textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- A Flood Watch and Wind Advisory will be in effect for the western, higher elevation portion of Joshua Tree National Park on Wednesday.

- A record high temperature for the date was set in Phoenix this afternoon.

- An unsettled weather pattern will carry us through the Christmas holiday and beyond, with gradually cooling temperatures and a couple periods of increased rain chances, one through tonight and another Wednesday afternoon into Thursday.

SHORT TERM /TODAY THROUGH THURSDAY/

For the third day in a row, a record high temperature has been broken at Phoenix Sky Harbor Airport. A temperature of 80F was recorded at the site, which breaks the previous record of 79F that was set in 1950.

The overall upper level pattern through Thursday will remain rather stagnant, with a broad area of ridging centered over TX/Gulf Coast states, and a longwave trough just off the West Coast. Satellite- derived total precipitable water reveals a couple features that will bring rain chances today/tonight and again tomorrow night into Thursday: 1) the influence of an initial shortwave currently moving inland and tapping tropical moisture, promoting a band of showers over Southeast CA that will continue eastward into South-Central AZ by the evening, and 2) a second shortwave offshore rounding the base of the longwave trough that is driving IVT values consistent with an atmospheric river and will make landfall Wednesday, influencing the forecast area mostly in the PM hours into Thursday morning.

Total rainfall accumulations with this initial round of showers are expected to be on the light side with the latest QPF from WPC showing most of the lower deserts receiving anywhere from a trace to a couple tenths of an inch, whereas the foothills and higher terrain areas north and east of Phoenix should observe locally higher amounts upwards of 0.30-0.40" thanks to the orographic enhancement. Some automated rain gauges in southeast CA have already reported on the order of 0.05" with the passage of the initial band of showers.

The initial shortwave will exit the region Wednesday morning. In its wake, GFS bufr soundings indicate notable midlevel drying and subsidence, ending rain chances temporarily for all areas but the mtns/southward facing slopes north and east of Phoenix, where scattered light showers may continue through the day. Attention then turns to the aforementioned second shortwave, which models have consistently advertised driving IVT values in excess of 500 kg/ms onshore along the LA/San Diego areas Wednesday morning and continuing inland into the Desert Southwest Wednesday PM into Thursday. An attendant upper level jet with this second shortwave will also promote breezier conditions and better forcing for precipitation. Though previous events prove the powerful impact of "rain shadowing", even over interior mountain ranges of Southeast CA, sufficient evidence existed to hoist a Flood Watch for the mtns/higher elevations of Western Joshua Tree National Park. Antecedent moisture brought into the region with this initial shortwave, coupled with jet dynamics and significant inland penetration of 250-500 kg/ms IVT values with the second shortwave should provide a window Wednesday afternoon for sustained moderate rainfall rates. HREF probabilities for >1" of rain over the mountains of far western JTNP are between 40-80%, and WPC QPF shows 0.8-1.65" totals. Meanwhile, similar HREF probabilities for wind gusts in excess of 40 mph in the same area justified a Wind Advisory Wednesday mid morning through the early evening.

Afternoon high temperatures both Wednesday and Thursday will remain above normal in the low to mid 70s. However, the bigger story will be the overnight low temperatures as given the highly anomalous moisture and abundant cloud cover that will be in place, readings will be quite warm for late December, in the upper 50s to around 60 degrees and potentially break record warm lows across all three climate sites (Phoenix, Yuma, and El Centro).

LONG TERM /FRIDAY THROUGH MONDAY/

By the end of the week and into the weekend, as the main trough axis pushes onshore across the western CONUS, the latest guidance now shows the base of the trough splitting off into a closed low off the southern CA coast in what is expected to develop into a rex blocking pattern across the West Coast. However, guidance then diverges significantly in the overall positioning of the closed low as this blocking pattern matures. Both the CMCE and EPS show the closed low becoming positioned well offshore with most of the Desert Southwest under strong ridging, while the GEFS positions the closed low much closer to the coast. The CMCE and EPS solution would keep the region dry through early next week whereas with the GEFS solution, subtropical moisture will be pulled into the region, resulting in increased precipitation chances once again as early as Sunday and continuing into early next week.

Temperatures heading into the weekend are expected to slightly cool off even more from the readings mid and late week with most of the lower desert communities likely stuck in the mid to upper 60s for afternoon highs, which would be close to seasonal norms for late December. A slight warming trend closer to 70 degrees will be possible heading into early next week, depending on the overall pattern evolution.

AVIATION

Updated at 0048Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: SHRA and lowered CIGs later this evening/tonight will be the primary aviation weather issues during the next 24 hours. Virga/sprinkles have now began to develop across the metro this evening with the main band of showers still across southwest Arizona. Expect -SHRA to push across the terminals around 5-10Z tonight, with CIGs lowering during this time to at least 7-8k ft, and perhaps as low as 5-6k ft. Model probabilities of CIGs <6k ft are only around 10%. There also may be FEW to SCT lower clouds down to 3k ft around mountain features. Showers should diminish by 15-17Z with SCT to BKN 7-8k ft clouds persisting throughout the rest of the day.

Winds will favor the west this evening, gusting upwards of 18-20 kts at times as showers move into the area. Winds will then switch around to the east later tonight and will continue through the rest of the TAF period. Southeast California/Southwest Arizona including KIPL and KBLH:

The main band of showers continues to shift east of the terminals with only a lingering VCSH possible this evening. CIGs down to around 6-7 kft are expected through tomorrow morning. However, FEW-SCT clouds around 3 kft may develop overnight with chances for MVFR CIGs climbing upwards of 50-60% for both terminals during this time. Winds will predominantly favor the E-SE at KIPL, while winds will vary between the E-NNE at KBLH before switching around to the south late Wednesday morning. Expect periods of light and variable winds to occur at both terminals throughout the period.

FIRE WEATHER

An unsettled weather pattern is expected to develop across the region beginning today and lasting through the end of the week, bringing higher moisture, increased rain chances, and cooler temperatures. A strong moisture push will spark scattered shower activity beginning this afternoon across the western districts, pushing into south-central AZ later this evening. Another round of rain is expected between Wednesday afternoon and Thursday morning. MinRHs today will range between 20-25% across the eastern districts to 35-50% across the western districts with excellent overnight recoveries of 75-100%. MinRHs will then jump in the 45-70% range Wednesday through end of the week before slightly decreasing over the weekend with excellent overnight recoveries of 75-100% continuing. Winds will remain light today with breezes picking up on Wednesday, especially across the western districts, with some periodic gusts in excess of 20 mph. Lighter winds are anticipated by the latter portion of the week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...Wind Advisory from 8 AM to 6 PM PST Wednesday for CAZ560.

Flood Watch from Wednesday morning through Wednesday evening for CAZ560.