textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- High pressure will center itself over Arizona and strengthen late this week resulting in record to near record temperatures and widespread major to extreme Heat Risk.

- Extreme Heat Warnings have been expanded to cover the entire forecast area starting Thursday, and continuing through the weekend.

- Drier air filtering into the region will limit any chances for showers and thunderstorms with only isolated coverage over the far eastern Arizona high terrain.

SHORT TERM /TODAY THROUGH FRIDAY/

A worked over atmosphere from yesterday's early morning rainfall along with a strengthening of the upper level ridge centered over New Mexico resulted in very little convection during the afternoon/evening hours yesterday. Temperatures were held down a few degrees due to the morning cloud cover and rain-cooled air, but the extra humidity still made it feel quite uncomfortable.

Drier air already working into southeast California and southwest Arizona will spread eastward through Thursday, lowering PWATs to slightly below normal for late July while surface dew points drop into the upper 40s to the mid 50s. The ridge center is also forecast to strengthen as it shifts directly over Arizona by Friday, increasing the subsidence across the region. The combination of the drier air and the stronger subsidence will essentially end monsoon activity across all but the far eastern Arizona high terrain. For today through Friday, any monsoon activity is likely to be confined to the White Mtns and maybe over portions of the Mogollon Rim.

The main forecast concern for the rest of the week will be the building heat and the expected widespread Major HeatRisk setting up over much of the area by Friday. Guidance has been consistent in showing the ridge strengthening from around 595dm currently to 600dm by Friday as the center shifts directly over Arizona. The expected strength of this ridge will reach record levels for this time of year over a good portion of the Southwestern U.S. by Friday. As the heat worsens over the next few days, humidities will lower somewhat but it should still feel a bit humid with daytime heat indices essentially mirroring air temperatures. High temperatures today should top 110 degrees across the western lower deserts to right around 110 degrees in the Phoenix area, but we are expecting to add 2 degrees each day through Friday or Saturday. Fairly widespread Major HeatRisk is expected beginning Thursday followed by widespread Major to localized Extreme HeatRisk starting Friday. Daytime highs should top 115 degrees over the hottest lower deserts by Friday with overnight lows increasing into the upper 80s to the mid 90s.

LONG TERM /SATURDAY THROUGH TUESDAY/

Dangerous heat will impact the region over the weekend and potentially extending into early next week. The peak strength of the upper level ridge is forecast to occur on Friday and Saturday before weakening Sunday into Monday. Even though guidance is forecasting record H5 heights on Friday and Saturday, forecast highs are barely reaching daily records. This is either due to the modest low level moisture remaining in place holding down daytime temperatures a bit, or the NBM is under forecasting temperatures. We are leaning toward the NBM deterministic forecast temperatures not being hot enough, but not enough to warrant nudging the forecast upward. It would not be surprising to have actual highs closer to the 75-90th percentile of guidance, or a couple degrees higher than the current forecast. We are likely to have some areas reaching 120 degrees across the lower deserts of southeast California on Saturday and Sunday, while the Phoenix area could conceivably reach 118 or even 119 degrees. Overnight lows will also provide little relief from the extreme heat with readings mostly in the upper 80s to the lower 90s to as high as the mid 90s in the urban core of the Phoenix metro.

Monsoon activity is expected to remain very limited on Saturday with little to no shot at any outflows nearing the lower deserts, but that may change by Sunday or Monday. As the ridge begins to weaken Sunday into Monday, guidance shows evidence of increasing low level moisture and more monsoon activity across eastern Arizona and northern Mexico. However, chances still remain fairly low on Sunday and Monday with high terrain PoPs of 20-30% and 10-15% into the Phoenix area during the evening hours. As long as the ridge continues to weaken early next week, monsoon convection should also increase, especially by the middle of next week as there are hints of a potential easterly wave moving into or near Arizona. As the ridge weakens and moisture increases, temperatures are likely to drift lower, but another day or two of Major to Extreme HeatRisk looks possible for Monday and/or Tuesday, especially across southeast California and southwest Arizona.

AVIATION

Updated at 1715Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns will exist through the TAF period under periods of FEW-SCT mid and high level cloud decks. Westerly winds will prevail through much of the period, with occasional late afternoon gusts into the teens. An increase in westerly winds may also occur late evening/overnight, acting to prevent or delay the typical E/SE switch. If winds do not switch E/SE overnight or early Thursday morning, a prolonged period of light and VRB winds may be favored instead.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns will exist through the next 24 hours under periods of FEW high cirrus and otherwise mostly clear skies. Winds at KBLH will continue to favor southerly directions with speeds 8-12 kts and afternoon gusts around 20 kts. At KIPL, winds will continue to oscillate between SE during the daytime and SW/W during the evening into the overnight hours, with speeds generally AOB 10 kts and a potential for a period of evening westerly gusts into the upper teens.

FIRE WEATHER

Strong high pressure will gradually build over the region through the rest of the week, while drier air also moves in from the southwest. These two factors will severely limit monsoon activity as very little activity is expected through at least Friday. Expect humidities to gradually lower over the next couple of days with MinRHs settling at 10-15% by Friday. Record to near record temperatures are also forecast for Friday and through the upcoming weekend. Winds will mostly follow diurnal patterns, but west southwesterly winds will be dominant much of the time with afternoon gusts of 15-20 mph at times.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530-532-537- 540-542>544-546-548-550-551.

Extreme Heat Warning from 10 AM Thursday to 8 PM MST Sunday for AZZ531-533>536-538-539-541-545-547-549-552>563.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ567-569-570.

Extreme Heat Warning from 10 AM Thursday to 8 PM PDT Sunday for CAZ560>566-568.