textproduct: NWS Phoenix

KEY MESSAGES

- Above normal temperatures will prevail starting Tuesday, resulting in widespread Moderate Heat Risk and areas of Major Heat Risk.

- Extreme Heat Warnings will begin Tuesday for portions of the lower deserts.

- Mostly quiet weather conditions are expected for the first half of the week, then chances for thunderstorms increase across Central and Eastern Arizona during the latter half of the week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

The RAP40 500MB analysis and GOES WV imagery continue to show the subtropical ridge positioned over northwestern TX, a large trough over NW CONUS. The Desert SW sits between these two features with a feint stream of upper level moisture being advected into the western lower deserts resulting in a few light showers/virga early this morning and lingering upper level clouds. Ensembles remain in good agreement that the subtropical high will begin to shift westwards, peaking over the Desert SW by mid week with H5 heights between 594-597 dam. 500 MB heights at this level are near climatological maximum values for the time of year. This will lead to afternoon temperatures 4-5 degrees above normal, resulting in temperatures between 105F-111F. Overnight lows will also provide very little relief as the lingering cloud coverage mentioned earlier will hinder radiative cooling, and dew points are expected to be well into the 60s across much of the forecast area and locally into the lower to middle 70s for areas such as the Lower CO River Valley and Imperial Valley. With all of these factors taken into consideration Major HeatRisk will make a return to most of the region tomorrow and even more widespread by Wednesday, with the overnight lows being the main driver as they are flirting with the record warm lows through the end of the work week. As a result, Extreme Heat Warnings have been issued for portions of the lower deserts staring on Tuesday (starting Wednesday for Lower CO River Valley, Desert Center, and La Paz County).

Little to no chance for showers/thunderstorms are expected through mid week under strong subsidence thanks to the subtropical high, with the exception being the far eastern high terrain. However, by Wednesday night, low level moisture ticks up with 1000-700 MB mixing ratios increasing to 10-12 g/kg. Enough instability, and this moisture in place, SC AZ lower elevations could develop isolated storms as early as Wednesday night.

LONG TERM /THURSDAY THROUGH SUNDAY/

Latest NBM/WPC guidance depicts the greatest coverage of Major HeatRisk across the forecast area on Thursday, with morning lows in the middle to upper 80s across the lower deserts and forecast highs 106F-110F. These temperatures during the hottest time of the year may not prompt extreme heat headlines, but as we head into meteorological fall, daily normals drop off noticeably, resulting in more sensitive Major HeatRisk thresholds than what folks may have gotten accustomed to over the last few months. It also makes sense that Thursday would be the warmest of the next several days, as thermal profiles would have had more time to respond to the near climatological maximum strength ridge overhead. Heat indices will likely also be at their highest values, peaking between 108F-117F across the lower deserts. Temperatures then begin a gradual downtrend heading into the weekend as the subtropical high weakens and is displaced from overhead.

As anticyclonic subsidence weakens, especially by late Thursday and into Friday, chances for lower desert thunderstorms become introduced across South-Central AZ (20-30%). A familiar monsoon thunderstorm evolution may begin to take place beginning Thursday and could last into the weekend, with afternoon thunderstorms focused over the eastern AZ high terrain, which will push outflows towards South-Central AZ in the evening that may be capable of sparking additional thunderstorms. Current thermodynamic profiles suggest all the typical monsoon thunderstorm hazards may come into play, including strong, gusty winds, blowing dust, localized heavy rain resulting in flash flooding, and frequent lightning. However, as we head into the weekend, the forecast confidence decreases substantially. WPC cluster analysis reveals large discrepancies, with close to 40% of ensemble members comprising a solution which shows negative midlevel height anomalies diving into the northwestern and north-central CONUS, possibly impacting the flow aloft as far south as the Desert Southwest and turning winds out of the west, beginning a drying trend. Other solutions comprise clusters which show subtropical ridging remaining in place somewhere near/over the Southwest US and keeping the influence of northern stream energy far enough north so as not to influence flow aloft significantly. Either way, drying will likely not be pronounced enough to preclude rain/thunderstorm chances once a sufficient forcing mechanism becomes introduced. Ensemble mean PWATs even remain near or above seasonal levels this weekend. Current NBM/WPC PoPs and QPF suggest higher/more widespread chances heading into Sunday, perhaps picking up on the influence of a distant/elongated shortwave trough axis rotating through from the Northwest US troughing that a good chunk of solutions show.

AVIATION

Updated at 1715Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather issues under occasionally mid/high cloud decks can be expected throughout the TAF period. The overall wind pattern will follow the typical diurnal tendencies with some occasional gusts near 20 kts this afternoon/early evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues under occasionally mid/high cloud decks can be expected throughout the TAF period. Winds will generally favor a west to north-northwest direction throughout the period with speeds generally under 10 kts along with periods of light and variable conditions.

FIRE WEATHER

Temperatures will warm to above normal levels through the middle of the week. Afternoon minimum humidities will generally range between 20-30% areawide through much of the week, with Tuesday expecting a larger area across the lower deserts between 15-20% before rebounding back near 20% Wednesday. Fair to good overnight recoveries are expected throughout the week. Fairly typical diurnal winds with occasional afternoon upslope breeziness can be expected through much of the week. Rain and thunderstorm chances will remain low and mostly limited to the eastern Arizona high terrain through midweek before slight chances become introduced to the lower deserts of at least the eastern districts during the latter half of the week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ... Extreme Heat Warning from 10 AM Tuesday to 8 PM MST Thursday for AZZ531-532-535>544-546-548>551-559.

Extreme Heat Warning from 10 AM Wednesday to 8 PM MST Friday for AZZ530-533.

CA...Extreme Heat Warning from 10 AM Tuesday to 8 PM PDT Friday for CAZ562-566-567.

Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Friday for CAZ569-570.