textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES.

- Temperatures will heat up several degrees above normal through at least the middle part of the week, with lower desert highs climb above 110 degrees most days.

- Extreme Heat Warnings are in effect today through Friday with 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/

Despite the subtropical ridge becoming the more dominant weather feature across the Southwestern U.S., we are also contending with a weakening easterly wave currently centered over far western Texas. This disturbance will eventually lose out to the ridge as it is forecast to shift southwestward over the next couple of days, but it is at least expected to clip southeast Arizona later today. As a result, some weak forcing across east-central and southeast Arizona in combination with some modest moisture advection should bring better coverage of showers and thunderstorms this afternoon/early evening. Hi-res CAMs mostly agree the bulk of this convection will move southeast of our CWA, but a few storms may impact southern Gila County. As the disturbance exits to the south southwest into Tuesday, convective potential is likely to shift more to the Mogollon Rim but with little expected impact for our area. Tuesday will also mark the start of low level moisture advection into southeast California and southwest Arizona from the south with even better and more widespread moisture advection occurring on Wednesday.

Temperatures have noticeably warmed up over the past couple of days with Phoenix, Yuma, and El Centro all reaching 110 degrees on Sunday. The subtropical ridge moving into the region has pushed H5 heights to between 594-596dm or well into the 90th percentile of climatology. Ensemble mean H5 heights are shown to stay fairly stable the rest of the week, but with the center of the ridge reforming over the northern half of Arizona by Wednesday. Given the little change in heights aloft and little if any influence on temperatures from convection through at least Tuesday, we have high confidence in forecast highs rising to 109-113 degrees across the lower deserts. The Extreme Heat Warnings across the bulk of the area go in effect later this morning with the coverage of Major HeatRisk expanding into mid-week.

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 1735Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns expected under few mid/high cloud decks can be expected throughout the TAF period. The overall wind pattern will exhibit the typical diurnal tendencies with speeds aob 10 kts. Thunderstorms over the higher terrain areas well east of Phoenix will likely send an outflow into the Phoenix area late this evening, most likely moving in after 03-04Z. The outflow will mostly serve as an easterly wind shift. No significant gusts are anticipated and no slantwise visibility reductions from dust expected with the outflow passage at this time.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under some passing mid/high cloud decks can be expected throughout the TAF period. At KIPL, very light westerly/variable winds this morning will shift out of the southeast early this afternoon before shifting back out of the west early this evening. At KBLH, winds will fluctuate out of the south to southwest. Overall wind speeds will remain aob 10 kts, although some modest afternoon/early evening gusts will be possible. A period of light variability will be common during the early morning hours.

FIRE WEATHER

Above normal temperatures with fairly stable moisture levels are expected early this week. Most lower desert communities will reach or eclipse the 110 degree mark over the next several days. Scattered showers and isolated thunderstorms are forecast for the high terrain through Tuesday with an increase in monsoon activity possible by Wednesday or Thursday. Afternoon MinRHs of 15-25% are expected through Tuesday before increasing to 20-30% starting Wednesday. Outside of thunderstorm winds, 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.