280 FXUS65 KPSR 111021 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 321 AM MST Thu Oct 11 2018 .SYNOPSIS... Above normal moisture will result in periods of shower activity the next several days. However, rain accumulations will generally be on the light side. Temperatures will likely remain at or below normal into next week as low pressure persists across the Desert Southwest. && .DISCUSSION... A weak short-wave trough is evident this morning on water vapor imagery across the Mojave Desert. Ahead of this system, clouds have been on the increase across central Arizona, associated with robust isentropic ascent (and enhanced pressure advection) around 305-310K. Latest consensus among the hi-res guidance is that isolated-scattered showers will develop this morning and continue throughout the day, most favored in the higher terrain of Maricopa County, where orographic influences will be maximized. Another short-wave trough diving southward along the central CA coast will close off overnight. This will transport deeper moisture associated with the remnants of TS Sergio into southern AZ. Depiction of reflectivity in the CAMs suggests that rainfall will generally remain light, except for a few embedded heavier cells Friday morning. Latest multi-model median indicates rain totals of only a few hundredths of an inch across much of the area. The highest rain totals are expected across far southern Yuma County, where up to a quarter of an inch of rain is likely, before the precipitation tapers off Friday afternoon. Focus will then shift back to the west as the aforementioned closed low migrates towards San Diego Friday night, which historically is a favorable position for low pressure systems. Vorticity-forced ascent will be on the increase, yielding an increase in shower activity initially across southeastern CA. The threat of rainfall will shift eastward into AZ during the day Saturday and given the anomalously high PWATS, there is a marginal risk of excessive rainfall. However, most locations will only see around a tenth of an inch of rain. Steeper mid-level rates will also support a slight chance of thunderstorms. Latest GFS/ECMWF have come into much better agreement for Sunday, indicating that another short-wave trough moving towards the Four Corners will absorb the aforementioned low-pressure system. Drier air will overspread the Desert Southwest as northerly flow develops around an anticyclone in the northern Pacific. However, model variability increases thereafter and continues through next week. Some GEFS and ECMWF ensemble members suggest a piece of the Four Corners trough will fracture, cut off and meander, so there is some potential for deeper moisture to return early next week. PoPs have been lowered but remain somewhat uncertain, though it is a good bet that near to below normal temperatures will persist, given the negative height anomalies. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: There are very few aviation weather concerns over the next 6 hours or so. However, by mid Thursday morning, there will be an increase in shower activity across Arizona, including the Phoenix terminal area. As such, vicinity showers will prevail throughout much of the TAF period. Showers, and even an isolated thunderstorm, at any terminal is possible but uncertain enough to leave out of the TAF at this point. Otherwise, ceilings will drop through the morning as low as 5k feet with little improvement throughout the day. The winds will also likely stay easterly for much of the day with periods of variability and even crosswinds. Southeast California/Southwest Arizona including KIPL and KBLH: There are minimal aviation weather concerns for tonight and through much of Thursday. Fortunately, much of the shower activity should stay west of the Colorado River as well as the lowest ceilings. Otherwise, winds will follow a typical diurnal pattern but with strong afternoon gusts. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Saturday through Wednesday: Below normal temperatures will continue through the weekend and into next week as persistent low pressure affects the southwest states. Relative humidities will continue above normal with afternoon minimums only falling into a 20-40% range. Periodic scattered showers will be common into at least early next week though overall amounts will be fairly light. Higher terrain areas of central Arizona will stand the best chance to receive wetting rains. There still is considerable uncertainty on how quickly conditions will dry out and unsettled, showery weather may continue into the middle of next week. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Hirsch AVIATION...Deemer FIRE WEATHER...MO/Hirsch