textproduct: NWS Phoenix

KEY MESSAGES

- Temperatures will begin a cooling trend reaching below normal levels over the weekend.

- Unusually strong winds will impact the region Friday through Sunday resulting in areas of blowing dust and critical fire weather conditions.

- More tranquil weather conditions and a slow warming trend will arrive during the middle of next week.

SHORT TERM /Today through Saturday/

Subtropical high pressure continues to be dampened and displaced into northern Mexico in response to increasing westerly flow in the northern stream and height falls approaching the western Conus. WV imagery depicts a subtle shortwave lifting towards northeast Arizona this afternoon coincident with lingering midlevel moisture. However, the strengthening westerly flow will allow mixing ratios to decrease closer to 7-8 g/kg across south-central Arizona (and markedly lower in the western CWA), but still sufficient for MUCape values around 500 J/kg across Arizona higher terrain locations. HREF output continues to highlight scattered storms over these higher terrain areas, however only clipping the eastern parts of Gila County. Over the next 48 hours, increased troughing with more robust height falls with surge into the SW Conus leading to more pronounced dry air and a gradual cooling trend, such that current Extreme Heat warnings will be allowed to expire as scheduled.

The rarity of the upcoming synoptic setup cannot be understated with extremely anomalous negative height anomalies near 4 normalized standard deviations below normal settling over the Great Basin through the weekend. This scenario resembles one more common for early May versus late June with intense zonal flow and a tightening pressure gradient descending into the Southwest. NAEFS U-wind component forecasts through the entire sfc-H7 layer Friday-Saturday essentially is at the seasonal maximum with raw values easily 30- 40kt. Given the mixing depths this time of year, this higher momentum airmass will be tapped with full insolation, and further exacerbated downstream of terrain features given the likelihood of compression and mountain rotors. Wind advisories have been issued for these preferred downstream locations where high profile vehicle travel may become difficult, and localized dense blowing dust may be common. In addition, Red Flag Warnings continue through the lower Colorado River valley and eastern parts of the CWA where combined wind gusts, low humidity levels, and very dry fuels receptive to fire could result in extreme fire behavior and uncontrolled wildfire growth. Even in areas not covered by the warning, an elevated fire danger will exist - especially Saturday when the most anomalous weather conditions will materialize.

LONG TERM /Sunday through Thursday/

A large Pacific trough will be the dominant weather feature across the Western U.S. through the first half of next week. The deep low with H5 heights briefly reaching near record lows by late June standards over the Great Basin Sunday will keep the pressure gradient abnormally tight. Strong gradient winds will become more focused across the eastern half of Arizona on Sunday posing a major fire weather threat. Gusts of 25-40 mph are expected to impact a good portion of the area as humidities hover into a 5-15% range. Red Flag conditions will continue over eastern Arizona Sunday where rapid spread and growth of any new or existing fires is possible. Guidance eventually shows the deep low gradually filling early next week, but remaining in place at least through the middle of the week. Winds will also diminish somewhat early next week, but breezy conditions are likely to persist.

The presence of the large trough will bring some relief from the heat this weekend with NBM forecast highs dropping below normal areawide by Sunday. Daytime highs anywhere from the upper 90s to just over 100 degrees are expected on Sunday and Monday before temperatures begin to slowly rise back closer to normal at some point during the latter half of next week. Overnight lows will also become quite pleasant with readings as low as the mid 60s across the western deserts to the low to mid 70s over the rest of the lower deserts.

AVIATION

Updated at 1745Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected through the TAF period. Winds will follow diurnal trends with gusts this afternoon upwards of around 20 kt. SCT to sometimes BKN mid-level clouds will last through portions of today with clearing expected by late tomorrow morning.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation concern will be breezy to windy conditions at each terminal this afternoon and evening, into tomorrow. W'rly and S'rly winds are favored at KIPL and KBLH respectively. Gusts 20-25 kt will then become common this afternoon into the evening hours. FEW to SCT mid- level clouds are likely to clear by tomorrow morning.

FIRE WEATHER

Elevated fire danger will continue today due to low humidity, very warm temperatures, and afternoon upslope/upvalley breezy conditions with critical conditions developing Friday. Red Flag Warnings have been issued for Friday through Sunday for much of the area. Expect MinRHs today into the single digits across the western districts, but closer to 20% in the east. These values will fall closer to 5- 15% starting Friday and stay in this range for several days. Winds will generally remain terrain driven today with upslope gusts during the latter half of the afternoon peaking around 25 mph. Winds will increase starting Friday with gusts into a 20-35 mph range and locally higher over higher terrain areas. Winds are likely to peak on Saturday with gusts commonly reaching 25-40 mph. These windy and dry conditions will combine with the very dry fuels to create a risk of extreme fire behavior. Temperatures will cool off this weekend reaching to slightly below normal starting Sunday. Winds are expected to gradually diminish next Monday into Tuesday.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST this evening for AZZ530-532- 533-537-540-542>544-546-548-550-551.

Red Flag Warning from 11 AM Friday to 8 PM MST Saturday for AZZ131.

Wind Advisory from 11 AM to 11 PM MST Saturday for AZZ530-533.

Red Flag Warning from 11 AM Friday to 8 PM MST Sunday for AZZ133.

CA...Extreme Heat Warning until 8 PM PDT this evening for CAZ562-566- 567-569.

Red Flag Warning from 11 AM Friday to 8 PM PDT Saturday for CAZ231.

Wind Advisory from 11 AM to 11 PM PDT Saturday for CAZ564>567- 569-570.

Wind Advisory from 5 PM Friday to 11 AM PDT Sunday for CAZ562.