327 NZUS99 KLMK 181417 WRKLMK Area Forecast Discussion National Weather Service Indianapolis IN 1221 AM EST Thu Jan 18 2024 .Key Messages... - Light snow Thursday morning for North Central Indiana, minor accumulations under an inch expected - Another round of light snow Thursday night into Friday morning with minor additional accumulations expected (1 to 3 inches total) - Arctic air returns this weekend, lows near zero Friday and Saturday night with below zero wind chills && .Update... Issued at 956 PM EST Wed Jan 17 2024 Observations (including ACARS sounding at KIND) show a very dry lower atmosphere this evening. More moisture is in the mid and high levels, as could be seen on satellite imagery. Strong winds in the mid levels are already creating some lift across northern portions of the area and across northern Indiana, but this has just created some of the mid clouds thanks to the dry atmosphere below. Coupled upper jet streaks along with a decent low level jet will impact the area later tonight, producing lift aloft. It will take a while to moisten up the lower atmosphere though, resulting in virga for a while. Eventually, moisture will be enough so that some snow may reach the surface, mainly in the northwestern forecast area. Will keep some chance category PoPs northwest, with dry conditions southeast. Little if any accumulation of snow is expected. Southerly winds and increasing clouds have kept temperatures in the 20s this evening. With thickening clouds overnight, nudged up low temperatures given recent trends. For past 12Z Thursday, uncertainty remains in the forcing and moisture profiles. However, with the coupled jet streaks around early, this may provide enough forcing for the likely PoPs in the far northwest 12-15Z or so. Thus, have made no changes, and will continue to monitor. && .Short Term...(This evening through Thursday) Issued at 302 PM EST Wed Jan 17 2024 Focus shifts to the next chance for light snow across the region late tonight into Thursday morning. Little change made to the overall forecast for the tonight and tomorrow period. An elongated area of low pressure in the Plains shifts eastward tonight extending northeastward into the Great Lakes. At the same time, Indiana will be placed in an area of enhanced lift within the right entrance region of a departing jet streak and in the left exit region of a small jet streak developing over the Lower Ohio Valley. An increasing low level jet (LLJ) ahead of this system will work to pump in lower level moisture northward into the state through the overnight hours. BUFKIT soundings indicate cloud bases gradually lowering through the night as saturation through the column increases. Rising motion at the nose of the low level jet and within the upper jet streak couplet will promote deeper saturation through the column and in the dendritic growth zone. By 3-6 AM tomorrow morning, precipitation processes within a growing iso-thermal layer will lead to light snowfall over northern portions of central Indiana. This forcing will slowly move to the SE along the LLJ axis but will weaken as diurnal LLJ strength wanes. Efficient dendritic growth will likely lead to high SLRs and some accumulation even with only a few hundredths of QPF. The current expectation with this round is 0.5 to 1.0 inches, with the highest snow totals over northern portions. With a jet couplet setting up over the region in addition to a strong LLJ, potential is there for some isolated areas of heavier snowfall to set up in bands. The most likely area for this to set up is across far north Central Indiana. Expect slick travel conditions Thursday morning, especially north of I-70. The best forcing for ascent weakens during the afternoon hours as the LLJ wanes and the jet streak couplet pushes off to the east. Soundings still indicate the potential for enough left over moisture behind the departing system for scattered flurries and/or drizzle through the rest of the afternoon. With southerly flow working to scour out the retreating arctic airmass, Thursday may be the "warmest" day of the week. While the pressure gradient and LLJ weaken, weak southerly flow will in the lower levels will result in highs in the mid 20s across the north and highs trying to reach the freezing mark in South Central Indiana. Another, stronger, system will be approaching Thursday evening into Friday giving another chance for minor snow accumulations. See the Long Term Forecast for more details on that next system. && .Long Term...