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 AZ, 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/

After a fairly benign weather pattern across the southwestern United States during the last week, big changes are coming for this upcoming weekend. Current water vapor satellite imagery as well as objective analysis show a large northern stream trough diving into the Pacific Northwest while Tropical Storm Rachel, being steered northeastward by another cutoff low is located several hundred miles to the south- southwest of San Diego. Rachel will continue to migrate north- northeastward throughout the day, making landfall along the far Northern Baja Peninsula by late tonight before accelerating north-northeastward through San Diego and western Riverside Counties and towards the Mohave Desert during the day Sunday.

South southwesterly flow will transport abundant moisture into the region throughout the day today, with the ensembles showing mean PWATs exceeding over 200-250% of normal by early this evening. The increasing moisture combined with the increasing jet dynamics from the west coast trough will support scattered showers and storms to break out. Latest hi-res guidance shows activity, mostly in the form of elevated showers, developing over southwest AZ starting late morning with scattered showers and storms developing across portions of south-central AZ through southeast CA during the afternoon/early evening hours. Some gusty winds and brief heavy rainfall will be the main threats with any storms that materialize as instability will be quite marginal with MUCAPE values at less than 500 J/KG. The chances for more robust convection will likely come late tonight into Sunday morning as better low-level moisture with the arrival of Rachel will support a window of 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.

As the remnants of Rachel approach southern CA combined with height falls from the large scale trough to the west, low-level winds will be increasing with a belt of 850 mb upwards of 50 kts. This will support non-thunderstorm wind gusts exceeding 40 mph starting after midnight across southeast CA into southwest AZ before the stronger winds spread further east into south-central AZ by the early to mid- morning hours Sunday. 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, 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.

After the couple of days of widespread highs in the triple digits, temperatures this weekend will steadily cool. Lower desert highs this afternoon will be about 10 degrees cooler compared to Friday with 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 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 This 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 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 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 this afternoon through Sunday afternoon for CAZ560-561-568.