textproduct: NWS Phoenix

KEY MESSAGES

- An unsettled weather pattern will carry us through Saturday, with temperatures cooling near to slightly below normal this weekend and chances for a few rounds of showers across South Central and Eastern Arizona.

- Some convective showers capable of producing small hail and lightning will be possible Saturday afternoon into the evening.

- Dry conditions return on Sunday, with temperatures gradually warming next week.

SHORT TERM /TODAY THROUGH SATURDAY/

Southwesterly flow aloft ahead of a closed upper level low off the SoCal/Northern Baja Coast has rapidly moistened the midlevels of the atmosphere over South-Central AZ. A few radiosondes launched from the office Thursday afternoon and evening revealed a notable increase in dewpoint temperatures from around a -20 to -15 deg C range at 00Z into a -5 to 0 deg C range by 03Z near the 700 mb level. This brought the profile to near saturation at and above 700 mb, consistent with the 9-10 kft AGL cloud decks that were observed and have since been maintained. This rapid moistening of the midlevels juxtaposed with upper level diffluence as the closed low slowly approaches the region led to spotty virga across a good chunk of the forecast area and even a light shower locally, with a trace of rain observed at Phoenix Sky Harbor Airport and a few automated stations recording measurable rain on the order of 0.01-0.02" in the East Valley. This spotty virga and isolated shower activity looks likely to continue through this afternoon as better quality moisture continues to be imported into the Desert Southwest.

The closed low will approach and eventually traverse the Baja Peninsula early Saturday. While the low remains to our southwest, PWATs are forecast to continue increasing, with the NAEFS mean values peaking around the 90th percentile of CFSR climatology across the forecast area and even exceeding the 98th percentile in the southeastern corner of AZ later today. Despite these climatologically significant PWAT values for this time of year, much of the moisture will remain in the midlevels through this afternoon, with top-down saturation eventually occurring this evening and overnight into early Saturday morning. This coincides with perhaps the best timing of upper level support for precipitation, as global guidance continues to show a period with weak, moist isentropic ascent across Central and Eastern AZ. Some amount of positive differential vorticity advection is no doubt also occurring as the closed low begins to open up with a second disturbance diving south from the Great Basin Saturday afternoon (helping to eject the initial closed low eastward over Sonora.) As a result, anticipate showers to become more widespread across the eastern CWA tonight into Saturday morning.

The second disturbance diving south along the Interior West and passing through Northern AZ late Saturday will act as a second focus for precipitation chances, sending a poorly defined cold front southward over the area. This disturbance will also bring colder air aloft, dropping NAEFS mean 500 mb temperatures below -20C. GFS bufr soundings show this as well, with midlevel lapse rates increasing and MUCAPEs nearing 100 J/kg Saturday afternoon into the evening. HREF mean MUCAPE values are perhaps a bit higher, between 100-250 J/kg across South-Central AZ. The right ingredients will be in place to allow shallow convection Saturday afternoon into the evening, with the main threats being small hail and perhaps some lightning.

Anticipate near normal temperatures today, with afternoon highs between the middle 60s and lower 70s across the lower deserts. Temperatures will be a few degrees cooler tomorrow, more noticeablyfor South-Central AZ than the western deserts thanks to cloud cover/shower activity.

LONG TERM /SUNDAY THROUGH THURSDAY/

The poorly defined cold front that sweeps the area from the north late Saturday into Sunday will usher in a cooler and much drier air mass, ending precipitation chances by early Sunday morning. Temperatures will drop noticeably from Saturday to Sunday, from near normal into a 2-5 degree below normal range. Morning lows are forecast to range from the mid 30s to mid 40s starting Sunday, with some of the typically colder rural, low-lying areas potentially flirting with freezing temperatures (32F or less) into early next week. NBM probabilities for freezing temperatures peak Sunday night/Monday morning around 20-50% in those rural and sheltered river valley locations, except slightly higher (50-80%) in the Tonto Basin.

Ensembles remain in excellent agreement that ridging off the West Coast will slide eastward and build over the Western US during the first half of next week, promoting a gradual warming trend. However, WPC cluster analysis reveals discrepancies on how strong the ridge will become, and whether any disturbances will undercut or weaken the ridge during the latter half of next week. Regardless, after the unsettled weather Friday-Saturday, expect quiet, dry weather to return Sunday and persist through at least the middle of next week.

AVIATION

Updated at 0556Z.

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

A weather system will bring slight chances (20%) for -SHRA tomorrow, but better chances for SHRA impacts will be after 05Z Saturday. so have only introduced -RA in a PROB30 in the KPHX TAF. Scattered virga and sprinkles are expected throughout the day tomorrow. While, virga and sprinkles are possible throughout the day, best timing looks to be between 12-17Z in the morning, so VCSH has been added to all TAFs. Despite the incoming weather system, VFR conditions are expected to continue through the current TAF period. CIGs will remain aoa 10 kft AGL through tomorrow evening, with FEW-SCT clouds around 6-8 kft moving in as early as tomorrow morning. By late tomorrow night into Saturday morning CIGs are expected to lower to around 6 kft. Winds will generally be out of the east, however they are expected to shift SW with any shower activity. Wind speeds will generally be aob 5 kt, with speeds increasing to 8-12 kt during the afternoon and early evening hours. Southeast California/Southwest Arizona including KIPL and KBLH:

No weather concerns will exist through the current TAF period under periods of thick mid/high level CIGs. Wind speeds will generally be aob 6 kt at both terminals with extended periods of nearly calm conditions amongst typical diurnal/nocturnal wind shifts.

FIRE WEATHER

Unsettled weather will carry us through Saturday, with temperatures cooling near to slightly below normal and chances for a few rounds of showers across South-Central and Eastern AZ. The best chances for wetting rains (up to 60-90%) will be focused east and southeast of the Phoenix area, especially over the higher terrain. MinRHs will increase from 20-40% areawide today to 30-50% Saturday, with even higher values over the higher terrain of the eastern districts. A weak cold front will sweep the area from the north late Saturday into Sunday, increasing northerly winds especially in the western districts along the Colorado River. As such, anticipate mostly light winds (under 15 mph) through Saturday morning, followed by locally breezy north winds Saturday afternoon into the evening, and lingering breeziness on Sunday. Afternoon minRHs will fall back into a 15-35% range for the eastern districts and 10-20% range for the western deserts Sunday onward.

PSR WATCHES/WARNINGS/ADVISORIES

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