textproduct: NWS Phoenix

UPDATE...Updated Aviation Discussion

KEY MESSAGES

- Dry and warm conditions are expected this weekend with highs mostly in the mid to upper 70s across lower deserts.

- A strong weather system arriving Monday and Tuesday should bring better rain chances, breezy to windy conditions, and noticeably cooler temperatures.

- Another weather system clipping the area Wednesday will bring additional breezy to windy conditions and rain chances, and yet another drier system Thursday into Friday will help keep temperatures cooler through the end of the week.

SHORT TERM /TODAY THROUGH SUNDAY/

As was anticipated, Friday's trough did not bring much in the way of rainfall across the lower deserts, with most areas receiving no measurable rain to at most 0.10" just east of Phoenix. The trough that brought the precipitation to the area is quickly being replaced by a transient 577-579dam H5 ridge, with drier more stable air. After a day with temperatures near to slightly below normal Friday, temperatures will warm back up this weekend as the ridge slides across the area. Temperatures are forecast to warm into the low to upper 70s across the lower deserts today - after a chillier start to the day in the 40s and 50s - under mostly sunny skies. There will be and increase in clouds Sunday, as the mid-level flow turns southwesterly again, ahead of the next coastal trough. Despite the clouds, temperatures are forecast to warm another few degrees, especially across south-central AZ where highs will reach the upper 70s, with a few spots touching 80 degrees. Winds will be calm to light through the weekend, which, combined with the dry and warm afternoons, will make for a pleasant weekend for outdoor activities.

LONG TERM /MONDAY THROUGH FRIDAY/

A large blocking high is forecast by global models to sit over the north-central Pacific and Bearing Sea through the next week to week and a half. This positioning will allow a series of several troughs to drop south along the U.S West Coast, bringing a few rounds of precipitation and breezy to windy conditions to much of the Western CONUS. Following the transient ridge this weekend, a large Pacific trough/low will dip far enough south to tap into some rich subtropical moisture and draw it up into the Desert Southwest beginning Monday. IVT values are forecast as high as 400-500 kg/ms in southern AZ by late Monday, with PWAT values up to 150-200% of normal. Models are in good agreement that the central low pressure will only drop as far south as San Francisco, CA before weakening and moving eastward inland, but with at least a couple embedded, fast-moving, shortwave troughs rounding the base of the parent trough. A very strong and deep upper level jet streak will also set up on the south side of the trough, with 250mb wind speeds forecast as high as 150-170kts directly over the local area.

Even though the coastal low stays well to the northwest of the area and the first shortwave rounding the base of the low takes a trajectory from LA up through southern NV, there will still likely be considerable impacts across the Desert Southwest. Perhaps most notably from wind, with the very strong jet overhead and low and mid level southwesterly winds increasing up to 30-50kts. Despite the track of the first shortwave, guidance still shows a fairly strong cold front dragging through the area later Monday through Monday night. This front, along with the jet dynamics, and strong upslope flow through the AZ terrain look to be the primary forcing mechanisms to drive scattered showers across the area. However, the window for this all coming together may be relatively short, especially for lower desert precip chances. Models show a mid and upper level punch of dry stable air above 600mb, cutting off the deep layer moisture, right around the same time, if not just before, the front moves across the area.

Latest NBM continues to hone in this window for precipitation with a west to east progression of PoPs, peaking around 70% in south-central AZ Monday night that then falls to under 20% by late-Tuesday morning. Given the shorter window and likely fast moving showers, QPF is not very high, with amounts anywhere from 0.00-0.25" across the lower deserts. Higher amounts are likely to occur across higher terrain areas north and east of Phoenix. Even though this system will be a bit colder, accumulating snow levels are likely to mostly stay above 6500 feet.

Tuesday, during the day, may bring some additional isolated shower activity with the second aforementioned shortwave rounding the parent trough and passing over the area. Another couple of troughs then look to clip through the region Wednesday and again Thursday into Friday, but the subsequent troughs will be working with less moisture and the forecast tracks of the troughs are not ideal for precipitation production across southern AZ and southeast CA. Energy with the Wednesday trough may miss just north. There is potential for light accumulating snow in western Joshua Tree NP Wednesday if the lower level Pacific moisture can penetrate the tall SoCal mountains. With tight pressure packing, the strong jet streak overhead, and strong west to east flow Wednesday, surface winds will likely be an impact again, especially across southeast CA where downsloping enhancements may lead to windy conditions and blowing dust.

Temperatures next week will easily be the coolest so far this month, but the NBM only shows readings falling a few degrees below normal starting Tuesday. Lower desert forecast highs from Tuesday-Friday are mostly in the middle to upper 60s. Once drier air and calmer winds develop later next week, some rural desert areas may see overnight lows dip into the 30s.

AVIATION

Updated at 1115Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation impacts expected through Sunday morning with clear skies lasting through the afternoon with increasing high clouds this evening. Winds will be light through the period with typical diurnal switches out of the west early/mid afternoon and back to the east early evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected over the next 24 hours. Winds will be light and try to resemble diurnal trends but it appears that period of VRB to calm conditions will be common through the forecast. Expect clear skies into the afternoon hours with increasing high clouds by this evening.

FIRE WEATHER

High pressure with warm, above normal, temperatures and dry conditions will prevail through this weekend. MinRH values this weekend will be around 20-30%. Occasional breeziness is expected through the weekend, especially across the western districts and the Arizona higher terrain on Sunday. Multiple weather systems will then impact the region next week, with greatest impacts expected Monday through Wednesday. The weather systems will bring a chance for scattered rain showers, mainly Monday night through Tuesday morning and on Wednesday. Breezy to locally windy conditions are also expected, with gusts up to 20-30 mph likely and 35+ mph in mountains and east-facing downslope areas. Winds will be strongest Monday-Wednesday. MinRH values are only forecast to improve to 25-35% through the first half of the week, with gradual drying during the second half.

PSR WATCHES/WARNINGS/ADVISORIES

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