408 FXUS65 KPSR 142115 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 215 PM MST SUN AUG 14 2016 .UPDATE...AVIATION AND FIRE WEATHER DISCUSSION... .SYNOPSIS... The weather pattern will allow for a transition to drier and more quiet conditions through early this week as moisture levels continue to trend downward. With this drying trend, temperatures will also increase with readings of a few degrees above normal. A return of modest monsoon moisture looks likely for the latter half of the week resulting in chances for showers and thunderstorms across southeast California and all of southern Arizona. && .DISCUSSION... The CWA remains under clear skies today and terrain driven CU/CB fields have been slow to develop throughout the day...evidence of the more stable and drier airmass that has transitioned into the region. Even the Rim was lagged in developing clouds, even slower than hi-res model forecasts progged. Dewpoints are running much drier than 24 and especially 48 hours ago with many sites dropping into the mid-50s as afternoon mixing gets underway with sfc temperatures cresting 100F CWA-wide. Today, along with the next few days, will transition into a low-grade Monsoon pattern with focus for the majority of storm activity over the mountains as LL dewpoints drop below and out of typical Monsoon levels and flow aloft becomes northerly. High pressure center continues to build off to our west, with strong subsident ML flow noted on water vapor imagery. Progs continue to call for the upper high to our northwest to sag southward as an area of low pressure dives south and into northern and central California. As this occurs, steering flow into our area shifts from north-northeast today to more of a northwest to north direction through Tuesday. This serves to advect even drier air into the area from the north and west, with the lowest PWAT values to move into portions of far southeast California. PWAT values fall to around one inch over the central deserts, and below 0.75 inches in the Joshua Tree NP area. With the relatively dry air in place and the unfavorable steering flow setting up over the area we will be looking at virtually no thunderstorm activity from Phoenix westward into SE California through Tuesday with just a slight chance of an afternoon storm over the higher terrain areas of southern Gila county - mainly east of Globe. Due to the drier air out west along with H5 heights around 591DM, we will also look for well above seasonal normal high temperatures over southeast CA and the western AZ deserts each day, with temps rising well above 110 degrees and near excessive heat warning levels. In fact, Imperial is expected to hit 115 today and Monday. Overall, areal coverage of temperatures reaching heat warning thresholds will stay just slightly below what we need to issue a warning but we will continue to highlight the threat via social media and weather stories. High temps across south central AZ will stay around seasonal normal levels each day and probably slightly above by a couple of degrees or so. The idea of very low grade monsoon conditions with little or no precip in the forecast is also supported by GEFS ensemble output; qpf from many members is non-existent, and those that do crank out some qpf keep things confined to southern Gila county for the most part. The modest upper trof that is forecast to set up over central CA early in the week is then forecast by operational models such as the GFS and ECMWF to slowly sag southwest and set up along the southern CA coast by the middle of the week. As this occurs, flow aloft becomes more southerly - actually is is southwesterly - but is still sufficient to pull modest amounts of moisture north and into the lower Arizona deserts and set up somewhat of a moisture corridor across much of the state. Some moisture tries to slide into the deserts of southeast California, but the overall orientation of the steering flow makes it hard for much significant moisture to reach those lower deserts. With the increase in moisture and a corresponding increase in instability and CAPE we expect low to moderate grade monsoon conditions to return by Wednesday, becoming more prominent by Thursday. As such we will continue to call for slight chances of afternoon and early evening thunderstorms mainly from the lower Colorado River eastward during the latter half of the week, with best chances over higher terrain areas east of Phoenix. High temperatures will fall off as the humidity and clouds increase with portions of south central AZ dropping slightly below seasonal normals by Thursday. Little was changed in the forecast during the extended period, however one small adjustment was made to slightly lower POPS each day due to a more westerly orientation of the steering flow ahead of the baggy low to our west. This is a very unfavorable steering flow direction in terms of advecting storms into any of the lower deserts. By Friday or Saturday there will likely be little if any convection in southeast CA and chances over the lower AZ deserts may be rather minimal as well. && .AVIATION... South-Central Arizona Including KPHX, KIWA, and KSDL: High pressure over the area is producing mostly clear skies. Winds are in the 10 to 15 kt range primarily from the east and southeast as the westerlies never fully kicked in today. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns through the taf period under mostly clear skies. Mostly southerly component with winds in the 10 to 15 kt range. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Wednesday through Sunday...High pressure will continue to dry out the atmosphere with min RHs in the 15 to 20 percent range increasing to 20 to 25 percent by Friday as a slight increase of monsoon moisture will move in across the region. Storm activity over the higher terrain may increase slightly later in the week over the eastern portions of Arizona. && .SPOTTER INFORMATION STATEMENT... Spotter activity will not be needed today and likely not be need through the early work week. Spotters are encouraged to report criteria through appropriate channels. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ Visit us on Facebook...Twitter...and at weather.gov/phoenix DISCUSSION...Nolte/CB AVIATION...Waters FIRE WEATHER...Waters