Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/08/20

Area Forecast Discussion
National Weather Service Louisville KY
821 PM EST Mon Dec 7 2020 .Forecast Update... Issued at 821 PM EST Mon Dec 7 2020 A very thin blanket of stratus persists over central KY and southern IN this evening. Cloudy skies should continue through much of the overnight. Northeast of a Madison IN to Liberty KY line, we could actually see more clearing by 08-12z Tue. Several models show a pocket of drier air in the 925-850 mb layer currently over northeast Indiana dropping south into the Bluegrass Region. Have updated the sky cover forecast to boost values overall, but with more SCT clouds over the Bluegrass after 08-09z. If that works out, temps in the Bluegrass will have no trouble dipping into the low to mid 20s. However, low to mid 30s will linger further west under the clouds. Tue will see an improving trend with the clouds gradually scattering out, but forecast confidence remains somewhat low on exact timing. && .Short Term...(This evening through Tuesday) Issued at 222 PM EST Mon Dec 7 2020 Widespread stratus has kept a "lid" on temperatures this afternoon and resulted in very minimal diurnal rises. Current temps at this hour are generally in the 30s, with the warmest temperatures west of I-65 where some breaks in the cloud cover have occurred. AMDAR and model forecast soundings reveal a very thin layer (1k-1.5kft in depth) of moisture trapped near the top of the boundary layer that has resulted in this persistent stratus. We should begin to see some of this stratus break up overnight as some drier air advects in and mixes out the lower atmosphere, though confidence in when and where this occurs remains low. In general, expecting more clouds than clearing for most folks the first half of the night, and then more gradual clearing after midnight into the early morning hours. Should clearing occur quicker than anticipated, will have to keep an eye out for fog potential as T/Td spreads will be minimal and winds very light to calm. Tomorrow will start the beginnings of a warming trend that will persist through much of the week. A ridge of high pressure will slide by to the south, and winds will turn more westerly to southwesterly during the daytime hours. Highs should climb into the low to mid 40s under mostly sunny skies. .Long Term...(Tuesday night through Monday) Issued at 245 PM EST Mon Dec 7 2020 Tuesday Night - Friday... Dry NW flow will dominate the upper pattern through mid week. This will keep our area dry with a steady trend toward milder conditions. Look for 50s on Wednesday. By Thursday, a shortwave ridge will slide overhead, bringing temperatures (55 to 60F) likely a full 10 degrees above normal for this time of year. Deeper SW flow takes hold by Friday ahead of a central CONUS trough. Depending on cloud cover and timing of precipitation onset, Friday could be 10 to 15 degrees above normal with readings in the low 60s. At this point, it does look like we will stay dry through the daylight hours with increasing clouds through the day. Low temperatures will also see a steady milder trend through the week. For example, Wednesday morning lows in the upper 20s and low 30s will give way to low to mid 30s by Thursday morning. Look for upper 30s to low 40s for lows by Friday morning as a light southerly flow and perhaps some increasing upper clouds keep things a bit milder. Friday Night - Saturday... A developing surface low is expected to move from the mid Mississippi River Valley up into the Great Lakes as we enter the weekend, developing into a mature cyclone as it does so. All of this will take place ahead of an amplified trough axis moving out of the central CONUS. Will continue to forecast likely to categorical pops for Friday night into Saturday as it appears a fairly widespread line of showers (perhaps a few rumbles of thunder) will move through our area ahead of these features. Expect cooler temperatures on Saturday given the clouds/rain, but overall the colder air won`t arrive until later in the weekend into early next week. Saturday Night - Monday... Overall confidence is a little lower into the late weekend/early work week time frame as some of the finer details are in question. There is still good agreement among the some of models that that the comma head portion of the mid latitude cyclone of the Great Lakes will work back across our CWA (especially the northern CWA). Should this occur, it would bring some light precipitation back into the forecast, as well as some rain/snow mix that would likely be driven by diurnal temperatures. For now, the forecast will reflect small chances for rain or snow into the late weekend/early next week with cooler temperatures mainly in the 30s and 40s. Monday morning could feature some lows in the 20s. && .Aviation...(00Z TAF Issuance) Issued at 702 PM EST Mon Dec 7 2020 Confidence has increased in borderline VFR/MVFR stratus lingering through most of the overnight period. A pocket of dry air/clearing currently over portions of Michigan and northeast Indiana could SCT out clouds at LEX in the 09-12z time frame. Otherwise, expect at least BKN ceilings to persist. Cloud bases will lower slightly overnight, increasing the likelihood of a period of MVFR given the current ceilings. MVFR ceilings may be able to persist during the morning hours after 12z at HNB/SDF, with VFR likely at all terminals by mid-afternoon. && .LMK WATCHES/WARNINGS/ADVISORIES... IN...None. KY...None. && $$ Update...EBW Short Term...DM Long Term...BJS Aviation...EBW
