806 FXUS65 KPSR 102201 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 300 PM MST Wed Oct 10 2018 .SYNOPSIS... Moisture will steadily increase across the region for the remainder of the week resulting in a prolonged period of occasional shower activity. Several waves of low pressure will aid in supporting rain into the weekend, though most locations will only receive periods of light rain. Low pressure will linger near the southwest states into next week resulting in more scattered showers and temperatures below the seasonal average. && .DISCUSSION... Several shortwaves continue to rotate about mean western Conus troughing with one dampened wave pushing east from southern California, a secondary deeper wave digging along central California coast, and a less descript yet critical wave diving south through British Columbia. Ahead of the former 2 shortwaves, modest moisture advection was commencing with a plume of objectively analyzed 8-9 g/kg mixing ratio streaming into western Arizona. Concurrently, steeper isentropic ascent was developing around the 305K layer and numerous convective towers can be noted on satellite imagery. Thus far, warmer air aloft has capped more robust vertical growth and HREF members (as well as HRRR) still depict only isolated showers from La Paz county and points north the remainder of the afternoon and evening. Isentropic ascent will shift east and become more prominent through a deeper 300K-305K layer across Maricopa County late tonight and early Thursday morning. Aided by some jet streak support and orographic ascent within a deeper southerly flow, rapid moistening will support isolated to scattered showers through Thursday afternoon. Much like today, warmer air in the 500-600mb layer may cap better convective growth and showers should remain on the lighter side with the best accumulations in terrain areas north and east of Phoenix. Sergio continues to have a ragged appearance on satellite suggesting more negative influence from cooler waters and stronger shear. Sergio's motion contains a strong easterly component and all indications show it will slide well south of our area and through northern Mexico. Nevertheless, some light rain associated with the outer reaches of Sergio (and a vorticity lobe) will be possible during the day Friday. Depiction on a few of the hi-res WRFs and even global models suggest a remnant rain band rotating as far north as the Phoenix area. At this time, it appears any more appreciable rainfall will be relegated to southern Arizona along the international border with much lighter amounts north of Interstate 8. The aforementioned British Columbia wave will have amplified off the central California coast by Friday afternoon in the process of cutting off from the main northern stream jet flow. The propagation and depth of this feature remains highly uncertain with ensemble members still ranging from the more progressive evolution to a circulation center spinning west of the Baja peninsula well into the middle of next week. The operational GFS and majority of its ensemble members have been the most consistent in hovering the circulation over the Baja through Monday before slowly ejecting the system northeast. Given the anomalously high moisture already in place, any lift will likely generate precipitation. The general consensus from both the operational GFS and ECMWF indicate the strongest Q-vector convergence will develop across southern California Saturday, then shifting into southern Arizona Sunday. Dynamic ascent may persist even longer across the area, but any rain that develops will likely not amount to much. The cutoff low may be even slower to move 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, but evidence is beginning to support an even longer period of isolated to scattered light showers over much of the forecast area even into Tuesday or Wednesday. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: Quiet conditions will persist through this evening with 5-10 knot winds gradually veering from southeast to south by 21Z and southwest by 23Z before becoming easterly overnight. Skies will remain mostly clear with potential for some scattered cumulus this afternoon with cloud bases staying above 8-10k feet. An increase in isolated to scattered shower activity with broken cloud bases lowering to 5-6k feet is expected to begin by 12Z tomorrow. These showers should continue through most of the morning and afternoon hours, with cloud bases possibly decreasing further after 18Z, but remaining VFR. A prolonged period of southerly crosswinds around 10 knots is also expected early tomorrow afternoon. Southeast California/Southwest Arizona including KIPL and KBLH: Minimal aviation impacts are expected for the next 24 hours aside from possibly a very isolated shower at KBLH this afternoon. However, most hi-res model runs keep this activity over western Arizona and far too isolated to warrant mentioning in their TAF. Otherwise, gusty southerly 20-25 knot winds are expected at KBLH this afternoon before subsiding during the evening hours. Winds at KIPL will veer from south-southeast to southwest by early evening and westerly overnight, generally remaining below 10 knots aside from a few stronger gusts between 02-07Z. 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...MO/Hirsch AVIATION...Hopper FIRE WEATHER...MO/Hirsch