST Sun Nov 24 2024

VFR cigs and vsbys will persist for most of the overnight period
with scattered showers late tonight, early Monday morning. Latest
models have MVFR cigs moving in after 09Z. Expect MVFR to develop
across all TAF sites by 12Z at the latest. IFR cigs with scattered
showers will develop by 15Z with periods of showers through most
of the day and IFR persisting through much of the TAF period.
There is some potential for breaks in the IFR Monday afternoon and
evening as low level moisture remains across southern lower
Michigan. Have left them prevailing due to the probability to
linger through 00Z.

&&

.MARINE...
Issued at 330 PM EST Sun Nov 24 2024

Expect a cloudy and rainy Monday with offshore flow. Winds will
shift tomorrow evening. Have hoisted marine headlines for
tomorrow evening into Wednesday. This is due to the strong
northwesterly flow behind a cold front that will swing through the
region. Strong winds upwards of 30kts will persist through
Tuesday before waining Tuesday night.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory from 6 PM Monday to 7 PM EST Tuesday for
     LMZ844>849.

&&

$$

DISCUSSION...Ceru/Thomas
AVIATION...Ceru/Thomas
MARINE...Ceru