textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES.

- Temperatures will remain several degrees above daily normals this week with lower desert highs climbing to 108-113 degrees and overnight lows in the mid 80s to low 90s.

- Extreme Heat Warnings are in effect through Friday evening due to widespread major HeatRisk.

- Thunderstorm activity will be mostly confined to the Arizona high terrain over the next few days, but chances of affecting the lower deserts should increase by Wednesday or Thursday.

SHORT TERM /TODAY THROUGH TUESDAY/

Latest mid-lvl WV imagery and 500 mb streamline analysis reveals a broad, expansive ridge of high pressure over the entire western half of the CONUS. Under this ridge, a weakness in the height field is apparent across NM and far W TX where an easterly wave/inverted trough is present. Although dry air is in place over south-central AZ, instability and lift associated with the approaching shortwave will provide a focus for shower and thunderstorm activity mainly across far east and southeast AZ this afternoon and early evening. The highest PoPs today will only be around 15-20% focused in S Gila County where a few isolated storms will be possible. High-res models suggest more widely scattered activity in the TWC forecast area, but the chances for any storms reaching the lower deserts is currently <10%. As storms begin to weaken early this evening, a residual outflow boundary could make it into far E Maricopa and Pinal counties, briefly causing winds to gusts around 20-25 mph and kicking up a bit of dust. Otherwise, the outflow will likely dissipate before making it into the Phoenix Metro. A gulf surge early Tuesday morning will foster low-level moisture advection from Mexico into southeast California and southwest Arizona. This will promote a slight increase of precipitable water values up to 1.2"- 1.3" across the lower deserts, which is still not optimal for convective activity. Isolated storms will again be confined to the higher terrain with a bit higher coverage along the Mogollon Rim and possibly S Gila County Tuesday afternoon, but chances for the lower deserts will remain <10%.

The unseasonably strong subtropical high will continue to dominate the weather pattern through much of this week with H5 heights remaining around 594-596 dam. Hghts of this magnitude are exceeding the 90th percentile of climatology for mid August. These positive height anomalies will result in high temperatures reaching 108-113 degrees across the lower deserts, or around 5-7 degrees above normal. An Extreme Heat Warning is now in effect for much of the forecast area and will continue through the rest of this week. Make sure to take all necessary heat precautions this week, especially if spending prolonged periods of time outdoors.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Forecast uncertainty has actually increased for the latter half of the week as guidance is now favoring slightly better moisture than previously shown while also repositioning the expected subtropical ridge center closer to the Four Corners area by Thursday. The forecast strength of the ridge has increased too, but if the ridge center is more removed from southern and central Arizona it will increase the convective potential. Although there is a good amount of ensemble spread for Wednesday and Thursday, the amount of members indicating QPF spreading into the lower deserts has more than doubled. Forecast mean PWATs are now shown to reach 1.3-1.5" Wednesday and 1.4-1.6" Thursday, or right around normal for the latter half of August. If the ridge does in fact end up moving far enough to the north, it should help to decrease the CIN over the area by Thursday. As of now, Wednesday should still involve mostly high terrain convection, but we can't rule out decent outflows pushing into the south-central Arizona lower deserts. Despite the uncertainty, storm chances for Thursday have definitely increased enough to go higher than NBM guidance. There are also indications of shear increasing by Thursday which could result in some stronger storms. However, we can't put a lot of stock in this so far as models have a poor track record with more mesoscale features as of late. Beyond Thursday, guidance mostly keeps with the near normal moisture levels across the area which means day-to-day somewhat low- end monsoon chances are likely to persist into at least next weekend.

The temperature forecast for the latter half of the week is also more uncertain, at least for eastern and central Arizona where higher chances for convection may eventually affect temperatures. NBM/WPC forecast temperatures have only slightly started to account for the potential increase in convection with lower desert highs roughly a degree or two cooler for Wednesday-Friday, but still easily high enough to keep the Extreme Heat Warning in place. However, it definitely is plausible the warning could end up being cancelled early across portions of Arizona if convection increases enough later this week.

AVIATION

Updated at 0500Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather concerns will exist through Tuesday night under a few mid/high cloud decks. Confidence in timing of wind shifts remains low as easterly outflow winds have nearly stalled east of KPHX, but still should incur a wind shift during the overnight and earlier than the past several days. Models also continue to exhibit directional variability Tuesday afternoon as winds revert back to a westerly direction, followed by the potential for another easterly outflow wind shift Tuesday evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather concerns will exist through Tuesday night under clear skies. Trends in wind speeds and timing of directional shifts will be similar to the past 24 hours featuring little to no gustiness, and extended periods of variability during the early morning hours.

FIRE WEATHER

Above normal temperatures with a steady increase in low-level moisture is anticipated over the next several days. Most lower desert communities will see temperatures topping out around 110-113 degrees, or around 5-7 degrees above daily normals. Widely scattered showers and thunderstorms are forecast for the high terrain through Tuesday with an increase in coverage expected on Wednesday. Better rain chances for the lower deserts will arrive by the latter half of this week. Relative humidity values will bottom out between 15-25% through Tuesday before increasing to 20-30% starting Wednesday. Overnight recoveries will also increase from fair to good starting Wednesday night. Outside of gusty and erratic winds near any thunderstorm, overall winds will follow the typical diurnal tendencies with some modest afternoon upslope gustiness.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ530>544-546- 548>551-553>555-559.

Extreme Heat Warning from 10 AM Tuesday to 8 PM MST Friday for AZZ560.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ562-565>567- 569-570.