IWX issues Area Forecast Discussion (AFD) at Dec 17, 5:46 AM EST447
FXUS63 KIWX 171046
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
546 AM EST Tue Dec 17 2024
.KEY MESSAGES...
- Variable cloud cover this morning increasing throughout the
day. Highs will be in the upper 30s and low to mid 40s.
- There are chances for rain and snow Wednesday, then chances
for snow with a second system Thursday afternoon into Friday
afternoon. Less than 1 inch of snow accumulation is expected
during this period. Highs in the mid to upper 30s and 40s
Wednesday will fall into the low to mid 30s for Thursday and
Friday. Lows will be in the 20s and low 30s.
- Lake effect snow showers are possible through the weekend. It
will be much colder, with highs only in the 20s and 30s, and
lows in the teens and 20s.
&&
.DISCUSSION...
Issued at 532 AM EST Tue Dec 17 2024
A cold front currently over west-central Lake MI will creep eastward
through the day, possibly washing out or stalling somewhere near
KSBN this afternoon/evening as flow aloft becomes slightly
ridged/more zonal. We'll see mainly an increase in cloud cover with
this front, with pretty dense overcast just along our northern
forecast area border and Lake MI/NW IL. How far does the cloud cover
make it? Does dry air (some gaps seen well upstream over
Minneapolis) start to mix us out a little bit once the front stalls?
For now have limited the more expansive cloud cover to the west and
north, eventually filling in elsewhere overnight with the approach
of the next system. Highs today will be in the upper 30s and low to
mid 40s. Lows tonight should fall into the low 30s.
Expect increasing chances for precipitation towards Wed AM, with yet
another cold front on our western doorstep--possibly also fizzling
out. A surface low develops in the plains this afternoon and
evening, then rides the cold front ENE into KY/WV/VA. As suspected
this southern system completely sideswipes our southern CWA, with
the best moisture transport focused towards the ENE. I suspect most
of our precipitation will be from this decaying cold front during
the day, which will have to overcome initially dry air ahead of it
for quite some time-leaving a little gap in the pops from NW to SE
(SE from WAA at the northern edge of the system to our ESE). Most of
the guidance has the front clearing the eastern CWA by 00z Thu, with
a surface high building in it's wake. There could be some
precipitation off the lake late Wed evening behind the front, but
otherwise dry conditions are expected Wed night through Thu
afternoon. Highs will be in the upper 30s and low 40s Wednesday
afternoon, so we'll see a mix of rain and snow. Minor accumulations
of up to about 0.5 inch are possible on grassy surfaces in areas
that see mostly snow. Lows Wed night will be in the 30s.
A clipper system will swing through Thursday afternoon into Friday
evening when an upper level trough takes shape over the Great Lakes
region and eventually the SE CONUS. As it moves eastward the surface
low weakens and by Saturday 00z will be merging with the system
riding up the east coast. Temperatures will be in the 20s and 30s
during this period, so we'll see all snow. The moisture with this
system isn't all that impressive 9models are bouncing around), so
don't expect much in the way of significant accumulations at this
point (currently have about 0.5 inch for this system).
This will transition to lake effect as 850mb temps behind the front
drop to -10 to -12 C. How much snow we get with this lake effect
snow from Friday evening into Sunday will depend on how the larger
scale flow sets up--and we have low confidence in that given the
merging systems Friday afternoon/evening. For now, tailored pops to
the wind direction--which is looking more of N-S orientation than
previous forecasts (which had a NW lean). The ECMWF actually has the
band setting up down the long axis of Lake MI (with a Lake Superior
connection Sat AM onward) with fairly decent delta theta-e's to work
with. The GFS has a more disjointed flow and keeps the band NW-SE or
N-S over predominantly Chicago's area. Bottom line is there is still
LES potential to monitor, but there is low confidence in where the
band sets up (and if it stays fairly still or moves around).
Highs will be in the 30s Friday, then fall into the 20s and 30s for
the weekend. Lows will be in the teens and 20s, coldest Saturday
night. LES should come to an end by 6z Monday at the latest (GFS has
nearly no LES for us).
&&
.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 545 AM EST Tue Dec 17 2024
Cigs will be the primary aviation weather forecast issue to resolve
for this forecast cycle. Stratocu deck continues to slowly sink
southward across northern Illinois and southern Lower Michigan
this morning. Near term guidance does suggest some continued
slow southward sink to 925 mb baroclinic zone, that should allow
these MVFR cigs around 2k feet to work into KSBN close to 12Z.
Given little movement in this baroclinic zone today and broad
synoptic scale subsidence helping to keep inversion in place,
would suspect these cigs may be tough to erode at KSBN. KFWA
remains more uncertain, with likelihood this terminal may be on
the edge of MVFR cloud deck. Probabilistic blended guidance
does suggest the 16Z-20Z timeframe would be greatest potential
for KFWA to experience MVFR cigs. Low clouds likely to linger
tonight, especially across far northern Indiana. Mid level
moisture recovery will develop tonight as a rather disjointed
system approaches from the west. Best low level moisture and
stronger low level baroclinic zone should focus south of
terminals with this system tonight, but given increasing
elevated moisture and mid level front, will hint at VCSH at both
terminals toward end of the period. In terms of winds, some
lingering 15 to 20 knot gusts are possible this morning but
should diminish in the afternoon as low level ridge axis sinks
across the Ohio Valley.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...Small Craft Advisory until 4 PM EST this afternoon for LMZ043-
046.
&&
$$
DISCUSSION...MCD
AVIATION...Marsili
Source:
IWX issues Area Forecast Discussion (AFD) at Dec 17, 5:46 AM EST---------------
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