textproduct: NWS Phoenix

KEY MESSAGES

- An unsettled weather pattern will materialize Friday and continue 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.

- Dry conditions return on Sunday, with temperatures gradually warming during the first half of next week.

SHORT TERM /TODAY THROUGH SATURDAY/

Current water vapor satellite imagery and RAP 500 mb analysis show a closed upper level low off the California coast, responsible for the thick layer of cirrus clouds streaming over the forecast area. As the low tracks southward along/just off the coast, anticipate another day of generally quiet weather today. The forecast still largely remains on track, with this initial closed low expected to eventually progress eastward over the Baja Peninsula and helps to increase moisture across the region through Friday night/early Saturday morning. PWATs should increase to 200-250% of normal for this time of year over the southeastern portion of AZ, lining up well with the area of highest PoPs/QPF. With this initial closed low, top down saturation of the atmospheric profile (as evidenced in GFS bufr soundings) will preclude much in the way of accumulating rainfall until at least Friday evening, with perhaps some isolated virga or light showery activity before this time. A period of weak, moist isentropic ascent ahead of this initial closed low will help force more widespread shower activity Friday night into Saturday morning, but again, better quality moisture and upper level forcing (coupled with terrain influences) will focus higher precipitation totals well to the east/southeast of the Phoenix Metro.

A second shortwave will dive south through the Interior West Saturday, shifting flow aloft from southwest to west northwest and providing a potential second focus for shower development over the region. However, compared to previous model runs, the placement of this second disturbance now looks further removed from the area, skimming northeast AZ and sending a poorly defined cold front southward over the region late Saturday. Lapse rates will not be quite as impressive, but forecast soundings show a favorable profile for some shallow convection (MUCAPEs peaking around 100 J/kg) Saturday afternoon into the evening. Showers capable of producing lightning and small hail cannot be ruled out. Through the event, average rain totals for those that see showers across the lower deserts should be light, on the order of 0.10-0.25" for the Phoenix Metro, with a gradient in QPF towards the east/southeast and up to 0.5-1.0" over the higher terrain.

Temperatures today will likely achieve around 5 degrees above normal for the date as we see some breaks in the thick high clouds and transient ridging ahead of the incoming closed low influencing the region. However, with the incoming moisture and thicker mid level clouds, expect a downward trend in temperatures through Saturday, more pronounced for South-Central AZ than the western deserts. Widespread afternoon highs in the 60s look likely for much of the lower deserts by Saturday, except for some of the typically warmer spots out west like Yuma and El Centro, which are forecast to reach the lower 70s.

LONG TERM /SUNDAY THROUGH WEDNESDAY/

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. Sunday and especially Monday are shaping up to be relatively cold mornings with this cooler and drier air mass in place. Lows are forecast to range from the mid 30s to mid 40s Monday morning, with many of the typically colder rural, low-lying areas potentially flirting with freezing temperatures (32F). Afternoon highs Sunday will be similar to that of Saturday, though perhaps a few degrees cooler for the western deserts.

Ensembles are 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 and dry conditions Sunday onward. 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, quiet, dry weather will return Sunday and persist through at least the middle of next week.

AVIATION

Updated at 0515Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and Southeast California/Southwest Arizona including KIPL and KBLH:

No major weather concerns will exist through Thursday night under periods of thick mid/high level cigs. Wind speeds under 10kt will be common across the region with extended periods of nearly calm conditions. Typical diurnal/nocturnal wind shifts will be absent or temporally limited with a light easterly fetch more prevalent around the Phoenix metro, and a light north wind across SE California terminals.

FIRE WEATHER

Dry conditions and above normal temperatures continue today. An unsettled pattern will take hold Friday and Saturday, cooling temperatures near to slightly below normal for the weekend, increasing humidities, and bringing chances for a few rounds of showers mostly to South Central and Eastern AZ. The best chances for wetting rains (70-80%) will be focused east and southeast of the Phoenix area. MinRHs will increase from 15-25% 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, increasing northerly winds especially in the western districts. As such, anticipate mostly light winds (under 15 mph) through Saturday morning, followed by locally breezy north winds Saturday afternoon into the evening. Afternoon minRHs will fall back into a 15-30% range areawide Sunday onward.

PSR WATCHES/WARNINGS/ADVISORIES

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