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Friday, December 8th, 2023 @ 9:30 AM MST by Malloy, J.
A large low pressure trough now occupies the western United States, yielding a reprieve from the unseasonable warmth experienced midweek in south-central AZ. Under the new weather pattern regime, conditions over the Desert Southwest are influenced by strong, cool, and dry northwesterly flow aloft through early Monday. The warmest portions of Phoenix today likely stay capped in the low 70s, upper 60s on Saturday, and then back to the low 70s Sunday and Monday. The prevailing cool, dry air mass keeps morning lows across Downtown Phoenix in the low to middle 40s each day. Saturday and Sunday are projected to be particularly dry with widespread dewpoints potentially falling in the single digits and precipitable water values below a quarter inch. With such a dry air mass and mostly clear overnights, wouldn’t be surprised if a few rural desert locations reach the upper 30s during this forecast period (e.g., Wickenburg, Buckeye, Gila Bend).
Of note is the evolving synoptic weather pattern with rapid succession of an exiting low pressure trough (today) followed by the brief development of robust surface high pressure over the Colorado Plateau region (Saturday/Sunday). The resulting tight near-surface pressure gradient for central AZ would support breezy to windy northerly/easterly flow along and downwind of significant terrain features, including the County, beginning tonight. Winds (occasional peak gusts 30-35mph possible) are most pronounced during the overnight and early morning hours for the higher terrain this weekend, while more intermittent breezes (15-25mph) for lower valley locations are likely restricted to the daytime hours when atmospheric heating and mixing potential is greatest. Model guidance is pointing to Sunday having the greatest coverage of breezier winds at the lower elevations. The regional wind event ends by Sunday night when another dry low pressure trough encroaches from the west to weaken the surface high pressure facilitating the windy conditions. The trough has little direct impact on our weather other than forcing temps to stay near seasonable values and ushering in partly cloudy skies.
Have a great weekend everyone!
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Normal High 67˚F (Record Warmest 84˚F in 1939; Record Coolest 46°F in 1978)
Normal Low 46˚F (Record Warmest 57˚F in 2021; Record Coolest 24°F in 1916)
Daily Record Precipitation (1.06” in 1992)
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Today’s Sunset – 5:20 PM
Daylight Hours - 10hrs 1 mins
Monday's Sunrise – 7:22 AM
Moon Phase – Waxing Crescent (20%)
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Weather Tutor
 Part of winter weather hazard awareness is knowing and protecting yourself and pets against “wind chill”. What is the science behind wind chill, though? An important aspect about the atmosphere to be mindful of is that it is constantly seeking equilibrium from an energy and moisture standpoint. The inherent spatial and temporal imbalance of temperatures and water vapor across the planet forces constant air mass energy transfers and water phase changes (e.g., condensation and evaporation). Everyday weather is the product of that perpetual battle in an environment seeking equilibrium! Now to why wind chill can be a significant health hazard. When I say air mass, that also means the modified micro air mass your body generates. Just like the atmosphere’s constant pursuit of equilibrium, your body desires the same! The air in motion has a certain temperature and if something in the environment is warmer (like you!) than heat is transferred to the air. Under cool, calm conditions your body may be able to keep up with the energy being lost to the surrounding air; however, faster cold winds is like a never ending reset button on your body’s attempt to gradually “warm” the air in contact with your skin. It’s a losing battle. Furthermore, dry and cold winds interacting with any sweat you have adds to your temperature deficit through evaporative cooling effects. Bottom line, bundle up if in cold and windy weather!
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