textproduct: NWS Phoenix

UPDATE

Updated 06Z Aviation Discussion.

KEY MESSAGES

- Quiet weather with well-above normal temperatures will continue today across the region.

- A pattern change will bring more clouds and wetter weather to the area beginning tomorrow afternoon and persisting through late week with increased rain chances and gradually cooling temperatures.

SHORT TERM /TODAY THROUGH TUESDAY/

Upper-level ridging is gradually shifting more into the Plains today, but the Desert Southwest still remains under its influence with 583-585dam H5 heights over AZ, which is above the 90th percentile for this time of year. With the strong high pressure influence, high temperatures this afternoon will once again be upwards of around 15 degrees above normal and pushing daily record highs. Phoenix broke its daily record high yesterday and will likely break today's 100+ year old record of 79 degrees with a high forecast to reach 82 degrees. There will be increased high clouds today, but forecast soundings suggest they will not be optically thick enough to have any significant influence on the temperatures.

Heading into tomorrow, the overall CONUS weather pattern will become more amplified with ridging east of the Continental Divide and a deep long-wave trough along the Pacific coast. This pattern configuration will put the Desert Southwest under deep southwesterly flow and help draw anomalous moisture into the region. The deepening trough will push a moderate-strong AR southward through CA and tap into some subtropical moisture. A strong push of moisture from the subtropics is expected tomorrow, with PWATs forecast to quickly rise from 0.3-0.5" early tomorrow morning to as high 1.1- 1.3" (~300% of normal) by late in the evening. A weak subtropical shortwave moving up with this initial push of moisture will likely aid in the development of a band of scattered gusty light showers starting out west over southeast CA and southwest AZ around midday and moving into south-central AZ by tomorrow evening through tomorrow night. Rainfall is expected to be light with this first push, with accumulations ranging from a hundredths to a couple tenths. Latest HREF has a 10-30% chance of exceeding 0.2" of rain through early Wednesday morning in the Phoenix area and increases up to 70-80% chance along the Maricopa- Yavapai county border.

Afternoon high temperatures tomorrow will be slightly cooler than today, with the highest degree of cooling, 5-8 degrees, expected in the western deserts, while south-central AZ only cools 1-4 degrees. The cooling will be driven largely by an increase in the specific heat capacity with the increasing moisture and cloud cover. With the only minor cooling in Phoenix tomorrow, there is potential to tie or break yet another daily record high. Latest NBM forecast has a high of 80 in Phoenix and the record is 79 (1950).

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 into the afternoon, 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). The IVT axis, associated with AR that will cause major impacts in SoCal, will eventually push further inland into the interior deserts later Wednesday through Thursday morning. While a lot of moisture will be squeezed out in the SoCal coastal ranges, both GEFS and ENS models show pretty high probabilities (60-90%) of IVT values >500 kg/ms reaching beyond the Colorado River during the mentioned timeframe. This will lead to the best rain chances of the forecast, with the latest NBM maintaining 70-90% PoPs from Southeast CA through south-central AZ late Wednesday through Thursday morning. Despite this outlook, there are still some model differences in the evolution of this pattern which could limit the coverage of rain showers in south-central AZ. A shortwave trough helping guide the AR in SoCal is expected to track NE'ward from SoCal through southern NV. This track will favor better forcing and rain bands to track more through the mentioned area and parts of northwest AZ. Deterministic models like the GFS, Canadian, and experimental RRFS show the main rain band breaking up on the southern end becoming more isolated or scattered as the IVT axis shifts further east into AZ. The ECMWF holds the band together more into south-central AZ.

The highest rainfall totals are projected across the upslope areas and mountains where orographic lift will be maximized. The latest QPF from WPC has rainfall totals ranging from over 1" across the foothills and terrain north of Maricopa and La Paz Counties and in western Joshua Tree National Park to between 0.20-0.50" in the Phoenix area and between 0.05-0.20" across the rest of the lower deserts. Rainfall forecasts will become more refined and potentially less uniform heading into the hi-res model window. The one constant in the QPF forecast is that the highest rain totals regionally will favor the terrain areas of Yavapai and Mohave Counties, with GEFS and ENS still highlighting these areas with 50-80% chances of >1" storm totals (Tue-Fri)

Rain chances will gradually decline Thursday afternoon through Friday as subsidence and drying moves in above 700mb. However, lingering trapped low-level moisture will still be enough to support at least some spotty showers in the higher terrain areas of AZ. A surface cold front and axis of moisture should also push into SoCal again Friday, but most if not all of the moisture is expected to rain out on the windward side of the mountains again. There is a little better confidence in the pattern evolution going into this weekend with the latest 12Z global models favoring a rex block developing along the West Coast. The block will involve the base of the long-wave trough splitting off into a closed low near to off the SoCal coast. Details regarding how this pattern will affect the Desert Southwest is a little less certain, with some models tapping more sub-tropical moisture and pulling it northward and others putting the region more under the influence of drier ridging.

With the unsettled pattern evolving mid to late week, temperatures are expected to gradually cool, but will remain above normal as afternoon highs will top in the low to mid 70s. With the abundant moisture and cloud cover that will be in place, overnight low temperatures between the Wednesday through Friday time frame will be quite warm for late December, in the upper 50s to around 60 degrees and potentially breaking record warm lows across all three climate sites (Phoenix, Yuma, and El Centro). Latest NBM has temperatures cooling further, closer to seasonal levels this weekend.

AVIATION

Updated at 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; No aviation weather concerns under high cirrus decks can be expected through tomorrow afternoon. Winds will remain light (generally AOB 8 kts). Winds will exhibit an easterly component through the overnight hours before going more S/SE'rly early tomorrow afternoon. By the mid to late afternoon hours winds will then go SW'rly.

By Tuesday evening (01-03Z), an area of virga and -SHRA will move into the region from the W/SW. Due to this VCSH has been added to the TAFs at this time. With this virga and -SHRA, lower cloud decks will also move in. CIGs are expected to remain around 7-8 kft, with FEW-SCT clouds as low as 4-5 kft. Winds will go W'rly with this virga/-SHRA. Wind speeds will be around 10-12 kt, with some occasional gusts into the 15-20 kt range possible. The virga/-SHRA is expected to exit around 06-07Z, with winds going back easterly overnight, but remaining around 10 kt. Southeast California/Southwest Arizona including KIPL and KBLH: Winds will continue to be aob 5 kt through tomorrow morning under BKN high cirrus clouds. By tomorrow afternoon an area of virga/-SHRA will move into the region. With VCSH in the KIPL terminal between 19-22Z and 21-00Z at KBLH. Winds will be SE'rly around 5-10 kt with the VCSH. CIGs will lower to around 12-15 kft with the VCSH, with FEW-SCT as low as 8-10 kft. After the virga/-SHRA exit winds will continue to be light SE'rly through the remainder of the TAF period and cloud decks will slowly scatter out.

FIRE WEATHER

High pressure across the region will continue to promote dry and tranquil weather today along with unseasonably warm temperatures as afternoon highs top out in the upper 70s to low 80s across the lower deserts. 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% tonight. A weather system is expected to affect the region beginning tomorrow, bringing higher moisture, increased rain chances, and cooler temperatures. The initial strong push of moisture will come during the day tomorrow, with afternoon MinRHs still in the 20-30% range across south-central AZ and 35-50% across western areas. Best rain chances will be between Wednesday afternoon and Thursday morning. MaxRH values tomorrow night then jump to 75-100% area- wide. In the following days, MinRHs will be in the 45-70% range each day.

PSR WATCHES/WARNINGS/ADVISORIES

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