Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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678 FXUS65 KPSR 210554 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1054 PM MST Fri Sep 20 2024 .UPDATE...Updated Aviation Discussion. && .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 0554Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: VCSH with potential brief -SHRA impacts is anticipated now through 8Z at the latest tonight. SCT to BKN clouds down to 7-8K ft is expected during this time period, with clearing thereafter. Winds will also shift S-SE during this time and prevail through the rest of the morning. There is another low chance (10%) for VCSH/VCTS Saturday morning, around the 15Z timeframe, but any activity may stay just north of the metro area. Confidence and probability is too low to include this in the TAF. An early W wind shift is expected Saturday, as early as 15-16Z. Wind speeds will remain light through the TAF period, mostly aob 8 kts. Southeast California/Southwest Arizona including KIPL and KBLH: No major weather concerns should exist through Saturday under periods of mid/high cloud deck tonight, then followed by mostly clear skies. SHRA in the region of KBLH, but outside the 10sm radius, may continue through 9-10Z in the region before fully dissipating. Current N winds at KBLH are expected to slowly turn counter-clockwise through the next few hours before becoming VRB. KIPL will favor a W winds tonight before also becoming VRB. VRB wind conditions are then expected at both terminals for much of Saturday. && .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...Benedict FIRE WEATHER...Young