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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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012 FXUS65 KPSR 191207 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 507 AM MST Fri Jul 19 2024 .UPDATE...Updated 12z Aviation Discussion. && .SYNOPSIS... Thunderstorm chances will persist across the area through at least the middle of next week with the highest probabilities over higher terrain areas of Arizona. Far better odds for thunderstorm survival into lower elevations will arrive over the weekend and early next week. Otherwise, widespread excessive heat will impact much of the region the next several days with temperatures better than 5 degrees above the daily normal. && .DISCUSSION... Early morning WV imagery and objective analysis depict an expansive subtropical anti-cyclone over the SW Conus with a 597dm H5 center anchored directly over central Arizona. Several shortwaves were rotating about the periphery of this high pressure bubble - most notably, one lifting north into southern/western Nevada, another diving southeast into northern New Mexico, and lastly, a far more robust inverted trough over northern Sonora. Even closer to the anti- cyclone center, there are very subtle perturbations such as an MCV emanating from a convective complex over SE Pinal County. Operational and emsemble members are in very good agreement showing this feature remaining quasi-stationary through the next 48 hours before weakening slightly and migrating northwest into central Nevada. With heights aloft nearing 2 normalized standard deviations above the seasonal mean, H8-H7 thermal profiles will approach the 90th percentile of climatology, or around +30C and +15C respectively. Historically, values of this magnitude equate to lower elevations surface temperatures around 115F, and forecast guidance spread is very narrow around this expectation yielding rather widespread major HeatRisk the next several days. It wouldn`t be impossible for outflow moisture intrusions or extensive residual clouds to disrupt the extreme heat, however no tangible evidence in model output suggests this outcome. Thus, confidence is excellent that temperatures 4F-8F above the daily normal will be common the next few days, through still a few degrees short of record territory. Thunderstorm evolution this afternoon/evening should be somewhat similar to the past couple days with activity concentrated over eastern Arizona higher terrain sending outflows downhill into lower elevations. HRRR output continues to be the most bullish in maintaining some form of deep convection into lower elevations, however this model has displayed a high percentage of false alarm forecasts the past few days. Given boundary layer mixing ratios are still hovering at a marginally supportive 9-10 g/kg, it will likely take multiple colliding outflows to invigorate new updrafts removed from higher terrain; and with some amount of convective overturning in SE Arizona, extensive storms wouldn`t be expected in this area discounting strong outflow emanating from this direction. As a result, the preponderance of evidence suggests limited chances of storms surviving too far from the mountains, and even NBM POPs seem a bit overaggressive. Convective potential begins to change over the weekend as the aforementioned subtropical high starts shifting to the northwest allowing better divergence aloft combined with favorable steering flow to direct midlevel ascent and deeper moisture intrusion into the CWA. Although better quality moisture will likely still remain removed from the forecast area Saturday with MLCape struggling to exceed 500 J/kg, HREF output indicates a greater concentration of storms in eastern Arizona thrusting outflow and deeper moisture westward during the evening/overnight. While a few lower elevation storms Saturday evening might be expected, this scenario looks to set the stage for a greater impact event Sunday. Forecast BUFR soundings suggest mixing ratios finally achieving a preferred 11-12 g/kg level yielding a "sweet spot" 1000-1500 J/kg MLCape juxtaposed with DCape in excess of 1500 J/kg. Increasing N/NE midtropospheric flow should direct storms initiating along the Rim into lower elevations with minimal inhibition and favorable synoptic conditions encouraging additional development. Strong, locally damaging winds and areas of dense blowing dust have historically been associated with similar scenarios. While assumed forward storm motion would be progressive limiting flooding issues, it cannot be totally discounted especially across recent wildfire incidents. Forecast confidence deteriorates quite rapidly for the remainder of next week as uncertainties in both the mesoscale and larger synoptic patterns grow substantially. Daily convective trends will almost certainly be modulated by previous days activity and ability for atmospheric recovery after the influence of convective complexes. However, there is convincing evidence that 10-12 g/kg mixing ratios will be maintained through at least the middle of the week resulting in a continuation of MLCape above 1000 J/kg and convective potential after peak heating. Adding to the uncertainty towards the middle of the week is a small handful of ensemble members indicating the Southwest subtropical high re-strengthening (most aggressive members near 600dm) and shifting back into northern Arizona resulting in warming aloft and larger scale subsidence. However, while the majority of ensembles also shift the high pressure center back southeast, its magnitude is far weaker in response to deep troughing approaching/entering the NW Conus. Given this model spread, mandated NBM forecasts are only advertising temperatures in a slightly above normal range with POPs essentially set near climatology, though the range of possible outcomes is far wider than the official deterministic NBM forecast. && .AVIATION...Updated at 1207Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation weather concern through the TAF period will once again be the potential for abrupt wind shifts associated with thunderstorm outflow boundaries this evening. Winds this morning will favor easterly directions through about mid morning, followed by a period of very light speeds (aob 5 kt) and variability as winds try to shift out of the west by late morning/early afternoon. Some westerly gusts to 15-20 kt will be possible late this afternoon. Slight chances (around 20%) for VCTS/TSRA will exist for Central Phoenix this evening, but chances are slightly higher to the north and east, thus VCTS was included for KDVT and KSDL starting at 02Z. Confidence in wind directions after that time is low as Hi-Res guidance indicates outflows making their way into the Greater Phoenix Area early this evening, but the most likely initial outflow boundary with push out of the N/NE. Outside of any outflows, winds should eventually settle out of the east once again during the early overnight hours tonight. Skies will remain FEW-SCT and occasionally BKN this evening as debris clouds from earlier convection move overhead. Southeast California/Southwest Arizona including KIPL and KBLH: Winds at IPL will favor SE through this afternoon before likely shifting SW-W this evening, with speeds mostly aob 10 kt sustained. There is low confidence (10% chance) of a southwesterly outflow making its way to IPL from any isolated convection that occurs over the mountains. Winds will continue to favor S-SW at BLH through the TAF period. Skies will be FEW to at times SCT, with passing mid and high level clouds. && .FIRE WEATHER... Scattered afternoon thunderstorms will be common over higher terrain areas of eastern districts through at least the middle of next week with storms becoming more likely in lower elevations and western districts early next week. The main threats with these storms will be strong, gusty outflow winds which could exacerbate issues with any ongoing wildfires or new starts. Winds outside the influence of thunderstorms will generally exhibit typical afternoon upslope tendencies with gusts commonly 20-25 mph. Minimum afternoon humidity levels will mostly fall in a 15-30% range through the weekend, albeit improving closer to a 20-35% range next week. Similarly, overnight recovery should also improve from a 20-45% range to a 30-70% range. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST Sunday for AZZ530. Excessive Heat Warning from 10 AM this morning to 8 PM MST Saturday for AZZ531>546-548>555-559. CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT Sunday for CAZ560>570. && $$ DISCUSSION...18 AVIATION...Whittock FIRE WEATHER...18