187 FXUS63 KIND 292032 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 332 PM EST Tue Dec 29 2020 .UPDATE... The SYNOPSIS...NEAR TERM...SHORT TERM...LONG TERM and AVIATION Sections have been updated below. && .SYNOPSIS... Issued at 332 PM EST Tue Dec 29 2020 A large two part storm system will impact central Indiana beginning late tonight and continuing into the first part of the weekend. The first part will be associated with a frontal boundary moving through the area...bringing rain early Wednesday into Thursday morning with perhaps a light wintry mix to end. After a lull in precipitation Thursday...the second part of the system will impact the region Thursday night and Friday with locally heavy rainfall and an increased threat for flooding. There is also potential for light icing across northern parts of central Indiana Thursday night before temperatures warm. Precipitation chances will continue into Saturday with drier weather returning late weekend into next week. && .NEAR TERM /Rest of This Afternoon and Tonight/... Issued at 332 PM EST Tue Dec 29 2020 Clouds continue to thicken this afternoon as moisture increases ahead of an approaching warm front. Still a few peaks of sun left in eastern counties but even there expect clouds will take over by 21- 22Z. Afternoon temperatures were in the 30s. Much of the near term will focus on table setting ahead of the first part of the multi-wave storm system set to impact the region into the first part of the weekend. The approach and passage of the warm front over the next several hours into this evening will bring a small threat for very light precip largely as sprinkles and/or flurries as a narrow band of moisture aligns with a stronger axis of isentropic lift at 290K and 295K levels grazing the northern Wabash Valley. Model soundings continue to show a broad layer of very dry air present below 10kft which should keep most precip developing near that axis of isentropic lift from reaching the ground which radar returns currently is ongoing over central illinois. Will carry slight chance pops for a few hours over far northern counties through about mid evening with dry weather returning thereafter as the warm front and associated lift shift north of the forecast area. The rest of the tonight will largely remain dry until the predawn hours as the deeper moisture plume associated with the cold front approaches from the west and northwest. Stronger isentropic lift will begin to overspread central Indiana after 09Z with a strong low level jet moving into the region. Will see rain increase substantially over the northwest half of the forecast area in the 09- 12Z time period as a result. Winds will become gusty later tonight as well with the approach of the front. Temps...temperatures will drop back briefly this evening before rising overnight as warm advection takes over. Expect many locations in the 40-45 range by daybreak Wednesday. && .SHORT TERM /Wednesday through Friday/... Issued at 332 PM EST Tue Dec 29 2020 Forecast challenges focus on the large two part storm system set to impact the region into the first part of the upcoming weekend. Model guidance remains in relatively good agreement on the main features over the next few days...although there has been a subtle shift back to the southeast with the second system set to track through the area on Friday. More on that below. An amplifying upper low will track across the northern Great Lakes on Wednesday before shearing out further to the northeast Wednesday night. This feature has come in stronger over the last day or two and will aid in pushing the cold front across the region late Wednesday and Wednesday night in tandem with a surface low tracking into the Great Lakes. Widespread rainfall will be ongoing across the northwest half of central Indiana at daybreak Wednesday and should encompass the rest of the area through midday. A combination of strong isentropic lift...broad diffluence aloft and a 60kt+ low level jet will transport deep moisture into the region will aid in widespread coverage of steady rainfall all day as the front moves into the forecast area from the west. The front will continue to progress southeast slowly Wednesday night with precipitation continuing along it. As colder air advects into the region from the north behind the front...there remains concern at areas of light mixing of precip types on the northern flank of the rain...most likely in the form of light snow. Any amounts that accumulate will be minimal...at generally just a few tenths of an inch and likely focused over the southern half of the forecast area in closer proximity to the retreating rainfall and deeper moisture. Expect little to no issues from a wintry mix through Thursday morning. The front will shift south of the Ohio River on Thursday with a brief but stronger area of high pressure spreading into the area. Now anticipating an extended period of mainly dry weather across all of central Indiana for much of Thursday Model soundings and RH progs continue do keep plenty of mid level moisture over the region and even as lower clouds depart in the morning...they are likely to be replaced by thicker clouds aloft as the second part of the system begins to organize over east Texas. The result will be a relatively cloudy and chilly New Years Eve for most with at least some potential for scattered light precip focused over the southern half of the forecast area. The second surface wave will ride northeast from the northwest Gulf of Mexico in tandem with a strengthening upper wave lifting northeast into the Missouri Valley early Friday and then into the Great Lakes Friday night. With a strong Gulf fetch developing on the lee side of the surface wave...widespread precipitation will spread north quickly Thursday