textproduct: NWS Phoenix

UPDATE

06Z Aviation Discussion

KEY MESSAGES

- Chances for light rain will heighten over Arizona this weekend with the best chances for accumulations focused over higher terrain locations.

- Temperatures will gradually warm closer to the seasonal normal through the middle of next week. - A Pacific low pressure system moving into the Southwest during the middle and end of next week will result in much better rainfall chances with the possibility of a few embedded thunderstorms, as well as cooler temperatures

SHORT TERM /Today through Sunday/

Pronounced Rex blocking currently over the western Conus will transition into a more progressive pattern over the next 48 hours as strong jet energy entering the East Pacific acts to deamplify the ridge axis while also forcing the absorption the associated southern stream cutoff low into the mean westerly flow aloft. The latter of these two processes will take much longer for completion resulting in a prolonged period of southerly moist advection through the H7-H5 layer yielding total column PWATS in excess of 1" across much of the CWA. This pattern configuration will also place south-central Arizona within a favorable ascent position in relation to an upper level jet streak able to tap some of this thick midlevel moisture. However, the sfc-H8 layer remain exceptionally dry due to persistent downsloping easterly winds, and this deep sub-cloud dry layer will need to be at least partially moistened before more notable chances of measurable rain can be achieved. Based on the synoptic ascent structure, a weakening easterly low level gradient, and trends in regional WV imagery, feel this evolution will be more effective than suggested by base NBM POPS, and have boosted the chances for light accumulations through Sunday morning.

Forecast uncertainty Sunday afternoon and evening is greater than usual for a 24-48 time frame with some measure of midlevel cyclonic flow hovering over south-central Arizona as the upper wave only slowly gets stretched and deamplifies. Moisture should gradually become scoured from the cloud layer as winds veer more southwesterly, but the process may be more delayed with the slow transition. Thus, its not out of the question a few light showers linger throughout the day, though becoming primarily focused in orographically favored areas east of Phoenix. By Sunday evening, the upper low will finally begin ejecting into northern Arizona, but even so, moist upslope through Gila County could still support a couple isolated light showers into Monday morning.

LONG TERM /Monday through Friday/

By early next week, high pressure will be well established over NE CONUS, while the aforementioned wave becomes absorbed into the mean SW flow head of the next incoming low pressure system diving down the west coast. Another low chance (20-30%) of showers may be manifested Monday morning across the higher terrain of south-central Arizona , however if anything does develop, total accumulations are expected to be minimal. By Tuesday, deep troughing and strong negative height anomalies will amplify and intensify along the west coast, however confidence in the timing and depth of this feature remains low to moderate. The deterministic ECMWF forecasts a slower and deeper solution yielding a more robust rainfall scenario, while the GFS deterministic provides a faster outcome lifting the cold core into southern Utah resulting in a more tempered rainfall potential. Regardless, the timing of the best rainfall potential is highlighting the Wednesday and Thursday time frame while pattern recognition and some instability parameters suggest the potential for strong thunderstorms ahead of the shortwave passage. Additionally, with the uncertainty of this next system, a wide spread in NBM temperatures have presented themselves throughout the week, though below normal highs are most likely.

AVIATION

Updated at 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected during the TAF period. Winds will continue to favor an E'rly component through Sunday evening, with perhaps some isolated gusts during the mid to late morning hours. KSDL would be most likely to see these occasional breezy conditions due to their proximity to terrain features. VCSH will continue to move over the region tonight, but outside of instances of erratic winds, operational impacts from this activity are not anticipated. Current TAFs have SHRA exiting the metro by 14-15Z at the latest, but timing may need to be adjusted in subsequent amendments if different forecast trends are realized. Abundant cloud cover will be present through Sunday morning but should begin to scatter out later in the forecast window.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Outside of extended periods of light and VRB conditions, winds will contain a W'rly component at KIPL, while KBLH experiences oscillations between NW and NE. Cloud cover will begin to scatter out overnight, with mostly clear skies returning by the back-half of the forecast window.

FIRE WEATHER

Wind speeds through the beginning of next week will be much weaker than the past several days with minimal gustiness. Light showers will affect much of the Arizona districts this weekend, and particularly over eastern districts, but the chance for wetting rain is low (~10%). Minimum humidity levels will improve closer to a 30- 50% range during the first half of next week, then likely rise further during the latter half of the week. Overnight recovery should be good to excellent. A Pacific weather disturbance will bring increased chances of wetting rainfall, breezy winds, and cooler temperatures during the latter half of next week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.