textproduct: NWS Phoenix

Update

12Z Aviation Discussion

KEY MESSAGES

- Record breaking lower desert high temperatures near or exceeding 100 degrees will be common through at least the middle of next week, with the hottest days being today and Saturday.

- Extreme Heat Warnings will remain in effect across the lower deserts through Sunday.

- The unprecedented afternoon heat will be dangerous, especially with any strenuous or long-duration outdoor activities without proper hydration and taking breaks in the shade, or air conditioning.

SHORT TERM /TODAY THROUGH SUNDAY/

The unprecedented March heatwave continues, with monthly high temperature records being surpassed once again at our three major climate sites (Phoenix, Yuma, and El Centro) on Thursday. Current 500 mb RAP analysis and midlevel water vapor imagery show the center of an impressively symmetrical and strong ridge of high pressure over Southwest AZ, which is responsible for the record setting heat across the Southwest US. Ensembles remain in excellent agreement that the strength of the ridge will wane going forward, but to a large degree it has already modified low level thermal profiles to develop a record warm (for the time of year) and very dry airmass over the region, one which will remain with us through at least the middle of the upcoming work week. Midlevel heights will decrease more noticeably this weekend, starting Saturday but especially on Sunday, as a few northern stream shortwave troughs overtop the ridge and act to dislodge the center of the anticyclone further east.

Forecast highs today are expected to be the hottest of the entire event, peaking in a 102F-108F range across the lower deserts. Despite midlevel heights falling on Saturday, lower tropospheric temperatures will not respond as quickly as the mid to upper levels, and so anticipate similar or perhaps 1F-2F lower highs on Saturday. Therefore, highs will continue around 25F above daily normals and 4F- 10F above daily records through Saturday. Ensemble mean H5 heights will bottom out on Sunday in a 583-585 dam range. This is still at or above the 97th percentile of climatology, but it should allow for highs to drop by several degrees, falling into a 96F-102F range across the lower deserts on Sunday. These temperatures represent values around 20F above daily normals and still above daily records for many locations (see Climate section for more details). Considering the climatological context, the timing of the event which coincides with a weekend, and many out-of-state visitors and even locals who are not acclimated to this level of heat, this event will continue to be impactful. Extreme Heat Warnings will remain in effect across the lower deserts through Sunday. It is true that we experience this level of heat every year in this part of the country, but it is much more typical for the month of June; do not forget that this level of heat is unprecedented this early in the year and can be dangerous if you do not take proper heat safety precautions.

Aside from the heat, winds aloft will increase somewhat across the region as the ridge is dampened and shifts slightly eastward this weekend. Ensemble mean 700 mb winds increase to around 15-20 kts out of the southwest, particularly late Saturday. While this is not impressive, deeper mixing during the late afternoon/early evening may be able to tap into this higher momentum air and produce gusts to around 20-25 mph across some of the area, particularly portions of Southeast CA like Imperial County.

LONG TERM /MONDAY THROUGH THURSDAY/

Of the next 7 days, Monday is shaping up to be the "coolest" (lower desert highs 94F-101F) thanks to a couple of factors. The ridge will not rebuild substantially until Tuesday over the Desert Southwest and lower level thermal profile will require stronger/sustained anticyclonic subsidence to warm further. Another factor is that guidance indicates something reminiscent of a Gulf Surge occurring Sunday night and possibly again Monday night, increasing moisture over a shallow layer near the surface. Dewpoint temperatures are shown increasing upwards of 45F-55F for the Yuma area and at times for the adjacent valleys (Imperial, Lower Colorado River, Lower Gila River). Dewpoints are forecast to increase at least into the 30s elsewhere across the lower deserts. It will be harder to realize widespread temperatures in excess of 100F especially on Monday thanks to these factors.

During the middle of next week, ensembles continue to advertise another significant yet more transient ridging feature building under an anticyclonically curved jet streak. Though the ridging will be lower amplitude compared to the current ridge, and anticyclonic subsidence will be shorter lived (likely peaking in its influence sometime Tuesday-Wednesday), H5 heights are forecast to rise into a 588-591 dam range - once again above CFSR climatological maximum values over a broad area. This will allow a gradual warming into the middle of the upcoming workweek, with coverage of 100F+ temperatures increasing. In summary, confidence continues to grow in a longer duration heatwave, with highs remaining between 15F-25F above daily normals and breaking daily records well into next week.

AVIATION

Updated at 1050Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation concerns are expected during the TAF period. Winds will continue to follow similar trends seen over the past 24 hours with speeds generally aob 10kt. Extended periods of VRB to calm conditions will be common, especially for the western terminals. Clear skies will dominate the region through Saturday morning.

FIRE WEATHER

Historic March heat and very dry conditions will continue into the weekend. Afternoon highs around 25 degrees above daily normals will be common through Saturday, falling to around 20 degrees above normal by Sunday. Afternoon MinRHs between 5-10% will persist through Sunday, increasing slightly into a 10-20% range for portions of the Western Deserts early next week. Overnight recoveries will be poor, around 15-35% through Saturday night, and then improve Sunday night and Monday night primarily for valley locations in the western deserts like the Yuma area, lower Colorado River Valley, Imperial Valley, and lower Gila River Valley. Winds will be light today, mostly below 15 mph, but late afternoon and early evening breeziness will increase this weekend, with gusts locally to 20-25 mph.

CLIMATE

Daily record highs through the middle of next week:

Date Phoenix Yuma El Centro ---- ------- ---- --------- 3/20 96 in 2017 99 in 2004 98 in 2004 3/21 97 in 2004 102 in 2004 100 in 2004 3/22 94 in 1990 98 in 2004 96 in 2004 3/23 93 in 1990 96 in 1990 93 in 1990 3/24 96 in 2025 97 in 2025 96 in 2025 3/25 99 in 2025 99 in 1896 99 in 2025 3/26 100 in 1988 99 in 1988 98 in 1988

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530>555-559- 561.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ561>570.