textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- Above normal temperatures with lower desert highs in the middle nineties will continue today before much cooler temperatures into a below normal category briefly materializes during the first part of the week.

- Breezy to locally windy conditions will materialize through the first part of the week with some modest rain chances across the Arizona higher terrain areas.

- A rapid warming trend to above normal temperatures is likely by the latter half of the week.

SHORT TERM /Today through Tuesday/

Early morning water vapor satellite imagery depicts a ridge of high pressure centered over the Desert Southwest while a robust cut-off low sits off the central CA coast. Under the influence of the ridge, afternoon high temperatures will continue to run a good 4-7 degrees above normal as readings top out in the middle 90s across most of the lower desert locations. As the cutoff continues to slowly march its way eastward throughout the day, the combination of gradual height falls and increasing jet energy will result an enhancement in the low-level wind field heading into this afternoon and evening with widespread breezy to locally windy conditions expected, especially across the southwest AZ and southeast CA as peak gusts top out in the 25-35mph range. Across the western half of Imperial County, mountain rotor action combined with downsloping flow will cause winds to be even stronger with gusts in excess of 40 mph likely to materialize at times heading into this evening. Given this scenario, Wind Advisories have been hoisted for much of the western half of Imperial County, including the Imperial Valley, for the evening and overnight period. The strong wind gusts may also result in areas of blowing dust affecting portions of the Imperial Valley, resulting in temporary reductions in visibilities.

The aforementioned cut-off low will slowly migrate inland into central CA on Monday before migrating southeastward into southern CA and AZ on Tuesday while transitioning into an open wave. As the low migrates inland, the combination of 500 mb height fields dropping into the 568-574dm range and more abundant mid to upper-level cloudiness will result in much cooler temperatures with afternoon highs 10-13F cooler than today as readings top out in low to mid 80s across the lower deserts. These temperature readings will be a good 5-10F below normal for early May. Enhanced jet energy will continue to support enhanced flow throughout the low levels and thus another day with afternoon/evening peak gusts between 25-35 mph can be expected. In addition, the slow migration of the low will also allow for some subtropical moisture to be advected across the southeastern half of AZ as PWATs rise to 0.7-0.9" or around 200% of normal. This moisture along with robust synoptic focusing will result in some scattered light shower activity to break out mostly across the higher terrain areas east of Phoenix late Monday night into the first part of Tuesday while across the lower deserts of south- central AZ, some light sprinkles/virga will be possible as the low- levels will remain too dry to support accumulating rainfall.

As the cold core moves directly overhead on Tuesday, afternoon high temperatures will be even cooler with most lower desert locations struggling to reach 80 degrees. This will be a solid 10 degrees below normal and will possibly be the coolest temperatures the region will observe until the fall. Breezy conditions will continue to remain in place for much of the region, however, wind speeds will be weaker as peak gusts top out between 15-30 mph.

LONG TERM /Wednesday through Saturday/

The main energy associated with the upper low will exit to the northeast on Wednesday with upper-level ridging building along the west coast. However, model guidance is showing a trailing piece of energy left behind that will likely develop into a cut-off low somewhere to the south of the region with differences noted with respect to the positioning and speed as it gradually progresses eastward through the latter half of the week. Nevertheless, given that this feature will be positioned far enough south, it will likely not have much in the way of sensible weather impacts for the region. With upper-level height fields steadily rising from the building ridge over the west coast, temperatures during the latter half of the week will rapidly warm from the middle 80s on Wednesday to middle 90s on Thursday. Additional warming into the triple digits is forecast as early as Friday and will likely be the start of an extended stretch of triple digit highs as the upper-level ridge gradually builds across the western CONUS.

AVIATION

Updated at 1118Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty winds will be the main aviation concern through the TAF period. Marginal breeziness will be common this morning (15-18Z) and once again in the evening (02-05Z) where gust will generally be between 20-30 kts. Easterly winds will prevail through this morning with a later than usual shift to the W this afternoon. A period of southerly crosswinds are expected prior to the westerly shift. SCT-BKN skies will progressively become OVC by midnight tonight.

A wildcard in the forecast is the potential for slantwise VIS issues for KPHX come morning due to a brush fire to the west of the terminal. Easterly morning winds are expected to take smoke mainly to the west of the airspace, therefore likely not impacting the site directly, but reductions in approach VIS cannot be completely ruled out.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation concern during the next 24 hours will be breezy to windy conditions this afternoon and evening. VRB winds at KBLH this morning will eventually become established out of the S by mid morning. Gusts 20-25 kt enter the picture by the early afternoon hours and are expected to prevail through the rest of the TAF period. At KIPL, E'rly winds will quickly switch to the W by the afternoon, with peak gusts potentially exceeding 35 kt by the evening. These enhanced gusts will increase the potential for blowing dust, which may result in reductions in VIS. However, confidence is too low to warrant any mention of sub- VFR visibilities in the TAF at this time. SCT-BKN mid to high clouds will be common through Sunday evening.

FIRE WEATHER

Dry conditions along with above normal temperatures will continue today. An approaching low pressure system will result in widespread breezy to locally windy conditions through the first part of the week and temperatures cooling into a below normal category Monday-Tuesday. Gusty afternoon winds combined with low humidities will likely produce elevated fire weather conditions today and Monday. As the low pressure system moves through the Desert Southwest, the chance of showers will increase late Monday into the first part of Tuesday across the far eastern districts, however, the probability of wetting rains will remain on the low side. Afternoon MinRHs between 10-20% will be common today before increasing into a 15-25% range Monday and into a 20-35% range Tuesday. Lighter winds as well as much warmer temperatures and lower humidities are then forecast for the latter half of the week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...Wind Advisory from 5 PM this afternoon to 8 AM PDT Monday for CAZ562.

Wind Advisory from 7 PM this evening to 3 AM PDT Monday for CAZ566-567.