562 FXUS63 KIND 181033 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 633 AM EDT Sun Apr 18 2021 ...Updated Aviation Discussion... .Short Term...(Today through Monday) Issued at 250 AM EDT Sun Apr 18 2021 Skies were mainly clear early this morning over much of central Indiana with pockets of mid level clouds focused over western portions of the forecast area. Winds have remained up a bit higher than previously thought and that has hampered temperature drops to this point. 0630Z temps ranged generally from the upper 30s to the mid 40s. The initial concern for the next few hours will be on frost accrual focused primarily across the northeast counties. Skies will remain clear through daybreak with light northerly flow allowing temps to fall into the mid and upper 30s. Will continue with the frost advisory in its current location through 13Z. Focus then shifts to a weak low pressure across central Illinois this morning with a trailing boundary. This feature will shift southeast into the forecast area this afternoon and evening. The surface wave and boundary will have a wave aloft aiding in forcing along with the development of a weakly unstable airmass over the region by the afternoon. With model soundings indicating steepening lapse rates through around 800mb as well...anticipate isolated to scattered convection during peak heating this afternoon and evening. Have introduced an isolated thunder mention...focused mainly south of Interstate 70 as well. Scattered convection will likely linger into the evening but expect all of the rain will be out of the forecast area prior to midnight as the upper wave axis moves off to the east. Skies will clear overnight as a combination of ridging aloft and surface high pressure expand into the Ohio Valley behind the upper wave. This will set up a warmer and breezy Monday as southwest flow strengthens ahead of a developing cold front that will drift south into northern Indiana by late day Monday. After a sunny morning... diurnal cu will form Monday afternoon with additional mid level clouds moving into the area by late day ahead of the southward sagging cold front. Model soundings show a well mixed boundary layer Monday afternoon with peak gusts at 25 to 30mph likely. Temps...An overall model blend will work quite nicely through the period as low level thermals support highs today in the low to mid 60s and on Monday in the mid to upper 60s. Could see some overachieving to temperatures Monday as the forecast area aligns within an area of compressional heating ahead of the frontal boundary. Lows tonight will be a touch warmer than recent nights with much of the area remaining in the low to mid 40s. && .Long Term...(Monday Night through Saturday) Issued at 250 AM EDT Sun Apr 18 2021 Primary focus for the period will be on the potential for snow Tuesday night and the colder temperatures mid-week. During the daytime hours on Monday, a weak surface front associated with an upper level low in southern Canada will be passing through the area and exiting by late Monday night. The airmass associated with the front is polar in origin and lacking moisture, so no impacts are expected other than a surface wind shift. During the daytime hours on Tuesday, a stronger trough will be digging into the Central Plains with a broad area of mid level ascent west of the forecast area. There may be a few rain showers across the area during the daytime hours with some indications of an area of frontogenesis at around 700mb, but current thoughts are the majority of the precipitation should hold off until the evening to overnight hours when the bulk of the forcing arrives. CAPE is non-zero during mid afternoon on Tuesday, so can't rule out a rumble or two of thunder, but confidence is far too low for a mention in the forecast at this time. By late Tuesday into Tuesday night, very strong CAA will lead to a period of plummeting temperatures with a drop from the mid to upper 50s down to near freezing over a period of 4 to 8 hours. Model guidance continues to show good chances for snow across the area Tuesday night, but with a surface well above freezing, any snow accumulations would have to be during periods of heavy snow. The NAM solution makes the most sense out of major model solutions with a sharp cut-off of snow accumulations across the northern counties vs heavier accumulations from the GFS and Euro. While 2-4 inches of snow may fall, those numbers won't be realized due to melting and the more likely outcome will just be the light accumulations in grassy and elevated surfaces with rapid melting occurring during the morning hours. Going into Wednesday and Thursday, much cooler temperatures arrive, but not as cold as models suggest due to a feedback loop from model guidance assuming there is snow on the ground. Do think a freeze is likely across the area, but don't buy the mid to upper 20s that models suggest for Wednesday night. The potential for cloud cover associated with a weak secondary low and fairly strong winds that should allow for some mixing down of slightly warmer temperatures off the surface which further lowers the threat for a hard freeze. Temperatures will begin to moderate Thursday into Friday before a clipper system moves in from the northwest bringing periods of light to moderate rain to the area Saturday. Headlines for either frost or freezes will likely be needed for Tuesday night and Wednesday night with temperatures dropping either near to below freezing. With active precipitation and fairly strong winds on Tuesday night, frost formation will be limited, but those conditions become much more favorable for Wednesday night other than the wind speeds remaining near 10 mph. Another period of frost looks probable Thursday night although temperatures are not expected to be quite as cool as the previous nights. && .Aviation...(12Z TAF Issuance) Issued at 633 AM EDT Sun Apr 18 2021 IMPACTS: - Scattered showers this afternoon and evening may produce brief restrictions at all sites but KLAF. - Otherwise VFR conditions expected throughout the forecast period. DISCUSSION: Mid and high level clouds beginning to drift into central Indiana early this morning as low pressure resides over central Illinois. The low and an associated boundary will drift into the region this afternoon and weaken in tandem with an upper level wave. The presence of steepening low level lase rates and weak instability will promote scattered convection this afternoon and evening focused mainly near and south of I-70. As the upper level wave shifts east tonight...any rain chances will end with skies gradually clearing by daybreak Monday. There is some potential for patchy fog during the predawn hours Monday but confidence is too low for inclusion at this time. Winds under 10kts will back from N/NW early this morning to southwest this afternoon and S/SW tonight. && .IND WATCHES/WARNINGS/ADVISORIES... Frost Advisory until 9 AM EDT this morning for INZ021-031-038- 040>042-049. && $$ Short Term...Ryan Long Term...White Aviation...Ryan