textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- Above normal temperatures with minimal day to day changes will exist through the majority of the week as afternoon highs reach 97 to 103 degrees across the lower deserts.

- An Extreme Heat Watch is in effect for portions of southeast California and southwest Arizona on Wednesday and Thursday as temperatures warm to near record levels and lead to major HeatRisk.

- Dry weather with zero rain chances will persist through Saturday with shower and storm chances returning to Arizona as early as Sunday.

SHORT TERM /TODAY THROUGH WEDNESDAY/

It has been another breezy day across southern AZ, but winds have not quite as strong as yesterday due to a weakening pressure gradient. Surface pressure to the east is lowering which is allowing the east-west gradient to relax. This weakening will continue through tomorrow morning. So, there will still be some breeziness in the morning tomorrow before the gradient fully relaxes and winds return to a more typical light diurnal pattern.

High pressure remains in place over the Southwest, with midday objective analysis showing H5 heights around 591dam, which is just above the 90th percentile based on sounding climatology. This high pressure, and mostly clear skies, will continue to support above normal temperatures across the region. Latest aircraft soundings out of Phoenix show little change in the low-level thermal profile over the last 24 hours. So, high temperatures this afternoon will be nearly identical to yesterday, with lower deserts topping out around 97-103 degrees.

Heading through tomorrow and Wednesday a weak shortwave currently over northern NV is expected to turn southeast and drop through northeast AZ on Wednesday while a strong anticyclone develops near the coast of SoCal. While the shortwave may lead to isolated showers and storms up along the Mogollon Rim Wednesday, the lower deserts are expected to remain dry. The shortwave will also help keep south- central AZ from seeing a significant warm-up, unlike areas to the west. Temperatures tomorrow will still be similar to today, if not a degree warmer, and then temperatures warm 2-4 degrees into Wednesday, with nearly all lower desert locations reaching 100 degrees in the afternoon. Temperatures will be hottest across southeast CA and along the Lower Colorado River Valley, including Yuma, where highs on Wednesday will reach the 104-107 degree range, which is upwards of 11-14 degrees above normal for early October and will be near daily records. An Extreme Heat Watch remains in effect starting Wednesday for the Imperial Valley, CA and Yuma, AZ.

LONG TERM /THURSDAY THROUGH SUNDAY/

A strong anticyclone is expected to persist near the SoCal coast through at least Thursday, which will keep temperatures well above normal and near daily records across SoCal. Temperatures are also expected to warm another 1-2 degrees across south-central AZ on Thursday, following the passage of a weak shortwave trough. Even south-central AZ will see high temperatures up to 10 degrees above normal (99-102 degrees) on Thursday. An Extreme Heat Watch remains in effect through Thursday for Imperial Valley, CA and Yuma, AZ, where probabilities of Major HeatRisk are highest (50-70%). This watch is likely to be upgraded to a warning tonight or tomorrow morning and Friday is being considered for the heat alert as well. Latest NBM forecast shows 30-50% Major HeatRisk probs on Friday for the same areas under the current watch.

The coastal anticyclone is expected to breakdown heading into Friday as the synoptic pattern shifts, with a Pacific low moving toward the CA coast, but low level temperatures are expected to remain very warm. Latest global ensembles show 850mb temps cooling by only a degree or two centigrade into Friday, which should only result in 1- 4 degrees of surface cooling from Thursday. Latest high temperature forecast for Friday has 101-106 degrees across southeast CA and southwest AZ to 97-101 degrees across south-central AZ.

As alluded to, the pattern will shift heading into the end of this week and this weekend with a trend toward troffing and negative height anomalies across the Western CONUS. While there is good confidence in the trough pattern developing, there is still a lot of model uncertainty/spread regarding how the trough pattern will evolve heading into the end of the weekend and start of next week. It does look like the Pacific low that will near the CA coast on Friday will dip south this weekend and interact or faze with TC Rachel while at the same time a trough will move out of the Gulf of Alaska and strengthen as it moves south down the northern Pacific coast. There are certainly a lot of eyes on what will happen with TC Rachel and whether or not it will help bring rain to the Desert Southwest, just like with TCs Odalys and Polo early last week.

Latest guidance has trended toward an eventual turn to the northeast by Rachel by the end of the weekend. While this trend does lend itself in favor of a push of moisture toward the Southwest CONUS, there is still a lot of uncertainty in the forecast this far out, especially with timing and evolution of the trough moving down the northern Pacific coast and how that may affect the remnants of Rachel. Current thinking is at least mid and high level moisture will eventually shear off or Rachel up toward the Southwest, which may bring showery activity as early as Sunday and then the more potent coastal trough/low will help drive additional shower and storm chances across AZ, while also sending a cold front and cooler temperatures through the region early next week. Again uncertainty is still high and it is ill-advised to buy into single deterministic long-range model runs that may show a strong tropical circulation pushing into the U.S.

AVIATION

Updated at 2315Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No weather issues will exist through Tuesday evening under a few midlevel cloud decks. Confidence is excellent that easterly winds will persist well into Tuesday afternoon with far less gustiness than the past few days. Confidence is increasing that wind directions will finally complete the traditional westerly switch late Tuesday afternoon/evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather issues will exist through Tuesday evening under clear skies. Winds will continue to oscillate between NE and NW with speeds under 10kt and extended periods of nearly calm conditions.

FIRE WEATHER

Dry conditions with well above normal temperatures will continue through the week, with afternoon high temperatures reaching up to 100-107 degrees across the lower deserts (hottest on Thursday). Breezy easterly winds will continue across southern AZ through this afternoon and Tuesday morning, with gusts up to 20-25 mph. Winds will return to their more typical light diurnal tendencies Tuesday afternoon through Thursday, with only minor afternoon upslope gustiness. A gusty easterly gradient wind is expected to develop again by Friday and last through Saturday. MinRHs will mostly range from 15-25% across the western districts to 20-35% across eastern districts through the week with fair to good overnight recovery (35- 60%) area-wide. Humidities will be lowest on Thursday when temperatures are likely at their hottest, but winds will still be light at this time, so there are no elevated fire weather concerns. No wetting rain expected through at least Saturday. Rain chances then go up by the end of the weekend/start of next week with increased moisture and a potential weather system moving through.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Watch from Wednesday morning through Thursday evening for AZZ532.

CA...Extreme Heat Watch from Wednesday morning through Thursday evening for CAZ566-567.