(Thursday night through Wednesday) Issued at 302 PM EST Wed Jan 17 2024 The primary forecast challenge through the long range continues to be cold temperatures, potential snowfall, and a pattern flip towards the end of the period. SYNOPTIC OVERVIEW Quasizonal upper-level flow exists across the CONUS today, with slight cyclonic curvature east of the Rockies. A large upper-level low pressure system is currently spinning across eastern Canada as well. Strongly negative teleconnections (NAO and AO) have lead to a blocking pattern which has caused this upper low and flow pattern to remain in place for the past several days. With the primary axis of the polar jet just to our south, arctic air has flowed freely southward contributing to the very cold weather we have seen. Continued blocking is shown by most guidance, so no relief from the very cold weather is expected through the weekend. A potent vort max embedded in the mean flow has made it onshore over the Pacific Northwest yesterday evening, and is expected to dive southeastward towards Indiana tomorrow. Favorable jet dynamics aloft (left exit), vorticity advection, and isentropic lift in the lower levels all point toward precipitation formation ahead of this system. Model soundings show saturation in through the column beginning Thursday evening continuing into Friday. Additionally, the profile through the dendritic growth zone is deep and contains a bulk of the upward motion. Dendrite production could be quite efficient, which may lead to higher ratios and thus snowfall rates. However, given the fast-nature of the system, maximum snowfall potential is likely limited to some extent. The various teleconnections do turn positive, however, by early next week. As such, a pattern shift is beginning to appear more likely. Developing split flow over the west coast is likely to initiate the shift, as a branch of the jet stream heads northeast into Canada and another dives southward into the southern Plains. Ridging builds in response across the southeastern US. Given our position relative to these features, southwesterly flow through the column is to prevail much of next week. With multiple waves rounding the southern stream trough over the Plains and then lifting northeastward within the southwesterly flow...Indiana is in a good spot for repeated rounds of precipitation throughout the coming week. Temperature profiles look warm, so mainly rain is expected though a few hours of freezing rain or sleet is possible at onset Monday night if the retreating cold is a bit too stubborn. WEATHER HAZARDS AND IMPACTS Regarding snowfall potential, widespread light to moderate snowfall appears likely across all of the area Thursday night into Friday. Heaviest snow should be concentrated along and just to the north of the parent vort max, which looks to take a track across central Illinois through central Indiana. As such, our northern counties should see the highest snowfall amounts with decreasing totals to the south. There has been a noticeable trend in latest guidance towards the north with this feature...with reduced amounts from Indianapolis southward. Should this trend continue, then even less snow is possible than currently forecast. Overall, from 1 to 3 inches, with a south to north gradient, is the current most likely scenario Thursday night into Friday. A reasonable worst-case scenario would be a stripe of 4 to 5 inches, mainly across our north. Regarding temps, strong cold air advection is shown in all guidance regardless of snowfall totals. Additionally strong winds of near 30 knots are depicted by various CAMs. Temperatures falling back into the teens and single digits, combined with the wind, may lead to prolonged sub-zero wind chills yet again Friday into Saturday. Sunday morning has the potential to be the coldest start, as surface high pressure slides in leading to great radiational cooling potential with fresh snow cover in place. Below zero ambient air temperatures are not out of the question. FORECAST CONFIDENCE Models are in unanimous agreement with the vort max diving southward tomorrow, bringing snow to the region. Confidence for snow is high. As for amounts, that is a different story. As mentioned before, there has been a noticeable trend northward today within various guidance. Given that guidance still hasn't settled on a track just yet...confidence in snowfall totals is only moderate. Still, even with the more northerly track at least an inch should fall across most of the Indy metro area northward. Confidence with cold and wind behind the system are high, and with the light fluffy high ratio nature of the snow it may get blown around quite easily. Next week, NAEFS and ECMWF situational awareness tables continue to show 99th percentile, or higher, 500mb heights and PWATs. Given the range we are looking at, this is quiet impressive and implies ensemble agreement in an anomalous set up. Teleconnections trending positive add weight to this, and confidence is unusually high regarding a pattern shift to warmer and potentially wetter conditions next week. Confidence is low regarding the fine details such as the track of individual waves of low pressure and precipitation timing/totals. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 1210 PM EST Tue Dec 5 2023 Surface low pressure centered roughly along the central IL/IN border this hour. Winds will become more westerly and then northwestly as this low moves eastward this afternoon. Have had some lingering low clouds with HUF/BMG briefly breaking out into just a mid deck. As we get to the late afternoon evening hours all sites should see low clouds return, with MVFR less than 2 kft and even IFR briefly. By daybreak Wednesday we could see some breaks in the cloud cover, but will not get too cute with TAF's at this point. && .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ Update...50 Short Term...CM Long Term...Eckhoff Aviation...Updike