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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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553 FXUS65 KPSR 142305 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 405 PM MST Wed Aug 14 2024 .Update...00Z Aviation Discussion && .SYNOPSIS... Quieter weather conditions will prevail during the latter half of the work week, with relatively stable daily high temperatures reaching up to around 110 degrees for the typically warmer lower desert communities through this weekend. High pressure will begin to shift back over the southern plains this weekend resulting in a slight increase in moisture and increased thunderstorm initiation chances mainly over Arizona high terrain into early next week. && .DISCUSSION... Current WV imagery and 500 mb RAP analysis depicts a trough moving eastward over the Intermountain West, with another upstream approaching the Pacific Northwest coast. The persistent Pacific Northwest negative height anomalies coupled with a strong westerly jet streak over the Great Basin region has acted to displace/dampen the subtropical high pressure over the region. An anticylonic circulation is in place overhead, with very dry air and subsidence aloft. The 12Z KPSR sounding this morning depicted a strong inversion between ~600-500 mb associated with the subsidence aloft, and model soundings show this feature persisting to some extent through the next several days, becoming less prominent later in the day on Friday. Considering the strong cap and drier air in place, shower/thunderstorm chances through the end of the work week will be near zero, but Friday may bring some very isolated development over the typical prominent terrain features of Southwestern Maricopa County, Northeastern La Paz County, and Gila County. Late Friday, the upper level pattern shifts such that upstream troughing amplifies just off the West Coast and the subtropical high builds as it centers itself over the southern plains/eastern New Mexico. This shift in the pattern will allow south/southeast flow to reestablish over Southern AZ, drawing moisture back into the region for this weekend. Global ensembles are in excellent agreement with respect to the trends in moisture through this weekend, suggesting PWATs (and lower desert storm chances) will peak late Saturday, though the GEFS shows the driest ensemble mean compared to the ECMWF and CMC ensembles. Despite the increase in moisture this weekend, forecast near-surface mixing ratios (peaking around 10-11 g/kg) will only be marginally supportive of lower desert storms, and much of the daily thunderstorm activity will be confined to the Arizona high terrain and some of the typical mountains/terrain features of South-Central and Southwestern AZ. NBM PoPs peak around 10-20% for the AZ lower deserts on Saturday and Sunday and around 20-40% for the high terrain east/north of Phoenix and for Southeast AZ. Early next week, the upper level flow pattern does not seem to change significantly, though the trough off the west coast and subtropical high both seem to shift slightly westward, and ensemble mean H5 heights modestly increase over the eastern CWA to around 595-597 dam. With increased subsidence aloft, convection would be even more confined to the AZ high terrain early next week, and lower desert PoPs decrease to 10% or less starting Monday. Temperatures will be quite stable through the next 7 days, with daily highs reaching around 108-112 deg F each afternoon across the typically hotter lower desert locales. These temperatures will lead to widespread Moderate HeatRisk through at least the middle of next week, so sensitive groups will be at risk of heat-related illness during the heat of the day, and all should continue to practice proper heat safety measures. As current global guidance indicates heights aloft increasing over the eastern CWA next week, areas of Major HeatRisk develop over the San Carlos area as early as Monday, however, subtle differences in the locations and orientations of the previously discussed synoptic features could lead to considerable changes in moisture, magnitude of heights aloft, and the subsequent temperature forecast, so excessive heat products are not necessary at this time. && .AVIATION...Updated at 2305Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation concerns are expected during the TAF period. Winds will follow typical diurnal trends, with occasional gusts 15-20 kts during the afternoon and early evening hours. Other than a FEW clouds around 10k ft, skies will be mostly clear through tomorrow afternoon. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are anticipated through the next 24 hours. Winds will follow familiar diurnal trends, with some periods of variability during directional shifts, especially at BLH. Mostly clear skies will prevail during the next 24 hours. && .FIRE WEATHER... Drier conditions and slightly above normal temperatures will prevail through the end of the work week, with only isolated (10% or less) chances for showers and thunderstorms returning late Friday. Over the weekend, a modest increase in moisture will lead to better storm chances and coverage. Today and Thursday, with drier air in place, afternoon MinRHs will bottom out around 10% for the western districts and between 15-20% for the Arizona districts. By Friday, these values will increase across the region to between 15-25%. Overnight recoveries will be fair at between 25-50% and increase up to 60% for portions of southern Arizona over the weekend. Wind patterns should follow a more typical upslope/drainage pattern through the next several days without much chance of gusty outflows. However, local afternoon upslope gusts of 20-25 mph will be common. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Whittock AVIATION...RW FIRE WEATHER...Whittock