textproduct: NWS Phoenix

UPDATE

06Z Aviation Discussion

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 0455Z.

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

No outflows are expected this evening, with the main concern, through tomorrow morning, being the possibility of VCSH/VCTS during the overnight/early morning hours (similar to the past couple nights). Light (aob 10 kt) diurnal winds are expected through early tomorrow afternoon, with current westerly winds going southeasterly overnight and back westerly around noon. Winds could go light and variable with any VCSH/VCTS activity overnight/early tomorrow morning.

Turning to tomorrow afternoon and evening the forecast becomes a lot more uncertain with VCSH/VCTS/TS/RA and HZ/BLDU with reduced visibilities all possible. Therefore, a PROB30 has been introduced at this time from mid tomorrow afternoon through tomorrow evening. Hi-res guidance shows storms starting to pop-up within the PHX Metro around 22Z. Gusty and erratic winds are expected with thunderstorm activity. Hi-res guidance shows a brief break in activity possible during the early evening hours, with another round in the mid to late evening hours. Models also show multiple colliding outflows moving through the metro tomorrow form all directions. Any outflows from the S/SE would also bring BLDU/HZ with them. Due to all the possibilities for tomorrow night and multiple outflows, confidence in the forecast, especially wind direction, is low. Winds are currently forecasted to settle back out of the east once storms subside, but they could remain highly VRB. SCT to at times BKN mid/high cloud decks will be common through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will tend to follow their typical diurnal patterns. Oscillating between southerly and southwesterly at KBLH and southwesterly and southeasterly at KIPL. Wind speeds will generally be aob 10 kt, with some gusts up around 20kt expected tomorrow afternoon at both terminals. FEW-SCT mid/high cloud decks will be common throughout the period.

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.

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