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
821 PM CST Mon Dec 7 2020 .UPDATE... 820 PM CST The only updates this evening were to increase sky cover and nudge overnight temperatures up a few degrees. Stratus is expansive, with moisture trapped under an increasingly robust subsidence inversion at 850 mb, and with low-level warm advection expected to subtly increase overnight, don`t envision this cloud cover going anywhere even through the day and evening tomorrow. This may have implications on our diurnal temperature range on Tuesday and have knocked a degree off our forecast highs as a result as well. Regional TDWRs continue to show some very light returns just offshore, so the inherited mention of scattered flurries into NW Indiana look good. IR channel is sampling -10 to -11 C cloud top temperatures and recent AMDAR soundings show similar readings, so imagine this should be sufficient for ice nucleation. Even if drizzle ends up being more predominant, surface temperatures at or above freezing point to limited to no impacts, especially considering the limited coverage. Updated products have been transmitted. Carlaw && .SHORT TERM... 228 PM CST Through Tuesday night... Forecast challenges remain focused on light lake effect precipitation, and then how quickly the extensive cloud shield upstream will clear. A mid week warm-up can also been seen on the horizon! We have been dealing with very light lake effect precipitation the last few days, today included, though the forcing today is overall weak. There is still some weak surface wind convergence in the benign surface pressure field that has supported some light precip -- mainly drizzle or a few flurries. The low level flow is now quickly turning around to the northwest this afternoon and will continue this evening. An axis of clearing to the west should start to fill back in. A sheared out vorticity axis will aid in some light precipitation continuing in northwest Indiana tonight, again largely flurries or drizzle given the marginal cloud layer temperatures. Surface temperatures will hold near or above freezing so road impacts will be minimal. Following the passage of a shortwave to our west tonight, height rises and breezy southwest winds in response to low pressure to our north attempt to erode the cloud cover tomorrow. Some guidance actually tries to scatter things out briefly behind the shortwave, which could happen, but would like to see some evidence of that occurring upstream in daylight hours, and that is not occurring. Forecast soundings hold strong as the suppression is not strong enough, though it will work to try and lower the inversion and cloud depths some. We will be waiting on advection to erode the clouds from the west. This may occur late in the day or even later to occur as clouds are still fairly extensive upstream at the moment. Northwest flow aloft will then dominate the upper air pattern for the next few days, but the main forcing mechanisms will avoid the region for a little while as upper level ridging will build in from the west. The jet stream will lift well north of the area which is where the storm track will remain, and a warm advection regime from the west will ensue from the west KMD && .LONG TERM... 218 PM CST Wednesday through Monday... After a warm and dry mid week period, focus is on our first chances of widespread precipitation in a few weeks. Have not made many changes to the scientific discussion on the long term as the forecast evolution between the main deterministic and ensemble suite of models is still fairly similar to the the earlier model suite, so will highlight a few quick things here and you can see the remainder of the discussion below. The EC is still the most progressive and has a zone of mixed precip on its back side late in the weekend, the GFS tracks the system overhead, and the GEM is tracking upstream of our local area with rain and now back side precipitation. Ensemble guidance spread is also fairly large. Therefore, we have held fairly close to the model consensus. This keeps the message as follows...Expect increasing chances for rain through the day Friday with increasing southwest