300 FXUS65 KPSR 101128 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 428 AM MST Wed Oct 10 2018 .UPDATE...Updated Aviation Discussion... && .SYNOPSIS... Persistent low pressure lingering over the region will continue to result in below normal temperatures through next week. Dry conditions will steadily give way to increasing moisture and rainfall chances as early as Thursday across portions of central Arizona. Deeper moisture associated with a decaying tropical system will mainly affect southeastern Arizona Friday. However, another low pressure system will move into the region this weekend, maintaining the threat of rainfall through early next week. && .DISCUSSION... Latest water vapor imagery reveals several short-wave troughs across the intermountain West and the eastern Pacific. A vort max currently off the central CA coast will move into southern California this afternoon. Latest suite of hi-res WRFs indicates enough ascent will be present ahead of the vort max this afternoon for isolated showers, particularly across western Maricopa County and eastern La Paz County. Isentropic ascent will become more prominent just a bit further east across Maricopa County late tonight and early Thursday morning ahead of another short-wave trough. Rapid moistening on the 305K surface will support isolated-scattered showers through Thursday afternoon, particularly across the higher terrain from Phoenix north and eastward where orographic influences will be maximized. Latest IR imagery shows that Sergio continues to produce convection in the northern semi-circle. However, its ragged appearance on satellite suggests it's already being affected by cooler waters and stronger shear. Sergio's motion already contains a strong easterly component and trends in the GFS/ECMWF ensembles continue to indicate it will slide well south of our area and through northern Mexico. Nevertheless, some light rain will be possible during the day Friday, especially across southeastern Arizona. Depiction on a few of the hi-res WRFs suggests a remnant rain band may reach as far north as the Phoenix area, though rain totals will generally amount to only a few hundredths of an inch. Focus will shift back to the west Friday night as another short-wave trough along the CA coast closes off and drifts towards San Diego Saturday night. Given the anomalously high moisture already in place, any lift will likely generate precipitation. Latest consensus from both the operational GFS and ECMWF indicate the strongest Q-vector convergence will develop across southern California Saturday and southern Arizona Sunday. But again, any rain that develops will likely not amount to much. The aforementioned closed low will become a cutoff early next week as a ridge in the north Pacific builds eastward into the Pacific Northwest. This pattern is notoriously less predictable, though it is a good bet that well-below normal temperatures will persist through at least the middle of next week, given the negative height anomalies. PoPs remain quite uncertain and will be highly dependent on the evolution and track of the meandering cutoff low. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: Quiet conditions will persist through this evening with diurnal driven light winds. Skies will remain mostly clear with potential for some scattered cumulus this afternoon; most bases should be 8-10k feet. More unsettled conditions will begin tonight with an increase in moisture and a chance of showers sometime around 12Z Thursday. Shower activity should be light and isolated to scattered and may briefly impact area terminals. A BKN mid level cloud deck should also form between 09-12Z Thursday with bases between 6-8k feet. Southeast California/Southwest Arizona including KIPL and KBLH: No real aviation concerns over the western deserts or at the TAF sites next 24 hours. Skies at KIPL will remain mostly clear with some cirrus while light winds will favor a westerly direction. For KBLH, some higher based cumulus decks are possible this afternoon with winds favoring the south and some gusts into the teens this afternoon. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Friday through Tuesday: Below normal temperatures will persist through the weekend as a series of low pressure systems affects the western states. Relative humidities will also remain well above normal, particularly late in the week as moisture associated with the remnants of Hurricane Sergio is transported into the Desert Southwest. Although the best chances for heavier precipitation will be confined to far southeast Arizona, a few isolated areas through central and western Arizona could see wetting rains late in the week. There is considerable uncertainty on how quickly conditions will dry out and unsettled, showery weather may return early next week. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Hirsch AVIATION...Kuhlman FIRE WEATHER...MO/Hirsch