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FXUS65 KPSR 042259 AFDPSR

Area Forecast Discussion National Weather Service Phoenix AZ 359 PM MST Wed Oct 4 2023

Update
Updated Aviation

SYNOPSIS
Dry and tranquil conditions will prevail the next several days as high pressure builds into the region. As a result, above normal temperatures will return to the region with many lower elevation communities likely reaching 100 degrees by Friday. A slight increase in moisture will occur over the weekend which may produce a few light showers over the eastern Arizona high terrain.

DISCUSSION
A higher amplitude flow pattern continues to materialize across the western hemisphere with deep negative height anomalies over southern Canada/Great Lakes and a quasi-stationary Rex block over the East Pacific/western Conus. The response across the forecast area still appears to be towards the linkage of high pressure centers such that H5 heights steadily build towards 588dm (above the 90th percentile of climatology) by tomorrow morning. Heights and tropospheric thermal profiles should be maximized Friday yielding the warmest day of the period with many lower elevation communities reaching or exceeding 100F (record high for Phoenix of 105F with ensembles suggesting a 25% chance of reaching).

A weakening trough west of central Baja with a series of low amplitude waves embedded in the subtropical jet will enter the region over the weekend helping partially erode the stronger ridging aloft. In addition to cooling temperatures a few degrees, this feature will aid in lowering pressures across NW Mexico while strong surface high pressure over the central/southern high plains supports an increased gradient through the eastern CWA. Marked SE low level flow beginning Friday morning will start importing moisture up the Rio Grande valley, however trends in recent model output suggests moisture flux far less than previously forecast. The preponderance of guidance now indicates low level mixing ratios only reaching 5-6 g/kg and PWATs falling short of 1" through Saturday. As a result, instability parameters now appear minimal with NBM POPs deteriorating substantially (even over higher terrain) and near elimination of thunder probabilities. In fact, the only remaining notable POPs now occur on Sunday across the Mogollon Rim when thicker mid/upper moisture arrives on the nose of a subtropical jet streak (likely only yielding virga across lower elevations).

During the first half of next week, model ensembles are in favor of some form of dampened ridging over the SW Conus only slowly shifting east in response to stronger wave breaking entering the Pacific NW.
This evolution should keep H5 heights around 582dm over the region with NBM forecast highs showing little spread Monday and Tuesday and remaining 4-8F above normal. Looking out even further, guidance primarily keeps some sort of ridging over the Southwest the remainder of the week ensuring temperatures generally remain in an above normal category.

AVIATION
Updated at 2300Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation concerns are expected during the TAF period. Winds will continue to follow light and diurnal tendencies, with periods of variability during the overnight hours. Skies will remain clear through tomorrow

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns are expected during the TAF period. Winds at IPL will be out of the NW this evening before backing to the W early tonight. BLH will also see NW winds during the evening and overnight hours before winds there veer to the N/NE late tomorrow morning. CLear skies will prevail through tomorrow.

FIRE WEATHER
Temperatures will continue to warm over the next few days reaching above normal levels and peaking late in the week. Dry conditions with afternoon minimum humidity levels mostly in the teens will give way to a modest moisture increase over the weekend where humidities rise towards a 15-25% range. Similarly, poor to fair overnight recovery of 20-50% the next couple days should improve closer to 35-70% over the weekend. While this moisture increase may result in a few light showers over higher terrain of eastern districts this weekend, wetting rainfall should not be expected.
Light winds will give way to enhanced easterly winds Friday and Saturday where some Arizona districts could experience occasional gusts 25-30 mph - especially over ridge tops and through terrain gaps.

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

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