IWX issues Area Forecast Discussion (AFD) at Dec 14, 6:35 AM EST387
FXUS63 KIWX 141135
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
635 AM EST Sat Dec 14 2024
.KEY MESSAGES...
- Increasing chances for rain or drizzle from southwest to
northeast starting late this afternoon and persisting into
Monday night. Expect around 1 to 2 inches of rain by Tuesday
morning.
- There could be a wintry mix with freezing rain at the onset
of precipitation this evening, especially east of US 31 and
north of US 6. Minor ice accumulations are possible. Roads may
become slick, so be cautious if travelling.
- Highs in the 30s today will climb overnight and Sunday to the upper
30s and low to mid 40s. Highs Monday and Tuesday will be in
the 40s and 50s, then fall back into the 30s and low 40s
Wednesday into Friday.
&&
.DISCUSSION...
Issued at 547 AM EST Sat Dec 14 2024
A surface low will make its way northeastward today from the Central
Plains, reaching northern IL by 12z Sunday morning as it occludes.
By the 21-00z time frame today, we'll have fairly strong divergence
on the nose of an 300mb jet and a potent vort max/upper low moving
into the western CWA. At 850mb we have a decent LLJ that looks to
take aim into our area-especially between 00z-9z, which brings in
deeper moisture and warmer temperatures (850mb temps rise to 3-4 C
between 0-6z especially, then linger between 0-2 degrees until 12-
15z). By 00z tonight, surface temperatures for most areas should be
in the upper 30s and low 40s. However, locations roughly east of US
31 and north of US 6 will probably linger in the low 30s before
rising into the mid to maybe upper 30s later tonight. Road
temperatures are pretty cold thanks to our recent arctic airmass-and
it may take some time before they warm up enough to prevent slick
spots from forming on the road as precipitation begins. This is
especially the case north of US 6 and east of US 31, where air
temperatures remain colder during the day. As warm air lifts in and
saturates, we'll see rain overspread the area from SW to NE [starts
after 5 pm ET, continues into late Sun Aft/Eve]. By the time the
first round of rain ends Sunday Aft/Evening, we should have around
0.5 to 1 inch of rain.
The tricky part of the forecast-and the part we'll need to monitor
closely in the coming shifts-is the warm nose at 850 mb combined
with surface air/road temps around freezing at the onset of
precipitation. Soundings suggest that for areas north of US 6, east
of US 31 we'll see a quick wintry mix of freezing rain (or sleet if
warm layer only 1-2C as some models suggest) before
precipitation turns to all rain into the overnight and road/sfc
temps warm. The key is how much ice accumulation (if any) can
we get before it melts and rain takes over. Was on the fence
with a winter wx advisory-as my confidence in seeing a quick
wintry mix with some slick spots on the road is low to medium.
As to whether it lasts more than 1-2 hours and we actually get
ice accumulations greater than a trace...confidence was low.
After collaborating with neighboring offices, opted to hold off
on any headlines as we monitor the influx of warm air and
evaluate the morning model guidance. For now, do have some light
ice accumulations in there, but confidence is low. Some of the
guidance suggests we remain dry in these areas (especially far
NE near Hillsdale, MI and Fulton, OH) until temps rise just
above freezing. We'll see!
Dry air will move in behind the decaying low, with light drizzle and
fog in the forecast for later tonight into Sunday afternoon. We'll
have a period of subsidence beneath the incoming ridge, which will
limit precipitation chances late Sun afternoon/evening before the
next chances arrive. Highs on Sunday will rise into the upper 30s
(far NE) to the mid 40s. Lows Sunday night will be around 40
degrees.
An upper level low will move into the Dakotas by 6z Monday, with the
surface low around MN. As the upper low moves east and fills, the
broader trough becomes more negatively tilted and crosses our CWA
Sunday night into Tuesday morning [surface low in James Bay]. This
will pull warm, moist air from the gulf area into our CWA on a
decent LLJ. 850mb temps rise to 7-8C Sunday night into Monday, with
surface temps rising into the 40s and 50s by Monday afternoon.
Thankfully, we'll see all rain with this system-with totals Sunday
night into Tuesday morning around 0.5 to 1 inch. A rumble of thunder
is possible south and west of US 30 but left out of forecast for now
given low confidence. The greatest totals will probably be south of
US 30 east of I 69.
Zonal flow/weak ridging moves in Monday night into Tuesday night,
with high pressure settled at the surface. Expect dry conditions
through late Tue night (possibly into Wed AM) before another system
develops along a sharp mid level trough in the central plains and
drifts eastward into KY/TN/WV/VA by 12-18z Wed, with the cold front
associated with the system to the north (over James Bay) swinging
through our area. Depending on the low track, precipitation
associated with this system to the south could sideswipe our area.
For now have a 20-50 percent chances for rain/snow showers. Highs on
Tue will be in the 40s with lows Tue night around 30 degrees.
Highs Wed will be in the 30s.
Models disagree from Thu to Sat with differences in the timing of
high pressure exiting/upper level wave moving through-but it looks
like the best forcing is further north. Outside of some 20-40
percent pops for lake enhancement in W-NW favored areas (depending
on the model-GFS is colder than the ECMWF) and from the passing
system, expect mostly cloudy skies and dry conditions. Highs will be
in the 30s until Saturday, when they drop into the mid-upper 20s.
Lows will be in the teens and 20s, coldest Friday night.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 630 AM EST Sat Dec 14 2024
No significant changes are anticipated for the 12Z TAFs with
primary forecast challenge centering on the timing of
deteriorating conditions this evening. Mainly just some high
and mid level clouds have been working across northern Indiana
during the overnight hours, although a few patches of low clouds
have been able to form just at the leading edge of the stronger
950 mb theta-e gradient across southern Lower Michigan. With
broad low level moisture advection ongoing, cannot completely
rule out a few patches of low clouds next few hours, but will
continue to hold with VFR conditions today. Large precip shield
early this morning is co-located with a broad low level pre-
frontal confluence zone marked by strong low level moisture
transport from western Arkansas into southeast Iowa. The
primary upper level circulation lags well back to the west
across the Central Plains, so this pre-frontal forcing area
will take some time migrating eastward. Have not made many
changes to the early evening arrival time of precip at
terminals, with what should be a sharp drop off in cigs as
deeper moisture axis shifts eastward, and an eventual trend to
IFR cigs late this period. Will maintain precip mention all of
liquid variety as low level profiles should be warm enough at
terminals to overcome initial wet-bulb effects. Have also
maintained LLWS mention tonight as the upstream low level jet
axis becomes focused from the Ohio Valley to the southern Great
Lakes this evening.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...Small Craft Advisory until 5 AM EST Sunday for LMZ043-046.
&&
$$
DISCUSSION...MCD
AVIATION...Marsili
Source:
IWX issues Area Forecast Discussion (AFD) at Dec 14, 6:35 AM EST---------------
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