Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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656 FXUS65 KPSR 202011 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 111 PM MST Fri Sep 20 2024 .SYNOPSIS... A low pressure system over southern California will track across Arizona tonight and Saturday. This will lead to isolated showers and thunderstorms into Saturday morning. The best chances are over eastern Riverside and La Paz Counties this afternoon and evening. Lesser chances will be over northern Maricopa, northern Pinal, and southern Gila Counties tonight into Saturday morning. This system will bring below normal temperatures for a short time followed by a warming trend early next week with desert highs in the 100 to 105 range Monday through Friday. && .DISCUSSION... An upper low is centered over far southwest CA early this afternoon. The combination of dynamical forcing and thermodynamic destabilization has been generating thunderstorms over/near Joshua Tree National Park. This was handled well by the hi-res models. Anticipate some development further east later on today over La Paz County expanding to portions of northern Maricopa, northern La Paz and Gila County tonight as the system tracks eastward. However, it appears the activity will be isolated per HREF and this is not surprising due to limited moisture availability as well as the timing of the colder air aloft (nighttime/morning for south-central AZ) for less optimal destabilization. For the Greater Phoenix Area, the window of opportunity is roughly 8pm tonight - 8am Saturday. But, coverage - if any - will be limited. The system will exit AZ by Saturday. With the passage of the system, temperatures will trend down (more noticeably today over SE CA and SW AZ; Saturday for south-central AZ). Breeziness picks up as well but speeds are expected to remain solidly below Advisory criteria for the large majority of the forecast area. For next week, a Pacific ridge slowly advances into the western CONUS leading to a warmup with highs at most lower desert locations in the 100-105 range Monday - Friday. Probability of reaching 110 any given day during the that time frame remains at or below 10%. Something of note is that follow-on short waves (subsequent to the system currently over SoCal) in concert with a bit of fold-over by the advancing ridge, are looking more likely to lead to a very weak upper low/weakness over/near AZ. For now, that would likely only result in slight chances of storms over the White Mountains on Wednesday. && .AVIATION...Updated at 1705Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: An extended period of southerly cross runway winds early/mid afternoon will be the greatest weather issue under occasional scattered cloud decks 080-100ft AGL. Confidence is moderate that a 150v220 wind component will be common through late afternoon before finally settling on the SW direction into the evening. Frequent gusts 15-20kt should develop through the afternoon before relaxing mid evening. Eventually overnight, a switch to an E/SE will occur with isold SHRA over mountains north and east of the terminals. A few SHRA may persist just outside the immediate Phoenix airspace Saturday morning with an earlier than usual flip back to SW by late morning. Southeast California/Southwest Arizona including KIPL and KBLH: No major weather concerns should exist through Saturday afternoon under periods of mid/high cloud decks. Confidence is good that W winds will prevail through much of the period at KIPL with some modest gusts possible this afternoon. Directions will be more variable at KBLH with a S component favored through early this evening. && .FIRE WEATHER... A drier and cooler airmass encompassing the region will result in below normal temperatures through Saturday. Stronger southerly to southwesterly will develop this afternoon, with gusts to 20-25 mph expected today, focused across higher terrain areas of the western districts and southcentral AZ. MinRH`s will hover around 15-30% region-wide over the next couple of days, with some slight drying first across the western districts starting Saturday and spreading eastward on Sunday, with MinRH`s more in the 10-20% range by the beginning of next week. Overnight RH recoveries will be fair to good across the region over the next couple of mornings, before degrading to generally fair to poor in spots going into next week. Warming temperatures to above normal are expected to develop late this weekend and persist through most of next week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...AJ AVIATION...18 FIRE WEATHER...Young