textproduct: NWS Phoenix

KEY MESSAGES

- Above normal temperatures with minimal day-to-day changes will carry through the rest of this week as lower deserts top out around 97 to 108 degrees.

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

- Dry weather will persist through the at least the end of the workweek before a potential cooler and more unsettled pattern arrives this weekend into early next week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

RAP analysis this afternoon shows a mostly quiet conditions across much of the western CONUS, with only a very weak trough present over the Intermountain West. Outside of this minor disturbance, high pressure persists over the region and will continue to do so through the middle portion of the week. 500mb heights currently stand around 588-591 dam, nothing too out of the ordinary for the Desert Southwest, but still 1-1.5 standard deviations from typical climatological normals during the front half of October. This relatively enhanced atmospheric thickness will allow for temperatures to warm further through the middle portion of the week, pushing readings to around 10 degrees or so above normal for this time of year, which translates to lower desert highs between 97-107 degrees. The higher end of the range will be collocated with the highest heights, which will be focused over southwestern Arizona and southeastern California. Although expected temperatures will be far from what we see during the peak of summer, they will still reach near record levels (see climate section), which will produce areas of Major HeatRisk. As a result, the existing Extreme Heat Watch has been upgraded to a Warning, which will go into effect starting late Wednesday morning.

LONG TERM /THURSDAY THROUGH MONDAY/

Though ridging aloft will begin to weaken during the back half of the week, low-lvl thermal profiles will lag behind a bit as 850mb continue to rise toward 26-27C into Thursday. This will result in sfc highs reaching 102-108 degrees across the western deserts, again nearing daily records in Yuma and the Imperial Valley. Major HeatRisk is expected to be most expansive on Thursday afternoon, leading to a spatial extension of the Extreme Heat Warning to include the entire Lower Colorado River Valley and a portion of the Gila River Valley in Yuma County including Dateland and Tacna. Portions of south-central AZ will also see highs climb to around 100-103 or around 10 degrees above normal on Thursday, although the HeatRisk is expected to remain in the Moderate category. Friday will be a transition day where a deepening trough over the Pac NW which in turn induces SW flow aloft over the West Coast. Although we will begin to see a reduction in overall hghts aloft on Friday afternoon, low-lvl thermal profiles will be slow to respond as 850 mb temperatures are still expected to reach 25-26C across the much of the forecast area equating to sfc highs around 100-105 degrees in most lower desert communities. Due to these temperatures and lingering Major HeatRisk for the Imperial Valley and Yuma area, the Extreme Heat Warning will continue through Friday.

We then turn our attention to an upcoming pattern shift heading into this weekend as most deterministic models and ensemble members are trending toward an overall troffing pattern setting up across the western CONUS. There is now better model agreement that a stronger and deeper trough will transition into a cutoff low over the Pac NW on Saturday and gradually sink southward into CA Sunday and Monday. Meanwhile, latest forecast guidance from the NHC shows an eventual turn to the northeast by Rachel this weekend with the remnants of the storm itself being pulled northeastward and phasing with the west coast trough. As this pattern transpires, there should be an increase in southwesterly flow aloft over the Desert Southwest which will begin to pull Pacific moisture into the region by late Saturday into Sunday. Coincidentally, rain chances will return to the much of the region with NBM PoPs now around 20-50% across south-central AZ on Sunday and even up to 40-60% across the foothills NE of Phoenix by Monday. There is still high uncertainty in rainfall amounts at this time but there is at least a chance of some higher totals exceeding 1.50" across AZ high terrain. As 500 mb hghts begin to lower ahead of the approaching trough along with increasing cloud cover and moisture, high temperatures will take a dive from the mid 90s on Saturday to the low to mid 80s by Monday.

AVIATION

Updated at 1720Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns under FEW occasional mid-level cloud decks can be expected throughout the TAF period. Winds today will return to a more traditional diurnal pattern with the westerly shift likely by mid-late afternoon. Wind speeds are expected to remain light, mostly below 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns will exist through the next 24 hours under mostly clear skies. Winds will generally remain light, with extended periods of variability and directions favoring a westerly component at times.

FIRE WEATHER

Dry conditions with well above normal temperatures will continue through the week, with afternoon highs reaching up to 100-107 degrees across the lower deserts (peaking Thursday). Winds will return to their more typical light diurnal tendencies today and persist into Thursday before increasing again Friday into this weekend. MinRHs will mostly range from 15-25% across the western districts to 20-35% across eastern districts through the week with fair to good overnight recovery (35-60%) area-wide. Humidities will be lowest on Thursday when temperatures are likely at their hottest, but winds will still be light at this time, so there are no elevated fire weather concerns. No wetting rain expected through at least Saturday. However, chances for wetting rainfall is expected to arrive by the end of the weekend/start of next week as a weather system begins to move into the region.

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 from 10 AM Wednesday to 8 PM MST Friday for AZZ532.

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

CA...Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Friday for CAZ566-567.

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