661 FXUS63 KEAX 121746 AFDEAX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE KANSAS CITY/PLEASANT HILL MO 1145 AM CST SAT JAN 12 2013 ...UPDATED AVIATION DISCUSSION... .DISCUSSION... Short Range (Saturday through Monday)... The main forecast concern centers on when/where the expected snow will fall. Earlier today the SREF began to shift the snow band further northwest. The 00z NAM and to a lesser extent GFS also latched onto this trend. A strong cold front is currently sliding southeast through the CWA sending temperatures below freezing. The degree of cold air advection brought in by a stiff northwest wind prevent temperatures from rising more than a few degrees. It is conceivable that some areas will hold steady for the better part of the day. A well defined positively tilted upper trough extends from the Upper MS Valley through the Desert Southwest. Attention is focused on the base of the upper trough where a shortwave and speed max will rotate rapidly northeast and into KS and OK by late afternoon. A southwest- northeast band of frontogenesis sloped between 850-650mb is expected to form early this afternoon from OK into east central KS and be enhanced by an impressive 300mb jet streak. A strong region of upward vertical motion co-located within a region of very moist air and negative equivalent potential vorticity (epv) will lead to rapid top- down saturation. This set-up favors banded precipitation which is characterized by narrow bands of enhanced snow rates. The models have trended to the northwest with the heaviest snowfall and have keyed in on a well-defined baroclinic zone centered around 700mb. What is keeping me from going with higher snow amounts is the best lift and negative equivalent potential vorticity are forecast to occur outside of the dendritic snow growth area. Should the models change their tune we will need to adjust snow amounts upwards. Have opted to issue a winter weather advisory for a relatively large area with the idea that later shifts may need to better define the region of heaviest snowfall as the bands of heavier snow form and/or the best lift and negative epv fall within the dendritic snow growth region. Felt it was best to go this route than have to play catch up later in the afternoon and issue advisories with short or non- existent lead times. This should be a rather quick snow event and by midnight the bulk of the snow should have moved out of the CWA. Cold high pressure will settle over the CWA Sunday and Sunday night. This combined with a fresh snow cover will require going a bit colder than the current forecast so have lowered temperatures a bit. MJ Medium Range (Tuesday through Friday)... Latter half of the forecast continues to be relatively quite. As Tuesday arrives a large trough will be shifting across the nation as a blocking ridge across the eastern Pacific holds strong well to our west. Resulting adjustment in the flow across the nation will evolve from the highly amplified pattern, currently in place, to a more broadly zonal flow by the end of the work week. With a dry surface high still oozing down through the Plains States, settling along the Gulf Coast, don't expect any precipitation as moisture will be long gone from our region. Otherwise, only issue to be watched looks to be temperatures. As the mean trough slides past the Plains the prevailing westerlies will shift from south of the forecast area to north during the tail end of the work week. This will allow for moderation of temperatures through the work week, though with a nominal northwest flow still nosing into the Northern Plains thoughts are that temperatures wont climb too high, staying within a category or two of seasonal normals. Cutter && .AVIATION... For the 18Z TAFs, recent model guidance still suggests the bulk of this evening's winter weather event will likely skate just east of the terminals with MKC seeings the greatest impacts of some accumulating snow. A signficant layer of dry air both above and below a narrow corridor of strong elevated frontogensis is the main prohibiting factor. VFR conditions will continue through 00Z with gradually lowering mid cloud. Ceilings should steadily fall thereafter. Basically have kept STJ dry, MCI on the fringe with MVFR to perhaps brief IFR conditions this evening, and MKC in the steadier snow this evenint with IFR conditions likely. Conditions should rapidly improve after around 07Z. Winds will remain northwest to north-northwest at around 10 knots. Bookbinder && .EAX WATCHES/WARNINGS/ADVISORIES... MO...WINTER WEATHER ADVISORY from 3 PM this afternoon to Midnight CST tonight FOR MOZ021-028-029-037-043-053. WINTER WEATHER ADVISORY from 6 PM this evening to 3 AM CST Sunday FOR MOZ016-017-022>025-030>033-038>040-044>046-054. KS...WINTER WEATHER ADVISORY from 3 PM this afternoon to Midnight CST tonight FOR KSZ057-060-103>105. && $$ WFO EAX