Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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451 FXUS65 KPSR 141147 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 447 AM MST Sat Sep 14 2024 .UPDATE...Updated 12z Aviation Discussion. && .SYNOPSIS... After a quiet day today with temperatures near normal, unsettled weather conditions are expected for Sunday and Monday as good chances for showers and thunderstorms cover much of southern and central Arizona. Dry and mild conditions are then expected starting next Tuesday with temperatures running close to 10 degrees below normal through around next Friday. && .DISCUSSION... High pressure has mostly vacated the region with the center of the high roughly 1000 miles west of Baja and a weak shortwave trough now approaching from the west. To the south, over the southern part of the Gulf of California, Tropical Cyclone Ileana continues to spin, but it is forecast to gradually weaken over the next couple of days before dissipating along the west coast of Mexico. Despite TC Ileana not having any direct impacts on our area this weekend, it will at least help to advect moisture northward into southern and central Arizona. Lower level moist southerly flow started across our area earlier on Friday with the moist flow now extending upward into the mid levels. This moisture advection is forecast to continue through Sunday with PWATs increasing up to around 1.5" and low level mixing ratios increasing to around 12 g/kg. The moisture advection today will largely go unnoticed as skies remain clear to mostly clear and afternoon highs reach to around normal. Starting Sunday morning, the atmosphere over south-central Arizona should be moist enough to promote some modest elevated instability and possibly some scattered showers and a thunderstorm or two. Going into Sunday afternoon, we should see our peak moisture and instability over the area with a largely uncapped sounding profile supporting 1000-1300 J/kg of CAPE. 0-6km Bulk Shear is also forecast to be between 25-35 kts, while DCAPEs are expected to be between 1000-1400 J/kg. This should to support a few strong to severe thunderstorms Sunday afternoon across south-central Arizona with severe winds and marginally severe hail possible. The severe threat should dissipate quickly during the evening hours, but the threat for isolated to scattered showers and thunderstorms are likely to persist through the overnight hours Sunday night into Monday. By Monday morning, the large scale forcing will quickly increase across the region as a strong Pacific trough tracks southeast through northern California into Nevada. This system should help to drive additional shower and thunderstorm development on Monday, but it will also help to pull drier air northeastward into Arizona throughout the day. The drier air will first surge in aloft and it is likely to hamper or even completely cut off convection west of Phoenix by noon Monday. However, there may be a short period during the early to mid afternoon hours that could bring some additional strong to severe storms focused over Pinal and Gila Counties to as far west as Phoenix. By Monday evening, the dry air is likely to push through all of the Arizona lower deserts ending any rain chances with only lingering chances over the higher terrain through mid to late evening Monday. Forecasting rainfall amounts for this upcoming weather event is actually fairly difficult as the showers and thunderstorms are expected to be fairly scattered in coverage. This is mostly due to the lack of large scale forcing with the Pacific trough mostly bypassing our region to the north. Given the ample moisture forecast to spread into south-central and eastern Arizona through Sunday, any stronger storms could drop decent rainfall amounts of 0.5" or more. The fast ~25 kt storm motion will largely prevent any one storm from causing any flooding issues, but any location that sees a series of storms over a period of any hour or two could see rainfall amounts of over an inch and may result in some localized flash flooding. The latest forecast average rainfall amounts still shows amounts ranging from 0.1-0.2" over western portions of Maricopa County, 0.25-0.5" in the Phoenix area, to upwards of an inch over portions of the higher terrain north and east of Phoenix. Near normal temperatures are expected through the weekend before highs finally drop into the 90s starting Monday. Forecast highs then drop into the upper 80s for Tuesday across portions of the western deserts to the lower 90s over the south-central Arizona lower deserts. Well below normal temperatures should then persist through the rest of the work week as a second strong but dry Pacific trough passes across northern portions of the region Wednesday into Thursday. Forecast highs mainly stay in the lower 90s through Friday with overnight lows mostly in the mid 50s to lower 60s across the western deserts to the 60s across much of the south-central lower deserts. Model guidance eventually signals a moderation of temperatures closer to normal for next weekend as a ridge builds off the West Coast displacing the trough to the northeast. && .AVIATION...Updated at 1145Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected through the next 24 hours. Winds are expected to follow mostly diurnal trends, though confidence remains low with respect to the W`rly shift this afternoon at KPHX. FEW to occasionally SCT cloud decks around 8-10 kft will be present through Sunday morning. After 12z Sunday morning, chances for VCSH/SHRA conditions increase as scattered, fast-moving convective showers are expected to develop and move northeast over the Greater Phoenix Area. Southeast California/Southwest Arizona including KIPL and KBLH: No weather concerns will exist through Sunday morning under mostly clear skies. Winds will favor a SE component at KIPL and are likely to shift out of the W for a period this evening, whereas S winds will be favored at KBLH. && .FIRE WEATHER... A more unsettled weather pattern is expected through Monday as moisture increases today ahead of an approaching weather system expected to impact the region on Sunday and Monday. Expect overall light winds today before winds increase on Sunday predominately out of the south. Humidities will be more elevated this weekend with MinRHs mainly between 20-30%. Sunday into early Monday should bring periods of showers and thunderstorms across south- central Arizona with CWR between 30-60%. Drier air will begin to push eastward through the area later on Monday, but chances for rain will linger from Phoenix and areas east of Phoenix through the afternoon. Dry air will fully push through the area on Tuesday as below normal temperatures take hold across the region. Dry conditions with temperatures running 5-10 degrees below normal are likely to persist well into the latter half of next week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Kuhlman AVIATION...Whittock FIRE WEATHER...Kuhlman