textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- Well above normal temperatures will persist through the end of the week as lower desert highs reach triple digits.

- An Extreme Heat Warning remains in effect for portions of southeast California and southwest Arizona through Friday as temperatures warm to near record levels, resulting in major HeatRisk.

- Dry conditions will persist the next several days before a more unsettled pattern arrives over the weekend with much cooler temperatures and increasing rain chances.

SHORT TERM /TODAY THROUGH FRIDAY/

Current regional analysis reveals broad, but somewhat messy, ridging over much of the western CONUS, while a weak embedded shortwave continues to meander around the Four Corners. The latter of these features, combined with some relatively decent mid-level moisture, may aid in the development of some very isolated and and inconsequential showers over the White Mountains this afternoon as it slides down into eastern Arizona. Outside of that however, dominant high pressure will promote dry conditions across the Desert Southwest as we transition to the back half of the week.

The main focus in the near term will be the continuation of well above normal temperatures. With heights aloft topping out between 587-590 dam this afternoon, a good 1-1.5 standard deviation above climatological normal for early October, along with 850mb temperatures peaking between 25-27C, triple digit highs will be quite common the next few days across the lower deserts. Readings for south-central Arizona will range between 98-103 degrees while slightly hotter observations near 102-109 degrees can be expected across the western deserts. Although these values are not atypical during late spring and summer, they will still be 10 or so degrees above normal for this of year while nearing daily records at Yuma and El Centro. This will result in areas of Major HeatRisk and thus Extreme Heat Warnings have now gone into effect for the Imperial Valley and Yuma, with the warning expanded to include the Lower Colorado River Valley and along I-8 in Yuma County for Thursday.

Although the ridge will begin to wane Friday, thermal profiles will be slow to respond which will allow for one more day of triple digit heat. Readings will not be much different from the days prior, but perhaps a degree or two "cooler" from Thursday. This will allow for the cessation of at least part of the Extreme Heat Warning, but with lingering Major HeatRisk for the Imperial Valley and Yuma, these areas will remain within said warning to finish out the workweek.

LONG TERM /SATURDAY THROUGH TUESDAY/

A significant pattern shift is still anticipated this weekend through early next week as the most of the deterministic and ensemble model guidance continues to show a broad troughing pattern setting up across much of the western third of the CONUS. Guidance continues to show a northern stream trough diving into the Pacific Northwest on Saturday before sinking southward over CA on Sunday. Meanwhile, a more southern stream shortwave trough is likely to move into the subtropical eastern Pacific and pick up the remnants of Hurricane Rachel as it moves northeastward and phases with the west coast trough. As this whole pattern evolution takes shape, the increasing south to southwesterly flow aloft will advect moisture into the Desert Southwest starting Saturday, with most of the moisture at first confined in the mid to upper-levels before better low-level moisture moves in on Sunday as the remnants of Rachel gets pulled in.

Coincidentally, rain chances will be on the increase as early as Saturday afternoon across south-central AZ before peaking Sunday into Monday across much of the region with the arrival of better low- level moisture. There continues to be a great deal of uncertainty in rainfall amounts as it will be dependent on the overall track of the remnants of Rachel and how it interacts with the west coast trough. It will likely take at least another couple of days to get a better idea on the rainfall perspectives as guidance comes into better agreement on how the overall pattern shapes up. In addition to the rainfall potential, winds will be another hazard to contend with as the combination of the height falls from the trough and remnant circulation of Rachel will likely result in widespread breeziness.

As 500 mb heights lower with the approaching trough along with the increasing cloud cover and moisture, much cooler temperatures are expected this weekend into early next week. Afternoon highs across the lower deserts will go from mid to upper 90s on Saturday to low to mid 80s by Monday and may potentially drop even lower on Tuesday with many locations stuck in the low 80s.

AVIATION

Updated at 2305Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No weather issues will exist through Thursday evening under a few afternoon cumulus decks over mountains. Timing of wind shifts with speeds under 10kt will be similer to the past 24 hours, although the switch to a west component late Thursday afternoon may be more subtle, or just result in a prolonged variable component.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather issues will exist through Thursday evening under clear skies. Extended periods of light and variable winds will continue with a tendency for directions to vary between northerly and westerly.

FIRE WEATHER

Dry conditions along well-above normal temperatures will continue through the end of the work week as lower desert highs reach into the triple digits. Winds through Thursday will generally be light and diurnally driven before increasing Friday into this weekend. Afternoon MinRHs through the end of the work week will range between 15-25% with fair to good overnight recoveries (35-60%). A more unsettled weather pattern is expected this weekend through early next week as a weather system affects the area with much cooler temperatures along with increasing moisture and chances for wetting rainfall.

CLIMATE

Record highs: Date Yuma El Centro ---- ---- --------- Oct 7 111 in 2024105 in 1991 Oct 8 107 in 1996 106 in 1996 Oct 9 106 in 2024104 in 1996

PSR WATCHES/WARNINGS/ADVISORIES

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

Extreme Heat Warning from 10 AM to 8 PM MST Thursday for AZZ530- 536.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ566-567.

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