textproduct: NWS Phoenix

UPDATE...Updated 12z Aviation Discussion

KEY MESSAGES

- Above normal temperatures with minimal day to day changes will carry through the rest of this week as lower desert highs top out around 98 to 107 degrees.

- An Extreme Heat Watch 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 with zero rain chances will persist through Saturday before a potential cooler and more unsettled weather pattern arrives Sunday into early next week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Latest GOES mid-lvl wv imagery and 500 mb objective analysis reveals a broad area of high pressure encompassing much of the Desert Southwest while a weak shortwave trough is progressing through the Intermountain West. Looking farther southwest, Hurricane Rachel in the E Pacific is churning well west of the tip of the Baja California and continues to slowly progress wwd into open waters. Moisture from Rachel will likely come into play later this upcoming weekend, providing the next decent chance for rain across AZ and southeast CA. Until then, dry conditions are expected through the remainder of this week.

We will continue a warming trend today and tomorrow as high pressure builds over SoCal with positive hght anomalies expanding over the entire forecast area. The aforementioned weak shortwave trough will slide through the Four Corners area over the next couple days, bringing isolated showers to the high terrain of E AZ, but south-central AZ will remain dry. The most pronounced positive 500 mb hght anomalies today will again be focused along and west of the Lower Colorado River Valley where highs are expected to top out around 99-104 degrees, which is a solid 8-10 degrees above normal for early October. Highs in the central AZ deserts will not be as hot as areas farther west, but are still expected to reach the upper 90s to around 100 degrees in many locations this afternoon. As mid-lvl hghts/thickness increase further on Wednesday, sfc temperatures will warm 2-4 degrees across the board, with nearly all lower desert communities reaching 100+ degrees in the afternoon. The hottest locations will again be along the Gila and Colorado River valleys as well as Imperial Valley where highs on Wednesday afternoon will reach the 102-105 degree range, which is upwards of 10-12 degrees above normal or near daily records. An Extreme Heat Watch remains in effect beginning on Wednesday for the Imperial Valley and Yuma.

LONG TERM /THURSDAY THROUGH MONDAY/

Ridging aloft will reach peak strength over the western half of the forecast area on Thursday with low-lvl thermal profiles responding as 850 mb temperatures climb to around 26-27C. This will result in sfc highs reaching 102-107 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 and therefore the Extreme Heat Watch has been expanded 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 into the low 100s 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. Since the latest NBM forecast shows a 50-70% probability of high temperatures reaching the Major HeatRisk category for the Imperial Valley and Yuma area, the decision was made to extend the Extreme Heat Watch for those areas through Friday evening.

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 1055Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns under a FEW occasional mid-level cloud decks can be expected throughout the TAF period. The overall wind pattern will return to a more traditional diurnal pattern with the westerly shift likely to occur during the mid/late afternoon hours with overall speeds aob 7 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns under clear skies can be expected throughout the TAF period. Winds will generally be light and variable with periods of calm conditions.

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.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Watch from Wednesday morning through Friday evening for AZZ532.

Extreme Heat Watch from Thursday morning through Thursday evening for AZZ530-536.

CA...Extreme Heat Watch from Wednesday morning through Friday evening for CAZ566-567.

Extreme Heat Watch from Thursday morning through Thursday evening for CAZ569-570.