textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- Dry and tranquil weather conditions with well above normal temperatures will continue across the region through Monday.

- An unsettled weather pattern is expected mid and late week during the Christmas holiday time period with increased rain chances and slightly cooler temperatures.

SHORT TERM /TODAY THROUGH TUESDAY/

Strong high pressure (589-590dam) remains in place just southeast of AZ this afternoon. This high continues to drive well above normal temperatures for the Desert Southwest. Lower desert highs will reach the upper 70s to around 80 degrees today. Highs will be within a couple degrees of daily records today, as well as tomorrow, with highest odds (~80%) in Phoenix. Increasing high clouds may be an inhibiting factor in record chances Monday.

Heading into Tuesday, the overall pattern will be in a transitional state as the upper-level ridge migrates further east to the Gulf states, allowing for a deep trough to settle just off the west coast which will kick-off a significant moisture flux into the Southwest U.S. heading into the Christmas Holiday period and likely persisting into next weekend. A strong and rapid push of subtropical moisture is expected throughout the day Tuesday, coming up from the southwest, with models showing PWATs going from 0.3-0.4" early Tuesday morning to 1.0-1.1" (250-300% of normal)by late in the evening. A shortwave trough moving up with this push of moisture is likely to help drive a band of virga and light showers through the region, moving from west to east, from late morning out west to south-central AZ Tuesday evening/night. Very little, if any, rainfall accumulation is expected with this initial wave.

Afternoon high temperatures Tuesday will once again be well-above normal, but cooled slightly, especially out west, due to the pattern change. Lower desert highs will reach the low to mid 70s in southeast CA and southwest AZ and mid to upper 70s in south- central AZ. There is even a potential that Phoenix will see another daily record high, given that currently the forecast high is 79 degrees with the record being 79.

LONG TERM /WEDNESDAY THROUGH NEXT WEEKEND/

Following the shortwave trough, with the initial surge of moisture Tuesday, there may be a brief break in shower activity Wednesday morning due to subsidence and drier mid-level air in the trough's wake. However, once the high-end moisture levels arrive there will be shower chances at nearly all times through at least Thursday (Christmas Day). PWATs are forecast to remain at or above 1" (above the 90th percentile) through Thursday, based on the global grand ensemble. A moderate-strong AR, with IVT values over 500 kg/ms, will shift further east into the interior Desert Southwest Wednesday afternoon through Thursday morning. Global ensembles consistently show this to be the time period when most rain will fall across the region and latest NBM PoPs have increased and are highest (70-90%) during this timeframe across southeast CA through south-central AZ. Heading into Thursday afternoon and Friday, a gradual decline in moisture is expected, with model soundings showing notable drying and subsidence above 700mb. This will subsequently result in a decline in rain chances.

However, with the lingering moisture in the boundary layer, below 700mb, isolated surface-based light showers may continue to develop, favoring the AZ high terrain, Thursday afternoon through Friday. Overall, dynamic forcing will be a bit limited Tuesday-Friday, with the primary trough and best forcing staying to the west and northwest. A lot of the forcing in the interior deserts will come from isentropic ascent and upslope warm-air advection. The richest low-level moisture with the AR will rain out mostly on the windward side of the mountains in SoCal, with a lot of shadowing into Southeast CA and parts of far southwest AZ. For these reasons the highest accumulating rainfall amounts will favor southwest- facing slopes and mountains where orographic lift is maximized. Latest QPF from WPC has storm total rainfall amounts ranging from a few hundredths of an inch in the southern-most deserts to around 0.50- 0.75" in Phoenix and along Phoenix's latitude to over 1" in the foothills north of Phoenix and in western Joshua Tree NP. Latest 12Z runs of the GEFS and ENS maintain the highest probabilities (70-90%) for storm total precip >1" across Yavapai, Mohave, and western Coconino counties. This is where most of the rain is expected to fall with this evening. There is also potential for high rainfall amounts in western Joshua Tree NP, which is being monitored for a potential Flood Watch in the next couple days.

The forecast becomes highly uncertain heading into next weekend as both the deterministic and ensemble model suite continue to diverge significantly on the overall strength, timing, and trajectory of the trough as it eventually migrates inland through the Desert Southwest. Depending on the overall strength and trajectory of the trough, another round of precipitation will be possible.

As this unsettled weather pattern evolves during the middle and latter portion of the week and into next weekend, temperatures are expected to gradually cool but will likely remain above normal in the low to mid 70s. With the abundant cloud cover and moisture expected to be in place, especially Wednesday through Friday, overnight low temperatures are expected to be quite warm for this time of the year, in the upper 50s to near 60 degrees. In fact, there is a potential for all three climate sites (Phoenix, Yuma, El Centro) to break record warm lows. Even more of a cooldown is not expected until the main trough fully moves through, likely toward the end of next weekend/early the following week, and even still temperatures may only drop to seasonal levels.

AVIATION

Updated at 2305Z.

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

No weather issues will exist through Monday evening under gradually increasing thicker high cirrus decks. Wind speeds and directions will be nearly identical to the past 24 hours with prolonged periods of nearly calm conditions.

FIRE WEATHER

High pressure across the region will continue to promote dry and tranquil weather, as well as unseasonably warm temperatures up to 10 to 15 degrees above normal through at least Monday. Winds will generally be light under 15 mph and tend to follow typical diurnal trends. Afternoon MinRHs will range between 20-35% with good overnight recoveries of 50-75%. A weather system is expected to affect the region mid and late week bringing higher moisture, increased rain chances, and cooler temperatures. The initial strong push of moisture will come during the day Tuesday, with MaxRH values Tuesday night jumping to 75-100%. In the following days, MinRHs will be in the 45-70% range.

PSR WATCHES/WARNINGS/ADVISORIES

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