textproduct: NWS Phoenix

UPDATE

00Z Aviation Discussion.

KEY MESSAGES

- Tranquil weather conditions and slightly above normal temperatures today will transition into a cool down over the weekend as a mostly dry weather system brings breezy to windy conditions to the region.

- Near normal temperatures will be common much of next week despite another potential weather system during the latter half of next week.

SHORT TERM /TODAY THROUGH SUNDAY/

Midday synoptic analysis shows quasi-zonal flow from the Eastern Pacific through nearly the entire southern tier of the Lower 48. Latest water vapor imagery also shows a few low pressure features in the Eastern Pacific including a weak shortwave moving onshore now through middle portions of CA and a larger low another 800 miles west over the Pacific. These features will influence weather conditions through this weekend as they progress through the West. Ahead of these features temperatures have warmed, with lower desert highs this afternoon forecast to top out near 90 degrees. The first of the two mentioned lows will not do too much to the local area besides help enhance westerly mountain top and sundowner downslope winds this evening across Southeast CA. For several hours this evening, beginning around 5-6 PM MST/PDT, HREF shows high probabilities (70-90%) for wind gusts >35mph in areas of Imperial County, including parts of the Imperial Valley. A Wind Advisory has been issued for the far southwest corner of Imperial County starting this evening, but held off on one for other parts of the county.

The second low mentioned eventually will move through the region later Saturday into Sunday and continue to drive breezy to windy conditions, as well as cool temperatures down and bring a low-end chance for rain. The wind and cooler temperatures will be the primary sensible weather impact this weekend for the lower deserts of southern AZ and southeast CA. Winds will be particularly strong across southeast CA and portions of southwest AZ Saturday evening through early Sunday morning, coinciding with the passing of a very strong jet max over the area, on the southern side of the low as it moves inland. Latest model guidance has 500mb winds peaking around 50-60kts and 110-120kts at 200mb, which is not completely uncommon to see this time of year, but still pushing above the 90th percentile of climatology. The timing of the jet moving overhead in the evening should help lead to strong mountain wave activity and sundowner downsloping winds across Imperial County. Latest NBM forecast has surface wind gusts peaking around 30-35 mph across much of southeast CA, however downsloping enhancements, mountain rotors will be capable of pushing peak gusts locally upwards of 40-50 mph. As a result, a Wind Advisory has been issued for most of southeast CA Saturday evening through early Sunday morning. The Advisory for the southwest corner of Imperial county will continue through Sunday evening. Widespread breeziness can be expected both days this weekend, but most areas outside of southeast CA will see peak gusts more in the 20-30 mph range. The strong winds Saturday night will be capable of generating localized dense blowing dust channels in Imperial County and may lead to very hazy conditions across even south-central AZ and the Phoenix area Sunday morning.

The main energy from the weekend disturbance is forecast to move through southern California Saturday evening and then across the northern half of Arizona Saturday night. This trajectory is not ideal for rain chances across the lower deserts and the deep mid and upper level moisture drying out, with a dry punch from the southwest ahead of the best forcing, is not great either. There will at least be some jet forcing, but with dry low levels (dew points in the 30s-40s) and the mentioned drying aloft, the prospects of rain at lower elevations is not great (10% or less). Still, some isolated to scattered light showers across the high terrain north of the Phoenix area with very little rainfall is possible (10-20% chance).

Temperatures will cool off over the weekend with highs falling back mostly into the mid 80s on Saturday with the help of the clouds and then only in the upper 70s on Sunday behind a cold front.

LONG TERM /MONDAY THROUGH NEXT FRIDAY/

Model uncertainty becomes an issue for later next week as yet another Pacific weather system begins to take shape well west of California early in the week. For our region, the exiting of the weekend system on Sunday will leave behind a drier air mass which will slowly modify and warm Monday into Tuesday. Both days should still bring nice weather and clear to mostly clear skies, but high temperatures are forecast to warm back to near 85 degrees on Monday and then 90 degrees Tuesday.

Over the past couple of model runs, guidance has put a little more doubt in the timing of the Pacific system for later next week. Models still mostly favor the system to begin approaching the region by next Tuesday/Wednesday, but it may become more cut off from the main flow which could delay the system to more like Thursday or even as late as Friday. The average of the ensembles generally indicate the system moving through our region later Wednesday into early Thursday, but some members hold off until late Thursday/early Friday. Early indications are for a bit better moisture this time around which should bring at least minimal chances for showers whenever the main shortwave moves through the area. The latest NBM/WPC PoPs give us 15-20% chances across the lower deserts to as high as 20-30% in the high terrain for the latter half of Wednesday into Thursday morning. The timing uncertainty could easily change this timeframe as well as any significant changes in forecast moisture.

AVIATION

Updated at 2350Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Occasionally gusty SW/W winds will be main weather issue through Saturday evening. Winds Gusts to 20-25 kts are expected to linger for a few couple of hours, but will diminish after sunset. Confidence remains high that the nocturnal easterly wind shift will occur tonight, though later than usual at KPHX. After sunrise Saturday morning, winds are expected to increase to around 7-8 kts out of the S-SE before westerly winds become established in the early afternoon. Westerly gusts in the 20-25 kt range will become common again through Saturday afternoon. FEW-SCT high cirrus decks today will be followed by increasing mid and high clouds tonight through Saturday morning, with CIGs lowering to 15-20 kft AGL.

Southeast California/Southwest Arizona including KIPL and KBLH: Increasingly gusty winds late this afternoon/evening will be the main aviation weather issue. Winds will remain westerly at KIPL and favor S-SW at KBLH. Gusts up to 20-25 kts will continue for the next few hours at KBLH while increasing closer to 30kt for a few hours after sunset at KIPL. Confidence is good that gusts will subside at KBLH tonight, but wind speeds between 10-15 kts with occasional higher gusts may be maintained overnight at KIPL. Mid and high cloud decks will increase through Saturday morning.

FIRE WEATHER

Dry conditions with relatively light winds are expected for much of today before breezy conditions develop tonight and become locally windy on Saturday. Wind gusts Saturday will commonly reach 20-30 mph in many locations to as high as 35-50 mph across portions of southeast California, especially Saturday evening through early Sunday morning. Expect MinRHs this afternoon between 10-15% before increasing to 15-20% Saturday. Overnight recoveries will improve over the next couple of nights, rising from 25-40% tonight to 40-60% Saturday night. A mostly dry weather system passing through the region Saturday night may bring some high terrain showers, but CWR is less than 10%. Seasonably breezy afternoon winds and drying conditions are forecast for early next week, but winds should fall short of creating widespread elevated fire weather concerns.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...Wind Advisory from 5 PM Saturday to 5 AM PDT Sunday for CAZ560- 563>568.

Wind Advisory until 11 PM PDT Sunday for CAZ562.