Area Forecast Discussion
Issued by NWS Indianapolis, IN

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154 FXUS63 KIND 191950 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 350 PM EDT Mon Apr 19 2021 .Short Term...(This evening through Tuesday night) Issued at 349 PM EDT Mon Apr 19 2021 FREEZE WARNING IN EFFECT FOR WEDNESDAY MORNING Expecting lows Wednesday morning of 28-31F, so have decided to upgrade Freeze Watch to a Freeze Warning for entire CWA, effective 1000 PM Tuesday through 1000AM Wednesday. The period will feature both a regime change from seasonably mild this afternoon to near-record cold late Tuesday night...as well as a rare, perhaps record, late-season accumulating snow. This afternoon`s breezy, yet pleasant spring-like weather will be the warmest of the workweek. A slow cold frontal passage tonight will precede the arrival of deepening trough over the upper Mississippi Valley. The front will stall over the Ohio Valley Tuesday...while the H500 trough`s leading vort max progresses from positive to neutral tilt, while approaching the region...resulting in cyclogenesis and a quick rain to mix to snow maker late Tuesday and Tuesday night. This evening`s frontal passage will diminish winds as they turn northerly. Increasing clouds will envelop the region, although only the northern zones can expect slight chance/chances of rain showers through noon Tuesday. Organized rain will develop over central Indiana Tuesday afternoon...from northwest to southeast...as the surface low forms while sliding west-to-east through Kentucky along the frontal boundary. By 00z Tuesday evening, likely chances of rain will have reached the southeastern corner of the CWA, while the arrival of colder air will have begun to mix-in wet snow north and west of Indianapolis. The mixture with wet snow will push into southern counties through the evening, with a complete changeover to snow expected along and north of the I-74 corridor by 6z. Precipitation intensity will be light to moderate...with embedded bands of moderate to briefly heavy snow, especially north and northeast of Indianapolis where frontogenesis aloft will be strongest. Snow will decrease in intensity and taper off from west to east in the pre-dawn hours. Confidence is building today for the snow threat given few dModel/dt changes...and the few small trends are indicating a slightly faster/colder push during the rather brief period of snow...with slightly more impressive QPF in the 0-6z period. Also favoring confidence in the event are the associated vort diving a tad farther south into the region, while sliding east through the southern Great Lakes...as well as the struggling dry conveyor on the back side of the system not pushing as far north/east into central Indiana...which would promote a slightly longer period of moderate snow Tuesday night. All the while temperatures will be dropping through the 30s Tuesday night. Snow will first struggle to accumulate due to antecedent warmth, but the dark of night, and adequate snowfall rates should overcome melting on grassy and elevated surfaces. At least a thin coating is likely for most locations, with 1-2" possible to the north and northeast of Indy where the longest duration and best chance of heavier bands will be found. Low confidence exists in intensity and location of said bands, and therefore also accumulation efficiency...although locally 2-3" is not out of the question at this time. However, once snow begins to taper off the warm ground will likely melt whatever snow pack exists from the ground-up, despite temperatures near/slightly below freezing...limiting what are already expected to be only minor impacts to morning commute. Nevertheless areas of slick roads will be possible early Wednesday morning, especially in the 9-11z time frame where road temperatures may briefly drop to the low 30s, mainly north of I-70. Temps tonight will drop to the upper 30s to near 40F. High temperatures Tuesday will range from the mid 40s around Lafayette to the low 60s along the southern tier....before readings drop to near 30F on the heels of the rain-to-snow event by Wednesday morning. The normal max/min for the short term period is 65/45. Record low for the Indianapolis Area Wednesday morning is 28F...current forecast low at KIND is 29F. The record daily snowfall amounts in Indianapolis for Tue/Wed are T/0.7". Tuesday`s trace (from 1953) is in jeopardy. && .Long Term...(Wednesday through Monday) Issued at 300 PM EDT Mon Apr 19 2021 Upper energy rotating through an upper trough will work with a surface trough to bring chances for precipitation on Wednesday. Could be a mix of rain and snow/graupel given the cold air aloft. Otherwise it will be a cold and brisk day with temperatures well below normal. Question then turns to how cold temperatures will get Wednesday night. GFS holds onto clouds and keep temperatures warmer, while others have less clouds and colder. Will go colder than the raw GFS numbers and close to consensus of others since expect some partial clearing. A hard freeze is possible, especially northeast. Will have to watch closely. Dry conditions will return Thursday with high pressure in control. Some frost is possible Thursday night with lower winds and some areas seeing temperatures in the mid 30s. Not quite phased northern and southern stream systems will bring rain chances to the area Friday and Saturday. High pressure will bring dry weather Sunday and Monday. Below normal temperatures will continue through much of the period, but with a moderating trend. By Monday though readings could be above normal and approaching 70. && .Aviation...(18Z TAF Issuance) Issued at 140 PM EDT Mon Apr 19 2021 IMPACTS: -W/SW wind gusts continuing at 20 to 30 kts this afternoon and early evening -MVFR ceilings develop overnight and drift south in spots as a cold front settles over the area -Winds will veer to northerly with the frontal passage, while diminishing to 5-7 knots by 12z Tuesday. DISCUSSION: VFR will continue to prevail this afternoon across central Indiana under high pressure, although robust west-southwesterly winds will also persist through early evening...courtesy the well mixed boundary layer, as well as a tightened surface pressure gradient per a cold front that is approaching from our north. The front will drop into the northern Wabash Valley this evening, then drift south slowly across central Indiana tonight...bringing expanding areas MVFR ceilings southward behind it. Showers will stay in the vicinity of KLAF after 00z this evening, especially to the north...although showers are not expected to effect any other sites through 18z Tuesday. Nevertheless at least moderate confidence exists that MVFR will be become established across the region during 10-14z early Tuesday. Behind the frontal passage, Winds will veer to northerly behind the frontal passage by 6z, although speeds will decrease to 5-7 knots by dawn Tuesday. && .IND WATCHES/WARNINGS/ADVISORIES... Freeze Warning from 10 PM Tuesday to 10 AM EDT Wednesday for INZ021-028>031-035>049-051>057-060>065-067>072. && $$ Short Term...AGM Long Term...50 Aviation...AGM

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