943 FXUS64 KTSA 302330 AFDTSA Area Forecast Discussion National Weather Service Tulsa OK 630 PM CDT Thu Sep 30 2021 .AVIATION... CONCERNING TAF SITES KTUL/KRVS/KBVO/KMLC/KXNA/KFYV/KFSM/KROG. Scattered thunderstorms will primarily affect NW AR TAF sites through 02Z with a few lingering showers possible this evening. Lower stratus/fog will likely develop overnight with the potential for dense fog, especially across NE OK, where low level winds will be the lightest. Fog will also be possible elsewhere, but may be more low clouds. Otherwise, ceiling heights will begin rising during the morning hours with the chance for scattered thunderstorms again Friday afternoon. && .PREV DISCUSSION... /Issued 307 PM CDT Thu Sep 30 2021/ DISCUSSION... A line of showers and thunderstorms has now raced well ahead of the cold front which is lagging behind on our western CWA border. The latest data from the mesonet shows winds from the east behind this ongoing line of storms with winds from the northwest behind the cold front. The front is expected to remain nearly stationary over the next several hours before slowly pushing into northeast Oklahoma with time overnight. In the mean time, the current line of storms is generally tracking (albeit slowly for the most part) slowly to the east. A gust front can be easily seen on radar outpacing the line and as a result, there has been a noticeable decrease in the intensity of these storms overall. With that said, storms that are keeping up with the gust front are exhibiting strong wind gust potential. The overall severe threat at this time appears to be relatively low, though there continues to be a nonzero chance for a damaging wind gust or two through the early evening especially for storms that can develop ahead or in close proximity to the gust front. In addition to the strong storm potential, the higher threat for the next several hours may be in the form of heavy rainfall. Radar has estimated rates of 2 to 4” per hour with some rates locally exceeding 5” per hour at times. This may lead to localized flash flooding concerns over the next few hours, especially where storms are slow to clear an area. Rain amounts with this line have been spotty but beneficial nonetheless. Some locations have seen nearly 4” of rainfall (eastern Pawnee county) as estimated from radar over the last 6 hours but there are locations that have only seen 0.25” Expect the spotty nature of locally heavy rainfall to continue through the remainder of the afternoon and early evening as this line passes. NW AR will likely see some additional activity late this afternoon and evening as the gust front and line of storms pushes eastward. There is some uncertainty with this as the current line of storms may lose steam and dissipate with the loss of peak heating. This aspect, will need to be watched but regardless, at least spotty activity is expected to continue across NW AR this afternoon and evening. By midnight, the vast majority of activity will have likely diminished. A lack of a decent low level jet will likely keep the majority of activity at bay. With that said, there appears to be marginal amounts of theta-e advection (closer to sunrise mainly) which may be enough to trigger some shower and storm activity. PWATs will remain elevated (in excess of 1.5” possibly closer to 2”) through the night. Any activity that does develop will still feature the threat for heavy to very heavy rainfall rates with some rates briefly approaching 3” per hour at times. Friday will see an upper low across the four corners region beginning its trek toward eject the central plains as it transitions to become a more open wave. As the wave approaches the region, so will better lift. This wave will also help to (initially) slowly push the cold front further into NE OK where it will stall once again. In terms of precipitation chances, that will likely depend on what happens with this afternoon’s storm complex. Subsidence in the wake of the MCS will likely prevail thus the reasoning for mostly dry weather or perhaps a lull may be the better term for much of the overnight and into tomorrow morning. Expect scattered to numerous showers and thunderstorms tomorrow afternoon across mainly the eastern half of the area where richer moisture characterized by higher PWAT values will reside. Synoptic lift will also start moving into the area as the aforementioned wave pushes into the region. With elevated PWAT values, locally heavy to very heavy rainfall rates will again be possible mainly over the eastern half of the area. Rates near 3” per hour will be possible. Some strong storms will be possible though the lack of upper level support will really preclude the strong to severe storm risk. With that said, some gusty winds will be possible. Friday night into Saturday will see the area within the warm conveyor region ahead of the upper low/opening wave ejecting onto the plains. A surge in PWATs and theta-e are noted ahead of the cold front during the overnight Friday. This will likely lead to warm advection showers and storms along with development of activity along the cold front that will finally get a push from the upper wave. The afternoon bears watching with the possible development of a triple point. If storms can fire ahead of the dryline (somewhere over central to eastern OK by the afternoon), there is enough shear through the atmospheric column that a severe weather threat would be supported. Showers and storms will continue to push eastward through Saturday evening and overnight along with the cold front and upper wave. Sunday will largely be a dry day for most as Nwly flow aloft moves into the area effectively drying the atmospheric column, though some diurnal showers and isolated storms will linger across the far eastern part of the area, mainly across northwestern Arkansas where some slightly elevated PWAT values will continue though not nearly to the extent that has or will be seen. Monday through the end of the upcoming week will see the system that moves through move to our east before cutting off. This upper low will become trapped under a mid level ridge with the polar jet well to north (highly unusual for this time of year) in Canada and with the low likely meandering around the eastern half of the country mainly within the southeastern US. All of the models are showing a retrograding aspect to the upper low but exactly how far west it may move is still in question. The GFS is by far the most aggressive with the low retrograding all the way back into our area which would bring much cooler and wetter weather than the current forecast suggests. The ECMWF and Canadian keep the low well to our east largely keeping the area dry with slightly above normal temperatures. This pattern leads to very high uncertainty thus the forecast confidence past Monday begins to really drop off. Snider && .TSA WATCHES/WARNINGS/ADVISORIES... OK...None. AR...None. && $$ AVIATION.....12