textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES.

- Temperatures will remain several degrees above daily normals through this weekend with lower desert highs ranging 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 in coverage through mid-week with chances returning to the south-central Arizona lower deserts as early as Wednesday.

SHORT TERM /TODAY THROUGH THURSDAY/.

Latest mid-lvl wv imagery and 500 mb streamline analysis reveals an elongated area of high pressure stretching across the western CONUS. Currently the ridge axis is situated over western AZ allowing 500 mb hghts to reach 596-597 dam across our forecast area. Hghts of this magnitude are above the 90th percentile of climatology for this time of year. As such, high temperatures will once again top out well above normal this afternoon, ranging from 108-113 degrees across the lower deserts. The ridge axis will begin to shift NE over the AZ/UT border on Wednesday which will promote a slight decrease in overall heights/thickness. Although highs will cool by a degree or two Wednesday and Thursday, apparent temperatures (heat indices) are actually expected to increase to around 109-117 degrees as dewpoints rise into the mid 60s both afternoons. Thus, widespread Major HeatRisk is expected to persist and the Extreme Heat Warning will continue.

Moisture is still expected to be very limited over south-central AZ this afternoon with PWATs hovering around 1.2-1.3". Therefore, only isolated high-terrain convection is anticipated across S Gila County. The northward migration of the subtropical ridge will allow for much better moisture advection into the region starting on Wednesday with PWAT values rising to around 1.5-1.7". This noticeable increase in moisture along with greater instability (ML CAPE 800-1000 J/kg) and a less robust capping inversion (ML CIN -80 to -100 J/kg) will allow for better odds of storms reaching the lower deserts by Wednesday evening with NBM PoPs now around 20-30%. The overall environment will also be favorable for longer- lived outflow boundaries and strong winds which could result in blowing dust across the favorable dust prone regions of N Pinal and SE Maricopa Counties. Almost identical environmental conditions are anticipated on Thursday, although convective potential across the lower deserts will be highly dependent on how much storm activity we see Wednesday evening. For now, NBM PoPs are currently 20-30% across southcentral and southwest AZ Thursday afternoon/evening which seems reasonable.

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 2345Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The only aviation weather concerns through Wednesday afternoon will be low confidence on an easterly outflow reaching the terminals this evening, causing an abrupt wind shift, followed by elevated westerly winds overnight and modest LLWS conditions (below TAF criteria). Confidence is low on exact wind directions Wednesday morning, however, they should relax to AOB 6 kts, followed by a typical increase in westerly winds late morning into the afternoon. Occasional late afternoon gusts between 15-20 kts are anticipated. Cloud bases will remain generally above 8 kft AGL through the TAF period, with FEW-SCT and occasionally BKN coverage of mid and high cloud decks.

Wednesday evening outlook: Conditions are more favorable for strong, gusty outflows to survive into the lower deserts and could spark additional SHRA/TSRA in the Phoenix Airspace. Chances for SHRA/TSRA are currently around 20%.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather concerns will exist through the next 24 hours under FEW-SCT high cloud decks. Winds at KIPL will favor southeasterly with speeds AOB 10 kts sustained, however, there is a chance for TEMPO SW-W winds with occasional gusts into the mid teens for a period this evening. Winds at KBLH will continue to oscillate between SSE and SW with occasional gusts to around 20-25 kts, especially by Wednesday morning.

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 afternoon high temperatures topping out around 108-113 degrees through the rest of this week. Widely scattered showers and storms are forecast for the AZ high terrain today with an increase in coverage, including the lower deserts expected Wednesday into Thursday. Relative humidity values of 15-25% today will increase to 20-30% starting Wednesday with continued fair to good overnight recoveries. 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>555-559-560.

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