Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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714 FXUS65 KPSR 242321 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 421 PM MST Sun Nov 24 2024 .UPDATE...Updated 00Z Aviation Discussion. && .SYNOPSIS... Temperatures will remain above normal across the region through the next week, with dry conditions prevailing and periods of mostly cloudy skies. A weather system will pass the Desert Southwest to the north during the middle of the upcoming workweek, but precipitation chances should remain confined to northern Arizona. && .DISCUSSION... The region still finds itself between a broad area of negative H5 height anomalies over the Pacific Northwest centered offshore, and a broad area of positive height anomalies centered over Baja California and into Sonora. The resultant tightened pressure gradient aloft has led to stronger zonal flow, with 500 mb RAP analysis showing speeds between 50-65 kt over the forecast area. This strong, zonal flow will continue to bring moisture-laden air in the middle and upper levels inland, increasing cloud cover over the course of the day that should persist through Monday. Thanks to the increased cloud cover, highs today across the lower deserts should be a few degrees cooler than what was observed yesterday, mostly in the middle 70s, and overnight lows will be a few degrees warmer. Similar highs are forecast Monday as well. Quasi-zonal flow should persist over the region through much of the upcoming workweek, though with slight variations in heights aloft, the strength of the flow, and moisture aloft. Ensembles are in excellent agreement that H5 heights will build during the latter portion of the day on Monday into Tuesday, peaking around 582-585 dam over southern AZ. This will shift the stream of moisture further north, temporarily clearing skies, and likely causing the warmest afternoon highs of the next 7 days to occur on Tuesday, in the upper 70s to around 80F, followed by the warmest overnight lows Wednesday morning. Latest NBM output shows widespread lower desert lows in the 50s Wednesday morning, with a 40-60% chance of temps staying at or above 60F overnight in the typically warmer locales. On Tuesday night, the return of thick cloud cover above the 500 mb level will also be a factor in keeping overnight lows relatively mild. Cloud cover will return as the Pacific Northwest low finally moves onshore, suppressing heights aloft, once again tightening the pressure gradient overhead, and thus drawing upper level moisture back over the region. This weak shortwave will cause little in the way of sensible weather impacts, though widespread breezy conditions may develop behind a dry cold front that is expected to pass over the area. Highs should drop a few degrees Wednesday, and flow aloft will shift out of the northwest in the wake of the shortwave trough as it passes north of the Desert Southwest. Cluster analysis shows decent agreement between ensemble membership even beyond midweek. All clusters depict a ridge over the Northeastern Pacific, undercut by a closed low well offshore centered approximately along 30 degrees north. Both features approach the west coast late week. With continued quasi-zonal flow during the latter half of the upcoming workweek, and an upstream source for moisture aloft (the aforementioned closed low), GFS bufr soundings indicate fairly persistent cloud cover. Latest NBM deterministic output reflects this, as forecast temperatures stay quite stable Thursday onward, with highs in the low to mid 70s and lows in the upper 40s and 50s for the lower deserts. These forecast temperatures are still around 5 degrees above average. Whether the closed low can bring appreciable precipitation chances to portions of the region next weekend is still a key forecast uncertainty. As of now, PoP remain low next weekend, mostly below 20% across the forecast area. && .AVIATION...Updated at 2320Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and Southeast California/Southwest Arizona including KIPL and KBLH: Winds will continue to follow their typical diurnal pattern with speeds generally remaining aob 8 kt. Extended periods of light and variable to calm winds can also be expected, especially during wind shifts. High cirrus clouds will continue with a SCT layer of clouds around 15 kft and a BKN-OVC cloud layer around 20 kft. && .FIRE WEATHER... Dry and warmer than normal conditions will persist across the region for the next several days. Winds will follow generally light and diurnal tendencies through Tuesday before some areas, mainly across parts of the Arizona high terrain and Lower Colorado River Valley, pick up some marginal breeziness Wednesday and Thursday. Regional moisture content will be on the increase today with MinRH values between 25-35% across the western districts. Values across the eastern districts will only be 15-20% this afternoon, but will increase to 20-25% by Monday afternoon. Fair to good overnight recoveries can be expected through the middle portion of the week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Whittock AVIATION...Berislavich FIRE WEATHER...RW