(As you might expect, this is a very long read)
Area Forecast Discussion
Issued by NWS Indianapolis, IN
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000 FXUS63 KIND 212004 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 304 PM EST Wed Dec 21 2022 .Short Term...(This evening through Thursday) Issued at 302 PM EST Wed Dec 21 2022 .This Evening - Thursday Daytime... Current satellite imagery shows high clouds streaming in from the southwest ahead of the developing system in the Plains. Light winds have turned easterly this afternoon as isallobaric response to strong upstream mid-latitude system begins. Weak warm advection is bringing temperatures again up to similar levels as yesterday into the upper 30s and low 40s. Latest guidance continues to show the 285-K isentropic surface moistening quickly after midnight tonight with ascent indicated by pressure advection on that surface, and low-level streamline analysis. Forecast soundings become saturated to a sufficient depth for drizzle, though a thin near-surface layer may lag with saturation delaying onset some. Meanwhile, warm advection should push temperatures above freezing right around or shortly after onset of light drizzle. In a reasonable worst case scenario a brief period of freezing drizzle at precipitation onset may occur but this is unlikely to be impactful due to (1) brief duration, and (2) minimal amounts. ===================================================================== SIGNIFICANT WINTER STORM EXPECTED THURSDAY AFTERNOON - FRIDAY ===================================================================== ************* Highlights: ************* WINTER STORM WARNING for counties along and north of I-70 from 3PM Thursday through 7PM Friday for: - Dangerous wind chills to -20F to -30F late Thursday into Friday - Period of high winds (40-55 mph) Thursday night through Friday - Confidence is increasing in the snow threat of 3 to 5 inches - Transition from rain to snow continues to trend earlier in evening beginning around 3 PM in western Indiana. WINTER STORM WARNING for counties in South Central Indiana south of I-70 and south of Indy Metro area 4PM Thursday through 1PM Friday for: - Dangerous wind chills to -15F to -25F late Thursday into Friday AM - Period of high winds (40-50 mph) Thursday night through Friday - Confidence is increasing in the snow threat of 1 to 3 inches - Transition from rain to snow continues to trend earlier in evening beginning around 4 PM in Southwestern Indiana ***************** Synoptic Pattern: ***************** Confidence continues to increase in a rapidly deepening low pressure system with a surface low pressure system tracking from northern Texas into the Midwest with rapid cyclogenesis as it crosses into Indiana and moves to the northeast. Model ensembles are getting a better handle on the timing and strength of this cyclogenesis with a deepening rate of 1-2mb/hr as it passes over central/northern Indiana. There is still some uncertainty as to the exact timing of rapid depletion, of which will impact where the heaviest bands of snowfall will form. A well developed cold front will pass through Thursday evening into Thursday night. There has been a trend in the higher res models to quicken the progression of the cold front, of which seems plausible. The sheer density gradient of the cold to warm air should allow for an efficient progression. Prior to frontal passage, an influx of surface moisture within weak southerly flow should all for extensive low level cloud cover. Precipitation will be likely, but a mid level dry layer will likely inhibit droplet growth. This should result in a more drizzle-like precip vs large rain drops. Temperatures will rapidly decrease following the passage of the cold front with precipitation continuing in an anafrontal region. A warm nose will wrap around the low, producing a TROWAL-like feature. This should become even more enhanced as the low deepens. Mesobands are likely to form in this region producing higher QPF. This warm nose will also prolong the depth of DGZ saturation allowing for high SLRs. This should lead to 1-2"/hr rates at times Thursday night into very early Friday morning. As the low moves off to the NE, central Indiana will be encompassed within an arctic air mass. The extreme cold temperatures in combination with some low level saturation and steep lapse rates between 1-3km could lead to snow squalls throughout Friday, especially over north central Indiana.Weak isentropic lift along the 280 K isentropic surface is evident Friday as the TROWAL remains over much of the Great Lakes region. This combined with frontogenetical forcing near the surface within a saturated layer should further promote the development of snow showers and snow squalls. This could add additional amounts to increase total snow accumulation. Any snow that does fall is likely to have lower SLRs due to the lack of dendrites and mechanical fractures of any snow flakes that due form. As mentioned, the low will rapidly deepen Friday into Friday night creating a very tight pressure gradient over the Great Lakes and Ohio Valley. This should result in high winds; sustained at 20-30MPH with gusts