000 FXUS65 KPSR 062148 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 248 PM MST Mon Dec 6 2021 .SYNOPSIS... The first of two weather disturbances will move through the region Tuesday bringing increased cloudiness with possibly some sprinkles and light rain. A second, colder weather system will enter the area Thursday into Friday yielding even better chances for widespread light precipitation. Temperatures will initially drop closer to the seasonal normal Tuesday, then fall to slightly below normal levels during the end of next week. && .DISCUSSION... We are beginning to see signals of a changing weather pattern as northerly flow aloft advects mid to upper moisture over the Southwest. This will result in more cloud cover this evening and tonight while temperatures remain well above normal. Currently, temps are mostly in the mid to upper 70s. A cooling trend will commence tomorrow as a quick moving shortwave, currently located along the southern fringes of the Gulf of Alaska, sweeps southward along the California coast, finally liberating our region from the grips of a persistent ridge. More clouds and decreasing heights support a rapid cooldown of 7 to 12 across southeastern CA and along the Colorado River where forecast highs will struggle to reach even 70 degrees tomorrow. The cooldown will be more subtle in south-central AZ with forecast highs settling in the low to mid 70s. Despite increasing moisture in the mid and upper levels, the sub- cloud layer will remain too dry for widespread rain. A few showers are possible in southeast CA, particularly across the higher terrain like Joshua Tree NP and the Chocolate Mountains, but total QPF is not likely to add up to much as NBM probabilities for 0.05" or more are less than 15 percent. For areas farther east, like Phoenix, chances for anything more than virga showers are less than 5%. After Tuesday, the development of a trough axis through the region seems increasingly likely as the aforementioned shortwave continues its southward journey along the Baja California coast. This setup would support deeper southwesterly flow aloft capable of tapping into subtropical moisture. The latest IVT guidance supports this and gives an 80% chance for an IVT of at least 250 kg/m/s pouring into the region Wed-Thur. Main takeaway: this is a positive signal moisture will be adequate for precipitation. Ensembles and cluster tools continue to show another perturbation moving through Thur/Fri that will eject troughing eastward, opening up the next period for relatively significant weather changes. For now, PoPs generally begin to increase through the day Thursday, peaking in the afternoon through early Friday morning. Activity will wane from west to east as the aforementioned trough axis moves through, but most areas in the CWA have at least 30-40% chance for rain. The referenced IVT guidance explains the higher PoPs for the Phoenix area, peaking at 65-75% Thu night/Fri morning, complemented by the 50% chance for 0.25" of rain. Lingering showers are possible later Friday but should mostly be out of the area Saturday. Otherwise, breezy to locally windy conditions are possible Thursday and Friday across the region with isolated occurrences of 30-40 mph wind gusts. By Friday, a much colder airmass will slink through the Intermountain West bringing the coldest temperatures so far this early winter as lower desert highs sink into the 60s Friday and Saturday. Friday and Saturday mornings will be the coldest periods with freezing temperatures possible along any remote lower desert river valleys, mountain peaks, and the higher terrain north and east of Phoenix. By Sunday, guidance suggests a quick return of high pressure ridging that will ultimately lead drier and warmer conditions for the start of next week. && .AVIATION...Updated at 1700Z South-Central Arizona including KPHX, KIWA, KSDL, KDVT: No major weather impacts will affect operations through Tuesday evening. Increasing cirrus cover will lower and eventually form cigs 12K-15K ft AGL by Tuesday afternoon. Otherwise, light east winds will attempt to become westerly this afternoon, but may remain light and variable versus sustaining a true west component. Confidence is good that a switch back to the nocturnal east direction should occur by mid evening; and a more distinct and consistent westerly component materializing Tuesday. Southeast California/Southwest Arizona including KIPL and KBLH: No significant weather issues will exist through Tuesday evening as SCT high cirrus steadily lowers into a 12K ft AGL cig Tuesday afternoon. Winds will be variable for prolonged periods today, but eventually should obtain a westerly component at KIPL and northerly direction at KBLH. Speeds should remain below 12kt. && .FIRE WEATHER... Wednesday through Sunday: Gradual increasing moisture Wednesday into Thursday will lead to considerable cloud cover and eventually decent chances for wetting rains later Thursday into early Friday across much of Arizona. Much cooler air will also accompany the system pushing temperatures to below normal starting Friday. After light winds on Wednesday, breezy to locally windy conditions are expected for much of Thursday and Friday. Generally quiet conditions should settle into the region over the weekend. Minimum humidity values be quite high Wednesday through Friday, mostly above 30-40% before lowering closer to 15-20% by Sunday. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...12 AVIATION...18 FIRE WEATHER...Kuhlman