textproduct: NWS Phoenix

UPDATE

Updated 12Z Aviation Discussion.

KEY MESSAGES

- Dry conditions along with temperatures running about 10 degrees above normal will persist through Friday.

- 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 areas of major HeatRisk.

- A much more unsettled weather pattern is expected this weekend through early next week as a series of weather systems affect the region resulting in much cooler temperatures, increasing winds, and elevated rain chances.

SHORT TERM /TODAY THROUGH FRIDAY/

Latest objective analysis shows a broad ridge of high pressure in place across the region while a very weak shortwave is situated near the Four Corners. This shortwave in combination with enough mid- level moisture will likely produce another round of isolated to widely scattered showers across the White Mountains once again for this afternoon. Otherwise, with high pressure continuing to remain the dominant feature over much of the region, dry conditions will persist through Friday.

With 500 mb heights ranging between 586-589 dam, afternoon high temperatures will once again be running about 10 degrees above normal across the region as lower desert highs will range between 100-103 degrees across south-central AZ and 102-109 degrees across the western deserts. With temperatures nearing daily records at both Yuma and El Centro, areas of Major HeatRisk will continue and thus Extreme Heat Warnings are in effect for the Imperial, Lower Colorado River, and Gila River Valleys as well as for Yuma. Even though the ridge will be on a weakening phase heading into Friday, low- level thermal profiles will not change that much and thus another day of widespread triple digits will continue, although likely a degree or two cooler from today. Areas of Major HeatRisk will continue to be prominent across the Imperial Valley and Yuma area and thus Extreme Heat Warnings remain in effect for these areas.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

A major pattern shift is still on schedule for the weekend through early next week as a troughing pattern will set up across much of the western third of the CONUS. A large northern stream trough is expected to dive into the Pacific Northwest on Saturday before diving into CA by Sunday. In the meantime, a shortwave trough sweeping across the eastern Pacific will pick up Tropical Storm Rachel with both systems to be steered northeastward out ahead of the large west coast trough. The latest track forecast from the National Hurricane Center continues to show Rachel tracking into the far northern part of the Baja Peninsula late Saturday night before accelerating north-northeastward through the Colorado River Valley during the day Sunday as a post- tropical storm. South to southwest flow aloft out ahead of the Pacific trough and Tropical Storm Rachel will advect significant moisture into the Desert Southwest during the day on Saturday with both the EPS and GEFS showing PWATs exceeding 200-250% of normal by Saturday evening. The increasing moisture combined with an upper- level jet streak moving through the Great Basin, increasing the upper-level forcing for ascent, will cause areas of scattered showers and isolated thunderstorms to break out, mainly over central and southern AZ Saturday afternoon/evening.

As both the Pacific shortwave and the remnants of Rachel move through late Saturday night through Sunday, even deeper moisture will be pulled into the region and this will likely be the timeframe for the best rain chances across the region with NBM PoPs at greater than 50-60% area wide. There continues to be quite a bit uncertainty with the rainfall amounts as it will be dependent on the eventual track of the remnants of Rachel and will likely have to wait for the hi-res guidance data to have better confidence on amounts. In addition to the rainfall activity, winds will likely be another hazard to deal with, especially for areas east of the track of Rachel as the guidance is depicting 850 mb winds to exceed 40-50 kts, which will likely translate to surface wind gusts in excess of 40 mph. As both the Pacific shortwave and the remnants of Rachel lift to the north late Sunday through Monday, subsidence left behind will result in decreasing rain chances during this period before chances increase once again, mainly for south-central and eastern AZ, on Tuesday as the larger west coast trough progresses eastward.

With the troughing pattern expected to take place this weekend through early next week along with the increasing cloud cover and moisture, temperatures are expected to take a nose dive. Afternoon highs across the lower deserts will go from the low to mid 90s on Saturday to low 80s on Monday, with many areas Tuesday and Wednesday likely topping out in the 70s.

AVIATION

Updated at 1110Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No weather concerns will exist through the TAF period under occasional midlevel cloud decks. Confidence is very good that light east winds will prevail through mid/late afternoon before attempting to become W/SW. However, this switch may not completed, and a variable component may occur for several hours before reverting back to easterly late this evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather concerns will exist through the TAF period under clear skies. Extended periods of light and variable winds will persist with a general trend for directions to vary between northwesterly (mainly at KBLH) and westerly (mainly at KIPL).

FIRE WEATHER

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

CLIMATE

Record highs: Date Yuma El Centro ---- ---- --------- 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 this morning to 8 PM MST this evening for AZZ530-536.

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

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