textproduct: NWS Phoenix

KEY MESSAGES.

- Temperatures will remain several degrees above daily normals this week with lower desert highs between 107-113 degrees and overnight lows in the mid 80s to low 90s.

- Extreme Heat Warnings are in effect through Friday evening due to widespread major HeatRisk.

- Thunderstorm activity is expected to increase through mid-week with chances returning to the south-central Arizona lower deserts as early as Wednesday.

SHORT TERM /TODAY THROUGH THURSDAY/.

Upper level ridging is becoming more established across the Southwestern U.S., but it remains elongated from west to east with a weak disturbance moving westward near the Mexican border of New Mexico and Arizona. H5 heights have climbed to around 596dm helping to push daytime highs to above 110 degrees across the majority of the lower deserts. Widespread Major HeatRisk is expected to persist with highs mostly between 109-113 degrees through at least Wednesday with a bit more uncertainty starting Thursday. Models continue to pivot more toward increasing moisture starting today and more widespread storm chances by Wednesday.

The elongated ridge is expected to form an established center somewhere across western Arizona tonight before shifting more over northern Arizona on Wednesday. This shift in the ridge is expected to allow for better moisture advection into southern and central Arizona from the southeast and off the Gulf of California. Moisture is still expected to be quite limited today with only high terrain scattered showers and isolated storms, but that should change starting Wednesday. Forecast low level mixing ratios are now expected to climb to a respectable 10-12 g/kg by Wednesday afternoon. The exiting disturbance across northern Mexico is also likely to help provide some upper level support for storms on Wednesday with southeast Arizona positioned near the right entrance region of a jet streak just to our south. The 06Z HRRR is starting to pick up on a better signal for storm activity late Wednesday afternoon across the AZ high terrain and into the south- central AZ lower deserts during the evening. Any storms will still have to fight against some weak to modest CIN over the lower deserts, but if that can be overcome there should be plenty of instability for further storm development. We have added 10-20% PoPs for Wednesday evening through the Phoenix area to account for this possibility. The environment will favor strong gusty winds on Wednesday with DCAPEs over 1500 J/kg likely.

A similar monsoon storm set-up is expected for Thursday, but this time the NBM recognizes the potential with more reasonable 15-30% PoPs across the area. However, if the convection is more widespread on Wednesday, storm chances Thursday may be too optimistic. Additionally, if we do have more storm coverage than anticipated, it is plausible (but not very likely) for temperatures to meaningfully cool off by Thursday or Friday.

LONG TERM /FRIDAY THROUGH MONDAY/.

Ensemble guidance generally agree the subtropical high will recenter itself near the Four Corners region on Friday and then gradually shift eastward through New Mexico this weekend. The position of the high should allow for fairly persistent south southeasterly flow into Arizona, likely maintaining moisture levels for much of the period. As the ridge shifts more to the east, an upper level trough may also attempt to nudge into western portions of our region by early next week. We expect at least day-to-day low-end monsoon storm chances each day, but there are no clear indications of any decent disturbances that may enhance chances. Above normal temperatures are forecast through at least the upcoming weekend, but as the ridge shifts more to the east temperatures are expected to drop off a bit with highs falling below 110 degrees by next Sunday or Monday.

AVIATION

Updated at 0900Z.

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

No major weather concerns will exist through the TAF period under a FEW/SCT mid/high cloud decks. Current easterly winds will make a slow turn to westerly this afternoon with a brief period of southerly cross winds expected prior to the westerly shift. Models continue to show potential for another easterly outflow wind shift this evening (around 03-04Z). However, this looks to be brief and winds may go back westerly (except at KIWA) as models also indicate a Gulf surge tomorrow evening making the winds go back southerly/southwesterly.

Southeast California/Southwest Arizona including KIPL and KBLH:

No weather concerns will exist through the TAF period under clear FEW/SCT high cloud decks. Trends in wind speeds and timing of directional shifts will be similar to the past 24 hours featuring little to no gustiness.

FIRE WEATHER

Above normal temperatures with a steady increase in low-level moisture is anticipated over the next few days. Most lower desert communities will see temperatures topping out around 110-113 degrees. Widely scattered showers and thunderstorms are forecast for the high terrain today with an increase in coverage expected Wednesday into Thursday. Relative humidity values of 15-25% today will increase to 20-30% starting Wednesday. Outside of gusty and erratic winds near any thunderstorm, overall winds will follow the typical diurnal tendencies with some modest afternoon upslope gustiness.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ530>544-546- 548>551-553>555-559.

Extreme Heat Warning from 10 AM this morning to 8 PM MST Friday for AZZ560.

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