textproduct: NWS Phoenix

UPDATE

Updated 12Z Aviation Discussion.

KEY MESSAGES

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

- Extreme Heat Warnings have been issued beginning 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/

Early morning midlevel water vapor imagery shows a trough pushing towards the Intermountain West from the Pacific Northwest, with an attendant SSW to NNE oriented jet streak originating over SoCal. This feature is helping to squeeze out some lingering, elevated light showers/virga across portions of the region which should mostly conclude by around sunrise. The Desert Southwest is already falling more so under the influence of a subtropical high centered approximately over the TX panhandle at this hour, which will become the predominant influence over local conditions as it shifts westward through the middle of the week.

Ensembles have come into much better agreement over the last 24-36 hours or so, with the consensus now advertising increasing midlevel heights as subtropical high pressure passes overhead and strengthens midweek. This will be accompanied by a return of easterly/southeasterly flow aloft, which will gradually allow better low-level moisture to push into the area from Sonora and support increasing thunderstorm chances during the latter half of the work week across at least South-Central AZ. In the short term, however, the main concern will be building extreme heat conditions. Ensemble mean H5 heights will increase upwards of 594-597 dam by Wednesday, near climatological maximum values for the time of year. Additionally, the increasing low level moisture through the middle of the week will bring dewpoint temperatures 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. This fairly rapid increase in surface dewpoints also likely reflects the influence of gulf surges returning as early as Tuesday night. Higher humidity and near climatological maximum H5 heights will support above normal temperatures, however, overnight lows appear to be the main driver for the areas of Major HeatRisk developing across the region. Afternoon highs around 5F above normal will become common across the lower deserts, but less effective overnight radiative cooling from the higher humidity and likely some mid-high level cloud cover overnight (whether it be debris clouds from Sonora convection or otherwise) will keep lows closer to 7F-10F above daily normals, providing little relief from the daytime heat. Apparent temperatures also near or exceed 110F across some of the typically hotter spots along the desert valleys of the western CWA. 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).

Rain and thunderstorm chances remain limited through Wednesday thanks at first to unsupportive low level moisture (7-9 g/kg mean mixing ratios between 1000-700 mb), then primarily due to anticyclonic subsidence centered directly overhead Wednesday into Thursday. Low level moisture and instability would otherwise likely be sufficient with a strong enough lifting mechanism to result in thunderstorm chances across South-Central AZ lower elevations as early as Wednesday. Expect storms to mostly be confined to the Southeastern/Eastern AZ high terrain through midweek. However, the change in the pattern during the middle of the week also brings the possibility of easterly waves providing a focus for thunderstorm development or convective complexes upstream/over Sonora to influence our area.

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 1130Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather issues will exist through the TAF period under occasionally thick mid/high cigs. Winds early this morning are light and variable at KSDL and KDVT. Winds should remain westerly at KPHX, but may briefly go light and variable at times this morning. At KIWA, winds may briefly go SE'rly between 14-16Z this morning. Winds will quickly reestablish a more defined W/SW direction heading by early afternoon. Some occasional gusts into the upper teens to around 20kt are possible, mainly at KPHX and KDVT. Winds should return to their typical diurnal tendencies tonight with a SE'rly component at KPHX and KIWA, a NE'rly component at KDVT, and light and variable to calm winds at KSDL.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues will exist through the TAF period under periodically thick mid/high cigs. Winds will generally favor a W/NW trajectory through the period, though periods of light and variable winds will be common.

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 following fair to good overnight recoveries. 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 CAZ566-567.

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