textproduct: NWS Phoenix

KEY MESSAGES

- A low pressure system moving through the region will result in scattered showers and thunderstorms later this afternoon through tonight along with cooler temperatures.

- Any thunderstorms will be capable of producing small hail, strong gusty winds, and brief heavy rainfall.

- High pressure will build into the region by midweek, leading to dry weather and rapid warming with record warmth possible starting by the end of the week.

SHORT TERM /Today through Wednesday/

Latest satellite imagery shows a potent cut-off low situated just off the west coast of the northern Baja Peninsula and will be the main weather maker later today. As the low gradually moves east northeastward, good moisture advection with PWATs rising to 0.8-0.9" (200-250% of normal) and increasing ascent in the form of upper- level divergence will lead to the development of scattered showers and storms. Latest HREF membership is in very good agreement in showing areas of widely scattered showers and storms first affecting portions of southwest AZ during the mid to late afternoon hours, mainly across Yuma County, before the focus of the activity shifts eastward into south-central AZ heading into the evening hours. While 0-6 km shear values are expected to be strong, in excess of 40 kts, which would suggest that some of these thunderstorms that do materialize could obtain some organized structure to them, the main limiting for a more organized severe threat is the instability as forecast soundings show CAPE values only peaking at 200-500 J/KG. Nevertheless, strong gusty winds in excess of 35 mph as well as small hail will be possible with any robust storms. Rainfall amounts for the most part will be limited given the overall widely scattered coverage and fast storm motion. However, some areas could still receive over 0.25" with any heavier thunderstorm. Scattered showers will remain a threat into the first part of Tuesday, especially across the higher terrain east of Phoenix, before ending sometime during the afternoon hours as the low shifts eastward into southern New Mexico/western Texas.

Under the influence of the low, temperatures through Tuesday will be noticeably cooler. Afternoon high temperatures today across the western deserts under the influence of more persistent cloud cover will only reach the middle 70s, whereas across the lower deserts of south-central AZ, afternoon highs will still manage to reach 80 degrees as more abundant sunshine is expected into the afternoon hours. Under the influence of lingering cloud cover and cool air advection on the back side of the departing low, afternoon high temperatures across the south-central AZ lower deserts are only expected to top out in the mid to upper 70s. More sunshine will allow afternoon highs across the western deserts to warm to around 80 degrees.

As the low fully exits the SW CONUS early Wednesday, strong upper- level ridging will move in from the eastern Pacific with rapid height rises expected as 500 mb heights rise above 580dam. As a result, temperatures will be on rise with afternoon highs rising into the mid 80s across most of the lower desert communities and will be the transition of what is expected to be a prolonged stretch of record warmth during the subsequent days.

LONG TERM /Thursday through Sunday/

Forecast confidence for the latter half of the week is excellent given remarkable ensemble agreement showing a low amplitude northern stream jet constricting poleward towards a Hudson Bay vortex allowing strong subtropical ridging to expand into the SW Conus. H5 heights will settle into a 585-588dm range through the majority of the period with only a marginal chance of heights/thermal fields dampening over the weekend due to shortwave energy digging into the Upper Midwest/Great Lakes. Ensemble guidance spread is extremely narrow and reflects climatologically anomalous tropospheric height/thermal measures. Surface temperatures 15F-20F above normal will become common by the end of the week resulting in widespread minor HeatRisk while also setting daily records (see Climate section). It would also not be out of the question for some the warmer, lower desert communities to achieve the first 100F of the season as the NBM 75th percentile easily covers this threshold. Just beyond this forecast period, essentially all ensemble members indicate even warmer weather as East Pacific blocking edges into the SW Conus and subsident ridging builds stronger. The ensemble guidance envelop is solidly above the 100F threshold for all lower desert communities portending a prolonged period of record setting warmth.

AVIATION

Updated at 1842Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation hazard for the forecast period will be the possibility of VCTS/TS and brief lower CIG and VIS this evening through tonight where the main TS impacts will be strong gusty winds and small hail. Before this evening's activity no significant aviation concerns are expected with VFR conditions. Wind directions are expected to favor a S/SW component through this afternoon with speeds generally aob 10 kts. SCT-BKN mid-lvl clouds, with bases around 7-10 kft, will develop rapidly by mid to late afternoon. Cloud bases are forecast to stay above 5-6 kft through the TAF period, but there is a 10-20% chance of briefly dipping to 3-4 kft in TS. Most of the SHRA/TS activity will shift east of the terminals by mid-morning Tuesday.

Southeast California/Southwest Arizona including KIPL and KBLH: The main potential aviation weather impact will be the development of SHRA/TS this afternoon and evening. Most of the activity will stay east of the CO river, with 10-30% chances at the terminals. Given the low chances, there is no VCSH/SHRA mention at KBLH in the TAFs. Winds will favor a E-SE component at KIPL and will predominantly favor S-SW at KBLH. Wind speeds at both terminals should generally be around 6-11 kt, before becoming light and VRB tonight through Tuesday morning. SCT-BKN mid-high clouds with lowest bases around 5-7 kft will persist through the evening hours.

FIRE WEATHER

A weather system moving through the region today through early Tuesday will bring cooler temperatures, increased humidity levels, and chances for wetting rainfall. Dry weather with unusually warm temperatures and low humidities will return during the latter half of the week. Minimum humidity levels will only fall into a 25-40% range this afternoon with good to excellent overnight recovery. Better than a 50% chance of wetting rain with embedded thunderstorms will exist in eastern districts through Tuesday afternoon before a drying trend enters the region. Conditions will trend markedly drier through the week with minimum afternoon humidities falling into a 5- 15% range by the end of the week while poor to fair overnight recovery of 15-30% materialize. Outside of thunderstorm wind gusts today, winds will be fairly light with only modest afternoon upslope gustiness limiting a greater fire weather danger.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.