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FXUS65 KPSR 281758 AFDPSR

Area Forecast Discussion National Weather Service Phoenix AZ 1058 AM MST Tue Nov 28 2023

UPDATE
18Z Aviation Discussion.

SYNOPSIS
High pressure aloft will continue to keep temperatures above normal today. The upper level pattern will begin to shift on Wednesday, as a shortwave trough brings breezy conditions to southeast CA and rain chances to the higher terrain of southcentral AZ. A secondary trough will rotate through the Desert Southwest late this week into this weekend, ushering in much cooler air and promoting better rain chances across much of the region. Drier conditions with rebounding temperatures can be expected early next week.

DISCUSSION
Another day of above normal temperatures is in store before a cooler and wetter pattern sets up through the remainder of this week. Latest IR WV imagery shows a highly amplified ridge situated over the Intermountain West with a deep upper low churning off the coast of OR/CA. Closer to home, we continue to remain under the influence of ridging aloft. In response to an increased mid- lvl height field over southcentral AZ, we will again see unseasonably warm temperatures today with highs topping out in the middle to upper 70s across the lower desert areas. Another round of high clouds will move over the region early this morning, but will gradually clear out through this afternoon. Low temperatures tonight are expected to remain slightly above normal in the mid 40s to lower 50s.

On Wednesday, we will begin to see a noticeable pattern change over the region as the afomentioned upper low off the West Coast moves inland and transitions to an open wave as it approaches the Desert Soutwhest. Increasing southwesterly flow aloft will result in breezy conditions across southeast California on Wednesday afternoon where gusts could reach 25-30 kts mainly in the Imperial Valley. Due to decreasing mid-lvl hghts over the forecast region, high temperatures will be a few degrees cooler Wednesday afternoon, but still around 2-4 degrees above seasonal averages. As the upper level trough continues to progress through AZ late Wednesday into early Thursday, there will be enough moisture and lift in place to generate light precipitation across the higher terrain areas of N Maricopa and S Gila Counties where PoPs have increased to around 40-60%. Elsewhere across the lower deserts, rain chances are much lower around 15-20%. Rainfall amounts with this first round of activity look to be light ranging anywhere from a few hundredth to around 0.10" at most. Any lingering precipitation should come to a close early Thursday morning as the first upper level disturbance continues to depart stage left.

Much cooler air will arrive on Thursday in the wake of the departing upper level trough as 500 mb hghts fall to around 556-558 dam. These negative hght anomalies will linger through the weekend as high temperatures will only manage to reach the the low to mid 60s across the lower deserts from Thursday-Sunday.
Latest EPS and GEFS ensemble members continue to show a secondary shortwave trough diving southward into the forecast region late Thursday night into Friday. The strength and depth of this system as well as available moisture will determine just how much precipitation we will see. Latest ensemble QPF totals across southcentral AZ during this timeframe have decreased from previous runs with latest GEFS mean around 0.05"-0.10" in the lower deserts up to 0.25" for the higher terrain whereas the EPS has slightly higher amounts ranging from 0.05"-0.25" across the lower deserts to upwards of 0.75" in the higher terrain. Even with these differences in model QPF, NBM PoPs remain highest on Friday with widespread 30-50% east of the Colorado River and up to 60-65% for N Maricopa and S Gila Counties. The forecast will likely continue to change and evolve so stay tuned for updates regarding rainfall potential.

Showers should clear out of the region early on Saturday with cool and drier conditions setting in through the remainder of the weekend. Ridging aloft will return to the western CONUS early next week, resulting in a warming trend beginning on Monday. Ensemble cluster analysis shows good agreement that ridging will continue through the middle of next week with temperatures climbing back above normal by Tuesday.

AVIATION
Updated at 1740Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns are expected through the forecast period. Somewhat elevated easterly winds will diminish as we enter the afternoon. Uncertainty exists regarding the typical westerly shift this evening. Guidance suggests it will be brief at KPHX and potentially KIWA with a more west-northwest tendency before easterly winds resume later tonight. SCT-BKN mid and high level clouds will scatter out this evening, with FEW low level clouds AOA 7 kft developing overnight.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are expected through the forecast period. Diurnal tendencies will prevail as northerly winds today switch to favor a westerly component first at KIPL early this evening, then at KBLH later tonight. Speeds will be light, generally remaining below 7 kts. FEW-SCT mid level clouds will clear out later this afternoon before mid and high level clouds return tomorrow morning.

FIRE WEATHER
Above normal temperatures are expected through Wednesday before cooler and wetter weather arrive late this week. Dry conditions will continue today with minimum humidity values ranging from 15-30%. Overnight moisture recovery will be mostly fair tonight and good to excellent beginning Wednesday night. 20 ft winds will remain mostly light and diurnal, with the exception of increasing breeziness across southeast California on Wednesday afternoon. An approaching weather system will result in increasing rain chances across much of the area beginning on Thursday and lasting through early Saturday. Chances for wetting rainfall will be highest across N Maricopa and S Gila Counties.

PSR WATCHES/WARNINGS/ADVISORIES
AZ...None.
CA...None.

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