textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- An unsettled weather pattern with a period remnant tropical moisture will settle across the region this weekend through early next week with temperatures cooling below normal.

- Strong, gusty winds will impact the region Sunday with widespread wind advisories in place as a result of gusts 35 to 55 mph.

- Numerous showers and embedded thunderstorms will periodically impact the area over the next several days with brief, heavy downpours.

- Weather conditions will dry during the middle of next week with temperatures rebounding closer to normal.

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 0540Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary weather issues will be SHRA and potentially embedded TSRA entering the Phoenix metro Saturday afternoon and evening, and subsequent the effect on winds as midlevel cigs develop and lower. Confidence is good that E/SE winds will prevail through Saturday morning as 10-15K ft AGL cigs fill in across the area. Scattered SHRA should enter the western portions of the metro early Saturday afternoon, and may allow winds to shift to a W/SW direction. Coverage of SHRA appears low enough to preclude a predominant mention, and will advertise a prolonged period of VCSH. A few embedded TSRA will also be possible heading into the evening, but odds only justify a prob30 mention at this time with potentially erratic, gusty winds. Confidence is better that an east component will settle back into the area Saturday night, and have kept a VCSH mention though its more uncertain how expansive SHRA will be.

Sunday outlook: High impact weather still appears to be arriving at all terminals Sunday mid/late morning through the afternoon. SHRA/TSRA should surge into the area with gusty south winds and MVFR/IFR cigs. The lower cigs and TSRA threat should be rather short lived over a 3-4 hour window late morning through early afternoon before lifting out of the aerodrome space. However, very strong S/SW winds will fill in behind the SHRA with gusts 30kt or higher throughout the afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH: Periods of SHRA with the potential for isolated TSRA along with erratic wind directions will be the main weather issues as thick midlevel cigs develop over the area. Winds should become more variable in direction overnight, then obtaining an E/SE component Saturday. Scattered showers would most likely impact KBLH by early afternoon as 10K ft cigs cover the area. At this point, a VCSH adequately describes the coverage, though enough odds for TSRA exist at KBLH to include a prob30. Uncertainty in winds and weather grow significantly Saturday night and Sunday as a period of high impact weather moves into the region.

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.

PSR WATCHES/WARNINGS/ADVISORIES

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