151 FXUS65 KPSR 092145 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 245 PM MST Tue Oct 9 2018 .SYNOPSIS... Persistent low pressure lingering over the region will continue to result in below normal temperatures this week. Dry weather will steadily give way to increased moisture and rainfall chances as the week progresses. Deeper moisture associated with a decaying tropical system will remain trapped in far southeast Arizona with just scattered lighter showers into central and western Arizona. A few showers could persist through the weekend and into early next week though much of the time will be dry. && .DISCUSSION... As one series of potent shortwaves lifts into the plains producing severe weather and flooding east of the Rockies, additional waves will drop into the Great Basin maintaining mean synoptic troughing through the western states. Meanwhile, Sergio has weakened into a tropical storm and is becoming even more disorganized over cooler water and increased shear. The latest hurricane models continue to steer the central circulation of Sergio well south of Arizona, and other than a modest increase in moisture preceding the the actual storm, Sergio will be essentially a non-event for the local forecast area. As opposed to Sergio, the main influence and biggest key to the forecast appears to lie with a shortwave digging along the West Coast and amplifying/cutting off the central/southern California coast during the latter half of the week. Several more subtle forcing mechanisms will materialize over the next 48 hours providing the chance for some isolated/scattered showers. First, a plume of moisture will stream into western Arizona Wednesday with increasing isentropic ascent around the 305K level. The pressure layers on this isentropic surface become fairly steep by mid afternoon and essentially all HREF and local high resolution members develop showers through Yuma and La Paz counties. POPs were increased through this area and further increases may be justified if more expansive coverage seem likely. The zone of isentropic lift shifts east towards Maricopa County on Thursday; and with the aid of orographic influences, additional isolated to scattered showers look like a good bet around and north of the Phoenix metro. Comparable warm air aloft above H7 should cap off the shallow convective elements though a persistent, deep southerly flow should allow regeneration of light showers through much of the daylight hours. All told, not looking at a tremendous amount of rainfall (likely 1/10th of an inch or less) and more of a nuisance than anything else. Both operational and ensemble members have been displaying a distinct trend over the past couple days with respect to the shortwave cutting off the California coast. That is, each successive run becomes slower and more retrogressive becoming completely cutoff from the main northern stream jet west of the forecast area. EVen the previously more progressive ECMWF has significantly slowed forward progression of this system. Ensemble members are squarely in favor of the slower operational GFS, and it would not be surprising to see future model iterations continue this slowing trend. Nevertheless, this problematic part of the forecast has serious ramifications for sensible weather as it may become an eventuality that the entire Friday-Sunday period remains dry for a vast majority of the forecast area awaiting the dynamic support/cold core to arrive with the cutoff. Have cut POPs through much of this period though had to maintain some mention given the uncertainty and moisture lingering within the region. It's probably more likely than not that POPs are cut even more through the weekend, but increased early next week as the low finally ejects through the SW Conus. && .AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL: Flat cumulus will be coming over the higher terrain of south- central Arizona the rest of the afternoon with some of the cumulus approaching the towering cumulus stage. No thunderstorms or showers are anticipated with those clouds. They will dissipate after sunset. Otherwise, just some minor cirrus moving in overnight before a batch of develops during the day Wednesday west of metro Phoenix. Surface winds will be light and favor familiar diurnal patterns. Southeast California/Southwest Arizona including KIPL and KBLH: Skies will be clear the rest of day before some minor cirrus moves through overnight. During the day Wednesday, anticipate a corridor of clouds (bases between 5kft - 10kft ASL) and possibly some weak embedded showers/sprinkles mainly over southwest AZ. Surface winds will be light and favor southerly directions. 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...MO AVIATION...AJ FIRE WEATHER...MO/Hirsch