textproduct: NWS Phoenix

KEY MESSAGES

- An unsettled weather pattern will prevail throughout the weekend as the remnants of Tropical Cyclone Rachel move into the southwest United States, resulting in heightened chances for showers and thunderstorms, widespread gusty winds on Sunday, and much cooler temperatures.

- Rain chances will continue for portions of the region, especially across South Central Arizona, early next week along with well-below normal temperatures.

- A drier and quieter weather pattern is expected by the middle and latter portion of next week along with gradually warming temperatures.

SHORT TERM /TODAY THROUGH SUNDAY/

An expansive area of abundant midlevel moisture has spread into the Southwestern US ahead of Tropical Storm Rachel, which can be seen in midlevel water vapor imagery quickly ejecting northeastward towards Northern Baja. A subtropical cutoff low intensifying to its west, well offshore, is also apparent and leading to an intenifying jet streak downstream. The combination of abundant midlevel moisture and upper level diffluence hes led to fairly widespread elevated showers and scattered thunderstorms across Southwest AZ and Southeast CA so far. Locally strong wind gusts and transient linear organization have been observed with some of these storms, despite the weak instability (RAP analyzed MLCAPE no more than 500 J/kg). Inverted-V GFS bufr soundings and DCAPE upwards of 1000 J/kg across much of the forecast area will mean that even weak storms are capable of wind gusts upwards of 35-50 mph and kicking up at least localized blowing dust, as was observed in Gila Bend late this morning. This activity will continue to push north and east with time, reaching portions of the Greater Phoenix Area over the next hour or so.

A window of time with an environment supportive of more robust and potentially organized convection is still expected to arrive late tonight into Sunday morning, as better low-level moisture with the arrival of Rachel will support briefly higher instability upwards of 500-1000 J/kg of MUCAPE. The higher instability combined with vertical wind shear in excess of 40 kts will support a severe threat, with SPC highlighting much of the area under a marginal risk. Damaging wind gusts will be the main threat, however, with low- level shear increasing above 20 kts along with 0-3 km Storm Relative Helicity over 100-200 m2/s2, there will be a non-zero threat for a brief tornado or two.

Perhaps the most widespread impact from the arrival of the remnants of Rachel will be gusty winds. Guidance is still in excellent agreement that the intensifying jet downstream from the cutoff low and remnant low/circulation of Rachel will support 850 mb southerly flow upwards of 40 kts near/just east of the Lower Colorado River Sunday morning. This is in excess of climatological maximum wind speeds at 850 mb for this time of year. Additionally, a dry slot behind the remnant Rachel circulation may add to the wind threat, as there will be additional mixing through the BL during the Day on Sunday, helping tap the higher momentum air aloft. As a result, expect fairly windespread gusts upwards of 35-45 mph across the forecast area Sunday, with locally higher speeds upwards of 50-60 mph (especially over higher terrain). Wind gusts this high could certainty result in areas of blowing dust in the dust prone areas. As the remnants of Rachel move northward into southern Nevada by late Sunday afternoon/early evening, winds will weaken as the gradient weakens.

In terms of rainfall amounts, given that much of the activity will be mostly scattered in nature, event-total rainfall amounts across much of the area will be under 0.75" with the potential that some areas see next to nothing. However, there can be some locally higher amounts in excess of 1" in areas that see a training of convection. The more favorable area for more widespread activity will be mainly across Joshua Tree National Park, close to where Rachel will track, and thus the potential will exist for more widespread amounts in excess of 1" with a Flood Watch in effect from this evening through Sunday afternoon.

Tempertures across the area will decrease over the weekend, with lower desert highs this afternoon about 10 degrees cooler compared to Friday and readings in the low to mid 90s. The more abundant cloud cover as well as rainfall activity will cause temperatures to cool another 7-10 degrees on Sunday as afternoon highs only reach the low to middle 80s.

LONG TERM /MONDAY THROUGH FRIDAY/

Unsettled weather will continue through early next week as the large- scale trough that will settle across the west coast over the weekend will gradually migrate eastward. The associated large-scale forcing for ascent combined with ample low-level moisture will support scattered showers and embedded thunderstorms during the day, mainly across the foothills and higher terrain areas north and east of Phoenix in upslope flow. Thereafter, as the main cold front associated with the trough moves through late Monday night into Tuesday morning, there will likely be a band of showers with embedded thunderstorms that moves through, particularly across south- central AZ, before rain chances end by Tuesday afternoon/evening as drier air filters in. At this point any of the rain that falls early next week will generally be of the nuisance type. Quieter weather with near zero rain chances can be expected starting Wednesday and will persist through the end of the week.

With the presence of the trough as well as more abundant cloud cover, well-below normal temperatures are expected during the first half of the week with many of the lower desert locations likely staying in the 70s for afternoon highs. Early morning lows will also be below normal with the coolest morning expected on Wednesday when many areas are expected to drop into the 50s with even some 40s across the higher terrain areas. Temperatures will then gradually rebound into the 80s by the latter portion of the week.

AVIATION

Updated at 1920Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Scattered showers and storms, strong winds, and potential slantwise visibility reductions are all in the forecast through the next 24 hours. Confidence in timing of everything in the latest TAF package is low to moderate. Showers and storms will be possible at just about any time, but with odds slightly better this afternoon through tonight and Sunday morning. SE winds are expected to veer SW this afternoon and then eventually back SE-E this evening, but wind directions may become variable this evening based on shower and storm activity. Storms will be capable of generating strong wind gusts and blowing dust. The most impactful time period will likely be Sunday morning between 14-19Z when a band of showers and storms are forecast to move through with lower CIGs down to 4-5K ft AGL and southerly wind gusts up to 30kt. Beginning with and following the band of rain, S-SW winds up to 15-20 kts and gusting up to 30-35 kts is expected through the afternoon, which may lead to a lot of lofted dust and reduce slantwise visibility.

Southeast California/Southwest Arizona including KIPL and KBLH:

Periods of SHRA with the potential for isolated TSRA along with erratic wind directions and strong wind gusts will be the main weather issues as thick midlevel cigs develop over the area. Winds will favor an E/SE component through tonight and then eventually S/SW by late Sunday morning. VCSH/VCTS chances will exist at both terminals through this afternoon and evening before drying. Very strong S/SE winds are expected Sunday morning, especially between 10-16Z with sustained speeds around 15-25 kts and gusts as high as 35-45 kts. There is some uncertainty if the stronger gusts will fully materialize in the morning, but winds will be strong enough regardless to generate blowing dust and impact slantwise visibility.

FIRE WEATHER

A much more unsettled weather pattern has taken shape this weekend as increasing moisture associated with the remnants of Tropical Storm Rachel brings increasing rain chances. Chances for wetting rain will continue to increase through Sunday morning with a greater than 50-70% chance of wetting rainfall area wide. Winds will also increase significantly, especially late tonight through Sunday with widespread gusts exceeding 30-40 mph. With the increasing moisture and cooler temperatures, MinRH values will increase from 25-40% today 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 foothills and higher terrain of the eastern districts, before quieter weather settles in by the middle of the week. Temperatures will steadily cool this weekend, with well-below normal temperatures expected for the first half of the next 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.

Flood Watch from 5 PM PDT this afternoon through Sunday afternoon for CAZ560-561-568.