textproduct: NWS Phoenix

KEY MESSAGES

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

- An unsettled weather pattern will carry us through the Christmas holiday and potentially beyond, with gradually cooling temperatures and a couple periods of increased rain chances, one early today and another this evening into Thursday.

SHORT TERM /TODAY THROUGH THURSDAY/

An impressive upper level trough is currently situated off the West Coast with the low center west of Washington and Oregon and the base of the trough extending as far south as 20 degrees N latitude, or equivalent to the southern tip of Baja. The base of the trough is also showing H5 heights at around record MIN for this time of year and this is forecast to continue through early Thursday.

For our region, we remain partially covered by ridging across eastern portions of the region and it is likely to stay that way as the deep trough is not expected to make any additional eastward progress. However due to the close proximity of the trough, we have seen strong moisture advection over the past 12 hours pushing PWATs to over 1" across southeast California and through most of the Arizona lower deserts. The moisture advection is also coinciding with the passage of a weak shortwave trough which has helped to spark off some light shower activity across portions of the region. This shower activity is forecast to move over eastern portions of Arizona later this morning before coming to an end during the afternoon hours.

The next round of precipitation will be associated with the main shortwave and upper level jet max coming onshore in California, but the trajectory of this band of rain will largely keep it northwest of our area as strong rain shadowing is expected into southeast California. However, Joshua Tree NP may see portions of this band with a period of moderate to heavy rainfall possible focused from late afternoon through around mid evening. A Flood Watch remains in effect for western portions of Joshua Tree NP from mid morning through this evening.

For the rest of the area, another batch of rain is likely to develop by late this evening, potentially lasting through Thursday morning. This band is likely to form somewhere across southwest or south-central Arizona in an area of increasing upper level forced ascent in combination with upslope low and mid level southwesterly flow. This band will be the main shot at rainfall across the lower deserts and it may involve a period of moderate rainfall and amounts over 0.25" in some spots. Additional periods of isolated to scattered showers are likely for the rest of Thursday with the main area of focus over the foothills and higher terrain areas of central Arizona.

Temperatures will take a bit of a hit today into Thursday, but due to the warm nature of the weather system and the fact the ridge is still over eastern portions of the region, readings will stay near 10 degrees above normals.

LONG TERM /FRIDAY THROUGH TUESDAY/

The deep Pacific trough is forecast to weaken quickly going into Friday with the system breaking apart into two pieces, one quickly ejecting northeastward into the Northern Rockies and another drifting southward off the coast of California. Drier air is expected to push into our region from the southwest Friday into the weekend with PWATs eventually falling to below normal across at least western portions of the region. What happens with the leftover energy from the trough and its track is still quite uncertain, but trends have been pulling it farther away from our region. If this continues, we are likely to see a break in the rain chances by the end of Friday and lasting through the weekend. A cooler air mass should eventually settle in over our region by Friday and last through the weekend with highs dropping into the upper 60s to the lower 70s.

Model uncertainty increases a good deal by early next week as the leftover trough energy may drift back closer to our region, potentially bringing another round of moisture advection and rain chances as early as Monday and lasting through the middle of next week.

AVIATION

Updated at 0559Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: SHRA and lowered CIGs through the overnight hours will be the primary aviation weather issues during the next 24 hours. Light shower activity continues to push across parts of the Phoenix metro this evening and will continue into the overnight hours. CIGs will lower during this time to at least 7-8k ft, and perhaps as low as 5-6k ft at times. 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 through the TAF periods will predominantly favor the east with speeds mostly aob 10 kts and periods of variability.

Southeast California/Southwest Arizona including KIPL and KBLH:

Shower activity has mostly diminished across southeast California, however, lower CIGs will continue into the overnight hours. CIGs down to around 6-7 kft are expected through tomorrow morning. Winds will predominantly favor the E-SE at KIPL, while winds will favor the E-NE 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 will result in gradually cooling temperatures, increased humidities, and periodic chances for light to moderate rain showers mainly through Thursday. MinRHs will improve dramatically today, ranging between 40-60% areawide and staying elevated into the weekend. Winds will mostly prevail out of the south to southeast through Thursday, with speeds generally 5-15 mph, and locally stronger gusts upwards of 20 to 30 mph across portions of Southwest AZ and Southeast CA today.

PSR WATCHES/WARNINGS/ADVISORIES

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

Flood Watch from 9 AM PST Wednesday through Wednesday evening for CAZ560.