textproduct: NWS Phoenix

KEY MESSAGES

- Above normal temperatures will prevail through the rest of the week, resulting in widespread moderate Heat Risk and areas of Major Heat Risk.

- Extreme Heat Warnings will remain in effect for portions of the lower deserts through the end of the week.

- Thunderstorms chances will continue to focus over Southeastern Arizona and the Eastern Arizona high terrain, with low chances for lower desert storms beginning Thursday and slightly better Friday.

- A pattern shift by the weekend will result in decreasing temperatures and eventually drying conditions from west to east across the forecast area.

SHORT TERM /TODAY THROUGH THURSDAY/

Midlevel water vapor imagery and 500 mb RAP analysis confirm that southeasterly flow aloft has established across the region, as subtropical high pressure quickly retrogrades from the Southern Plains into the Desert Southwest. Abundant midlevel moisture is still apparent in the wv imagery, with some lingering elevated showers/sprinkles moving through Southwest AZ into Southeast CA ahead of a remnant mesoscale vortex. Additionally, a gulf surge moved through yesterday evening, resulting in dewpoints across a good chunk of the area surging into the 60s and even lower 70s for locations closer to the Gulf of California. The main question for the remainder of the week will be whether the combination of better moisture profiles and instability will help bring thunderstorm chances to the lower deserts. Moisture and instability will be sufficient, but will there be a sufficient lifting mechanism to trigger lower desert storms? Latest guidance is not optimistic for the next few days (through Thursday). Under anticyclonic subsidence with the subtropical high shifting almost directly overhead later today, inhibition will likely be too strong (CIN around -70 to -100 J/kg or lower) for thunderstorms to develop anywhere but over higher terrain of Southern Gila/Northern Pinal/far northeastern Maricopa Counties. Additionally, low level mixing ratios look to drop into a 9-10 g/kg range due to the shallow gulf surge moisture mixing through the PBL. However, distant outflows making their way into South- Central AZ lower deserts from the east and/or southeast looks likely. This, combined with another gulf surge may help to further moisten the lower levels tonight, setting the stage for slightly better chances/coverage of storms Thursday. CAMs do also show the potential for scattered altocumulus showers to develop tonight if a sufficiently strong gravity wave propagates through the forecast area from earlier convection over Sonora/Southeast AZ. Latest NBM came in with <10% chances across all but the high terrain of S Gila County Thursday, which seemed a bit too low given the weaker inhibition under a diminishing subtropical high. Thus, increased chances slightly for Thursday late afternoon/evening.

For the past couple mornings, 12Z PSR soundings have indicated several near-saturated layers in the middle and upper portions of the profile, consistent with the patchy mid and high cloud decks that have been draped across the forecast area. This moisture had been imported from a period southwesterly flow, but now, debris clouds from upstream convective activity to the east/southeast will likely provide at least partial cloud cover for the next few nights. With higher surface dewpoints and continued periods of overnight cloud cover through the next few days, inefficient radiative cooling will result in very warm overnight lows in the middle to upper 80s across the lower deserts, around 7F-10F above normals. Daytime highs will commonly reach a 4F-7F above normal range under the influence of the subtropical ridge, peaking between 105F-111F across the typically hotter lower desert locales. These temperatures translate to widespread Moderate HeatRisk with pockets of Major HeatRisk, primarily driven by the very warm overnight lows. It is also important to note that the higher humidity now in place will make the heat less tolerable and potentially more dangerous, with heat indices peaking upwards of 108F-118F during the latter half of the work week. As a result, Extreme Heat Warnings will remain in effect for portions of the lower deserts through the end of the week.

LONG TERM /FRIDAY THROUGH TUESDAY/

The upper level pattern will begin to shift Friday, as troughing enters the Northwest US and displaces the subtropical high east of the region this weekend. Flow aloft will transition back out of the southwest, ending the brief, more typical monsoon flow pattern experienced during the middle of the work week to something more akin to a transition season setup. However, before more pronounced southwesterly drying begins in earnest this weekend, weaker flow aloft and perhaps the best moisture profiles of the week over South- Central AZ (near-surface mixing ratios 11-12 g/kg) will present an opportunity for at least one active monsoon thunderstorm day across the lower deserts (if the highly conditional threat is not realized on Thursday). KPHX GFS bufr soundings, which admittedly have been very sensitive to subtle changes in low level moisture, indicate a balance of CAPE/DCAPE around 1500 J/kg late Friday afternoon into the early evening, with weak (greater than -50 J/kg) CIN for several hours. On Saturday, thermodynamics also look similarly favorable, with the cap almost completely disappearing. Both days, the threat is once again highly conditional, as there is no obvious upper level ascent mechanism.

Ensembles continue to disagree on how far south the jet will dip with the incoming Northwest US troughing this weekend, however, it will likely remain far enough removed that jet dynamics do not come into play to influence thunderstorm chances locally. Regardless, moisture will not be entirely scoured out of Eastern/Southeastern AZ, and daily monsoon thunderstorm activity will be maintained over prominent terrain features of Eastern AZ, Southeastern AZ, and Sonora into next week. Eventually, if the subtropical high can reassert itself east of the region and/or troughing weakens and retreats north, daily convection to our south could push better moisture into South-Central AZ and once again give our area a shot at more widespread thunderstorm chances by the middle of next week. Anticipate temperatures to decrease back into a near normal range over the weekend as a result of the pattern shift, and possibly slightly below normal next week.

AVIATION

Updated at 1125Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather concerns will exist through tonight under occasional mid/high cigs, patchy virga, and isold SHRA. Winds this morning are expected to shift mostly out of the east southeast, but some variability is likely. Directions should switch back to W/SW by noon with limited afternoon gusts.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather concerns will exist through tonight under occasional mid/high cigs and patchy virga/SHRA. Confidence in wind trends is only moderate with more of a southerly during the day, but extended periods of nearly calm conditions.

FIRE WEATHER

Above normal temperatures will prevail through the end of the work week, then temperatures are likely to decrease closer to normal over the weekend. Afternoon minimum humidities will generally range between 18-30% areawide through much of the week following fair to good overnight recoveries between 35-70%. Fairly typical diurnal winds with occasional afternoon upslope breeziness can be expected through Thursday, followed by an increase in southerly to southwesterly winds Friday into this weekend. 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 late week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ532.

Extreme Heat Warning until 8 PM MST Thursday for AZZ531-535>544- 546-548>551-559.

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

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ562-566-567.

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