518 FXUS65 KPSR 012007 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 105 PM MST Sun Jun 1 2025 .DISCUSSION... Key Messages: - Showers and isolated thunderstorms will sweep across the area through tonight with some lingering higher terrain shower activity Monday - Another round of isolated light showers possible Tuesday night and Wednesday - Temperatures falling below normal the next couple days, however quickly rebounding near to slightly above normal the latter half of the week Early afternoon WV imagery reveals a compact, nearly cutoff upper circulation center spinning along the northern Baja coast while an elongated, positively tilted trough entering the Pacific NW will act to dislodge and absorb this cutoff feature over the four corners area in the next 48 hours. Concurrently, deep moisture profiles originating from a remnant tropical system continue to be pulled poleward in a robust meridional tropospheric wind profile with objective analysis suggesting 9-10 g/kg boundary layer mixing ratios hovering along the international border. Satellite and objective analysis also indicates impressive jet level divergence and strong ascent juxtaposed with this theta-e surge portending numerous rounds of showers propagating through the forecast area. While the anomalous moisture flux will support an unusually high POP forecast for early June, limiting factors for more expansive, heavier convective based rainfall include poor lapse rates owing to the tropical nature of the incoming airmass and extensive thick cloud cover muting insolation. Thus, embedded thunderstorms, while prevalent, may not be as prolific through much of the area awaiting the actual passage of the upper cold core this evening and overnight realizing modest deep layer instability. Potentially, the more robust thunderstorm activity may be possible in an area arcing across SE California/SW Arizona late this afternoon and evening where MLCape/MUCape 500-1000 J/kg have the best odds of being achieved. Should this outcome materialize, a few stronger storms would be possible with gusty outflow winds (20-40% chance of 35+ mph gusts), patchy blowing dust, small hail, and heavy downpours. Overall, the majority of the CWA should see some accumulating rainfall with HREF membership only leaving out portions of Riverside County on the NW side of the upper low where ascent mechanisms are not nearly as robust. In general, HREF mean output indicates around 0.10" base total with better than 0.50" accumulations under isolated thunderstorms and in preferred upslope locations. That said, it wouldn`t be out of the question for some very localized areas to receive 1.00"+ accumulation, but would likely require training thunderstorm echoes with odds less than 20% and representing the upper end of the ensemble spread. The upper low will have lifted into northern Arizona by Monday morning with strong evidence of only a few lingering showers rotating about the southern periphery of the vorticity lobe. Any remaining showers or new development should migrate almost exclusively to higher terrain locations Monday afternoon. While automated NBM POPs and QPF are generally acceptable forecast parameters over the next 18-24 hours, elevated values persist far too long with model artifacts providing an unrealistic outlook Monday, thus have trimmed this mandated NBM output to align closer to HREF output and conceptual thinking. Models remain consistent in breaking off a vorticity lobe from the aforementioned Pacific NW trough into a quasi-cutoff feature traversing the California coast, then ejecting across the Southwest during the middle of the week. While some low level moisture may remain available with this system passage, the majority of deep tropical moisture will have been scoured away resulting in limited rain chances. There also continues to be substantial ensemble spread with respect to the depth and forward propagation speed to this system yielding low forecast confidence to a scenario already subject to greater uncertainty given the climatological rarity of this synoptic setup. Have cut back NBM POP magnitudes and areal expanse during this time frame given the preponderance of ensemble output along with a recent precedent for egregiously deleterious NBM POP forecasts. While some measure of broad troughing will likely hover over the SW Conus during the latter half of the week, moisture will finally be completely scoured from the forecast area as subtropical ridging begins to strengthen over south TX/northern Mexico. The primary forecast uncertainty will be tied to the amount of retrogression this ridge will obtain into the Southwest and the magnitude of midlevel heights advecting into the region. Ensemble means suggest H5 heights increasing somewhere in a 584-591dm range resulting in temperatures rebounding back near the seasonal normal. Should heights solidly above 588dm be realized, then forecasts towards the upper end of the guidance envelop (several degrees above normal) would come to fruition yielding a return to widespread moderate HeatRisk by the end of the week. && .AVIATION...Updated at 1800Z. South Central Arizona including KPHX, KIWA, KSDL, KDVT: Lowering cigs, periods of showers, and gusty winds will be the main weather concerns this afternoon into tonight. Winds have shifted southwest across the terminals and begun to gust 20-25 kts, which will persist into the afternoon as another round of more robust showers and maybe a few embedded thunderstorms progress through the Phoenix Metro. This VCSH/SHRA will persist for around 4-6 hrs before slowly shifting NE of the region by mid to late evening. Confidence remains low (<30%) in cigs falling below 6 kft at KPHX, however could be a brief reduction to MVFR cigs and/or vsby if showers become heavy enough. After the showers pass, expect winds to shift E/SE from the rain cooled downdrafts. Overnight, CAMs suggest another potential area of SHRA/TSRA developing northwest of the terminals and drawing nearer before moving off to the north early Monday morning. Confidence is too low at this time to include in the TAF for any site other than KDVT. Southeast California/Southwest Arizona including KIPL and KBLH: Periods of SHRA and potentially TSRA later this afternoon, as well as gusty, erratic winds will be the primary weather concerns. A few strong thunderstorms may come within the vicinity or impact the terminals late this afternoon into the early evening, with better chances at KIPL. Winds will predominately be out of the south for much of the period with KIPL favoring SE and KBLH more often out of the S. Gusts to around 20-25 kts will be common at at KBLH through early this evening. Winds speeds will generally be lighter at KIPL than KBLH, though if a thunderstorm develops/moves sufficiently close to KIPL (chances peak around 30%), gusts in excess of 30 kts would be possible, and confidence in the forecast directions would be lower. Expect gradually clearing skies overnight and no major aviation weather concerns Monday. && .FIRE WEATHER... A very unusual wet, late spring weather disturbance will move through the districts tonight and Monday yielding better than a 50% chance of wetting rains. Chances for thunderstorms (10-30% chance) will also linger into Monday which could produce isolated areas of heavier rainfall. Additional light showers with lower wetting rain potential will arrive Tuesday night and Wednesday. Otherwise, afternoon minRHs will range from 25-40% Monday with temperatures much below normal. As temperatures warm closer to normal by Wednesday, minimum RH should fall into the teens, then closer to single digits by the end of the week as temperatures warm further. Excellent overnight recovery above 70% Monday will deteriorate into a poor to fair range of 20-40% by the end of the week. Outside of erratic, gusty winds near showers and storms early in the week, winds should be fairly typical for late spring with the usual afternoon upslope gustiness. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...18/Young AVIATION...Whittock/Kuhlman FIRE WEATHER...18