textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion.

KEY MESSAGES

- A slow moving weather system will bring cooler temperatures and breezy to locally windy conditions through Monday as well as late Monday rain chances.

- Weak high pressure will settle back into the region during the middle of the week leading to drier and slightly warmer conditions.

SHORT TERM /TODAY THROUGH MONDAY/

Large scale troughing continues to impact much of the Western U.S. with the second in a series of three weather systems currently moving into northern California. The main impact for the Desert Southwest today will be another day of breezy to windy conditions with afternoon/early evening wind gusts commonly reaching 20-30 mph. Drier air remains positioned across the region as the weather system is still too far removed to the northwest. Guidance also shows the second disturbance quickly weakening today as it moves eastward into the Great Basin, while a third piece of energy moves southeastward into the base of the trough tonight. Aside from the increased winds today, temperatures are also expected to at most reach into the normal range with highs mostly in the lower 80s across the lower deserts. Today should be our coolest day since around March 11th.

By Monday afternoon, a modest shortwave trough is forecast to move into southern California, a combination of the second and third pieces of energy. Like today, Monday will again be another breezy to windy day, but with the PV energy moving through the region later in the day it should result in some scattered shower activity. The lack of decent moisture will still be a limiting factor for rain chances, but models have at least maintained or even slightly increased chances with the latest run. Forecast soundings by Monday evening show a shallow moist layer centered around 10K feet, but little moisture above and below this layer. Considerable dry air in the boundary layer is expected to limit measurable rainfall to mostly higher terrain areas, but even good a portion of the lower deserts should receive some trace amounts from late Monday afternoon through Monday night. PoPs have increased slightly from the previous forecast package with 10-15% chances over southeast California and southwest Arizona, 20-30% within Phoenix, to as high as 40-50% over portions of the high terrain.

LONG TERM /TUESDAY THROUGH SATURDAY/

The weather disturbance will begin exit to the east during the first half of Tuesday with any limited lingering rain chances across far eastern Arizona. Drier air will begin to move in from the west on Tuesday allowing for skies to become clear to mostly clear. Temperatures will however continue to be below normal with the upper 70s to lower 80s on Monday carrying over into Tuesday.

Weak upper level ridging is then expected to move through the region on Wednesday helping to boost daytime highs back into the mid 80s, or right around normal. The weather pattern for the rest of the week should stay relatively quiet, but a mostly dry Pacific trough is likely to track by just to our north late in the week. Ensemble guidance is still a bit uncertain with this system, but for now has it tracking across the Four Corners area. It is quite unlikely it will result in any rain chances, but we very well could get another day or two of breezy conditions around Thursday or Friday. The NBM has also responded by dropping highs on Friday back into the lower 80s, but this will not last long as incoming ridging favored for next weekend should eventually push readings back to above normal.

AVIATION

Updated at 1120Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concerns over the next 24 hrs will be another period of gusty winds this afternoon and evening. Until then, expect more of a S-SE component through early this morning followed by a switch back to W-SW around 17Z-18Z. Speeds will gradually increase to around 10-12 kts with gusts up to 20-25 kts by this afternoon. High clouds will thicken throughout the day becoming SCT-BKN with bases aoa 12 kft. There may be a brief period of virga showers this evening, mainly after 00Z, but no impacts are anticipated.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation concerns at both TAF sites will be gusty winds, especially this afternoon and evening. Winds will remain westerly at KIPL and south to southwesterly at KBLH through the TAF period, although periods of calm and vrb will be possible at KBLH early this morning. Increasing winds with gusts up to 20-30 kts will become common at both terminals by this afternoon and persist well into the evening. SCT-BKN high clouds will continue to be common through the period.

FIRE WEATHER

An unsettled weather pattern will continue through the first part of this week resulting in near to below normal temperatures and slightly elevated humidities. MinRHs through Monday will mostly average 20-25% with overnight recoveries of 40-70%. Winds will somewhat follow diurnal patterns, but with more of a predominant southwesterly direction and gusty afternoon and evening winds of 25-35 mph over much of the area. Chances for light showers will be possible later Monday and Monday night, but CWR will remain below 10%. Weak high pressure will then settle back over the region during the latter half of the week leading to drier conditions and temperatures staying close to normal.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.