textproduct: NWS Phoenix

UPDATE...Updated 12z 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 Monday/

The Rex block pattern that was positioned over west CONUS has already begun to break down as the high pressure is now over the Northern Plain States. The high pressure will continue to quickly move into NE CONUS by late this evening. Meanwhile the cut off low to our SW has already begun to loosen into an open wave. As the low continues to open and begin to be absorbed into the main westerly flow aloft decent moisture advection through the mid levels will occur, leading to PWATS in excess of 1" across much of the CWA. This is near 200-225% of normal, or above the 90th percentile of NAEFS mean precipitable water, very unusual for this time of year. Additionally, as the low opens it will place south central AZ in a favorable position for associated positive vorticity, aiding in the ascent of the aforementioned moisture. As a result slight chances for isolated showers will continue to linger, with PoPs between 15-30% stretching between southern Maricopa County through Pinal County and into the higher terrain east of the Phoenix metro through the morning hours into the afternoon. By this 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. By Monday afternoon most of the lower deserts PoPs are <10% while 20-30% persist over the eastern high terrain, however if anything does develop, total accumulations are expected to be minimal.

Large spread in NBM temperatures remain for today and Monday with the uncertainty of showers, and depth of H5 heights associated with the low. Regardless, high temperatures today are forecasted to be in the mid 80s across the lower deserts. By Monday, ahead of the next low pressure system, H5 heights will increase as a ridge builds over eastern CONUS, in turn heights aloft increase slightly over the Desert SW raising temperatures a few degrees into the mid to upper 80s through mid-week.

LONG TERM /Tuesday through Saturday/

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 continues to be low to moderate. The deterministic's have started to come into better agreement compared to 24 hrs ago but still show key differences. The ECMWF forecast continues to show a slightly slower system however much shallower than before. Meanwhile the GFS deterministic remains mostly unchanged, with the exception of now leaning towards a more positively tilted trough than compared to the ECMWF, and the previous GFS runs, spreading out any vorticity advection and hindering better chances for storm development. Even with these differences compared to previous runs, the ECMWF continues to forecast a more more robust rainfall scenario in comparison to the GFS. Either way, Wednesday and Thursday continue to be the best window for rainfall potential, while pattern recognition and some instability parameters suggest the potential for strong thunderstorms ahead of the shortwave passage. Lingering PoPs continue into Friday morning before expecting to dry out by the weekend.

Additionally, with the uncertainty of this next system remaining, a wide spread in NBM temperatures continue throughout the week, most notably on Thursday with a spread of 11 degrees between the 25th and 75th percentile. Even with this large of spread below normal highs are likely late this week heading into the weekend.

AVIATION

Updated at 1120Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns are expected throughout the TAF period. Wind directions will maintain an easterly component throughout the period with speeds generally aob 10 kts. VCSH will remain a threat through the mid-morning hours, however, operational impacts should remain minimal. Abundant mid-level clouds will remain in place through the morning hours before scattering out heading into the afternoon hours.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected throughout the TAF period. At KIPL, winds will generally maintain a westerly component while at KBLH winds will fluctuate between the north-northwest to northeast. Overall wind speeds will generally be aob 6 kts along with extended periods of light and variable conditions. SCT mid- level cloud decks early this morning will give way to mostly clear skies by the mid to late morning hours.

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.