400 FXUS63 KLSX 240551 AFDLSX AREA FORECAST DISCUSSION...UPDATED AVIATION NATIONAL WEATHER SERVICE ST LOUIS MO 1151 PM CST Tue Dec 23 2014 .UPDATE: Issued at 821 PM CST Tue Dec 23 2014 Digesting the 00Z NAM and HRRR runs and combined they continue to support the going forecast with a band of rain developing overnight along and east of the Mississippi River, then mixing/changing to snow from west to east during the morning hours. Accumulation will be most likely under the heaviest and most persistent band of snow which looks to roughly align from KBLV to KTAZ. Only minor changes were made to the forecast as the placement of the mesoscale snowband and final adjustments to the forecast will be a nowcasting exercise by the next shift. Issued at 650 PM CST Tue Dec 23 2014 Quick update this evening to lower POPs, as it appears that other than some areas of drizzle across southwest Illinois, the better shower activity will not develop until after midnight. Still appears that the majority of the precipitation should fall as rain tonight, perhaps mixing with snow very late on the western fringe of the precipitation that develops overnight. Hope to have another update out by 10 PM after some of the 00Z model guidance arrives regarding any changes needed to the snowfall forecast for tomorrow. CVKING && .SHORT TERM: (Tonight through Wednesday Night) Issued at 251 PM CST Tue Dec 23 2014 The main forecast issue remains the impending storm system and potential for accumulating snowfall. The models have come into much better agreement with mass fields than they exhibited 24 hours ago. Yet, there are still some large issues with thermal fields, qpf, and speed of the system that are resulting in lower than normal confidence and higher than normal uncertainty. The overall scenario is unchanged. The upper low is in the development process within the southern portion of the deep positively tilted trof across OK. The guidance is fairly well clustered lifting the upper low northeast into southwest MO by daybreak Wednesday to near St. Louis at 18z and into the lower Great Lakes region by 00z. In response the surface low currently located along the front in LA will deepen and move through the TN and OH Vallies over the next 24 hours. Initially the warm conveyor belt and mid level frontogenetical forcing will be the key players, with rain presently across AR increasing in coverage and spreading northeast into southeast Missouri this evening and northeastward into far east central MO and southwest IL overnight. Model consensus indicates the thermal profiles should begin cooling sufficiently on the western fringe of the rain shield after 09z for the rain to mix and begin changing to snow. After 12z Wednesday the large scale forcing/ascent really ramps up in association with the ejecting mid/upper low and this should result in an upswing in coverage and intensity of precipitation from east central MO into south central and central IL, and accelerated changeover to snow. All of the CWA should be entirely snow by 18z, as the deformation zone precipitation begins shifting away from the CWA. Snow amounts are still a tough call and there is lots of room for error. The mean track of the H85 low would place the most-favored axis of highest precipitation from around Belleville to Decatur, but during some of the time the rain will be changing to snow. Also surface temperatures will be in the 32-35F range and SLRs are expected to be lower than normal with a wet snow expected. The new forecast has the highest amounts along the aforementioned corridor with a general 2-3 inches. I wouldn't be surprised to see locally higher totals, especially as you head northeastward into central IL. Given potential the impacts of slushy/slippery roads and reduced visibility on holiday travel, we have gone ahead an issued a winter weather advisory even though these amounts are generally below those we typically consider for an advisory. Some residual non-accumulating light snow/flurries may linger from east central MO into IL during the first part of the evening, however this will be a short window. The system continues its quick departure with ridging aloft already by 12z Thursday and low level warm advection underway after 06z. Glass .LONG TERM: (Thursday through Next Tuesday) Issued at 251 PM CST Tue Dec 23 2014 Pronounced warm advection and backing flow aloft in response to new upstream amplification will dominate Christmas day. The snow cover will likely be short-lived as temperatures moderate above normal Thursday into Friday. A deep and slow progressive long wave trof will then dominate the upper air pattern into the weekend featuring separate northern and southern stream components. Present indications are we should see a cold front passage on Friday night with an attendant threat of precipitation. An overrunning pattern could then set-up Saturday into Sunday with snow possible in the deeper cold air, however the models are quite variable in their depictions of the important features. Glass && .AVIATION: (For the 06z TAFs through 06z Wednesday Night) Issued at 1124 PM CST Tue Dec 23 2014 Tricky low confidence fcst for Wed. A cold front has pushed thru all the terminals this evng. CIGs should slowly lower to IFR by Wed mrng and then remain in place thru the end of the fcst prd. An upper lvl disturbance will force a sfc low across the deep south to strengthen overnight as it moves to the ern Grt Lks by tomorrow evng. An area of precip will dvlp and move towards the STL metro area late tonight. There is some uncertainty about how far west the precip shield will extend. Current guidance indicates that the precip should mix with and potentially change over to snow for a few hours Wed mrng, though new model solutions this evng are beginning to trend away from this scenario. There is some question about when or where the changeover to snow will occur. It is possible that KSUS may not see much precip at all while KCPS has accumulating snow during the late mrng into early aftn. Am not confident that any terminal in the STL metro area will see snow. If that occurs, then VSBYs are too low. Am not inclined to change the fcst until the precip dvlps. There is another upper lvl disturbance that will track into cntrl MO Wed mrng and then push east thru the aftn producing some addtnl VFR -RASN. Any precip should come to an end by evng. Specifics for KSTL: An area of precip is expected to dvlp south of the area and move north into the terminal prior to sunrise. Precip is expected to mix with and potentially switch over to all snow during the late mrng. Am not confident in this scenario as new model guidance this evng has trended away from this solution but, am inclined to leave the basic details intact until the precip dvlps and details become better defined. If the switch over to snow does not occur then VSBYs are too low tomorrow aftn. The main precip shield should lift north of the terminal by early aftn. Occnl VFR -RASN should continue thru the remainder of the aftn in the wake of this system and as another upper level disturbance approaches from cntrl MO. IFR CIGs should hang tough thru 6Z. 2% && .LSX WATCHES/WARNINGS/ADVISORIES: MO...WINTER WEATHER ADVISORY from 6 AM to 6 PM CST Wednesday FOR Jefferson MO-St. Charles MO-St. Louis City MO-St. Louis MO. IL...WINTER WEATHER ADVISORY from 6 AM to 6 PM CST Wednesday FOR Bond IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL- Madison IL-Marion IL-Monroe IL-Montgomery IL-St. Clair IL- Washington IL. && $$ WFO LSX