258 FXUS63 KLSX 180815 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 315 AM CDT Tue Jul 18 2017 .SHORT TERM... (Through Late Tonight) Issued at 314 AM CDT Tue Jul 18 2017 Focus today continues to be temps, dewpoints and resultant HI values. Mdls are in general agreement for today, tho some important differences do exist. The upper ridge is progd to strengthen and build ewd into the region today, with the 850mb thermal ridge also building ewd slightly. The sfc ridge will retreat slightly allowing the weak sfc fnt to move nwd with winds becoming sly to perhaps even swly across wrn portions of the CWA. This presents the forecast problem for today. Dewpoints are expected to pool along the fnt similar to Mon. Have raised dewpoints across wrn and into nrn portions of the CWA where the fnt is expected to linger, tho may still be too low across more nrn portions of the CWA. Also raised temps across this region with mdls with warmer 850mb temps and sfc winds becoming more sly. However, some uncertainty remains exactly where the warmer temps and warmer dewpoints will reside. Still, believe that areas where temps are lower will be offset by higher dewpoints and still result in HI values around 105. Have therefore converted the entire area previously in a watch for today to an Excessive Heat Warning and continued this warning through Sat. Otherwise, isod to widely sct TSRA will again be possible thru the central portions of the CWA. More than enuf CAPE will be available, but little to no shear and such warm temps shud prevent any storms from becoming severe. Tilly .LONG TERM... (Wednesday through Monday) Issued at 314 AM CDT Tue Jul 18 2017 Main weather story and focus in the extended will continue to be the dangerous heat wave expected to last through at least Saturday. Mid/upper level anticyclone will slowly migrate eastward from the south-central Plains toward the Mississippi River by Friday afternoon. Some weakening of the anticyclone and associated ridge axis occurs this weekend as flow aloft becomes more northwesterly. Some hope for an end in at least the oppressive heat sometime either Sunday, or more likely, for Monday. Plenty of uncertainty in temperatures on Sunday and Monday however. This is due to timing differences in the cold frontal passage with the previous 12Z ECMWF and current 0Z CMC being on the faster end of the guidance envelope with the GFS and new ECMWF being quite a bit slower. A slower frontal passage would likely yield higher temperatures than currently forecast for Sunday and the necessity of heat headlines potentially being extended another 24 hours. Forecast high temperatures slowly warm Wednesday through Saturday as the 850-hPa thermal ridge slides southward into the CWA. Highs are expected to range from the mid 90s to near 100 degrees on Wednesday and warm to the upper 90s to mid 100s on Saturday (warmest conditions each day within the St. Louis urban area). Similar to previous shifts, leaned a few degrees above guidance for high temperatures through Saturday. Heat index values will generally be in the 100-110 range, with warmest values on Friday and Saturday. Will leave rest of watch remain for now, but most of rest of CWA will likely need to be expanded to warning. Exception looks to be across southeast Missouri counties where dewpoints each afternoon will be lower than remainder of CWA. Appears these counties may need an advisory vs. a warning with peak heat index values Wed-Sat in the upper 90s to low 100s. Also the possibility that forecast dewpoints each afternoon are a couple of degrees too high for portions of east- central and southeast Missouri downwind of the Ozarks. Forecast southwest wind direction appears favorable for downslope off of the Ozark Plateau which may lead to deeper mixing and the aforementioned possibility of lower dewpoints. However, this in turn may lead to higher ambient temperatures so going heat index values in the current forecast look reasonable either way. Which should come as no surprise given the anomalous ridging across the mid-Mississippi Valley, dry weather is expected through at least Saturday. Do have PoPs coming in Sunday-Monday associated with a frontal passage. Vegetation in many areas have already become stressed after the previous stretch of heat last week. With this upcoming heat wave likely more intense and a longer duration, area vegetation including crops will become further stressed. The bi- state area will definitely be in need of some widespread rainfall next week. If the warm/dry weather continues too much longer, the onset of drought will become an increasing concern. Gosselin && .AVIATION... (For the 00z TAFs through 00z Tuesday Evening) Issued at 618 PM CDT Mon Jul 17 2017 Isod TSRA possible at KCOU for the first few hours of TAF period. Expect fog to develop late tonight across much of the region, impacting KCOU/KSUS/KCPS. Otherwise, winds will be light and generally nely thru tonight. Winds will gradually veer to eventually become sely to sly on Tues. Tilly && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Excessive Heat Watch from Wednesday afternoon through Sunday evening for Crawford MO-Iron MO-Madison MO-Reynolds MO- Saint Francois MO-Sainte Genevieve MO-Washington MO. Excessive Heat Warning from noon today to 8 PM CDT Saturday for Audrain MO-Boone MO-Callaway MO-Cole MO-Franklin MO- Gasconade MO-Jefferson MO-Lincoln MO-Marion MO-Moniteau MO- Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO-Shelby MO- Warren MO. Excessive Heat Watch from Wednesday afternoon through Saturday evening for Knox MO-Lewis MO. IL...Excessive Heat Watch from Wednesday afternoon through Sunday evening for Randolph IL. Excessive Heat Warning from noon today to 8 PM CDT Saturday for Madison IL-Monroe IL-Saint Clair IL. Excessive Heat Watch from Wednesday afternoon through Saturday evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL- Fayette IL-Greene IL-Jersey IL-Macoupin IL-Marion IL- Montgomery IL-Pike IL-Washington IL. && $$ WFO LSX