textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Above normal temperatures will prevail through the rest of the week, with Extreme Heat Warnings in effect for portions of the lower deserts.

- Daily chances for scattered showers and isolated thunderstorms will continue to be focused over the Eastern Arizona high terrain. Any chances for lower deserts should be confined to late Friday and again Sunday into Monday.

- Cooling temperatures over the weekend into early next week should eventually push readings back into the normal range by around next Tuesday.

SHORT TERM /TODAY THROUGH FRIDAY/

The high remains centered over southwestern AZ and southeastern CA with generally easterly flow aloft over the region. Mid to high level moisture has stayed put with 1000-700 mb mixing ratios still ranging between 10-12g/kg across the lower deserts and into the eastern high terrain. While we have plenty of moisture (PWATs 140-160% of normal), with a strong lower level subsidence inversion in place convection will struggle to have any development for most areas, especially after SE AZ had extensive activity last night, which may have overworked the atmosphere too much for anything developing tonight. This is reflected in the latest CAMs and HREF showing <10% chance of activity in the far eastern high terrain, but near a 30% chance of light showers later this afternoon in western Imperial County, with convection already visible on satellite developing in Northern Baja.

The peak strength of the ridge occurred late yesterday, translating to peak temperatures today with the highs between 106F-110F in SC AZ and 108F-112F in the western lower deserts and along the Colorado River Valley. Additionally, the lower deserts of southeast California are expected to feel especially unbearable as heat indices will be up to around 115 degrees today. With above normal temperatures and more humid conditions be sure to take extra care when spending time outdoors. The Extreme Heat Warning remains in effect through this evening for SC AZ, and the Phoenix Metro, and through Friday evening along the Colorado River Valley, Yuma area, and southeastern Imperial Valley. The upper level ridge will continue to weaken today into Friday, likely weakening enough to lower temperatures a good 2-4 degrees across south-central Arizona starting Friday and 1-3 degrees over the western deserts.

Friday should also bring another influx of moisture from the south southwest, pushing area low level mixing ratios more into a 11-12 g/kg range. However, this may still not be enough to spark off more widespread convection as the the subsidence inversion will still be in place (albeit slightly weaker) and the steering flow is expected to turn out of the southwest. We are expecting better storm coverage across the eastern Arizona high terrain on Friday, but with a southwesterly steering flow it will be hard to get convection to fire over the lower deserts. The latest NBM run continued to show PoPs between 25-25% over SC AZ and eastwards, however with the atmospheric conditions expected to be in place PoPs have been reduced to 5-15% with the higher range being mostly confined to the east of Phoenix. If storms do actually get going over the south- central Arizona lower deserts Friday evening, they will be within a high DCAPE environment which means strong winds will be a possibility.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Forecast confidence for the weekend into next week remains quite low, especially when looking at moisture levels and rain chances. Guidance does agree the subtropical ridge will get pushed to the east of our region over the weekend as a Pacific trough moves through the northern tier states, but the GEFS is stronger and farther south with the trough compared to the EPS. The models also differ in the amount of moisture and rain chances during the period with the GEFS favoring better rain chances from Phoenix and areas east of Phoenix over the weekend, while the EPS favors better chances by Tuesday into the middle of next week. Needless to say, these differences do not inspire much confidence in the available NBM/WPC PoPs.

The latest forecast places little if any PoPs on Saturday and Saturday night, which seems fairly reasonable, but then seems to be under doing chances late Sunday. The increased PoPs of 20-40% then seem too high for Monday before gradually lowering during the middle part of next week. Lately, guidance has had a tendency to place fairly high PoPs during the 4-8 day range, just to back off as it gets into the short-term. It would seem everything would have to come together nearly perfectly with the trough passing by to our north close enough, but not too close to scour out what moisture we have in the low/mid levels. Additionally, if the subtropical ridge becomes more dominant, then we will have to worry about too strong of a subsidence inversion. Given the fairly high model uncertainty, the forecast is likely to change.

The temperature forecast is also of lower confidence with a fairly wide spread starting Monday. At the moment, NBM forecast temperatures call for readings more in the normal range by early next week, but given the higher model spread we could end up with readings slightly below or even slightly above normal. Guidance at least mostly agrees on the 'coolest' temperatures falling on next Tuesday and Wednesday before trending upward again during the latter half of next week.

AVIATION

Updated at 0525Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

The only aviation weather issue will be low confidence on winds Friday evening, with the possibility of distant TS outflows causing sudden gusty winds out of the SE or NE. Winds will continue to favor SW tonight through around 10-11Z, with a period of gusts into the upper teens possible. A somewhat typical, if delayed a bit, southeast wind switch is expected before sunrise followed by a switch back to SW/W between 17-19Z. Occasional afternoon gusts to around 15-18 kts are likely. Chances for TSRA/SHRA are too low (10% or less) to include in the TAF Friday late afternoon/evening, and the associated impacts on winds are quite uncertain this far out and dependent on strong outflows progressing through the Phoenix airspace. However, if an outflow does reach the terminals, it will likely emanate from an easterly direction (SE to NE). FEW-SCT mid and high cloud decks will persist through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will continue to oscillate between SE and SW at the terminals with speeds 8 to 15 kts sustained (periods with lower speeds likely at KIPL). A few periods with gusts between 20-25 kts are possible at KBLH, one Friday afternoon and another in the evening after a lull. The possibility of another distant TS outflow reaching KIPL cannot be ruled out late Friday afternoon/early evening. Periods with FEW-SCT and occasionally BKN mid and high cloud decks will continue.

FIRE WEATHER

Above normal temperatures will continue into the weekend with modestly elevated humidities. Rain chances will improve by Friday, but still mostly favor the Arizona high terrain. Afternoon minimum humidities of 17-25% today will improve to 25-30% for Friday and Saturday. Overnight recoveries will remain fair to good at 40-70%. Expect fairly typical diurnal winds with little if any breeziness today, but with 20-25 mph afternoon and early evening gusts on Friday and Saturday, mostly in the western portion of the CWA. Chances for isolated showers and thunderstorms are likely to remain possible across the eastern districts through at least early next week while temperatures eventually fall back into a normal range.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ530-532-533.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ562-566-567- 569-570.