Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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365 FXUS65 KPSR 151811 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1111 AM MST Sun Sep 15 2024 .UPDATE...Updated Aviation Discussion. && .SYNOPSIS... A weather system will approach the region from the northwest today while moisture increases across eastern Arizona. This will result in modest chances for showers and thunderstorms focused more over the higher terrain east of Phoenix. Chances for rain will continue into Monday, but once again the best chances will be over higher terrain areas. Dry and mild conditions are then expected starting Tuesday with temperatures running five to ten degrees below normal through Friday. && .DISCUSSION... Talk about a dramatic shift in rain chances. Model trends over the past 12 to 24 hours show considerably less moisture getting advected northward into southern and central Arizona with much of this moisture now slated to be mainly east and southeast of the Phoenix area. This is likely because TC Ileana has dissipated earlier than expected with little to no convection now associated with the disturbance. As a result, the moisture advection into Arizona will be much less than what was expected with PWATs now likely only maxing out around 1.3" later today. Given the less moisture, instability will also be much more limited with HREF MUCAPEs falling well short of 1000 J/kg over much of the area. Looking at the latest guidance, there will still be a roughly 20-40% chance of showers and thunderstorms starting this afternoon as far west as Phoenix but more so to the southeast and east of Phoenix. The expected lower instability will also reduce the potential for severe weather impacts. The incoming Pacific trough from the northwest will help to increase winds aloft and thus we will still have modest shear, but the limited instability will play a bigger role in the reduced strong to severe storm potential. Guidance still shows the potential for scattered showers and isolated thunderstorms through Monday morning, but now the bulk of the activity is expected to be east of the Phoenix area. The drier air that was supposed to start moving eastward through the Phoenix area by Monday afternoon now looks more likely to move through by sunrise Monday morning. This could end rain chances altogether for Phoenix on Monday, but the CAMs still show a few isolated showers possibly around the eastern part of Phoenix through Monday afternoon. The best chances for convection on Monday should again fall over the higher terrain east of Phoenix with still some potential for strong winds aloft to get mixed down to the surface with any stronger thunderstorms. The expected lower moisture and less unstable environment means QPF amounts have also gone down dramatically. The latest guidance shows barely any QPF from central Phoenix and areas west to maybe 0.1" over northern and eastern portions of Phoenix. Higher QPF amounts upwards of 0.5-0.75" are shown across higher terrain areas of Gila County. Localized higher amounts of over an inch will still be possible over the higher terrain, but the potential for heavy rainfall and any flooding is also noticeably lower than it was last night. The drier air pushing eastward across Arizona should then mostly end rain chances over eastern Arizona by around midnight Monday night. The down trend in temperatures has stalled out a bit now that the rain chances have lowered. Forecast highs for today are now mostly just over 100 degrees across the lower deserts, including the Phoenix area. As the trough enters northwestern portions of the region on Monday, there will be some cooling over the western deserts as highs drop into the lower 90s, but Phoenix may still be able to reach 100 degrees Monday. The trough moving into the region will also bring increased winds Monday with gusts ranging from 30-40 mph across southeast California to 20-30 mph across south-central Arizona. As the trough lifts to the northeast on Tuesday, the cooler air should finally push through the rest of the area with highs topping out in the upper 80s across the western deserts to the lower 90s in the Phoenix area. Through latter half of the work week, we will still stay under a troughing pattern with another Pacific system tracking southeastward through California into the Desert Southwest later in the week. We may see a little bump in moisture ahead of the weather system Wednesday into Thursday, but for now we are not anticipating any chances for rain. The system will at least reinforce the cooler air across the region as forecast highs should stay in the upper 80s to lower 90s through Friday, while overnight lows dip into the upper 50s to upper 60s. Higher model uncertainty is seen thereafter with a stronger ridge building off the West Coast, but yet another potential trough diving southward toward our region. If the ridge does end up moving into our region, we may see temperatures creep toward the 100 degree mark again, but if the trough wins out temperatures may stay below normal for a few more days. && .AVIATION...Updated at 1810Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation weather concern through the period will be the potential for some showers and isolated thunderstorms starting this afternoon and again going into the overnight period. Hi-res guidance has backed off on the amount of shower/storm coverage over the Phoenix area with the greatest focus of activity expected to remain to the east/southeast of the terminals. There will at least be the potential for some isolated activity this afternoon and thus have added mention of VCSH in the TAFs, with confidence being too low to mention VCTS anywhere. Any thunderstorms that were to develop will have the potential to produce gusty, erratic winds with about a 30% chance of seeing some gusts in excess of 30 kts. Another around of activity will be possible by around midnight, continuing through the overnight hours. Winds will continue to pick up out of the south through the rest of this morning while gradually veering toward the west/southwest this afternoon. Winds are favored to switch back to the east tonight with some hints that an outflow stemming from convection across southeast Arizona may travel into the metro area and help to switch winds east. Gusty southerly crosswinds around 18-20 kts are expected to develop tomorrow morning starting around 16Z before gradually veering toward the southwest with gusts climbing to around 20-25 kts. SCT-BKN clouds mostly aoa 8 kft will continue throughout much of the period. Southeast California/Southwest Arizona including KIPL and KBLH: Winds at KIPL will mostly favor the west through the next 24 hours, while winds favor the south at KBLH. Winds speeds will be elevated through the period with some gusts upwards of 20-25 kts expected this afternoon at KBLH and this evening at KIPL. Otherwise, mostly clear skies will persist. && .FIRE WEATHER... A weather system will brush by the region through Monday, while moisture is expected to increase enough across the eastern districts to bring chances for showers and isolated thunderstorms from this afternoon through Monday afternoon. The best chances for wetting rains will be over the higher terrain east of Phoenix where chances peak around 50% Sunday night. Winds will predominately be out of the south starting this afternoon through Monday morning before turning southwesterly and becoming gusty Monday afternoon. MinRHs through Monday will range from the mid teens across the western districts to 20-30% over the eastern districts. 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 through the rest of the work week before temperatures push back into the normal range next weekend. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Kuhlman AVIATION...Smith FIRE WEATHER...Kuhlman