FXUS63 KIND 040601 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 101 AM EST Wed Dec 4 2024 .KEY MESSAGES... - Low chance for freezing drizzle over mainly the southwest half of central Indiana late tonight and early Wednesday - Snow showers Wednesday night with some travel impacts possible. - Wind chills as low as -5 degrees Thursday morning - Warming trend this weekend and rain early next week && .FORECAST UPDATE... Issued at 943 PM EST Tue Dec 3 2024 Stratus remains stubborn across the forecast area this evening but it is steadily diminishing from the southwest and southeast as warm advection and return flow has developed on the back side of high pressure centered over the central Appalachians. Temps have moved little this evening under the stratus...largely holding within a range from the upper teens to the mid 20s. Bit of a complex forecast overnight as warm advection strengthens and interacts with a narrow axis of moisture currently moving across Missouri. The stratus accompanying the moisture plume has been slowly decreasing in coverage as it moves into the mid Mississippi Valley but several of the CAMs have that increasing over the next 4- 6 hours as it approaches the region...influenced by an increase in winds within the 925-850mb layer and a noted uptick in a band of isentropic lift that is weak but present. Subsidence above the boundary layer will aid in a strengthening but shallow inversion and while the stratus over the region currently will continue to lift to the northeast and likely lead to a period with mainly clear skies after midnight...the arrival of the stratus to the west will expand back across the forecast area after about 07Z. The subtle uptick in isentropic lift noted above in tandem with the increasing warm advection regime presents a non-zero threat for patchy drizzle to develop. While temps will slowly rise overnight... readings will remain in the mid to upper 20s during the predawn hours and through daybreak with any drizzle freezing to surfaces on contact. Will continue the freezing drizzle mention focused especially over the southwest half of the forecast area...but have expanded the mention slightly further to the north and east towards daybreak. Still think any fzdz will remain generally south and southwest of the Indy metro but will continue to monitor trends over the next few hours and adjust as needed. Any amounts will be light and the overall freezing drizzle will be patchy at best...but even trace amounts will be enough to cause some slick spots on roadways. Focus for fzdz will largely exist between 08 and 13Z from west to east. Zone and grid updates out. && .SHORT TERM (This evening through Wednesday)... Issued at 228 PM EST Tue Dec 3 2024 A 1038mb surface high is still centered just SW of central Indiana, most of which is collocated with a low stratus layer. Within this area of high pressure, subsidence and weak winds have acted to reinforce the stratus layer, keeping it around through the afternoon and should continue to remain into the evening. There are already some hints at mixing over NE KY, of which could erode SE IN stratus earlier than currently in the forecast. This increase in cloud cover led to a dampened diurnal curve this afternoon, which has slightly suppressed afternoon highs along and south of the I-70 corridor. This will also suppress nocturnal cooling as well with temperatures staying in the 20s overnight. Surface high pressure and low level ridging is expected to push southeast this evening and overnight, resulting in continued warm air advection just above the surface. Surface temperatures will be slow to respond however, so a strong near surface inversion is expected to develop. A low level moisture surge is pushing north tonight below broader scale subsidence. This is expected to result in a relatively thin low stratus layer over SW portions of the state. As mentioned WAA will increase overnight, and could induce some lift through this stratus layer. HREF models are widely spread on the magnitude of moisture advection into central Indiana as well as have a wide spread in location of saturated layer with respect to vertical lift. With this said there is a low chance(20-30%) of this saturated layer coinciding with the vertical lift. If this occurs light drizzle may begin to develop. Usually this would not be impactful, however with surface temperatures lagging beneath the WAA, this drizzle may freeze on contact with frozen objects/ground. Overall this shouldn't result in more than a few hundredths on ice accumulation, but that can still produce slick spots on roadways. Greatest chance of occurrence will be over SW central Indiana between 4AM and 7AM. Once this stratus layer pushes off to the east, the rest of Wednesday should be rather quiet as temperatures