Sunrise 5:39 AM   Sunset 7:42 PM Moonrise 3:24 PM   Moonset 3:02 AM

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FXUS65 KPSR 182030 AFDPSR

Area Forecast Discussion National Weather Service Phoenix AZ 130 PM MST Sat May 18 2024

SYNOPSIS
A weather disturbance will slide primarily north of the region early next week allowing temperatures to retreat closer to the seasonal normal. This system will also encourage occasionally breezier conditions during the afternoon and evening hours with Monday having the strongest winds and greatest impacts. Very tranquil weather with near normal temperatures will prevail during the second half of the week.

DISCUSSION
Local shortwave ridging was being eroded and dislodged eastward early this afternoon as a well defined vorticity center and associated subtropical jet core punches into the SW Conus. This negative PV center can be linked to a portion of the base of a central Pacific Rex block which is currently in the process of decaying. While this block has resulted in a split flow regime over the western US, the resumption of more progressive flow early next week will allow the southern circulation center of the block to phase with northern stream Gulf of Alaska energy into a full latitudinal western trough. Although this energy will eject into the plains during the middle of the week, some measure of broad, low amplitude troughing should be maintained across the western Conus the remainder of the week.

Over the next 24-48 hours, anti-cyclonic and subsident midtropospheric flow will be replaced by a weakly cyclonic pattern with H5 heights gradually falling ahead of the aforementioned incoming shortwave. With excellent model agreement and narrow guidance spread, this evolution will shave a few degrees from a persistence forecast Sunday, then more so Monday where readings will have retreated to near the seasonal normal. With exceptionally dry air precluding any rainfall chances, strong gusty winds will be the greatest concern and impact, as well timed height falls, a tightened pressure gradient along a frontal boundary, and midlevel jet energy become coincident over the forecast area Monday afternoon. Although gusts 20-30 mph will become common as early as Sunday afternoon, more pronounced gusts 25-40 mph appear likely Monday as mechanical mixing taps 25-35kt winds in the H8-H7 layer. Unfortunately, recent changes in mandated NBM initialization have degraded deterministic wind forecasts with a distinct low bias at lower elevations, however NBM probabilities reveal greater chances of a far stronger outcome with better than a 75% chance of advisory criteria being met across much of SE California. While not as intense as SE California, stronger gusts into south-central Arizona will result in a heightened fire danger when combined with low humidity levels and receptive fuels.

From midweek through the weekend, a near persistence forecast appears most applicable as the region oscillates between flat ridging and low amplitude shortwaves passing north through the Great Basin. During this time frame, H5 heights should waver little in a narrow range of 576-580dm with minimal uncertainty among NAEFS membership. Thus, forecast confidence is very good depicting steady daily temperatures not terribly far from climatology with dry air continuing to dominate the region. Deep mechanical mixing will also continue to tap momentum above the H7 layer resulting in frequent afternoon/early evening gustiness, however there is little evidence from NBM probabilities of anything much more than 20-30 mph gusts, sans the typically windiest locations of far SW Imperial County.

AVIATION
Updated at 1730Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: VFR conditions with no aviation concerns expected through the forecast period. Winds are beginning to shift out of the WSW, but there could still be an hr or two of southerly crosswinds before ultimately becoming westerly. There will be an increase in breeziness this afternoon with speeds up to 15 kts and gusts in the 20-25 kt range. Winds will diminish after sunset and return to east again after midnight. High clouds will exit the region, with skies becoming clear through the rest of today.

Southeast California/Southwest Arizona including KIPL and KBLH: VFR conditions and no aviation concerns besides breezy winds are anticipated through the forecast period. Winds will remain WSW at KIPL and S-SW at BLH, becoming elevated and gusty this afternoon.
Both terminals will see speeds up to 15-20 kts with gusts up to 25- 30 kts. Gusts could linger past sunset at KIPL. Skies will remain clear.

FIRE WEATHER
A disturbance moving mostly north of the districts early next week will force temperatures to cool closer to the seasonal normal while also producing a period of strong, gusty winds. Throughout the week, minimum afternoon humidity levels will fall into the single digits at lower elevations and the teens across higher terrain areas. This will follow poor to fair overnight recovery of 20-40%. Widespread wind gusts as high as 30-35 mph may be common Monday afternoon yielding an extended period of near critical thresholds when combined with low RH and dry fine fuels. Land managers should be cautious for rapid, uncontrolled spread of any ongoing fires or new starts Monday. Otherwise, afternoon gusts of 20-25 mph should be more representative the remainder of the week resulting in a slightly elevated fire danger.

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

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