textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion.

KEY MESSAGES

- Breezy to locally windy conditions return over the weekend with the strongest gusts across Southeast California and the Arizona high terrain.

- Elevated to critical fire weather conditions this weekend, particularly on Sunday for the Eastern Arizona higher terrain, where a Red Flag Warning will be in effect.

- Afternoon high temperatures 4 to 6 degrees above normal through Saturday will drop into a near normal range Sunday and below normal Monday, followed by a warming trend through the rest of next week.

SHORT TERM /TODAY THROUGH MONDAY/

Broad cyclonic flow will become established over the Western US this weekend as an upper level trough currently moving inland over the Pacific Northwest enters an amplification phase. Ensemble guidance has been picking up on the impacts of two distinct shortwaves that will swing through the Desert Southwest as a result of this pattern evolution, the first of which can be seen in midlevel water vapor imagery over Central CA, and a second, more potent system associated with a stronger core of vorticity and negative midlevel height anomalies which will dive southward and swing just north of the area Sunday night into Monday. The first shortwave will quickly progress through the region later today, bringing a quick-moving axis of higher momentum air aloft with NAEFs mean 700 mb flow around 25 kts. Under clear skies and peak surface heating this afternoon into the early evening, it would not be surprising for much of the area to experience a period with gusts up to around 20-25 kts, as mixing heights commonly stretch up to 700 mb or higher this time of year. As this first shortwave quickly ejects northeast of the area, global guidance depicts a considerable decrease in surface pressure over the Colorado River Valley and later over the Four Corners, creating a strong MSLP E-W gradient across the San Diego Mountains that can support strong gap flow and mountain rotor activity downstream over Western Imperial County. As a result, a Wind Advisory will go in effect mid-late afternoon for the typically prone SW corner of Imperial County. This strong E-W MSLP gradient across the San Diego Mtns will be sustained as the second system dives south Sunday and deepens the surface low pressure across the region, resulting in a prolonged windy period for that area through the rest of the weekend but especially during the evenings.

The primary hazard this weekend will be elevated and locally critical fire weather conditions, especially Sunday PM, however, midlevel height packing over Northern/Central AZ will not be maximized until the overnight hours. Thus, peak flow aloft will occur overnight and stronger winds will primarily be confined to Northern/Eastern AZ higher terrain. NAEFS mean 700 mb wind speeds will peak upwards of 30-35 kts, above the 90th percentile across much of the state. There will be a window where relative humidities drop below 15% as wind gusts increase through Sunday afternoon into the evening, leading to critical fire weather primarily over the Eastern AZ high terrain. As a result, a Red Flag Warning will be in effect Sunday afternoon through much of the evening.

On the backside of the second system, a front will be sent down the Lower CO River Valley resulting in windy conditions across portions of Southeast CA Monday morning. Gusts will peak mid- morning, then decrease through the rest of the day as regional pressure gradients relax. Wind headlines may be needed for this in future updates. An additional impact of this tandem of systems this weekend will be cooling temperatures through Monday. By Monday morning, ensemble guidance indicates a much cooler airmass over the forecast area, with mean 850 mb temperatures bottoming out around 9C-12C, around the 10th percentile of climatology. Monday will likely be the coolest of the next 7 days, with afternoon highs only reaching the middle to upper 80s for the typically warmer spots across the lower deserts, between 5F-10F below the daily normals.

LONG TERM /TUESDAY THROUGH FRIDAY/

As the core of the weekend system departs northeastward, some measure of broad cyclonic flow over the Western US will remain and likely evolve into quasi-zonal flow through the middle of the upcoming work week. Heights aloft will rebound closer to seasonal averages and so temperatures are expected to warm through the middle of the week, with dry conditions continuing. Meanwhile, a strong midlevel ridge will remain nearly stationary off the Pacific Northwest Coast through much of next week, with most global guidance showing a subtropical jet forming under the ridge and developing a few weak disturbances, a few of which may be sent downstream and near/over the Southwest US. Regardless of the details and how weak troughing over the West-Central CONUS eventually resolves, essentially all solutions currently show the region remaining completely dry, with periods of mid-high level clouds and gradually warming temperatures through the work week.

AVIATION

Updated at 1750Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concerns will continue to be a period of elevated cross-runway winds at KPHX followed by gusty W-SW winds this afternoon. Latest sfc obs show sustained S-SE winds around 10-11 kts. Winds will gradually shift around to the SW through this afternoon with gusts increasing to around 20-22 kts at all terminals. Expected westerly flow to prevail through this evening with gusts subsiding after sunset. The typical diurnal easterly shift is expected to occur at KIWA and KPHX late tonight. By early Sunday morning, winds will begin to shift back out of the south and become elevated up to 10-11 kts with gusts developing again by the afternoon. Skies will remain mostly clear over the region.

Southeast California/Southwest Arizona including KIPL and KBLH: Periods of gusty winds will be the main aviation weather issue throughout the TAF period. At KIPL, winds will maintain a westerly component while at KBLH winds will fluctuate from the south to southwest. Winds will increase at both terminals by this afternoon with gusts reaching 20-25 kts at times. At KIPL, a period of stronger gusts can be expected during the early evening hours and may exceed 30 kts, with around 70% chance for peak gusts >35 kts between 00Z-04Z. Wind directions of strongest winds will not be ideal to bring dust plumes through KIPL, but some hazy evening skies may still develop. Winds will relax at KBLH after sunset, but will remain elevated at KIPL overnight. Mostly clear skies will prevail over SE California.

FIRE WEATHER

A weather system has moved onshore along the Pacific Northwest and will deepen over the Great Basin resulting in increased winds and elevated to locally critical fire weather conditions across the area this weekend. Afternoon/early evening wind gusts will commonly peak between 20-35 mph today, with locally stronger gusts in Southeast CA. The strongest gusts by Sunday will likely be confined to Eastern AZ higher terrain, where gusts between 25-40 mph will be common. A Red Flag Warning will be in effect starting Sunday 11 AM MST for Southern Gila County and Tonto NF foothills. Afternoon minRHs mostly between 8-15% today will increase to 10-20% areawide Sunday. Overnight recoveries between 30-50% tonight will improve primarily for the eastern districts Sunday night to around 40-60%.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Red Flag Warning from 11 AM to 11 PM MST Sunday for AZZ133.

CA...Wind Advisory from 2 PM this afternoon to 11 PM PDT Sunday for CAZ562.

Wind Advisory from 4 PM this afternoon to 11 PM PDT this evening for CAZ566.