IWX issues Area Forecast Discussion (AFD) at Dec 18, 6:20 PM EST674
FXUS63 KIWX 182320
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
620 PM EST Wed Dec 18 2024
.KEY MESSAGES...
* Periods of light snow expected Thursday night into Friday. Snow
accumulations of a half inch to 2 inches expected, best chances
north of US 30. Minor travel impacts possible Friday AM.
* Lake effect snow showers, locally heavy, Friday night into
Saturday morning may create difficult travel, mainly in La Porte,
Starke and Berrien counties.
* Colder temperatures this weekend will give way to a moderating
trend and periodic chances for rain next week.
&&
.DISCUSSION...
Issued at 146 PM EST Wed Dec 18 2024
Marginally cold nw flow behind this morning's shortwave may allow
for a few lake effect flurries across northern portions of the
forecast area into this evening. Somewhat breezy and seasonable
otherwise with a brief period of low level ridging supporting
light/variable winds by later tonight and Thursday morning.
Attention by later Thursday into Friday with turn to the track and
evolution of a Pacific Northwest shortwave trough expected to
progress quickly east-southeast in hybrid clipper fashion during
this time. Models continue to depict two distinct vorts embedded
within the shortwave trough, with the lead vort leading to a band of
warm advection driven snow breaking out across MN/WI, and
eventually MI later tonight through Thursday evening. This
blossoming area of light snow may clip our sw MI zones Thursday
aftn-early eve with chance PoPs retained here.
The trailing, secondary vort, and associated low level reflection,
then becomes the primary features of interest into Thursday night
and Friday locally within the base of the mid-level trough. This
should allow light snow to fill in farther south into at least the
northern half of the forecast area during this time, with cold
advection and concurrent steepening lapse rates leading to healthier
snow showers on the backside of the sfc low tracking east-southeast
through far northern IL/IN on Friday. Ptype should remain
primarily snow north of the sfc low track, with lulls in precip
and a mix with sleet and drizzle along and south of the track.
As expected at this forecast range, there does remain spread in
the exact track of this system and the two distinct vorts,
resulting in low confidence on potential snow amounts and minor
impacts in time for the Friday AM commute. An averaging out of
the deterministic and ensemble model systems suggests a 1-2"
type snow event north of US 30, with less than 1" to the south.
Chilly northerly flow with 850 mb temps dropping to M12/M13 celsius
will bring lake effect snow back into play Friday night into
Saturday. Lake-850 mb temp differentials into the upper teens and
lake induced inversion heights to 7-8 kft (into at least lower
portions of the dendritic growth zone) hint at the threat for a
relatively quick thumping of LES within a developing single
convergent band Friday night. Where this band settles in remains
uncertain given lingering timing/track differences of the hybrid
clipper and how the low level wind field evolves in its wake.
Something to watch going forward regardless, especially for La
Porte/Starke IN and southwest Berrien where a headline level LES
event could emerge. This weakens and possibly shifts a bit east then
during the day on Saturday as winds begin to back/diminish and drier
air overspreads.
A moderating trend is expected next week as pacific air overtakes
the lower 48. There will also be periodic low chances for rain in
this regime as several upper level waves take aim on the region.
&&
.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 620 PM EST Wed Dec 18 2024
Postfrontal CAA supporting widespread MVFR stratus that will
likely persist through much of the period. Some minor dry air
advection attempts to erode stratus later tonight with a period
of BKN040 conditions possible. However, hi-res models suggest
lower ceilings once again during the day Thursday given abundant
boundary layer moisture trapped below strong inversion. Yet
another shortwave will bring more precip and low stratus
Thursday night just beyond this forecast window.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...Small Craft Advisory until 3 AM EST Thursday for LMZ043-046.
&&
$$
DISCUSSION...Steinwedel
AVIATION...AGD
Source:
IWX issues Area Forecast Discussion (AFD) at Dec 18, 6:20 PM EST---------------
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