955 FXUS63 KLSX 312150 AFDLSX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE ST LOUIS MO 350 PM CST Wed Dec 31 2014 .SHORT TERM: (Through Late Tonight) Issued at 350 PM CST Wed Dec 31 2014 West-southwest flow has developed across the area on the northeast periphery of the Arctic high which is now centered over northeast Oklahoma. Southwest flow will continue tonight, with variable cloudiness. Clouds over southwest Missouri are spreading east between 850-700mb at this time. There isn't much in the way of model guidance that's handling the clouds right now, but indications are that the clouds will overspread much of our southern CWFA before midnight and then should thin out somewhat overnight as some drier air punches south at those levels in cyclonic flow aloft. Between the southwest flow and the clouds, expect temperatures to be between 5-10 degrees warmer tonight than last night. Carney .LONG TERM: (Thursday through Next Wednesday) Issued at 350 PM CST Wed Dec 31 2014 A stagnant pattern of a longwave RIDGE over western North America and a TROF over eastern North America is expected to persist...with some ebbs and flows...over the next seven days. This pattern will be very favorable for regular intrusions of Arctic air. (Thursday - Friday) A well moderated airmass will be in place during this period, featuring seasonable temps with maxes in the 30s and mins mainly in the 20s. There is much better model agreement on how to handle the initial wave that kicks out from the SW CONUS storm, maintaining the position of the weak upper RIDGE to our southwest and thereby shunting the light pcpn from this to our south. Have maintained slight chance PoPs for parts of SE MO and S IL...south of a Farmington MO to Chester IL line...Thursday night and Friday for what could be a light wintry mix, consisting of mainly FZRA and PL, but it will be tough to get anything measurable as it stands now. (Friday Night - Saturday Night) The models have maintained their good consistency on track, while tightening up significantly on the timing as well. The overall track has trended a tad more northerly as well, with a new surface low triple-point to track thru SE MO and S IL into northern Indiana by late Saturday evening. There will be two phases to this system, the first will be primarily driven by WAA with the weak upper RIDGE axis expected to pass thru to our north on Friday night, this should give the full go-ahead to the pcpn set to advance northward. Reasonably good confidence exists on the northern edge of the pcpn reaching I-70 by daybreak Saturday with decent evidence of a more western surge pushing thru central MO and into northeast MO by this time as well. Pcpn will then overspread the entire area for a period on Saturday morning with a dry slot then poking thru late morning and afternoon. Pcpn-types with this first phase will be primarily liquid, with some frozen on the northern edge. The location of the 32F isotherm will be critical to the forecast with FZRA being a likely possibility for most locations for a short time, and considering how the surfaces may be decently cold from this last Arctic airmass, another thing to consider is the possibility of FZRA for a longer period of time. The second phase of the storm will begin to take shape for our region on Saturday evening with a weak deformation zone of pcpn that will swipe thru, mostly for areas to the north and west of STL metro. For a period during the evening, the warm wedge of air aloft will be strong enough to maintain the predominant liquid nature of the pcpn, with some frozen perhaps holding on in the far north around UIN, but with passage of the upper system center and surface cold front, this warm wedge is then anticipated to collapse rapidly turning the pcpn-types into a R versus S question for late evening and overnight. Snow will have the best chance of being seen where this cold air can reach first before the def zone pcpn exits, meaning a COU-UIN axis. Putting it all together, it looks like minor ice accums from FZRA of less than a tenth of an inch are likely for a broad swath of the forecast area late Friday night and Saturday. Snow accums, perhaps as high as 3", could be seen in parts of NE MO and west-central IL, but it is walking a fine line depending on how far north that warm air aloft gets. (Sunday - Next Wednesday) Another intrusion of Arctic air will follow the storm on late Saturday night and Sunday morning and persist into Monday. A weak system will squirt thru Monday night and early Tuesday, warranting a low PoP for snow, with another Arctic invasion set for Tuesday. Below normal temps...if not well-below normal...look to be the rule here with any pcpn types, chances as meager as they are now, set to be snow. TES && .AVIATION: (For the 18z TAFs through 18z Thursday Afternoon) Issued at 1124 AM CST Wed Dec 31 2014 VFR flight conditions and west-southwest flow will prevail through Thursday as high pressure remains in control of the region. Specifics for KSTL: VFR flight conditions and west-southwest flow will prevail through Thursday at Lambert as high pressure remains in control of the region. Carney && .LSX WATCHES/WARNINGS/ADVISORIES: MO...NONE. IL...NONE. && $$ WFO LSX