Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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000 FXUS63 KIND 182140 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 440 PM EST Sun Dec 18 2022 ...Updated Aviation Discussion... .Short Term...(This evening through Monday) Issued at 237 PM EST Sun Dec 18 2022 - Clear and Cold Tonight - Increasing clouds on Monday Surface analysis early this afternoon shows a large area of high pressure stretching through the Mississippi River Valley, nudging eastward toward Indiana and the Ohio Valley. Westerly surface flow remains in place across Indiana and temps were climbing through the 20s. GOES16 shows the last of the departing clouds over far eastern and northeast Indiana while sunshine was found across the western half of the state. Water vapor imagery shows NW flow in place across Central Indiana, with subsidence in place. The water vapor imagery showed Indiana caught between two large low pressure systems, one over NE Ontario and the other over the American southwest. Tonight... Clear skies and cold conditions are expected tonight. Weak ridging aloft is expected to continue to provide subsidence tonight as the surface high pressure system builds east across Indiana. Forecast soundings and time heights continue the trend of a dry column with subsidence. This will lead to clear skies and light winds, ideal for radiational cooling. Given this along with the low dew points in the teens will trend toward lows in the middle teens. Monday... Models suggest the surface high will be centered over Indiana as we start on a cold Monday morning. Aloft the models push some of the forcing dynamics over the American Southwest toward the Tennessee River Valley by late Monday, but the best forcing looks to remain well south of Indiana. The ridging aloft is shown to begin to exit to the east of Indiana along with the associated surface high pressure system by aftrenoon. At that point the 290K GFS Isentropic surface begins to show the beginning of lift arriving on the backside of the departing ridge amid warm air advection, however moisture is very limited. Thus will just lead to invading high clouds as the afternoon progresses. Forecast soundings and time heights both indicate the beginning of a top down type saturation, indicative of the arrival of high clouds. Thus we will trend toward clear in the morning but increasing clouds in the afternoon. Given the expected warm air advection will trend highs to the lower and middle 30s. && .Long Term...(Monday night through Sunday) Issued at 237 PM EST Sun Dec 18 2022 Tuesday through Wednesday... The beginning of the long term features a quasi-zonal pattern in the wake the deep low departing to the northeast. Strong 1030mb surface high pressure builds over the region Monday night, then becomes centered over the Northeast by midweek maintaining influence over the region and keeping conditions mainly dry. Quiet yet cold conditions persist Tuesday as a weak system passes just north of Indiana. Weak low pressure in the Plains falls apart considerably as it interacts with the aforementioned strong high, with marginal if any support aloft. With such a disorganized system and such strong surface high pressure maintaining influence near the surface, no impacts are expected other than increased cloud cover. Wednesday looks to be another quiet day with a weaker upper level ridging regime behind the trough passage. Moisture levels in the low to mid levels look to remain without a strong advective push, leaving cloudy weather over central Indiana. This continued cloud cover will likely lead to more seasonable temperatures with highs in the mid 30s and lows in the upper 20s both Tuesday and Wednesday. ********************************************************************* ********************************************************************* Thursday through Saturday(Potential Winter System)... Confidence remains high in the potential for an impactful low pressure system to pass through the Ohio Valley sometime between Thursday morning and Friday. With that said, there is still significant variability amongst both ensemble members and various deterministic runs regarding details for exact track and timing of the system. *************************************** Differences in Deterministic Solutions: *************************************** First, lets discuss the deterministic runs. The last few GFS runs have done a better job of representing the expected very anomalous cold sector within this system. This along with a gradual shift westward of the upper level ridge axis over the western CONUS has allowed for solutions closer to the expected conceptual model discussed in previously (see below). This leads to increased confidence that a deep low pressure system will develop over the Great Lake or Ohio Valley late next week. There will still be a significant spread in the snowfall maxima over the next several runs amongst all deterministic models, of which leads to plenty of uncertainty on snowfall expectations Thursday and Friday. Current trends in deterministic guidance point to a track further north and west, possibly tracking right through Indiana... making the forecast even more challenging with large gradients in temperature, wind, and P-type possible through the state. One trend of note is a slight delay in pressure depletion as the low tracks west to east. This is likely due to the lack of true lee cyclogenesis, and with organization of dynamic forcings becoming more aligned later in its life cycle. This will be something to watch over the next several days to see if this trend continues. Previous discussion... The GFS has continued to be an outlier, mainly due to the position of the ridge axis over the Western CONUS. The American model has had the ridge axis over the intermountain west, leading to a shorter wavelength and therefore a neutrally tilted amplified trough over the Midwest.This solution would result in more impacts over the eastern US versus the Midwest. Of note is a slightly weaker thickness gradient prior to cyclogenesis. With the high