Scientific Forecaster Discussion
000 FXUS63 KLSX 180905 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 405 AM CDT Wed Sep 18 2019 .SHORT TERM... (Through Late Thursday Afternoon) Issued at 404 AM CDT Wed Sep 18 2019 Early this morning, an upper level RIDGE dominated the Mid- Mississippi Valley, while Tropical Depression Imelda was now inland near Houston, Texas. At the surface, a large area of high pressure continued to be centered over the Saint Lawrence Valley with its influence extending all the way westward into our region. The extreme western periphery of this high pressure area continued to be represented by a "backdoor" frontal boundary draped over much of eastern MO, just west of the Mississippi River. A few thin high cirrus clouds were passing through, with mainly clear skies. Temperatures are once again much above normal for this time of year, with readings in most locales in the upper 60s to lower 70s. Dewpoints were only a degree or two lower. The upper RIDGE is expected to maintain its strength and remain centered near western KY through Thursday afternoon. At the surface, the "backdoor" frontal boundary is expected to remain nearly stationary today, oriented NW-SE across eastern MO, and will result in a similar setup from Tuesday afternoon: MLCAPEs between 1500-2000 J/kg, very weak 0-6km bulk shear of less than 20kts, no CINH, and weak moisture convergence near the frontal boundary. Will maintain the slight chance PoPs for eastern and central MO for this afternoon. There is even some signs of this frontal boundary maintaining enough integrity to do a repeat on Thursday afternoon. This should be the only chances for rain during this period. The remnants of "Imelda" are expected to drift north into the Arklatex region by the end of this period, but this position is expected to have no impact on our region...yet. MOS continues to exhibit a cool bias with respect to max temps in this current pattern and have forecasted maxes a few degrees above MOS values, resulting in upper 80s to lower 90s today (very close to persistence), and similar values again on Thursday. Min temperatures tonight will likewise reflect persistence, ranging from the mid 60s to around 70. TES .LONG TERM... (Thursday Night through Tuesday) Issued at 404 AM CDT Wed Sep 18 2019 Will begin to see a gradual cooling trend with high temperatures beginning on Friday and increasing pops as the upper level ridge shifts southeast of the region. The remnants of Tropical Depression Imelda will lift northeastward into our area Friday afternoon and night. Low level moisture convergence will be increasing over east central and southeast MO on the nose of a southwesterly low level jet Friday night. A moist southwesterly flow will become established this weekend with low-mid level warm air advection and increasing upper level divergence ahead of weak southwest flow shortwaves. The highest pops and QPF will likely occur from Saturday night through Sunday evening as the ECMWF model has a deepening upper level trough approaching the region with an associated cold front dropping southeastward through our forecast area Sunday night into Monday morning. Although the operational GFS model is weaker with the upper level trough and associated cold front, it still depicts relatively high QPF across our area. With high precipitable water values over the region there will be the potential for heavy rainfall possibly resulting in flooding, although flash flood guidance values will be initially quite high due to the lack of rainfall the past week over our area. Still uncertainty this far out where the axis of heaviest rainfall will set up and the highest rainfall amounts expected. Cooler high temperatures can be expected this weekend due to the cloud cover and the precipitation with highs close to seasonal normals by Sunday. There is a lot of forecast uncertainty Monday and Tuesday due to significant model differences. The ECMWF model keeps it dry as a surface ridge moves eastward through our area after fropa. The operational GFS keeps low QPF over parts of our region along and north of a weak warm front. For now will lower the pops by Monday, but keep slight chance to low chance pops going for most of our area through Tuesday. GKS && .AVIATION... (For the 06z TAFs through 06z Wednesday Night) Issued at 1042 PM CDT Tue Sep 17 2019 Mainly dry and VFR conditions are expected through the period. The only exception will be late tonight into early Wednesday when patchy fog will reduce visibilities to in the MVFR/possibly IFR range, mainly in river valleys. Winds are expected to remain light through the period. SPECIFICS FOR KSTL: Dry and VFR conditions are expected through the period with light winds. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None.