textproduct: NWS Phoenix

UPDATE

00Z 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 2350Z.

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

No major aviation concerns are expected through the forecast period. Winds will follow familiar diurnal trends with a westerly direction expected through sunset. After relaxing slightly around sunset, westerly flow will increase for a few hours late tonight as a gulf surge arrives, delaying the the typical east shift until 10Z-12Z. FEW to SCT mid- and upper- level cloud decks will be present over the region through Friday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern will be the potential for gusty outflow winds from decaying thunderstorms near at KIPL this evening. Gusts may reach 20-25 kts for few hours until subsiding later tonight. Outside of any thunderstorm related outflow, winds are expected to follow diurnal trends at both terminals under FEW to sometimes SCT mid- and upper- level cloud decks.

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.

Extreme Heat Warning until 8 PM MST this evening for AZZ531- 535>544-546-548>551-559.

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