quickly rise beneath strong WAA. Broken cloud cover is likely within this WAA, but precipitation should be minimal until a shortwave trough approaches from the north. Most of the precipitation associated with this low should be after 00Z (7PM) but a light rain or even rain/snow mix is possible as the frontal boundary nears, mainly over far NE portions of central Indiana. Further discussion on precipitation after 00Z Wednesday evening is in the Long Term. && .LONG TERM (Wednesday night through Tuesday)... Issued at 228 PM EST Tue Dec 3 2024 Steep low-level lapse rates and residual moisture behind the cold front will result in snow showers Thursday night, until the mid- upper level trough axis passes later in the night. These could be briefly/locally intense causing travel impacts from some accumulation and visibility reduction. Post-frontal winds of 20-25 mph with gusts 35-40 mph are possible. Strong cold advection and MSLP gradient supporting 10-20 mph winds early Thursday morning will lead to wind chill values as low as -5F. Models have collectively trended more anomalous and therefore stronger with the Great Lakes shortwave trough departing on Thursday. Resulting sensible weather changes for us will be minor, but it strengthens the case for lingering stratus with perhaps stronger more cyclonic low-level flow remaining until Thursday night. Residual moisture below subsidence inversion this time of year often is more capable of stratus than models explicitly indicate. Will trend sky cover up through the day Thursday. Thursday night, midlevel clouds tied to a low-amplitude shortwave trough may limit radiative cooling. If these are less prevalent or delayed, this would increase the radiative component resulting in colder temperatures overnight. Cold advection with tempered radiative component would still result in low-mid teens for minimum temperatures, but with less wind and thus wind chill values not as cold as Wednesday night. Multi-model ensemble shows mid-upper ridging and anomalous mean heights this weekend, signaling a warming trend. We should return to near normal temperatures Saturday and exceed normal values Sunday into Monday as ridge becomes optimally positioned for warmth ahead of deepening central CONUS trough. Ensemble clusters start to fall out of phase by early next week with a more progressive GEFS- weighted camp and lagging EPS-weight camp. As the trough moves east, models differ in handling of a southern stream shortwave that will move northeast ahead of it. There does appear to be a strong IVT signal with broad moisture advection into the region and ~2-sigma PWAT anomalies. This should support a decent rain event sometime in the Sunday night through Tuesday morning window. Right now, multi-model ensemble mean is generally 0.50-1.00 inch, with some outlier members over 1.00 inch. Higher amounts would be possible if the moisture trajectories aren't more suppressed into the Ohio Valley. Too much chaos in the ensemble data with southern stream shortwave characteristics at the moment to narrow amounts down. In the Day 8-14 period it appears we'll see a colder trend initially followed by a signal in the medium-range models of a warming trend late in that period. && .AVIATION (06Z TAF Issuance)... Issued at 101 AM EST Wed Dec 4 2024 Impacts: - MVFR ceilings becoming VFR early morning - Low Chance -FZDZ at KHUF and KBMG early Wednesday morning - Low level wind shear impacting all terminals predawn through mid morning Wednesday - W/SW winds gust speaking at 25-30kts Wednesday Discussion: MVFR stratocu deck is finally eroding from SW to NE with the back edge of it along the I-74 corridor. Expect a brief hour or two period of relatively clear skies before the next area of VFR stratus tracks east across Indiana. This second area of stratus could produce patchy light FZDZ at both KBMG and KHUF for a couple hours with an increasing shallow inversion present but confidence in occurrence remains low at this time. With how cold surface temperatures are, it would not take much FZDZ to result in slick surfaces. The expansion of a jet at 925mb late tonight supports the potential for a 3-5 hour period with low level wind shear impacting the TAF sites after 08-09z. As surface winds increase later this morning, courtesy of a tightening surface pressure gradient ahead of an approaching cold front, low level wind shear concerns will diminish. Wind gusts will peak at 25-35kts this afternoon as winds veer from southwest to westerly by late day with the frontal passage. There is potential for a period of snow showers immediately behind the frontal passage this evening that may result in periods of MVFR or lower vis and cigs. && .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Ryan SHORT TERM...Updike LONG TERM...BRB AVIATION...CM