000 FXUS65 KPSR 282138 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 237 PM MST Thu May 28 2020 && .SYNOPSIS... Strong high pressure building over the region will bring temperatures around 10 degrees above normal through this weekend, with the heat persisting longer across south-central Arizona. Slightly cooler, but still above average temperatures are likely early next week as an area of low pressure slowly moves towards the west coast. Increasing moisture may also bring a slight chance of thunderstorms to southern Gila County. && .DISCUSSION... Broad anticyclonic flow aloft is firmly in place across the western CONUS, with a ridge centered across southern California into the western Great Basin. This feature is forecast to gradually expand eastward into Arizona over the next 24 hours, with a subsequent gradual warming trend expected in the lower troposphere with 700-mb temps warming to near 15C. A bit of mid- level moisture has also pushed into the region this afternoon as a weak shortwave brushes by to the north and east, enough to trigger few-sct showers across the Rim County and White Mountains at this hour. Ensemble guidance continues to depict a very narrow temperature spread for the next few days, with the warmest temperatures expected on Friday, which will likely be the hottest day of this heat event, as temperatures top out above 110 degrees across most of the Arizona deserts. A further increase in moisture is expected across eastern AZ on Friday as mid-level winds become southeasterly, with PW values approaching 1.0 inch across south-central Arizona. This increase in mid-level moisture and instability (a few hundred joules of CAPE) appears it will be now enough to allow the development of isolated TS activity across the higher terrain of South-Central AZ on Fri afternoon/evening. Although the potential for wetting rainfall is very low, there is still the concern that these thunderstorm could produce gusty outflow winds and lightning-caused fires. Into the weekend, the above-normal temperatures will persist across south-central Arizona, remaining above excessive heat thresholds as highs top out around 110 degrees for Phoenix. More substantial cooling will occur across southeast California and southwest Arizona as an upper-level trough approaches the California coast. The strengthening southerly low-level and mid- level flow will maintain seasonably high moisture values across southern and eastern parts of the state, with continued convective buildups possible on both Saturday and Sunday afternoons. The Excessive Heat Warning remains unchanged with this forecast package, and will expire on Sunday evening for south-central Arizona. The large-scale pattern evolution remains fairly uncertain into early next week, particularly with regards to the inward progression of the upper trough and increasingly compact associated upper low, and the strength and persistence of the upper-level ridge across the Southwest. Monday is still likely to remain very hot, with perhaps a couple degrees cooling over Sunday. However, a gradual increase in moisture will occur amidst strengthening southerly low-level flow. GEFS ensemble mean favors a more eastward trough progression, with modest height falls and cooling spreading eastward by Monday. This solution would favor an increase in convective coverage in the Monday-Thursday period, especially across the higher terrain north and east of Phoenix. However, couldn't rule out an isolated storm over the lower deserts within this regime, and greater potential for outflow. The latest (12Z) ECMWF/EPS output is now beginning to trend towards the GEFS output, giving somewhat more confidence that a trend towards cooler/more unsettled weather will occur, with both the GEFS/EPS now showing more-and-more of their respective ensemble members even showing measurable rainfall at Sky Harbor on/after June 3rd. However, given the still-large spread between the ensemble members, confidence continues to fairly low especially for a typically benign early June climatologically. && .AVIATION...Updated at 1720Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Mostly clear skies with a few mid-level clouds between 15-20 kft. Wind directions will continue to transition to the west. From 21z through around 2z, wind speeds will become more pronounced with gusts into the teens. Speeds will diminish overnight and easterly drainage winds will redevelop. Southeast California/Southwest Arizona including KIPL and KBLH: Clear skies through the TAF period. Winds will favor the southeast most of the period at KIPL with speeds near 5-10 kts. Winds will favor the south at KBLH, with a period of gusty conditions from around 20Z to 2Z. && .FIRE WEATHER... Sunday through Thursday: Much above normal temperatures on Sunday to gradually cool closer to the seasonal average by the middle of next week. There is a slight chance of an isolated thunderstorm in eastern Gila County on Sunday and Monday afternoons, albeit with little to no accompanying rainfall. Somewhat more expansive shower and thunderstorm activity may be possible towards the middle of next week in eastern districts, however confidence in this outcome is low. Minimum afternoon humidity levels falling to around 10% this weekend likely to rebound closer to 15% by the middle of next week. Overnight recovery will mostly fall in a 20-35% range though could gradually improve. Winds may be a bit gusty Sunday and Monday, though well below critical thresholds. Nevertheless, fire danger will remain heightened with the prolonged period of very hot, dry weather. && .CLIMATE... Record highs this week: Date Phoenix Yuma ---- ------- ---- May 27 111 in 1983 114 in 1951 May 28 113 in 1984 116 in 1983 May 29 112 in 1910 115 in 1984 May 30 114 in 1910 120 in 1910 May 31 109 in 2012 113 in 2012 && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST Sunday for AZZ534-537>556- 559>561. Excessive Heat Warning until 8 PM MST Friday for AZZ530>533-535- 536. CA...Excessive Heat Warning until 8 PM PDT Friday for CAZ561>570. && $$ DISCUSSION...Percha/Rogers AVIATION...Feldkircher FIRE WEATHER...Percha/Rogers/MO CLIMATE...MO