up to 55MPH. High winds are expected to stay elevated until Saturday, when high pressure begins to build, weakening the pressure gradient *********** Impacts: *********** Snow/Precipitation: Timing of the arrival of the arctic air looks to be between 3 PM and 7 PM Thursday with rapidly crashing temperatures. We expect a quick transition from rain to snow immediately after the frontal passage. Confidence in the exact timing is increasing as latest guidance speeds up the arrival of the front. Confidence is also high that temperatures will drop 20-30 degrees in a matter of hours. A flash freeze is likely for whatever residual precipitation is on the ground after the arrival of the colder air. This in combination with a quick transition to fairly high snow rates will lead to a fast degradation in road conditions. Exact timing of heaviest snow looks to be the few hours following the frontal passage Thursday evening. Snow rates Thursday night could reach 1-2" leading to vary hazardous conditions for anyone on the roads. Current thoughts are that snow totals will be around 3-5 inches in areas north and west of Indianapolis with a gradual gradient towards 1-3 inches across south central Indiana. The additional snow on Friday will be focused across north central Indiana mainly along and north of the I-70 corridor. While additional accumulations are possible Friday, it will be very hard to measure exact snowfall amounts as snow will likely be blowing and drifting significantly. Cold: As mentioned, temperatures will quickly drop 25-35 degrees following frontal passage Thursday night. Winds then will also pick up through the night with wind chills of 20 to 25 below zero by daybreak Friday. Temperatures will remain near steady state through the day Friday with increasing winds and wind chills approaching 30 below zero. Near zero to below zero temperatures are then expected Friday night and Saturday night with wind chills staying below -10 through Christmas. The length and severity of this cold will have compounding impacts with a 72 hour period of continual subzero wind chills. With the combination of ground snow and CAA, daytime highs will struggle to rise out of single digits this weekend. Wind: Wind speeds will begin to pick up immediately after the frontal passage with gusts of 35-55 mph through the overnight hours. Strengthening near surface level lapse rates combined with the increasing pressure gradients will then allow for even stronger winds going into the afternoon hours with occasional 50-60 mph gusts. Those stronger gusts will relax after sunset, but remain in the 30-45 mph range through the overnight hours, especially north of Indianapolis. Winds of 25-35 mph will then continue into Saturday. *********** Conclusion: *********** A deep low pressure system is expected to progress through the Great Lakes region Thursday through Saturday. This will provide widespread precipitation over central Indiana late Thursday through Friday night. Initially, p-type will be rain/drizzle, but by late Thursday it will transition to snow. The transition will be quick with the heaviest snow falling shortly after the transition. Dangerously cold wind chills, damaging winds, and blowing snow will lead to treacherous travel conditions Friday with lesser impacts going into Saturday. If planning to travel Thursday night or Friday, it is time to find an alternative plan. Regarding headlines; continuing the watch for all of central Indiana. While snow amounts may not reach the standard criteria, especially towards Vincennes and southern Indiana, the overall impacts of the winds, snow, and cold warrant the upgrade to a Winter Storm Warning. The warning also covers the full period of high weather impacts with a period where the snow is the primary impact before the period where the wind and cold becomes more impactful. && .Long Term...(Thursday night through Wednesday) Issued at 302 PM EST Wed Dec 21 2022 * Dangerous wind chills to -15F to -25F Friday night - Sunday Morning * Strong wind gusts as high as 25-45 mph through Saturday Evening The very intense, stacked winter storm system, with central pressure as low as 964mb Friday night along the Ontario-Quebec border...will slowly wobble northward through Quebec while filling...while the corresponding very broad, amplified upper trough surrounding this system envelops the eastern two-thirds of the CONUS...through early next week. The associated, amplified arctic surface high`s axis will remain positioned from interior Canada into the Gulf of Mexico...until a de-amplification of the upper pattern during Christmas Day/Night allows the surface ridge to transition eastward to the Ohio Valley. The next baroclinic system, crossing the north- central CONUS towards the end of the long term, should promote increasing southerly flow into the middle of next week. Lingering snow showers/flurries...and more importantly dangerously cold/ blustery conditions promoting blowing snow through Christmas Eve...will then transition to a generally dry and moderating pattern through the last week of 2022. Friday night - Saturday... The long term will begin with the VERY COLD temperatures as the axis of west-to-east, arctic cold-air advection holds the CONUS` lowest H850 temperatures, near -25 Celsius over the CWA and southeast across the Ohio Valley. Temperatures are expected to hold near zero Friday night. Saturday`s combination of slightly less CAA and clouds likely scattering out through the afternoon should allow temperatures to rebound closer to 10F...although at least a slight north-south gradient in temperatures should exist following greater snow and cloud cover over northern zones. 