textproduct: NWS Phoenix

KEY MESSAGES

- Generally dry conditions along with temperatures around 10 degrees above normal will persist today.

- An Extreme Heat Warning remains in effect for western Imperial County and Yuma through this evening.

- An unsettled pattern will take shape this weekend along the West Coast and drag the remnants of Tropical Cyclone Rachel into the Southwest United States, resulting in heightened chances for showers and thunderstorms, widespread gusty winds on Sunday, and much cooler temperatures.

SHORT TERM /TODAY THROUGH SATURDAY/

Fairly quiet conditions will round out the work week before several weather systems interact to bring unsettled and regionally impactful rain, wind, and severe weather potential over the weekend. Today, the influence of broad anticyclonic flow remains over the Desert Southwest, with H5 heights analyzed between 584-586 dam at this hour. Though heights are gradually eroding today, thermal profiles have yet to respond meaningfully, and thus 850 mb temperatures upwards of 24C-26C (above the 90th percentile of climatology) will continue to support highs around 10F above daily normals. Yuma and El Centro will once again be within a couple of degrees of daily records, with areas of Major HeatRisk still continuing. Therefore, Extreme Heat Warnings remain in effect for the Imperial Valley and Yuma areas through this evening.

Over the Eastern Pacific, Tropical Storm Rachel and a subtropical shortwave/cutoff low have begun to interact well offshore the Northern Baja Peninsula, turning the tropical system's track northeastward. South southwesterly moist flow will increase in earnest Saturday and especially by Saturday night, with NAEFS mean IVT near/exceeding climatological maximum for this time of year. Moisture and upper level support are likely to peak Saturday night/early Sunday morning as the remnants of Rachel quickly eject northeastward near/along the Lower Colorado River, resulting in the most widespread coverage of showers and storms. However, before the peak timing for rain chances, scattered showers and thunderstorms are expected to erupt as early as late Saturday morning/Saturday afternoon. Strong, gusty winds and localized heavy rainfall remain the main threats with this initial activity. SPC has issued a Marginal risk for severe storms Saturday into early Sunday morning, and indicates at least a very low chance for a tornado or two. The strengthening wind fields do support this, with GFS bufr soundings also showing a brief window with higher MUCAPE (upwards of 500-1000 J/kg), EBWD 30-40 kts, SFC-1 km shear above 10 kts, and LCLs lowering below 2000 m. Supercellular storms modes do look like they will be supported by the environment, at least for a temporary window of time Saturday night into Sunday morning. However, it is important to keep in mind this is a highly volatile forecast, with small upstream changes resulting in large downstream forecast shifts. Regardless, with any organized storm modes that develop Saturday PM into Sunday, the primary and more widespread threat will be for damaging wind gusts.

LONG TERM /SUNDAY THROUGH THURSDAY/

The more impactful weather is expected on Sunday as the remnants of Rachel move through. According to the latest official forecast track from the National Hurricane Center, Rachel is expected to accelerate north-northeastward along the vicinity of the Lower Colorado River Valley. An area of heavy rainfall is likely to follow along and just to the west of where Rachel tracks. Even though the official NHC forecast has the system tracking along the vicinity of the Lower Colorado River Valley suggesting that the heavy rainfall may fall across portions of southeast CA, there is a lot of spread amongst the guidance in the track as some solutions have Rachel tracking near the southern CA coast whereas other solutions have the system tracking as far east as southwest AZ. Therefore, the rainfall forecast is still highly volatile and could still significantly shift depending on the eventual track. The greater impact with Rachel will be the potential for strong winds as a belt of strong 850 mb winds in excess of 40-50 kts is likely to set up along the eastern quadrant of Rachel with much of the guidance showing gusts exceeding 40-50 mph across much of southwest into south-central AZ, especially early Sunday morning through Sunday afternoon. Wind gusts this high could certainly result in areas of blowing dust across the dust prone areas. Given the strong wind potential, a wind advisory has been hoisted from eastern Riverside and Imperial Counties through all of southwest and south-central AZ. In addition to the wind threat, there will also be the potential along the eastern quadrant for multiple rounds of widely scattered showers and storms to materialize. There is a potential for some of these storms to reach severe levels given that strong vertical shear in excess of 50 kts will be present, however, the severe potential will be dependent on how much instability will be present.

Even though the remnants of Rachel will be well north of the area by Sunday evening, unsettled conditions will continue right through early next week as the main west coast trough moves through the region bringing additional chances for showers and isolated thunderstorms, with the highest chances across the foothills and higher terrain areas to the north and east of Phoenix from upslope flow. Thereafter, drier and quieter weather is expected by the middle of the week as the main trough axis lifts to the northeast.

Under the presence of the troughing pattern, well-below normal temperatures are expected during the first half of next week with lower deserts highs going from the low to mid 80s on Sunday to mostly 70s Tuesday and Wednesday. Early morning lows will also take a tumble with the coolest morning expected on Wednesday as temperatures are expected to bottom out in the 50s across much of the region with even some 40s across the higher terrain areas.

AVIATION

Updated at 1744Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Main weather concern will be VCSH and -SHRA beginning to develop near the end of the current TAF package. Until then, confidence is excellent that east winds will prevail well into this afternoon with potentially a few late morning gusts 15-20kt, albeit spatially and temporally limited. Very low confidence remains regarding wind directions later this afternoon and early evening where some terminals may obtain a light southerly/southwesterly cross runway component, or potentially become light and variable for a short time. Areas of virga and possibly a few isolated SHRA are expected to develop, further complicating wind directions. Regardless, directions should quickly switch back to E/SE by the mid evening timeframe. Cloud decks will begin to thicken to BKN- OVC as the system approaches but should remain aoa 10kft, or at least above VFR conditions.

Southeast California/Southwest Arizona including KIPL and KBLH: Main weather concern will be VCSH and -SHRA beginning to develop near the end of the current TAF package. Wind speeds will be mostly aob 10 kt at both terminals. At KIPL, current westerly winds will go southerly this afternoon then back westerly this evening. At KBLH, current northwesterly winds will go southerly late this morning, southwesterly this evening, then back southerly overnight. Gradually increasing midlevel cloud decks with bases aoa 10 kft will continue to develop throughout the period.

FIRE WEATHER

A much more unsettled weather pattern will develop over the weekend as increasing moisture associated with the remnants of Tropical Storm Rachel brings increasing rain chances. The chances for showers and storms increase starting Saturday afternoon/evening and continues into Sunday with a greater than 50-70% chance of wetting rainfall area wide. Winds will also increase significantly, especially late Saturday night through Sunday with widespread gusts exceeding 30-40 mph. With the increasing moisture and cooler temperatures, MinRH values will increase from 25-40% Saturday to 45-70% on Sunday along with good to excellent overnight recoveries. Another weather system will bring additional chances for wetting rainfall early next week, mainly for the eastern districts, before quieter weather settles in by the middle of the week.

CLIMATE

Record highs: Date Yuma El Centro ---- ---- --------- Oct 9 106 in 2024104 in 1996

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST this evening for AZZ532.

Wind Advisory from 2 AM to 5 PM MST Sunday for AZZ530>533-535- 536.

Wind Advisory from 8 AM to 5 PM MST Sunday for AZZ534-537>563.

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

Wind Advisory from 2 AM to 5 PM PDT Sunday for CAZ560>570.