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FXUS65 KPSR 091207 AFDPSR

Area Forecast Discussion National Weather Service Phoenix AZ 507 AM MST Sat Dec 9 2023

UPDATE
Updated 12Z Aviation Discussion.

SYNOPSIS
Near normal temperatures will be common behind a cold front this weekend as breezy conditions affect higher terrain areas of SE California and south-central Arizona. High pressure builds back into the western U.S. early next week resulting in a slight warming trend. Precipitation chances will be neglible through next week.

DISCUSSION
Pattern amplification was evident across the Conus per early morning WV imagery as a series of deep negative PV anomalies carve out a pronounced trough over the plains and Midwest. Concurrently, high pressure ridging was building along the West coast producing a pattern configuration very favorable for strong surface pressure rises across the Great Basin. Objective analysis depicts a tightening pressure/thermal gradient across northern AZ, and nocturnal influences should allow further intensification of the thermal gradient allowing the cold, dense airmass to surge south through the morning. Observational data has yet to sample any notable gustiness, though HRRR output shows the strongest wind signal shortly after sunrise with the initial burst of mechanical mixing. Overall with jet energy and height falls already exiting east of the forecast area, the signal for expansive strong wind gusts is not optimal with advisory level winds likely only impacting the most susceptible exposed locations later this morning.

For the majority of the forecast area, cooler temperatures will be the most evident feature behind the cold frontal passage this weekend. Despite midlevel heights already beginning to rise this afternoon, forecast confidence is very good that the shallow, cold boundary layer airmass will result in readings very near the seasonal normal. The greatest impacts should be reserved for rural sheltered valleys of SW Arizona and higher terrain communities where morning lows Sunday may near or fall below freezing. Enhanced northeast winds around terrain gaps and outside of decoupled valleys adds a significant complication in assessing exact areas more likely to flirt with 32F. At this time, no headlines have been issued opting towards extensive messaging of the potential for localized frost/freeze issues, however a few isolated zones identified as not yet reaching freeze status this year may justify a warning Sunday morning.

Ensemble members remain in excellent agreement with respect to the overall western hemisphere pattern evolution next week featuring further amplification of longwave features towards a blocking type regime. As negative height anomalies become established over the Gulf of Alaska and northern Atlantic basins respectively, broad ridging and a poleward retreat of the northern jet will dominate the Conus. However, this pattern will also favor some form of a cutoff low developing over the four corners/Southwest during the middle of the week constituting a Rex block configuration (albeit, not necessarily a stationary feature). While uncertainty exists regarding the intensity and precise location of the aforementioned cutoff low, this weak vorticity center should effectively dampen height rises and cap potential warming such that daily temperatures should remain steady around 3F-6F above normal throughout the week.

Given the overland trajectory of this deepening vorticity center into an already dessicated atmosphere, rainfall chances will be almost non-existent. Not surprisingly, forecast confidence deteriorates rather significantly next weekend as models attempt to resolve the upstream blocking pattern. In general, some form of troughing should be approaching the California coast, though it remains to be seen whether shortwave energy surges inland with a Pacific moisture intrusion or lifts north with frontal decay and high pressure holding over the Southwest.

AVIATION
Updated at 1205Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: A northeasterly gradient wind aob 10 kt will prevail over the area through most of the TAF period. At KPHX current W winds will become northeasterly late this morning then become easterly early tomorrow morning. At KIWA, winds should briefly become southwesterly by 14Z. While the winds at this time should predominantly be southwesterly, they will be light aob 4 kt and may become variable at times. They will then switch back to the NE this afternoon before becoming easterly early tomorrow morning.
At KSDL and KDVT, current N-NE winds will become more easterly this afternoon before switching back to the NE this evening. FEW to SCT high clouds will be common through the TAF period.

Southeast California/Southwest Arizona including KIPL and KBLH: Current light northwesterly winds at KIPL will become more northerly late this morning and increase in speed. By late this morning winds will gust 15-20 kt and continue into the evening.
After 01Z winds will weaken and become northwesterly again aob 6 kt. At KBLH winds will be northerly throughout the TAF period.
Wind speeds will pick up through the morning with sustained gusts of 25-30 kt by the late morning and continuing the evening. After 01Z winds will weaken again and be aob 11 kt. FEW to SCT high clouds will be common through the TAF period.

FIRE WEATHER
With temperatures remaining near normal this weekend, a period of gusty northeast winds will impact much of the region. The strongest winds should funnel down the lower Colorado River valley, as well as affecting ridge tops and terrain gaps across eastern districts. This will result in a locally elevated fire danger with dry fuels and low humidity levels. In fact, minimum afternoon humidity levels will fall primarily in a 10-15% range through early next week following only poor to fair overnight recovery of 15-35%. Lighter winds will return by the middle of next week with the potential for a modest moisture increase.

PSR WATCHES/WARNINGS/ADVISORIES
AZ...None.
CA...Wind Advisory until noon PST today for CAZ564-565-568.

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