night. Initially...soundings show a deep layer of subsidence and drier air through the lowest 10-15kft which will take time to saturate...likely needing stronger lift coming north with an intense low level jet that will peak over the Ohio Valley early Friday. Surface temperatures are likely to slip back to just below freezing levels Thursday evening...especially over the northern half of central Indiana. While a brief period of sleet near onset is quite plausible considering the evaporative cooling that will be ongoing to saturate the boundary layer...expect precip type to transition relatively quickly to liquid...with freezing rain becoming a concern for the northern counties as temperatures will likely be at 30-32. Warm advection will steadily strengthen overnight through daybreak Friday with the approach of the low level jet...but potential is there for some light icing generally north of an Attica-northern Indy suburbs-New Castle/Muncie line. Considering temps just below freezing and saturated ground from rains late tonight into Wednesday...icing impacts are likely to be somewhat mitigated. Precipitation will fall as all rain Friday with the surface wave tracking up into the region. Expect continued widespread rainfall with the 70+kt low level jet traversing the area. There has been a subtle increase in elevated instability across the area Friday that may eventually warrant the inclusion of isolated thunder into the forecast. For now...will keep thunder out and continuing to monitor trends. Rainfall coverage will become more scattered by late day Friday as the low lifts through the area. By Friday evening...am expecting 1-2 inches of rain with locally higher amounts total from the two part system. Temps...low level thermals support leaning towards the warmer MAV guidance for highs Wednesday ranging from the mid 40s to the lower 50s. As the front moves into the area from the west during the afternoon temps will begin to fall. Much cooler on Thursday with temperatures likely to hold in the mid and upper 30s. Undercut guidance with expected cloud cover all day. Friday has now come in warmer with low level thermals supporting highs for the first day of 2021 in the 50s and maybe even threatening 60 degrees in southern counties. Expect lows mainly from the upper 20s to the mid 30s through the period. && .LONG TERM /Friday Night through Tuesday/... Issued at 332 PM EST Tue Dec 29 2020 The period should start as the long-duration storm system's second part is passing central Indiana...transitioning steady rain to only slight chances of light rain/drizzle. At least moderate breezes should continue Friday night as the gradient surrounding the system's occlusion moves off into the eastern Great Lakes. Precipitation should remain all rain as the main event tapers off Friday night with temperatures staying above freezing. Despite the main storm system's departure to our northeast, broad upstream troughiness over the southern Plains will then slowly cross the Mid-West through the remainder of the weekend. The broad remnant H700 dry conveyor from the strong occluded front of Friday's system should cut off any further Gulf moisture from subsequently reaching Indiana...and moderate confidence exists best mid-level PVA should stay to the south side of the CWA. While these features appear to be guiding NBM POPs <10%...decided to introduce slight chances of precipitation across much of the region Saturday from pre- dawn to evening hours. The remnant cold conveyor will include a broad ribbon of mid-level RH over 90%...and it appears the rearrangement of the atmosphere into a weak open-wave pattern should have just enough surface convergence to shake out a few light rain/snow showers...as temperatures likely stay above 32F. Weak high pressure will build back into the region Sunday night, finally bringing a return to partly/mostly clear skies. Light to moderate south to southwesterly breezes and generally partly cloudy skies early next week will then allow a slow moderation into above normal readings, as dry conditions prevail. High temperatures should be in the upper 30s to low 40s this weekend...and the 40s early next week. Overnight lows are expected to be in the mid to upper 30's Friday night...and the mid 20's to low 30's Saturday night through Monday night. && .AVIATION /Discussion for 292100Z TAF Update/... Issued at 332 PM EST Tue Dec 29 2020 Only minor adjustments needed for mid afternoon update. 18Z discussion follows. IMPACTS: -LLWS 40-55 kts 6-18z -MVFR ceilings by 10-12z -Rain begins 10-14z -IFR ceilings with steadier rain arriving by 14-17z DISCUSSION: Significant rain event on tap for Wednesday across central Indiana will be preceded by thickening clouds today...although MVFR ceilings should not arrive until pre-dawn. Rain coverage and intensity will increase through the morning, deteriorating flight rules to IFR ceilings (and MVFR visibility), which should last though 18z. LLWS developing 6-11z, should then drop below 40kt during 15-18z. KIND is expected to return to MVFR after 18z. Winds will continue at generally 7-13 kts with gusts as high as 20 kts along/north of I-70 through 12z...while continuing to veer from the current ESE/SE direction to southerly by 9z. Winds will increase Wednesday morning to 10-15 kts with gusts 20-25 kts...while further veering to SSW/SW. CONFIDENCE: -High confidence in winds shifting to southerly late tonight -High confidence in MVFR ceilings arriving with rain by 12z -Moderate confidence in IFR ceilings beginning 14z-17z -Moderate confidence in 45-50 kt LLWS 11-18z at KIND...and in 40-50 kt LLWS 8z through at least 15z at all other TAF sites && .IND WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SYNOPSIS...Ryan NEAR TERM...Ryan SHORT TERM...Ryan LONG TERM....AGM AVIATION...AGM/Ryan