San Tan Valley, AZ 85143 Weather

Update

Updated 12Z 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

The center of the upper low that brought some scattered showers and a few thunderstorms on Saturday is still hanging out over Arizona, but more over northern portions of the state. As it continues to weaken and lift northeastward today, any rain chances will exist only across northeastern Arizona. Drier air will also fill back in from the southwest today with generally clear skies across the lower deserts. Temperatures will also begin to trend upward with the exiting of the upper low with readings into the lower 80s for Phoenix to around 85 degrees in Yuma and El Centro.

A quiet weather pattern will then stay over the Southwestern U.S. during the first half of this week with quasi-zonal westerly dry flow lasting through Wednesday. Upper level heights will also stay near normal over the next several days allowing near normal temperatures under clear to mostly clear skies. NBM forecast highs do show readings nudging a bit 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.

Long Term

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.