Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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446 FXUS65 KPSR 170510 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1010 PM MST Mon Sep 16 2024 .Update...06Z Aviation Discussion && .SYNOPSIS... A weather disturbance will shift northeast of the region by Tuesday, but not before yielding a chance for showers and thunderstorms across eastern Arizona tonight. Dry and seasonably mild conditions will spread over the area through the middle and end of the week with forecast temperatures hovering 5 to 10 degrees below normal. Readings should moderate closer to normal over the weekend. && .DISCUSSION... Afternoon WV imagery and objective analysis depicts a seasonally deep negative height anomaly spinning over central California with an impressive jet streak punching inland towards the forecast area through the trough base. While the primary cold core and most pronounced height falls will remain north of the CWA as this system ejects eastward, the initial midtropospheric front was leaking into the area, and along with jet energy was aiding in large scale synoptic ascent. Within the boundary layer, ample moisture has surged north with 12-13 g/kg mixing ratios per in situ aircraft sampling resulting in MLCape 500-1000 J/kg within an environment of only weak inhibition. However, this area favorable for convective development will be battling drier air filtering eastward and steadily eroding the low level moisture. The best chances for showers and storms will align over higher terrain east of Phoenix where orographic effects of strong, moist S/SW flow will be optimized this afternoon and evening. However, even a few fast moving showers will be possible (~25% chance) through the central and eastern portion of the metro mostly tied to assistance from terrain escarpments. Through SE California, the combination of onshore flow and a deepening marine layer along with veering H8-H7 zonal flow crossing the San Diego mountains will promote mountain rotors and sundowner winds through western Imperial County. Areas of blowing dust will need to be monitored originating from source regions in recreation areas east of Borrego Springs. A gradual weakening of gusts should materialize later overnight as thermal and pressure gradients relax. Well farther east into southern Gila County, scattered showers may persist well into Tuesday morning as moist ascent persists ahead of the incoming midlevel front tapping lingering modest instability. HREF membership depicts the potential for multiple round of fast moving showers impacting the far eastern CWA before the trough axis and substantially drier air finally propagates through the state late Tuesday morning. Ensemble members remain in very good agreement the remainder of the week maintaining deep longwave troughing over the western Conus with a couple impressive negative height anomalies filling into the trough base over the SW Conus. Unusually low H5 heights in a 578- 584dm range will support an extended period of below normal temperatures beyond the middle of the week. In fact, a reinforcing shortwave with even more pronounced cool air and H5 heights as low as 572-576dm will spill into the region late in the week. Despite the passage of these weather disturbances, a very dry environment will remain in place resulting in no chance of rainfall. Model guidance spread suggests only minimal uncertainty with temperatures solidly 5F-10F below normal for much of the week. In fact by the latter half of the week, morning lows in the outlying suburbs of the Phoenix metro may touch into the upper 50s; and it would not be inconceivable for central Phoenix to fall into the upper 60s (last time KPHX was in the 60s was May 22nd). Larger model spread exists over the weekend regarding the potential downstream progression of mean troughing, or maintenance of lower heights over the west. At this time, ensemble mean output moderates H5 heights with temperatures rebounding back closer to normal over the weekend though temporal trends suggest a slightly delay in this warming. && .AVIATION...Updated at 0510Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation concerns are expected during the TAF period. Winds are not expected to fully switch to the E tonight, but there will likely be some moments of SSE/SE`rly winds, especially at KIWA and KDVT. Typical light and diurnal trends will resume by late morning/early afternoon Tuesday, with occasional afternoon W`r;y gusts near 15-20 kts. FEW to maybe SCT low-level clouds will prevail through the period, with the lowest bases around 7-9k ft. Southeast California/Southwest Arizona including KIPL and KBLH: Winds are beginning to relax, but lingering breeziness will be seen at IPL through about 09Z. From there, winds will be light and favor a W`rly component with periods of variability at BLH. Clear skies will prevail through Tuesday evening. && .FIRE WEATHER... A weather system will pass to the north of the region tonight bringing scattered showers and isolated embedded thunderstorms mainly over the high terrain of the eastern districts. Thereafter, a much cooler and drier airmass will spread across the region with temperatures 5F-10F below normal for the majority of the week. As a result, minimum afternoon humidity levels in the teens will be common in lower elevations and closer to a 20-30% range across higher terrain. Overnight recovery will range widely from fair to good at 30-70%. As temperatures warm closer to normal this weekend, humidity levels will decrease further with single digit readings more likely in lower desert communities. Winds should not be unusually strong with only some afternoon upslope gustiness. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...Wind Advisory until midnight PDT tonight for CAZ562-566-567. && $$ DISCUSSION...18/Whittock AVIATION...RW FIRE WEATHER...18/Whittock