12/23 Record lows at Indianapolis are -15 (1989) and 1 (1983). Strong westerly flow will also continue as the weekend begins across central Indiana, although there will also be a slow decrease in intensity here as the deep cyclone nudges northward over through Quebec and the broad surface high continues to slowly advance into the Midwest. Sustained winds are expected to trend from around 20- 25 mph Friday evening to 15 mph by late Saturday...while strongest gusts trend from 35-45 mph to 20-35 mph, with these ranges including lighter winds towards the Ohio Valley and the noticeably stronger gradient towards northern Indiana. The snow pack will continue to contribute to blowing snow, especially near/north of the I-70 corridor where both greater overall snowfall and wind speeds are expected...as well as especially Friday evening when winds will overall be stronger. Wind chill values, while perhaps having rebounded slightly from what should be their minimum values Friday morning...are still expected to be dangerously low Friday night through at least Saturday. Confidence is increasing in the aforementioned anomalously low temperatures and strong winds...with resultant wind chill values Friday night as low as -20F to -25F...and only rebounding to -10F or so during the day Saturday. Wind Chill headlines are possible in upcoming updates as this secondary threat to the main winter storm gets closer in time. Lingering flurries are certainly possible after the sun sets Friday, especially along the region`s northern tier...yet confidence is increasing in no additional measurable snowfall for most locations as the broad head of moisture aloft continues to recede northward with the overall departing system. Blowing snow, however, will be a continued concern, especially Friday evening when the long term`s winds will be strongest...blowing snow may lower visibility and be hazardous to travel at times...with impacts to road conditions likely greatest on north-south roadways. The blowing snow threat should diminish through the day Saturday between decreasing winds and the hopes that at least partial sun promotes a modest crust atop the snow pack. Saturday night through Wednesday... A progression into a slow and steady moderation will then take shape from Christmas Day and into the middle of next week...as the center of the very broad arctic high finally slides over the region Sunday, and continues to drift to our southeast next week, bringing at least modest southwesterly return flow. The broad subsidence under this ridge will promote generally dry conditions. Although the less- amplified, yet persistent eastern-US troughiness should include a weak short wave heading southward over the central US...which could bring low/no impact snow/snow showers around the Sunday night-Monday timeframe. Very cold conditions will continue through the holiday weekend, with morning lows expected to trend from near zero Christmas morning...to the 20s by the midweek. Afternoon highs in the teens on Christmas Day should yield to better southerly flow into the midweek should finally bring temperatures near to above the 32F mark. && .Aviation...(18Z TAF Issuance) Issued at 1246 PM EST Wed Dec 21 2022 Impacts: * Lingering HZ and MVFR ceilings at LAF * Deteriorating ceilings with drizzle developing overnight, reduced visibilities at times * East-southeasterly winds gradually becoming southerly * Arrival of strong cold front with a quick change to snow towards the end of the IND TAF period Discussion: Low clouds have been diminishing through the morning so far, having cleared at all terminals except for LAF where some haze and MVFR ceilings remain. High clouds have increased today, and east-southeasterly winds are expected to gradually become southerly ahead of an approaching cold front. Eventually, MVFR ceilings will arrive as rich low-level moisture streams in from the south. Some patchy drizzle may develop as soon as 06z which may reduce visibility overnight. As moisture increases, scattered rain showers are likely at all terminals by 12z tomorrow. IFR conditions may develop as rain and drizzle become more widespread, along with lowering ceilings. These conditions are expected to persist until the arrival of a strong cold front late in the TAF period. The intense front will see a rapid transition to snow, and abrupt wind shift to northwesterly, along with a sharp drop in temperatures. && .IND WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning from 3 PM Thursday to 7 PM EST Friday for INZ021-028>031-035>049-051>057. Winter Storm Warning from 4 PM Thursday to 1 PM EST Friday for INZ060>065-067>072.
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