textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Saturday with areas of Major to Extreme Heat Risk, despite temperatures slowly decreasing.

- Today will bring low end chances for showers and isolated thunderstorms, with a low probability for strong to severe outflow winds and blowing dust focused in Southeast California.

- An approaching low pressure system will bring much better chances for rain, including a chance of strong to severe storms across Southern Arizona mainly on Sunday and Monday.

SHORT TERM /TODAY THROUGH SATURDAY/

Analysis of midlevel WV imagery reveals that the flow aloft is in the process of shifting from southeasterly to southerly/southwesterly as Pacific troughing impinges on the forecast area from the northwest. The upstream troughing has begun to impact the subtropical ridge more noticeably, displacing the high eastward with some signs of weakening early this afternoon compared to previous days. This will become much more apparent tonight into Saturday, as ensembles continue to be in excellent agreement that H5 heights will drop from the current 594-597 dam closer to 591-594 dam tomorrow. Midlevel moisture transport from over Sonora is noted in the water vapor imagery as well and has become apparent in the 12Z PSR sounding with PWAT analyzed at 1.57", up from 1.26" yesterday. Low level moisture, however, remains generally unsupportive (marginally supportive at best) for the formation/maintenance of lower desert thunderstorms, with 1000-700 mb mean mixing ratios still below 10 g/kg, but hovering close to 9 g/kg across much of the area.

Through the rest of today, moisture looks to remain fairly constant, with midlevel subsidence still evident in area GFS bufr soundings, which makes sense given anticyclonic flow aloft. However, with this amount of moisture, only one outflow boundary passing through an area may be enough to moisten the low levels sufficiently to support deep moist convection if followed by another lifting mechanism (second outflow boundary). Inverted-V soundings and DCAPEs around 1000-1500+ J/kg areawide will mean that distant thunderstorms over AZ high terrain and portions of Southeast AZ are likely to form long- lived outflows, pushing into South-Central AZ lower deserts during the evening. The most likely outcome for the Greater Phoenix Area this evening would be another round of isolated-scattered elevated showers and storms forming later in the evening or overnight from earlier outflows or distant gravity waves. As steering flow transitions out of the southwest, particularly across the western CWA, there will be better chances for decaying thunderstorms to move off the Baja Spine into Imperial County mid to late this afternoon. HREF probabilities indicate a potential for additional development of thunderstorms over the Southeast CA deserts and even 10-30% neighborhood probabilities of 58+ mph wind gusts near storms, but these storms will be feeding off a decaying source of instability as southwesterly flow rapidly dries the lower levels. A 10% neighborhood probability for severe gusts is also indicated over prominent terrain of Central/northeastern La Paz County late this afternoon into the evening. If this comes to fruition, expect areas of blowing dust to cause reductions in visibility that could become hazardous to motorists/aviation users across portions of Southeast CA and Southwest AZ (mainly La Paz County).

What occurs Saturday afternoon/evening is still quite uncertain as it will be more of a conditional type environment for storm development. Moisture is expected to have increased enough to bring MUCAPEs up to around 1000 J/kg into South-Central Arizona, but not enough to erase all of the CIN. The question is where exactly will showers and storms develop Saturday afternoon and if a large enough cluster can develop to force a strong or multiple outflows into the lower deserts. It is looking more encouraging for storm potential as the amount of CIN does not look too high and any decent outflow should overcome it. We will also start to see some upper level support from the incoming trough. Latest HREF consensus is for storms to eventually congeal over Central Pima County then trend northwards over the Tabletop Wilderness and southern Pinal County, sending outflows northward towards the Greater Phoenix Area during the early evening, with isolated- scattered development of elevated showers/storms behind those outflows. Saturday should also be the last of the extreme heat with forecast highs dropping to 107-111 degrees before a much more noticeable cool down begins Sunday.

LONG TERM /SUNDAY THROUGH THURSDAY/

Sunday and/or Monday are still looking quite interesting for thunderstorm potential with the environment potentially supporting some severe storms. Some ingredients for strong to severe storms are expected to be present, like upper level support and sufficient instability, but wind shear will be somewhat lacking. Moisture is forecast to improve enough to support a nearly uncapped environment across south-central Arizona Sunday and potentially again Monday afternoon with MUCAPEs as high as 1500-1800 J/kg. Both days have the potential to bring scattered to numerous showers and thunderstorms across south-central and eastern Arizona. Strong to localized severe wind gusts, blowing dust, heavy rainfall, and flooding all look possible, but as stated above the threat may be localized due to the fairly weak shear which is oftentimes needed to sustain strong storms or clusters of storms. We should have a better idea on how this strong to severe storm threat may evolve by Saturday/Saturday night. Guidance then suggests some drying will occur by Tuesday which should decrease rain chances substantially through the middle of next week.

Temperatures will drop into or even just below normal early next week, but highs are still forecast to top 100 degrees each day across the lower deserts. Once the drier air moves into the region by the middle of next week, we expect temperatures to quickly warm to a few degrees above normal but with little chance of reaching 110 degrees again.

AVIATION

Updated at 1735Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

A gusty southerly outflow with some lofted haze this evening will be the primary aviation weather issue during the next 24 hours. In the meantime, winds will gradually shift out of the west by the early to mid-afternoon hours with a period of southerly winds before the eventual westerly shift. Overall speeds through this afternoon are forecast to remain aob 10 kts. Thereafter, guidance continues to show a southerly outflow from convective activity in southeast AZ propagating into the area, sometime between 03-06z, with some gusty winds potentially upwards of 25-30 kts as well as some reduced VIS due to lofted HZ/BLDU. After the outflow passage later this evening, VCSH is possible during the overnight period, mainly between 06-12z, with minimal operational impacts expected. SCT to at times BKN mid/high cloud decks will be common through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Isolated TS activity, especially at KIPL, this afternoon will be the primary aviation weather issue during the next 24 hours. Winds will fluctuate between the southeast to west at KIPL and south to southwest at KBLH. Heading into this afternoon, TS activity over the Baja Spine may drift into the Imperial Valley, affecting KIPL, however, there continues to be uncertainty in the overall coverage and intensity of the TS activity and thus a PROB30 remains in the latest TAF package. If a TS would to materialize, gusty winds and reduced VIS due to HZ/BLDU will be the main impacts. KBLH may see an outflow from the activity to the southwest, but confidence remains too low for inclusion in the latest TAF package. FEW-SCT mid/high cloud decks will be common throughout the period, with occasional BKN decks with any convective activity.

FIRE WEATHER

Hot and relatively dry conditions will continue into this weekend, but with slight chances for isolated showers and thunderstorms. Chances then increase considerably by Saturday night/Sunday, especially across Southern and Central Arizona. Afternoon minimum humidities will range between 17-25% Saturday and 15-30% areawide Sunday following fair to good overnight recoveries. Winds will be relatively light and follow diurnal tendencies through tonight before south to southwesterly winds increase this weekend, particularly across the western deserts, where afternoon gusts increase up to 20-30 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Saturday for AZZ530>555-559- 560.

Extreme Heat Warning until 8 PM MST this evening for AZZ556-561- 562.

CA...Extreme Heat Warning until 8 PM PDT Saturday for CAZ562-565>567- 569-570.

Extreme Heat Warning until 8 PM PDT this evening for CAZ561-563- 564-568.