textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- Temperatures for the upcoming week will generally be within the normal range for late October with lower desert highs mostly in the mid eighties.

- Dry and tranquil weather conditions will last through the middle of week.

- A return to inclement weather with increased rainfall chances is likely towards the end of the week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

The center of the low continues to linger across Northern Arizona, but behind the low, as is continues to track to the NE, drier and warmer conditions are expected for the lower desert. On the RAP40 500MB analysis ridging can already be seen building over the coastline of southern CA and stretching into the Intermountain West. However the increasing H5 heights will be somewhat suppressed as another upper level low, currently positioned just off the coast of Washington, will push inland starting early Monday. This low is expected to stay in northern CONUS, and not dive south towards the Desert SW, however as mentioned before this will help suppress H5 height aloft from building over the lower deserts, resulting in near normal heights for this time of year and quasi-zonal westerly dry flow lasting through Wednesday.

With that being said temperatures will be in the low to mid 80s today, then starting tomorrow increase to the mid to upper 80s, lasting through Wednesday. NBM forecast highs do show readings nudging more toward the upper 80s for Tuesday and Wednesday, but there is currently only a 25% chance (for KPHX) at reaching 90 degrees this week. Meanwhile morning lows will be in the low to mid 60s.

LONG TERM /THURSDAY THROUGH NEXT SUNDAY/

Models continue to suggest a more organized large scale troughing pattern setting up later this week with the parent low setting up along the British Colombia coast before diving southward into the Pacific Northwest by Thursday. A secondary shortwave or weak closed low may also form west of California Wednesday into Thursday, more associated with the sub-tropical jet. What is likely to be more important is the eventual phasing of the polar jet over the Pacific Northwest with the stronger subtropical jet by next Friday. Both the EPS and GEFS support this phasing, while also strengthening the subtropical jet to near record strength for this time of year just west of Baja on Friday. Guidance then supports a fairly strong weather system (with H5 heights within the 10th percentile of climatology) developing within the base of the trough by next weekend. Although we are at around 5-7 days out from this next expected weather event, there is enough model evidence for us to begin worrying about another significant precipitation event. A bit more than 50% of the ensemble members favor the trough deepening just off the West Coast Friday into next weekend, likely pushing a modest to strong atmospheric river (AR) somewhere into the Southwestern U.S. next weekend.

As the trough first begins to set up during the middle part of the week, there is high confidence in strong northward moisture advection in the upper levels out of Mexico Wednesday night into Thursday. This should be followed by lower level moisture advection later Thursday into Friday. Although the timing may change and assuming the positioning of the subtropical jet is correct, shower chances may begin as early as late Thursday and carry into Friday under broad divergence aloft and modest upslope southerly flow. This first period of rain chances very well could change and confidence is still quite low in how much rain could fall.

Scattered mostly high terrain rain chances may continue into Saturday before the main weather system traverses the region at some point Saturday night and Sunday. Additional moisture advection is likely to occur, associated with the AR, right ahead of the weekend system, but it is too early to know where exactly the best moisture and forcing will end up. Even if we do not get a direct hit, ensemble guidance is heavily favoring at least a partial hit from the AR late Saturday into Sunday. Preliminary QPF numbers show upwards of an inch may be possible over the south- central Arizona lower deserts to upwards of 2" possible in the high terrain north and east of Phoenix. These numbers very well could change and seem to lean more on the optimistic side. If the upper end of precip guidance comes to fruition, locations across central Arizona are expected to have to worry about rising stream and river levels again, but it is doubtful there will be big flood concern due to the longer duration of the event. Given we are still outside the winter season, this weather system will be on the warmer side with snow levels likely staying above 9-10K feet.

AVIATION

Updated at 2315Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns expected throughout the TAF period. The overall wind pattern will exhibit the typical diurnal tendencies with overall speeds aob 7 kts along with extended period of light variability to even calm conditions. Clear skies through this evening will give way to some passing high cirrus cloud decks during the overnight hours.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns expected throughout the TAF period. Wind speeds will generally be light aob 6 kts with a westerly component favored at KIPL and a northwest to northeasterly component favored at KBLH. Extended periods of light variability to even calm conditions can be expected. Clear skies will give way to some passing high cirrus cloud decks later this evening.

FIRE WEATHER

High pressure will settle back into the region over the next couple of days leading to drier conditions and temperatures returning into the normal range. Minimum humidity levels will fall into a 20-40% range this afternoon and then 15-35% on Monday. Overnight recoveries will remain good at 45-70% across the area. Winds will remain light each day favoring typical diurnal patterns. A shift in the weather pattern late this week and into next weekend may bring about cooler, wetter, and breezier conditions.

PSR WATCHES/WARNINGS/ADVISORIES

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