Portage, Michigan 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

808
FXUS63 KGRR 180857
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
357 AM EST Sat Jan 18 2025

.KEY MESSAGES...

- Lake Effect Snow Begins Saturday

- Very Cold Temperatures Early Next Week

- Moderating Temps and Additional Snow Chances Late Next Week

&&

.DISCUSSION...
Issued at 256 AM EST Sat Jan 18 2025

- Lake Effect Snow Begins Saturday

Light rain across our southeastern CWA will change over to snow over
the next few hours before diminishing mid-morning. This
precipitation is associated with a cold frontal boundary that marks
the start of our arctic air intrusion.

Behind the front 850mb temps to fall into the negative mid teens
today which generating sufficient over-lake instability for lake
effect snow showers. However, 700mb ridging keeps inversion heights
aob 5kft through the day keeping the showers weak. Accumulations
will be light, under 1 inch, through the evening and mainly confined
to the lakeshore given a NNW flow pattern.

The arrival of a mid-level shortwave leads to more robust lake
effect snow beginning Saturday Night. This will be a long duration
snow event with two phases.

The first phase begins tonight with the arrival of the mid-level
wave increasing inversion heights to 10-15 kft. However, this also
coincides with the arrival of the coldest air (850mb temps in the
minus 20 to 30 range) causing the DGZ to fall to the ground below
the cloud layer. NNW flow becomes northwest by Sunday evening
keeping the most significant lake effect snow shower activity to
the lakeshore counties. Accumulations through this time will be on
the order of 4-6 inches where the heavier bands set up with locally
higher amounts possible. Have issued a Winter Weather Advisory for
all of our lakeshore counties beginning this evening for this
portion of the event.

Sunday evening the eastward progression of a plains high causes
winds to become more westerly and marking the transition to the
second phase. Combined with sufficient low-level instability lake
effect snow showers will expand further inland through later
Tuesday. Given this is 36-48 hours from now it is a bit soon to
consider any headlines for this second phase of the event.

Throughout the event small flakes will reduce visibilities, and when
the dry snow combines with 20-25 mph wind gusts blowing and drifting
snow will only make this worse. Travel impacts are likely between
the snow, blowing snow, and bittier cold limiting road salt`s
efficacy. By the time all is said and done Wednesday it is possible
some areas near the lakeshore see double digit snowfall. However
given this is spread out over several days, accumulations over any
given 24 hour window look to be limited to high-end advisory levels.

- Very Cold Temperatures Early Next Week

As alluded to in the snow section, cold will also create impacts for
the first half of next week. Behind the aforementioned cold front
temps fall into the single digits this afternoon and likely remain
there through Wednesday afternoon. Overnight lows fall near to below
zero Tuesday and Wednesday mornings, especially where any breaks in
lake effect cloud cover develop.

Areas away from lake influence will likely see wind chills of -15F
to -25F so Cold Weather Advisory potential will need to be evaluated
for subsequent shifts. While still bitter, wind chills will be
slightly "warmer" near the immediate lakeshore where lake influence
and cloud cover will be highest. There is a 20-30% chance that wind
chills in areas near the US127 corridor dip below our -25F Extreme
Cold Warning criteria if sufficient clearing occurs. Given cloud
cover uncertainty, the coldest wind chills being 72+ hours out,
and low probability of occurrence, will not be issuing an Extreme
Cold Watch. However this potential will need to be monitored over
the coming days.

- Moderating Temps and Additional Snow Chances Late Next Week

The main upper-level trough moves east Wednesday allowing highs to
climb back into the 20s for the back half of the week. Additional
snow chances are possible as several shortwaves cross the area, with
850mb temps near -10 providing enough instability for lake
effect/enhanced snowfall. Given 1000-850mb flow will be
predominantly from the southwest, the northwest portion of the CWA
will have the greatest snow chances.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1243 AM EST Sat Jan 18 2025

The low level jet responsible for the  wind shear at the TAF
sites earlier, was pulling away from the region. A cold front will
track southeastward through the TAF sites in Southwest Lower
Michigan for the remainder of the night. The winds will shift and
become northwest behind this front. An area of light
precipitation could occur along this frontal zone. Towards the
colder side, light snow will be possible and on the warmer side,
light rain. By 12z, enough cold air will have filtered in for any
remaining precipitation to be in the form of snow. Conditions
become favorable for scattered bands of lake effect snow Saturday
into Saturday night. Confidence with those bands impacting any TAF
sites was too low to feature much in the way of impacts, however
close monitoring will be needed.

&&

.MARINE...
Issued at 351 AM EST Sat Jan 18 2025

High confidence in conditions hazardous to small craft continuing
through at least Tuesday afternoon as strong winds keep waves high
with gusts of 25-30 knots. Given the very cold nature of the
incoming cold air starting later Sunday through at least Tuesday
afternoon and higher winds/waves periods of moderate to heavy
freezing spray are likely. In collaboration with surrounding
offices, have issued a Heavy Freezing Spray Watch for all
nearshore waters given the high confidence freezing spray setup.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 7 PM this evening to 1 AM EST
     Monday for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from 7 AM this morning to 11 PM EST Tuesday
     for LMZ844>849.
     Heavy Freezing Spray Watch from 1 PM EST this afternoon through
     Tuesday afternoon for LMZ844>849.

&&

$$

DISCUSSION...Thomas
AVIATION...MJS
MARINE...Thomas