IWX issues Area Forecast Discussion (AFD) at Dec 14, 2:09 PM EST
269
FXUS63 KIWX 141909
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
209 PM EST Sat Dec 14 2024
.KEY MESSAGES...
- Onset freezing rain brings accretion potential in especially
Branch, Hillsdale, Steuben, and Williams counties, where a
winter weather advisory has been issued.
- Rain takes over later tonight after a transition away from
freezing rain. A second opportunity for rain arrives early
Monday morning and continues through much of the afternoon.
- Cool highs on Sunday in the 40s achieve the 50s on Monday
before falling back to more seasonable highs in the 40s on
Tuesday.
- Additional chances for precipitation occur Wednesday, when
snow could mix in with rain. Temperatures cool mid to late
week with below normal temperatures possible.
&&
.DISCUSSION...
Issued at 200 PM EST Sat Dec 14 2024
WPC analyzed a 1050 mb surface high pressure across northern New
York at 12z this morning. Analysis has shifted this eastward and
models continue to shift it eastward over the next day or two so
that it doesn't appear to stall. This is important because a
sprawling lingering portion of the high pressure system might
indicate a hard to move pool of cold air. Out in front of an upper
low pressure system comes moisture this evening and overnight. Two
big questions remain: Can we moisten the column while sub freezing
surface air remains and can we warm quick enough to limit the amount
of freezing rain accretion at the surface? Looking at the cross over
between sub freezing sfc temps and precipitation shows 3 or 4 hours
where Hillsdale, Williams, and Fulton OH could see hazardous road
conditions with counties to their west more like 1 to 3 hours. East
winds remain 10 kts sustained overnight with some gusts between 15
and 20 kts possible allowing for evaporational cooling slowing
warming. Current radar imagery shows a quick saturation with 30 to
40 dbz within the band of moisture, but a pretty stout wall of dry
exists out in front of it. That kind of precipitation rate also
would act to limit accretion as big drops splash and disrupt the
calm accretion process. A 10 degree dew point difference existed
between the moisture slug and just out in front of it. After
collaboration with neighbors, have opted to issue an advisory for
the areas where the greatest confidence for accretion exists. An SPS
covers other areas where some confidence in freezing rain
accumulation also resides.
Once the warm rain overtakes the freezing potential across area
tonight, it continues into the morning. Less of a mid level dry slot
exists in mid levels of the atmosphere on most recent model runs so
not expecting as long of a period of drizzle on the back end of this
rain. And rain could just shut off all together Sunday morning
instead. Surface temperatures look to warm above freezing even at
Hillsdale county before sunrise. Total QPF still looks to range
between 0.5 and 0.75 inch with this first batch. It has some large
scale ascent with the main band of rain, but low level dew point
values are rather low.
Sunday afternoon should see some drying as the main area of
vorticity associated with the upper low departs the area. But quick
on its heels is the next upper trough that brings our next batch of
rain late Sunday night/Monday morning. We won't have to worry about
wintry precip with this one as temperatures reach a floor around 40
degrees overnight. This one looks to have a little bit more of a
convective component with a few hundred Joules of MUCAPE available,
but large scale ascent actually passes south of the area, which
could eat into rainfall totals some. The best location for any
precipitation appears to be in our southeast where totals
between 0.5 and 0.75 inch are possible by Monday evening. This
trough also looks to tilt neutral to slightly negative as it
swings through, which may help bring additional moisture to the
area.
Another area of high pressure looks to keep the area dry from Monday
evening through Tuesday night. At this point, another trough moves
in. The ECMWF run appears to have come a little closer to the GFS,
although it is a little farther north of it yet. The main issue is
the previous departing trough deepens into an upper low, which
shunts the next disturbance southward, but how far? As a result of
this model disagreement, there still exists quite a bit of spread on
solutions with wintry precip possible at least at the onset and
departure of precipitation. The GFS is much stronger with the cold
punch behind this low pressure system and would bring a better lake
effect signal with it for Wednesday into Thursday when compared to
the weaker ECMWF.
&&
.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1225 PM EST Sat Dec 14 2024
Existing TAFs were in good shape and in line with the latest
guidance. Therefore, only a couple of small changes were made. A
surface low near Kansas City is gradually drifting northeast
while the eastern extent of its rain shield if struggling
against dry, easterly flow off lingering Northeast US high
pressure. The rainfall onset was slowed at KFWA as a result.
Cross sections and high resolution guidance favor a quick onset
of rain once saturation is reached and a period of moderate
rain with IFR conditions. Drizzle could linger beyond 15z at
KSBN, while rain tapers off at the very end of this TAF period
at KFWA as the low departs.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory from 8 PM this evening to 4 AM EST
Sunday for INZ007.
OH...Winter Weather Advisory from 8 PM this evening to 4 AM EST
Sunday for OHZ001.
MI...Winter Weather Advisory from 8 PM this evening to 4 AM EST
Sunday for MIZ080-081.
MARINE...Small Craft Advisory until 5 AM EST Sunday for LMZ043-046.
&&
$$
DISCUSSION...Roller
AVIATION...Brown
Source: IWX issues Area Forecast Discussion (AFD) at Dec 14, 2:09 PM EST (https://mesonet.agron.iastate.edu/p.php?pid=202412141909-KIWX-FXUS63-AFDIWX)
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