textproduct: NWS Phoenix

UPDATE

06Z Aviation Discussion

KEY MESSAGES

- Temperatures around five degrees above normal will result in localized major Heat Risk through Thursday with Extreme Heat warnings remaining in place.

- Upper level moisture streaming into the region will continue through Thursday resulting in expansive cloud cover, virga, and a few isolated showers and thunderstorms.

- Drier, cooler, and breezier conditions will move into the region late this week resulting in areas of very high fire danger.

SHORT TERM /Today through Friday/

Early afternoon WV imagery depicts a sprawling anti-cyclone centered over southern New Mexico with SW flow, subtle vorticity centers, and enhanced midlevel moisture lifting through southern California and Arizona. Steep 7-8 C/km lapse rates juxtaposed with saturated ascent have yielded 100-200 J/kg MUCape over the region resulting in expansive virga and isolated showers/storms. This activity should continue to fester through this afternoon before shortwave forcing lifts into northern Arizona this evening. Over the next 48 hours, the subtropical high will continue to weaken while shifting south into SW TX/northern Mexico allowing the local flow pattern to become more westerly, gradually scouring away the midlevel moisture source while simultaneously introducing modest height falls/cooling with a strengthening jet core.

Objective analysis suggests the H5 anti-cyclone center near 596dm with H7 temperatures around +15C encroaching into south-central Arizona this afternoon. Despite these measures typically supporting late June high temperatures easily in a 110F-115F range, the abundant midlevel cloud cover and increased boundary layer moisture with 8-9 g/kg mixing ratios have been detrimental to mixing depths and full realization of the surface temperature potential. However, the increased moisture profiles have forced low temperatures towards the upper reaches of the guidance envelop restricting the amount of overnight relief from the heat. Thus, the Extreme Heat warnings continue through Thursday despite only isolated instances of major HeatRisk centered in the Phoenix urban corridor. Otherwise, steep midlevel lapse rates and lingering moisture could conceivably create another round of virga/elevated showers tonight and Thursday morning, however forecast data shows reduced ascent and more limited saturation. Drier air will begin sweeping into the region throughout the day, but not totally removed such that several HREF members indicate isolated storms materializing over eastern Gila County in the afternoon.

By Friday afternoon, large scale troughing will surge into the western Conus with the trough base anomalously extended into southern California resulting in seasonally unusual midlevel height falls across the CWA. Enhanced westerly jet energy and strong onshore flow will characterize the synoptic flow; a pattern more common in early May vs. late June. Other than initiating a cooling trend, this evolution will foster the inland translation of stronger winds given deep mixing depths tapping a higher momentum airmass aloft. Given the rarity of this pattern for late June, impacts may be rather large and multi-faceted given the forecast winds through the sfc-H7 layer. Initially across southeast California, a downslope wind component and 40+kt H9-H8 winds will favor areas of blowing dust and local advisory level wind speeds in the typical locations, and combined with low relative humidity levels will yield the potential for critical fire weather conditions where more dense fuel beds exist along the lower Colorado River valley. Farther east into Arizona, winds may not initially be quite as strong, however with energy release components and burning index percentiles near their maximum in this very anomalous weather setup, fire danger may become very high. Fire Weather Watches continue for these areas with a high likelihood of future upgrades to warnings.

LONG TERM /Saturday through Wednesday/

Forecast concerns regarding fire danger will continue through the weekend as wind speeds increase further and much drier conditions persist. The large Pacific trough will only gradually shift eastward with strong negative height anomalies diving into the Great Basin allowing maintenance of cyclonic flow and lower heights into the SW Conus. Thus, persistent height falls from the incoming trough will continue Saturday while a strong pressure gradient becomes established. Guidance shows unseasonably strong gradient winds lasting through at least Saturday, and potentially into Sunday with afternoon gusts of 20-30 mph across the entire area, and locally higher gusts of 30-50 mph. Gradual drying of the boundary layer will also lower daytime humidities to between 5-15%. The combination of the low humidities and the strong gusty winds are likely to make for dangerous fire weather conditions. A Fire Weather Watch remains in effect for the Lower Colorado River Valley through Saturday evening and extended through Sunday evening across eastern Arizona high terrain where winds will be slower to relax while fuels remains very receptive.

Enhanced downsloping winds with areas of blowing dust appear likely again Saturday when the combination of height falls and onshore flow are optimal for mountain rotors and hydraulic jumps inland. In addition to the fire weather concerns along the lower Colorado River valley, a myriad of other wind/dust advisory products may be necessary given the synoptic setup. Otherwise, although H5 heights quickly decrease to below 588dm, temperatures will be somewhat slower to respond over the weekend, though eventually falling 4F-8F below normal by Sunday. The latest NBM/WPC forecast highs shows readings only in a 98F-104F range across lower elevation population centers of the forecast area, then only warming a few degrees during the first half of next week as H5 heights remain sequestered in a 585-588dm range.

AVIATION

Updated at 0500Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected during the TAF period. Winds will follow diurnal trends will gusts during Thursday afternoon upwards of around 20 kt. THough it appears unlikely, some lingering virga cannot be ruled out around the airspace, but any radar echoes that are observed should not create any impacts at the terminals. SCT to sometimes BKN mid-level clouds will stick around through Thursday evening.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation concern will be breezy to windy conditions at each terminal Thursday afternoon and evening. Until then W'rly and S'rly winds should be favored at KIPL and KBLH respectively. Gusts 25-30 kt will then become common later in the period, with stronger gusts upwards 35 kt possible at KIPL. Some virga showers will may observed Thursday morning, mainly near KBLH, but should not result in any operational impacts. Other than a FEW mid- level clouds, skies should be mostly clear during the next 24 hours.

FIRE WEATHER

Locally elevated fire weather conditions due to low humidity and afternoon upslope/upvalley breeziness will continue through Thursday. Temperatures will be several degrees above normal while MinRHs values around 15% decrease into the single digits across the western districts starting Thursday. Winds will generally remain terrain driven with upslope/upvalley gusts during the latter half of the afternoon into the early evening peaking around 25 mph Thursday. Areas of critical fire weather conditions are likely Friday through weekend as gusty southwest winds increase across the region combined with humidity levels settling in a 10-15% range. A Fire Weather Watch continues for the Lower Colorado River Valley Friday-Saturday, and had been extended Friday through Sunday for the Tonto NF/Southern Gila County districts given the rarity of this weather scenario for late June.

PSR WATCHES/WARNINGS/ADVISORIES

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

Fire Weather Watch from Friday morning through Saturday evening for AZZ131.

Fire Weather Watch from Friday morning through Sunday evening for AZZ133.

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

Fire Weather Watch from Friday morning through Saturday evening for CAZ231.