likelihood of a rather anomalous cold sector within this system this leads to the belief that this solution may be misleading. On the other hand is the remainder of the deterministic suite (Canadian, ECMWF, UKMET). These three are much more aligned on the ridge axis placement further west over the Pacific Coast. In return, the wave length is larger, allowing for a more negatively tilted upper trough and less subtropical jet influence. The polar jet in this instance in piece-wise with a strong jet streak upstream of the trough and a supergeostrophic jet streak near the ridge over the Northeast. A coupled jet streak provides an efficient environment for surface cyclogenesis, as seen near the surface of these various solutions. All three of these solutions also seem to better represent the anomalous Arctic air expected in the cold sector, increasing baroclinicity and the rate of surface pressure depletion. Another point which increases confidence in these solutions is the better run to run consistency (especially with the ECMWF) regarding track of the both surface and upper level features. While the latest run have trended a few hours slower with the arrival of the storm system, the track has been consistent showing a track right through the state. ********************************** Ensemble and Probabilistic Output: ********************************** These deterministic solutions can help dissect the probabilistic output given through various ensemble members of each parent model. With the conceptual model chosen (longer wave length, negatively tiled trough, coupled polar jet), we can narrow the expected solutions to those that align. While there is still a wide range of solutions even through this process, trends are indicating a deep surface cyclone over the Midwest or Ohio Valley Thursday into Friday. This leads to a greater likelihood of an impactful winter storm within the vicinity of central Indiana. The track of the surface low will still have a significant influence on the severity and type of impact central Indiana will witness. Any track south of central Indiana will lead to a greater chance of significant snow fall, whereas north or through central Indiana will limit snow, but increase rain and even thunderstorm chances. This process also shows a wide array of temperature outcomes for Thursday into Friday, ranging from low 50s in the warm sector, to mid 20s in the cold sector. For now, temperature forecasts are more aligned with the ensemble mean, but the result with much more likely be on the lower or higher extremes (10th or 90th percentile). No matter the track of the low, there does look to be very efficient surface pressure depletion within a highly baroclinic system. This will allow for a large pressure gradient over the region, pushing very cold air into central Indiana Friday, and continuing through the weekend. There is still some disagreement on the extent of this cold air mass, with an ensemble from -10F to 5F degrees on Saturday morning. Probabilistic output shows temperatures 3 standard deviations below normal, with a large shift of tails, indicating a potentially very anomalous cold outbreak. Snow pack will also have an influence on low temperature potential, of which would be impacted by low track as discussed previously. No matter the track, confidence is increasing in a period of high winds around the time of the wind shift to W/NW flow as very cold air aloft is advected in with a strengthening sting jet on the south of the low. High winds Friday night through Sunday along with the cold temperatures could lead to very dangerous wind chills. ********************************************** Winter Storm and Arctic Outbreak Conclusion: ********************************************** Through a diagnostic synoptic process, discussed above, trends are leaning towards an impactful low pressure system with a low track ranging from Chicago to the Ohio Valley. Many potential solutions and outcomes remain in play including the potential for significant snowfall sometime between Thursday morning and Friday evening. There are still many factors that will influence the forecast in the coming days ans this system will continued to be monitored very closely. Despite uncertainties regarding the potential winter storm Thursday into Friday, there are a few areas where forecast confidence is very high. No matter the track of the upcoming storm system, confidence remains very high that some of the coldest air of the year will arrive by the holiday weekend with temperatures potentially falling below zero! Any snowpack will further allow temperatures to plummet into the negatives and possibly into record territory. Along with the cold air will likely be dangerously cold wind chill with values possibly below -20F. High winds and/or snow could also lead to power outages further enhancing the potential risks. Take precautions now to prepare for the upcoming cold weather this holiday weekend. Areas of higher confidence: * Period of high winds with a wind shift to the NW Thurs into Fri * Extremely cold arctic air this weekend with temperatures falling below zero * Dangerous wind chills possibly down to -20F to -30F this weekend * Periods of snow showers on the backside of the system Fri into Sat Areas of lower confidence: * Exact storm track and where areas of heavy snowfall could set up * Snowfall amounts remain very uncertain at the moment, higher confidence in accumulating snow further north and west * Temperatures Thursday into Friday, could vary significantly across short distances through the state depending on track of the system && .Aviation...(18Z TAF Issuance) Issued at 439 PM EST Sun Dec 18 2022 Impacts: - VFR Conditions are expected through the TAF Period. Discussion: High pressure and a mostly dry column will provide clear skies tonight. Then, VFR flying conditions will continue but cirrus will be increasing as the upper flow flattens. Weak surface pressure gradient will keeps winds under 10 knots. Direction will start off WSW but back to S and then SE late Monday. &&
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