457 FXUS65 KPSR 082114 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 214 PM MST Mon May 8 2017 .SYNOPSIS... An unusually strong low pressure storm system for early May will continue to bring unsettled conditions for the Southwest United through the middle of the week. Precipitation chances will peak early Tuesday morning into Tuesday afternoon across most of south-central and southwest Arizona. Temperatures will be well below-normal as afternoon highs struggle to get out of the 70s. Clearing skies and a substantial warming trend will develop through the second half of the week. && DISCUSSION... Upper low near the mouth of the Colorado River has temporarily stalled, and is undergoing an eastward and eventual northward shift in motion. A long fetch of southerly flow on the eastern periphery of the low has advected mid-level moisture northward into the eastern half of Arizona, as evidenced by a thickening cumulus field this afternoon. The SPC mesoanalysis picked up on the diurnally enhanced area of destabilization with a large area of MLCAPE values around 250-500 J/kg across eastern Arizona, but this has dissipated somewhat with increased boundary-layer mixing. Low-level moisture is still lacking, but convective trends show enough instability to produce isolated showers and a few thunderstorms across the higher terrain north and east of Phoenix. In fact, with steering flow out of the southeast, it is not inconceivable we could see a couple showers or thunderstorms moving into the Phoenix area late this afternoon and evening. Rainfall amounts would likely be light, with the primary threat being localized gusty winds. As such, PoPs have been raised a bit through this evening across south-central Arizona. The upper low will continue to track northward and will move into southern Arizona by tomorrow afternoon. Precipitation chances will increase overnight and remain relatively high through Tuesday afternoon, and could see the development of a band of thunderstorms with locally heavier periods of rain. However, widespread rainfall amounts should generally remain at or under one quarter of an inch. For the Phoenix area, rainfall chances increase after 06Z (11 PM MST) and will remain elevated through 18Z, which is consistent with both the SREF ensemble plumes and most high-resolution model guidance. Much cooler temperatures and increased cloud cover can also be expected Tuesday, with highs 20-25 degrees below normal for the date. The upper low will begin accelerating northeastward into the southern Rockies by Wednesday, with lingering cloud cover and weak showers persisting into Wednesday morning, before drier air moves into the region. Upper ridging will build in the wake of this system, resulting in a substantial warming trend Thursday and Friday. Temperatures are forecast to rise into the upper 90s Friday and Saturday. Fortunately, the above-normal temperatures may be short-lived as another storm system looks likely to take shape across the western CONUS into early next week. && .AVIATION... South-Central Arizona Including KPHX, KIWA, and KSDL: Area of strong low pressure well south of the state will continue to circulate clouds and moisture up into central Arizona. Widespread TCU fields will continue over eastern Arizona, with SCT to BKN coverage 10-12kft working into the Phoenix metro from the east. Enough instability and moisture are coming together to generate scattered small/light high based showers over the eastern mountains and potential for showers into the terminals, at least VC, will start this early afternoon and continue through sunset. Primary threat/concern with the mostly high-based showers will be strong gusty winds in the proximity of them, with some short-lived 30kts gusts possible. Longer lived/wetter rains and lowering CIGs to develop overnight and for the AM traffic Tuesday. Southeast California/Southwest Arizona Including KIPL and KBLH: Large area of Pacific low pressure will continue to circulate just south of the border before beginning to lift northeastward into Arizona Tuesday. SCT to BKN mid level clouds will drift westward into southeast CA along with some weak, high-based showers through the evening. Wrap-around activity along the north side of the low will support at least VCSH mention for KIPL and KBLH for periods tonight. Winds will vary off of northerly headings, but at time be so light VRB may result. Any passing shower activity could generate short-lived wind gusts, 20-25kts possible. Aviation discussion not updated for amended TAFs. && .FIRE WEATHER... Thursday through Monday... Drier and warmer conditions are expected with a return to above normal temperatures. Minimum relative humidity Thursday will range from 15-20 percent, then fall to near 10 percent Friday through Sunday. Afternoon southwest winds 10 to 20 mph, but increasing Saturday and Sunday into the 15 to 25 mph range as another upper trough approaches from the west. Humidities to make mostly fair overnight recoveries. && .CLIMATE... Record cool high temperatures Sunday through Tuesday: Date Phoenix Yuma ---- ------- ---- May 7th 70 in 1965 72 in 1964 May 8th 66 in 1930 64 in 1912 May 9th 67 in 1912 74 in 1922 && .SPOTTER INFORMATION STATEMENT... Spotter reports of gusty winds and blowing dust may be needed early this week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ Visit us on Facebook, Twitter, and at weather.gov/phoenix DISCUSSION...Rogers AVIATION...Nolte FIRE WEATHER...Nolte/Vasquez CLIMATE...MO