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278 FXUS63 KLSX 171730 AFDLSX AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE ST LOUIS MO 1230 PM CDT Wed Sep 17 2014 .UPDATE: Issued at 1230 PM CDT Wed Sep 17 2014 Main batch of rain from MCS should remain to our southwest the remainder of the day with scattered showers being generated further north thru the MS river area, in part from influence from the MCS, slow to dissipate and wind down. In fact, it will probably take much of the afternoon, and it is not a given that something may still linger into the evening although looking pretty meager at the present time. Clouds will be very slow to clear out today, and probably only most effectively in the far eastern counties of our CWA (southwest IL) late this afternoon. Look for this clearing trend to continue from the east as high pressure`s easterly flow keeps chipping away, and once the rain clears out, should also be more effective in booting the clouds out too. Only in central and southeast MO will it be very tough, probably too tough, to fully clear out tonight. Temps will end up being cooler than previously thought, due to clouds, and have most max temps remaining in the 60s today. TES && .SHORT TERM: (Through Late This Afternoon) Issued at 347 AM CDT Wed Sep 17 2014 No changes to the Dense Fog advisory issued earlier for our south central IL counties. Some patchy dense fog is occurring further to the west into east central MO but this appears to be primarilyl confined to river valleys, and high clouds should be moving in shortly to limit the dense fog threat along the Mississippi River. Otherwise, concern for today is shower and thunderstorm threat, and increasing WAA and moisture advection finally initiated convection between 07-08z over east central KS. Low level theta-e progs suggest that this area will continue to grow in coverage (and intensity) over the next few hours, with the activity making an e-se surge during the morning generally along and especially south of the Interstate 70 corridor. While forcing and moisture advection with the low level jet would suggest most of the precipitation threat would be this morning, there is some indication in both the hi-res and synoptic models that additional development will occur this afternoon as the increasingly unstable air over the Plains interacts with the rain-cooled dome from this mornings convection. Because of this will continue PoPs this afternoon, especially over southern sections of the CWA. Still working on temps, but believe much of the area may struggle to warm up due to clouds and the potential of rain-cooled air, especially if the convection can linger for any length of time this afternoon. Welcome to the cool season... Truett .LONG TERM: (Tonight through Tuesday) Issued at 347 AM CDT Wed Sep 17 2014 There will be a continued potential for convection mainly across southeast MO tonight in an area of weak low-mid level warm air advection north of a weak warm front, and where ample low level moisture and instability exists. The threat for any convection should end by Thursday afternoon as a strong surface ridge builds southward through the Great Lakes region and into MO and IL with slightly drier and more stable air filtering southwestward through our forecast area. Warmer temperatures can be expected on Friday as an upper level ridge shifts eastward into MO with southerly surface winds on the western periphery of the surface ridge extending southwest into southeastern MO. Shortwaves will break down and flatten the upper level ridge on Saturday, and this coupled with a deepening upper level trough across the northern Plains and Great Lakes region will allow a cold front to drop southeastward through our forecast area late Saturday night and Sunday. The ECMWF model is now slower with the progression of this front compared to the GFS model. The chance for convection will return beginning Saturday afternoon as the shortwave approaches, then continue Saturday night and Sunday as the cold front moves through our area. Most of the precipitation should end by Sunday night, although the ECMWF model does still have some precipitation continuing until Monday across our area due to the slower progression of the surface cold front and 850 mb front and as shortwaves drop southeastward through our area on the backside of the Great Lakes upper level trough. Much cooler temperatures can be expected beginning Sunday night and continuing through Tuesday night as a fairly strong surface ridge moves slowly eastward through the region with upper level troffing centered over the Great Lakes region leading to upper level northwesterly flow over our area. The models drop the 850 mb temperatures to 4-8 degrees C across the forecast area by 12z Tuesday. GKS && .AVIATION: (For the 18z TAFs through 18z Thursday Afternoon) Issued at 1230 PM CDT Wed Sep 17 2014 Region of sub-VFR ceilings, whose eastern edge is near the MS river, will begin to erode this afternoon and shrink back to the west as easterly flow combined with dissipating SHRA kick in more. This will result in some intermittent MVFR ceilings across STL metro sites with much more persistent sub-VFR conditions at COU, unfortunately. May eventually see a break in COU very late this afternoon and into parts of this evening, but looking like IFR conditions are likely once again later tonight there. Depending on how well we clear out further east into STL metro and UIN, may see some fog development in an otherwise favorable setup with light east winds and damp ground. Threw out initial numbers on VSBYs with this being understood conditionally--if clearing is more substantial in those areas, will need to go more widespread IFR VSBYs. A return to VFR is anticipated across all sites by midday Thursday, except perhaps COU. Specifics for KSTL: Intermittent MVFR ceilings likely for the next 2-3 hours before easterly flow and time of day should push the eastern edge backward. Period of VFR then expected for late this afternoon and early this evening before some fog is expected to develop. How well we clear out will determine how bad the fog gets as this is an otherwise good setup for it given light easterly flow, lack of previous day mixing, and recent rainfall. A return to VFR then expected late Thursday morning. TES && .LSX WATCHES/WARNINGS/ADVISORIES: MO...NONE. IL...NONE. && $$ WFO LSX