winds. Cooler air will wrap into the system leading to some lower probabilities of light snow accumulation on the back side of the system Saturday night into early Sunday. KMD Previous discussion unchanged... Focus in the long term portion of the forecast period remains on dry and warmer than average weather during the middle of the week, then the continuing prospects for a storm system to affect the region Friday into the weekend. Broad ridging is expected to be in place across the central CONUS by Tuesday evening with the main upper jet core and storm track off to the north and northeast of the forecast area. The upper ridge is progged to gradually flatten as it shift slowly to the mid and lower Mississippi Valley through Thursday. While the greatest mid-level heights and highest surface low pressure will remain well to our southwest and south, northern IL and northwest IN will enjoy a period of dry west-northwesterly downslope flow off the northern high plains as well as significantly milder temps as we head toward mid-December. How warm our temps rise Wednesday and Thursday look to be limited primarily by shallow boundary layer mixing with low sun angle and short daylight hours at this time of year, and by any significant cloud cover that might occur. While some periodic passing mid and high clouds are likely along the southern periphery of the upper jet, west-northwest flow without low level cold advection is not often a low-cloud/stratus producer for us. While forecast 925 mb temps would support temps approaching 60 in some spots, the shallow boundary layer mixed-depths noted in forecast soundings through mid-week suggest it would be more reasonable to expect that we`d be limited to the upper 40s far north and low to perhaps mid-50s south. Not too bad however, considering our normal highs are in the mid and upper 30s at this point in the season. As we`ve been noting the past couple of days, a weak cold front does look to sag into northern IL late Wednesday or Wednesday night which would bring an easterly component to surface winds into Thursday, which would likely limit temps near the lake across far northeast IL. As the latter part of the week approaches, attention turns to an expected amplification of an upper trough across central North America, which would likely turn our weather inclement into next weekend. The evolution of this long wave trough is dependent on the phasing of several pieces of energy during the Thursday night through Saturday period, and global models continue to struggle with the finer details of this process at this distance. Overall, models have trended toward a slightly stronger storm system over the past couple of days. The GFS and GEM deterministic 00Z runs have the surface low tracking well northwest of the cwa, across MN and northwest WI through midday Saturday, while the ECMWF remains weaker and farther south with the low track, taking it across northern IL. It should be noted that 00Z ensembles from the GFS and GEM are a little farther south than their deterministic runs, while the ECMWF ensemble is similar in track but significantly deeper than its deterministic output. Despite the model differences at this distance, guidance is in general consensus that as precipitation develops into the area from the west on Friday our thermal profile will be warm enough for rain. In fact most guidance keeps temps steady or slowly rising in southerly flow ahead of the developing surface low through Friday night. A more northerly track may even favor some thunderstorm potential in the warm sector. Colder air then looks (at this distance) to wrap in around the departing system on Saturday, with the potential for lingering precipitation to mix/change over to snow before ending or tapering off to showers in its wake. A low track farther to the south could mean a quicker changeover to snow and greater potential for accumulation before precip tapers off. Blustery and colder conditions with some flurries or snow showers could linger Saturday night into Sunday. Again, this remains several days away at this point so expectations will likely change over the next several days as elements of this system become more clear. Ratzer && .AVIATION... For the 00Z TAFs... An expansive area of stratus over the Upper Mississippi Valley will remain trapped under a strengthening subsidence inversion through the TAF period. High-end MVFR ceilings at TAF issuance will gradually lower into low-end MVFR levels by Tuesday morning, and remain at low-end MVFR levels into Tuesday night. A window of IFR ceilings cannot be ruled out Tuesday morning, particularly at RFD. Light VRB winds favoring the northwest quadrant will become west under 10 knots overnight, before veering southwest with gusts of 15 to 20 knots by late Tuesday morning. Kluber && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: