325
FXUS63 KGRR 101112
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
612 AM EST Fri Jan 10 2025

.KEY MESSAGES...

- Accumulating Snow Today into Saturday Morning

- Arctic Air With Continued Snow Chances Early Next Week

&&

.DISCUSSION...
Issued at 325 AM EST Fri Jan 10 2025

- Accumulating Snow Today into Saturday Morning

Snowy road conditions and slower travel speeds are likely over much
of the area later this morning and tonight. A pretty standard-fare
event for January but impacts worthwhile to mention. This will occur
in two parts. First, a steady 2 to 3 inch snow for most, beginning
late this morning and ending by the middle of the night, as mid and
upper level moisture streaming in from the southwest is lifted by
phasing upper-level troughs. There may be southwest-wind lake
enhancement which would favor a swath of over 3 inches by evening in
parts of Muskegon, Ottawa, western Allegan, and possibly southwest
Newaygo. Second, during the overnight into Saturday morning,
northwest-wind lake effect snow showers should result in more
intermittent heavier bursts of snow with highly variable conditions.
Combining both phases of this event, spotty totals over 6 inches are
plausible in lakeshore areas west of US-131.

The HREF mean QPF for the synoptic contribution inland is generally
0.15 to 0.2 inches, though there is a 20 to 60 pct chance of over
0.2 inches (local minimum perhaps near M-66). The moisture layer is
fairly deep and extends above and below a moderately thick DGZ with
gentle lift throughout. Snow/liquid ratios should be fairly normal,
between 12:1 and 15:1 per Cobb snow tool and NBM. The southwesterly
lake effect during the afternoon-evening will be in the vicinity of
a slow-moving surface trough where surface convergence is maximized
along a wind shift. The southwesterly lake effect convective layer
is fairly low-topped and may not extend into the DGZ, but should
provide some precip production regardless. Once the surface trough
passes east, deeper cold air extending above 850 mb in northwest
flow will grow lake effect showers into the DGZ. Bursts of high
snow/liquid ratio snow with over an inch per hour are possible late
tonight in some lakeshore areas. By late Saturday morning, the HRRR
has been a more robust model in giving parts of the lakeshore
south of Muskegon QPF over 0.5 inches and possibly isolated amounts
over 0.75 inches, which would indicate potential for localized
amounts well over 6 inches.

- Arctic Air With Continued Snow Chances Early Next Week

A slow moving upper level trough will bring our next winter system
to the area Sunday into Monday and a shift to a colder pattern
through the remainder of next week.

Sunday we`ll be in a regime of warm air advection with some
convergence provided by the low level jet and additional lift from
rounds of positive vorticiy advection. Guidance has been
highlighting a dry slot at 700 mb with better saturation in the
lower levels. Looking at soundings there is a concern that we will
lack cloud ice resulting in freezing rain chances. Therefore have
introduced slight chances for freezing rain to the forecast until
Sunday night. This could impact snow amounts, but overall several
inches is possible mainly along and west of US-131 through Monday.

The front and colder air will move through sometime early Monday
morning with westerly winds throughout the day. The upper level low
over the Great Lakes will advect another vorticity maximum through
the area Monday evening with winds gradually turning to the
northwest through the day Tuesday into Tuesday night. The flow off
of the lake along with colder temperatures will continue snow
chances through the week with better bursts timed with the
shortwaves and positive vorticity advection Monday night into
Tuesday, potentially Wednesday morning, and toward the end of the
workweek.

Our warmest day will be Sunday with temperatures in the upper 20s to
low 30s. Highs then drop in the teens for Tuesday with 20s expected
the remainder of the week. Lows are expected to be in the single
digits to teens with Monday night into Tuesday morning being the
coldest.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 612 AM EST Fri Jan 10 2025

Slight adjustments to the timing of snow and visibilities from the
previous forecast. MVFR ceilings will gradually spread over the
area with snow starting between 12-15Z for most sites with the
eastern terminals more between 15-18Z. Once snow starts expect
reductions in visibility along with ceilings dropping to IFR
values or lower at times. As mentioned before MKG will likely see
the greatest impacts due to lake enhancement which could result in
snowfall rates around an inch per hour at times. Elsewhere
snowfall rates around 0.25 to 0.50 inch per hour are expected.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 10 AM EST Saturday for MIZ037-038-
     043-044-050-056-064-071-072.
     Winter Weather Advisory from 10 AM this morning to 1 AM EST
     Saturday for MIZ051-052-057>059-065>067-073-074.
MARINE...Small Craft Advisory until 7 PM EST this evening for LMZ844>849.

&&

$$

DISCUSSION...CAS/RAH
AVIATION...RAH