136 
FXUS63 KIND 240800
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
300 AM EST Sun Dec 24 2023

.Key Messages...

- Dense Fog slowly improving through this morning
- Well above normal temperatures through Tuesday
- Widespread light/moderate rainfall expected Christmas day/evening
- Light precip continues on and off through the week, some snow 
  possibly mixing in at times after Thursday

&&

.Short Term...(Today and Tonight) 
Issued at 300 AM EST Sun Dec 24 2023

Widespread dense fog with pockets of drizzle continue across 
especially the northern half of the forecast area early this 
morning...but there has been a trend towards improving visibilities 
across south central Indiana over the last few hours as drier air 
and slightly better boundary layer mixing develops with southeast 
flow. Temperatures have moved little since Saturday...with 07Z temps 
largely in the upper 40s and lower 50s.

The primary focus for the short term will be on the fog this morning 
and the ongoing dense fog advisory. There will be marked 
improvements as stronger southerly flow will finally break the lock 
holding the inversion in place...but it will take time. Once the fog 
does lift...expect a rather pleasant and mild Christmas Eve with the 
sun even likely to make an appearance later today under an 
amplifying ridge aloft. Clouds will return tonight with rain to 
follow in the predawn hours Christmas morning as low pressure over 
the central Plains pivots into the Missouri Valley.

First and foremost is the fog and headline decisions going forward 
with the morning forecast package. Satellite...obs and webcams all 
show the dense fog remains widespread particularly near and north of 
Interstate 70. Visibilities have recovered to levels at or above 3SM 
across our southern counties and there does appear to be an ongoing 
trend for subtle improvements further north from Terre Haute east 
through Greensburg. 

Will continue to collaborate with neighboring NWS offices and 
monitor obs/webcams...but giving serious consideration to shaving 
the far southern row of counties out of the Dense Fog Advisory with 
the main morning forecast package set to go out between 08-09Z. 
After that...will monitor and make adjustments to the headlines as 
needed with further improvements in visibilities expected to the 
north through the morning as the inversion weakens and areal 
coverage to fog weakens. While coverage will slowly diminish from 
the south...model soundings remain supportive of areas of dense fog 
to linger through mid morning in parts of the northern half of the 
forecast area. The expiration time of 16Z for the advisory looks 
good taking the above into consideration.

The fog will fully diminish by midday with the lower stratus to 
follow into the afternoon as the increased boundary layer mixing 
aided by stronger southerly flow will knock out the persistent 
inversion that has been in place since Friday night. With ridging 
aloft amplifying ahead of the storm system over the Plains...model 
soundings and RH progs are supportive of cloud coverage breaking and 
a period with sunshine for a few hours this afternoon in a broad 
warm advection regime. 

Dry weather continues tonight with ideal conditions for Santa and 
his reindeer on their journey as mid level clouds will slowly 
increase through the night. Deeper surge of moisture arrives predawn 
Monday courtesy of a low level jet and stronger isentropic lift. 
Rain will expand into the Wabash Valley and progress northeast after 
08-09Z Monday...with much of the rest of the forecast area seeing 
rain after daybreak Christmas Day.

Temps...the potential for sunshine this afternoon combined with the 
warm advection will enable temperatures to surge to levels of 20-25 
degrees above normal. Low to even mid 60s are on the table for areas 
near and southwest of I-74 with upper 50s further to the northeast. 
Lows tonight will remain in the upper 40s and lower 50s.

&&

.Long Term...(Monday through Saturday)
Issued at 300 AM EST Sun Dec 24 2023

Synoptically, the 250mb flow pattern is quite complicated. A 
pronounced trough is noted in the polar jet over the Rockies. 
Guidance depicts this trough inducing cyclogenesis over the Great 
Plains, roughly over Oklahoma, with the resulting low tracking 
northward. As the system occludes early Tuesday, the parent trough 
should pinch off from the northern stream and become more 
reminiscent of a cut-off low. Enough influence from the stronger 
subtropical jet should allow this feature to trek eastward over the 
course of the week. Forward progress should be slow, however, and 
the cold unstable air mass associated with the stacked low should 
provide enough instability for precip chances through Friday. Vort 
maxes rotating around the broader feature could locally enhance 
chances / amounts, but details this fine are not yet resolved in 
guidance.

Model trends in the past 24 hours have been minor, but continue to 
show an overall northwestward tendency regarding the upcoming 
system. Surface low placement at peak strength now resides, on 
average, in northwest Iowa at 00z Tue. Changes to the forecast in 
Indiana are also minor, but reflect lower QPF values as the best 
forcing drifts further north and west. Rainfall onset still remains 
around 12z to 15z across the CWA, from south to north, with precip 
rates/amounts decreasing with northward progression. Ensemble QPF is 
around a half of an inch now, and the current forecast will reflect 
that. Rain should arrive along an advancing cold front, with a 
potent low-level jet feeding moisture. Thermal profiles suggest 
rather stable low levels, so momentum transfer downward is not 
anticipated to be efficient. Blended guidance suggest wind gusts 
over 30kt Monday, but this may be overdone. 

As mentioned before, the rest of the period beyond Christmas looks 
active...at least in terms of PoPs. Overall amounts are expected to 
be light, as our main source of precip is the weakly-forced unstable 
air mass lingering overhead. Tuesday into early Wednesday sees the 
lowest PoPs, as the system's dry conveyor is progged to be over the 
region. Though some members of guidance now hint at slower frontal 
progression with lingering showers well into Tuesday. PoPs begin to 
rise again late Wednesday through Friday when the core of the 
stacked low arrives. Additionally, thermal profiles suggest that 
some snow may mix in at times. Temperatures at the surface appear 
marginal, at best, so accumulations are not currently expected. 
Though it bears watching should a more potent vort max rotate around 
the broader low at the right time.

Temperature trends through the period will be downward. Very warm 
conditions Monday will trend towards average (upper 30s for highs) 
as the stacked low drifts overhead. Some hints in guidance of even 
colder temperatures arriving in the day 8-14 range. Ensemble spread 
widens considerably by Dec 30th, but retains a faint signal of 
prolonged troughing over the Great Lakes / Eastern CONUS into 
January.

&&

.Aviation...(06Z TAF Issuance)
Issued at 1153 PM EST Sat Dec 23 2023

Impacts: 

* LIFR to VLIFR conditions in fog will continue through Sunday 
  morning
* VFR conditions returning Sunday afternoon

Discussion: 

Widespread dense fog continues across the northern half of central 
Indiana late this evening with subtle improvements to 2-3SM noted 
further south at KBMG and KHUF. The fog and very low ceilings will 
remain the primary impact for aviators the rest of the overnight and 
into Sunday morning.

The onset of an increase in southerly flow on Sunday will finally 
enable the stout inversion to mix out with improvements expected 
through midday first with visibilities then ceilings. By the 
afternoon...VFR conditions will return along with some sunshine as 
drier air advects in under ridging aloft. Mid level clouds will 
begin to increase from the southwest again Sunday night in advance 
of low pressure moving through the central Plains.

Winds will remain less than 10kts through the forecast period.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 11 AM EST this morning for INZ021-
028>031-035>049-051>057-062>065.

&&

$$

Short Term...Ryan
Long Term...Eckhoff
Aviation...Ryan