textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- High pressure over the Southern Plains will cause breezy to windy conditions for portions of South Central Arizona Sunday morning, with the strongest gusts over higher terrain east of Phoenix.

- Temperatures will warm this weekend, reaching 5 to 10 degrees above daily normals Sunday into early next week, which translates to widespread lower desert highs in the nineties.

- Periods of breezy to locally windy conditions and cooler temperatures are expected by the middle of the upcoming work week.

SHORT TERM /TODAY THROUGH SUNDAY/

Two large-scale features are apparent in midlevel water vapor satellite imagery early this morning that will be relevant to local weather this weekend into next week: 1) a broad upper level low over the Northeastern Pacific that is deepening as it approaches the West Coast, and 2) downstream ridging shifting eastward over the Western CONUS. Ensembles remain in excellent agreement that the upper low will settle just off the West Coast late this weekend, with downstream ridging centering itself over the Rockies. Meanwhile, in the wake of a departing shortwave that is now over the Central CONUS, surface high pressure is shifting east of the Great Basin early this morning. This area of surface high pressure will build southward along the front range of the Rockies later today, eventually reaching Western TX/Eastern NM Sunday morning, resulting in a tightening of regional pressure gradients especially late tonight (peaking around 12Z Sunday). As a result, Hi-Res and global models are highlighting an abnormally strong easterly wind Sunday morning, with mountain and ridgetop wind gusts pushing 40-50 mph east of Phoenix. The breezy easterlies should mix to the surface across south-central AZ after sunrise, but with gusts mostly peaking in the 20-30 mph range. From an event like this, it would not be surprising to see localized blowing dust in the typically prone areas and hazy conditions from lofted dust spreading further west as the day goes on.

With the influence of upper level ridging becoming more pronounced over the weekend, lower desert high temperatures will rise from the upper 80s to around 90F this afternoon into the middle 90s by Sunday. Lower desert highs in the middle 90s represent values around 5F-10F above daily normals. Morning lows may also be unusually warm across South-Central AZ Sunday morning and even overachieve from the current forecast, thanks to the strong E/SE gradient winds and adiabatic warming from downsloping off the high terrain to our east.

LONG TERM /MONDAY THROUGH FRIDAY/

The overall pattern will remain mostly unchanged Monday, with upper level ridging still the main influence on our weather locally, allowing continued lower desert highs in the 90s and fairly quiescent conditions. However, the southeasterly gradient wind event Sunday will have pulled midlevel moisture into the region, and some of the global guidance is picking up on a weak shortwave disturbance dislodging from the upper low off the west coast and ejecting overtop the ridge. This weak, transient forcing coupled with remnant midlevel moisture could be enough to spark convective showers over the far Eastern AZ high terrain (White Mtns, possibly the Rim) Monday afternoon.

By the middle of the upcoming work week, the pattern will begin to shift. The upper low off the West Coast will push inland over the Western US, albeit in a weakening phase. Ensembles are in good agreement on this evolution, but details such as the N/S displacement of the low and the exact timing/speed of its eastward progression remain unclear. On a large scale, a broad area of positive midlevel height anomalies looks to establish off the Pacific Northwest/British Columbia with negative height anomalies to its south and east. This is in response to a split jet regime, where much of the Western and Central CONUS falls under the broad cyclonic flow of the poorly defined southern stream. As a result, we can expect cooler temperatures (likely closer to seasonal levels, in the 80s across the lower deserts) Wednesday onward across the forecast area, and periods of breezy to locally windy conditions as shortwaves progress through the Desert Southwest. The first period of increased winds will likely be Tuesday, especially for the Western CWA, as the initial, broad upper low moves inland and packs heights fields over the Southwest US.

AVIATION

Updated at 1135Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No major aviation concerns are anticipated through the forecast period. Winds will switch out of the east early this morning, becoming elevated for a few hours with gusts into the upper teens possible at all terminals. Winds will then gradually shift out of the SW later this afternoon with speeds predominantly aob 8 kts. Skies will remain mostly clear through most of the day, with FEW to SCT high clouds arriving over the region after sunset.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns throughout the TAF period. At KIPL, winds will favor a W/N direction with speeds aob 6 kts. At KBLH, expect a gradual transition from NW to NE winds with speeds also remaining generally aob 6 kts. Skies will remain mostly clear this morning, with FEW-SCT high clouds beginning to overspread SE California by the afternoon.

FIRE WEATHER

Dry and occasionally breezy conditions will prevail through the next week. Winds will be much lighter, generally below 15 mph, on Saturday, and then a strong easterly wind will develop across south-central AZ Sunday morning. The strongest winds will be over the higher terrain of the eastern districts, but shortly after sunrise, gusts to 20-30 mph are likely to become common across the lower elevations. Given the time of day, RH values will be above critical levels. Still, some locally elevated fire weather conditions may exist across south-central AZ midday through early afternoon Sunday. Afternoon MinRHs below 15% and at times in the single digits will be common through the next 7 days, with minor day to day variations. Overnight recoveries will offer little relief tonight, with values between 10-30% areawide. Overnight recoveries improve slightly by Sunday night, but still fall in a poor to fair category into early next week.

PSR WATCHES/WARNINGS/ADVISORIES

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