203 FXUS63 KIND 210435 RRA AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 1135 PM EST Sun Dec 20 2020 .UPDATE... The AVIATION Section has been updated below. && .SYNOPSIS... Issued at 315 PM EST Sun Dec 20 2020 Some patchy fog is possible again tonight as the low clouds return across the area. A mix of rain and snow is possible tomorrow, mostly across north central Indiana as a clipper system quickly moves through. No snow accumulations are expected at this time. High pressure will then briefly build in for midweek before a much stronger system arrives Wednesday into Thursday with rain likely and a few snowflakes possible as the system exits on Thursday. In the aftermath of the system, the coolest temperatures of the season are expected with lows in the teens Christmas morning. && .NEAR TERM /Overnight/... Issued at 1011 PM EST Sun Dec 20 2020 Higher clouds beginning to spread into the region from the west this evening while low stratus approaches from the southwest. 03Z temperatures were in the 30s with generally light southwest winds. Primary focus for the update is on the low clouds and fog concerns developing late tonight into early Monday. While the higher clouds will continue to overspread central Indiana from the northwest ahead of low pressure over the upper Midwest...monitoring the cloud deck that has been steadily progressing northeastward from the lower Ohio Valley this evening. CAMs analysis continue to advect this area of clouds and slightly deeper moisture into the forecast area over the next several hours...first reaching the lower Wabash Valley around or shortly after midnight...then expanding northeast towards the Indy metro by 08-09Z. Model soundings show lingering low level moisture becoming trapped beneath an inversion and with winds remaining light...expecting visibilities to drop in fog as the stratus deck moves in. Could be an axis of locally dense fog somewhere across the southeast half of the forecast area prior to daybreak and have beefed up fog concerns for late tonight and early Monday. As winds pick up Monday morning and some light precipitation arrives...any locally dense fog should diminish rather quickly. Zone and grid updates out. && .SHORT TERM /Monday through Wednesday/... Issued at 315 PM EST Sun Dec 20 2020 Any ongoing fog will quickly dissipate by mid morning with cloudy skies ahead of a clipper system that is expected to bring rain and perhaps a few light snow showers to the area during the daytime hours. Time heights show a brief period where mid level moisture will be sufficient for some precipitation, especially across the northern counties where that moisture is highest along with the forcing associated with the clipper system is strongest. A weak dry layer just off the surface may delay precipitation onset, but that should quickly saturday. Confidence is fairly high in the rain/snow chances across the southern counties with lower confidence south of 1-74 where moisture is very marginal. With that in mind, will have a transition from a rain/snow mix to all rain during the morning hours with best chances north and east of I-74. In areas further south, will keep just SChc categories with sprinkles across the southern counties where moisture looks too marginal for measurable precipitation. In addition to the rain, pressure gradient forces along with a strengthening 850mb jet will bring gusty winds in excess of 30 mph to the area during the morning hours and continuing through the day. Soundings show that conditions will be nearly dry adiabatic up to around 825mb which should lead to fairly efficient mixing down of the 40kt LLJ. Chances for precipitation will wrap up during the evening as the clipper system rapidly exits the area. Cold air advection related to the exiting clipper system and broad northwesterly flow will keep clouds across the northern counties through the day Tuesday with generally clear skies to the south. These clear skies will allow for temperatures to rise into the mid 40s on Tuesday with high pressure building across the area. This high pressure though will rapidly exit the are Tuesday night as a strong upper level system approaches from the west. During the day Wednesday, strong southerly flow will bring much above average temperatures to the area with highs in the low to mid 50s expected. Highs will climb into the mid 40s to low 50s on Monday followed by the low to mid 40s on Tuesday. Temperatures will then reach into the low to mid 50s on Wednesday. Lows will fall into the upper 20s to low 30s on Monday and Tuesday nights. && .LONG TERM /Wednesday Night through Sunday/... Issued at 245 PM EST Sun Dec 20 2020 Models trending faster with an Arctic system for late Wednesday into Christmas Eve. Broad and deep upper trough will move from the Plains to the Great Lakes and Ohio Valley Christmas Eve and Christmas. Models are having difficulties with where and when upper low will develop and where and when the deep forcing will spinup surface lows along the associated front. Best guess is the front will move through sometime Wednesday night and possibly as late as Christmas Eve morning. Regardless, deep moisture courtesy strong 50 knot low level jet and potential jet streak interactions support widespread PoPs by 00z Thursday with chance PoPs Wednesday afternoon. Precipitation will likely be winding down late in the Day Christmas Eve. Critical low level thicknesses and Bourgouin Method suggest the rain showers will quickly turn to snow showers in the wake of the front and from northwest to southeast Wednesday night into Christmas Eve. However, dry air works in aloft which would suggest only minor snow accumulation. Winds will also be an issue as strong surface pressure gradient will allow for 25 knot gusts on Christmas Eve. Strong cold advection in the wake of the front will make it feel like single digits Christmas morning. Good confidence high pressure and drying column will result in a dry but cold Christmas. Then, look for temperatures to moderate as the high moves off to the east and a low pressure system moves east towards the Great Lakes and Ohio Valley. $$ .AVIATION /Discussion for 210600Z TAF Issuance/... Issued at 1135 PM EST Sun Dec 20 2020 IMPACTS: Flying restrictions will develop late tonight with fog and low stratus. Expect IFR and lower at times conditions in the predawn hours with improvement in ceilings Monday afternoon. Gusty westerly winds will develop on Monday with peak gusts at 30-35kts. DISCUSSION: High clouds increasing across the region late this evening with an area of low stratus approaching from the southwest. Expect these ceilings to expand into central Indiana in the next couple hours and eventually overspread all of the terminals except KLAF by 08-10Z. Additionally...the presence of a lingering shallow inversion with light winds and low level moisture suggest visibility restrictions in fog are likely to develop late tonight as well...especially at KBMG and KIND. A weak surface wave will dive southeast through the Great Lakes on Monday...bringing scattered light precipitation in the morning followed by increasingly gusty westerly winds Monday afternoon with a tight surface pressure gradient and a well mixed boundary layer tapping into strong winds at 850mb. Gusts are expected to peak at 30-35kts on Monday afternoon. Some clearing will develop late day but not expecting low clouds to fully diminish until later in the day with additional clearing Monday night. CONFIDENCE: High confidence in MVFR ceilings late tonight into Monday. Medium to high confidence in a period of IFR and lower ceilings at KBMG and KIND in the predawn hours through the morning. Low to medium confidence in fog potential late tonight and early Monday at KBMG and KIND. High confidence in gusty winds developing Monday. && .IND WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SYNOPSIS...White NEAR TERM...Ryan SHORT TERM...White LONG TERM....MK AVIATION...Ryan