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