textproduct: NWS Phoenix

UPDATE

18Z Aviation Discussion.

KEY MESSAGES

- A slow moving weather system through early next week will bring much cooler temperatures, breezy to locally windy conditions, and late Monday and early Tuesday rain chances.

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

SHORT TERM /TODAY THROUGH SUNDAY/

A more unsettled weather pattern is expected over the next few days as multiple shortwaves rotate through a larger scale trough situated over the Western U.S. and the eastern Pacific. For our region, a very weak mid-level disturbance is currently moving northeastward across southeast California into western Arizona. Although moisture remains very limited, some virga showers and/or mostly sprinkles will be possible through around sunrise across Maricopa County. This weak feature will lift into northern Arizona later this morning as another batch of drier air surges in behind leaving clear skies for the rest of today.

Now that the large scale trough has mostly moved into our region, temperatures today will benefit from the lower heights, as highs are only forecast to reach into the mid to upper 80s across the lower deserts. The next more organized shortwave disturbance will reach northern California tonight and slowly move inland reaching Nevada later on Sunday. The closer proximity of this disturbance will help to tighten the pressure gradient over the region on Sunday as height falls also increase. This will result in more widespread breezy to windy conditions by Sunday afternoon, but guidance has mostly backed off on Advisory level winds. Afternoon/early evening wind gusts Sunday are likely to reach 25-30 mph for most areas to as high as 30-40 mph across southeast California. Temperatures will dip even further on Sunday with highs barely reaching 80 degrees across southeast California to closer to 85 degrees in the Phoenix area.

LONG TERM /MONDAY THROUGH FRIDAY/

Guidance continues to show the late weekend disturbance over the Great Basin merging with a third shortwave quickly diving down the back side of the large scale trough on Monday. The exact track and timing of this last shortwave feature is still a bit uncertain, but for now it is favored to move through central and northern Arizona Monday evening with lingering troughing lasting through the first part of Tuesday. As with the other disturbances, the lack of good moisture will be an inhibiting factor for rain chances. Models do show an increase in moisture in the mid- levels on Monday, but it is likely to be quite marginal with forecast mixing ratios only in a 4-6 g/kg range. Considerable dry air is forecast above and below this 750-650mb moisture layer, so any showers that do develop are not expected to produce much if any rainfall. The latest NBM PoPs are largely unchanged from yesterday's runs with evening/early overnight 10-15% chances for the south-central Arizona lower deserts to 20-30% over the high terrain northeast of Phoenix.

Monday will also be another breezy to windy day with widespread gusts of 20-30 mph to as high as 30-40 mph across portions of southeast California and over the Arizona high terrain. Temperatures should bottom on Monday with lower desert highs mostly in the upper 70s, or 5-8 degrees below normal. Overnight lows Monday night may also dip into the 40s for some locations, but lingering elevated winds and cloud cover may keep most spots from dipping below the low to mid 50s.

Once the shortwave trough exits to the east Tuesday morning, drier air will surge into the area starting Tuesday afternoon. Other than a lingering 10-15% chance of a light shower across far eastern Arizona Tuesday morning, sunny skies and pleasant weather conditions are expected across the area. Quiet weather with near normal temperatures are then favored for the latter half of next week under a zonal to broad troughing upper level pattern.

AVIATION

Updated at 1740Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: VFR conditions will persist with gusty winds at times. Overall light and variable winds midday will become dominate from the west and become gusty for a period late this afternoon and early evening with occasional gusts to around 20kts. Winds will make the typical diurnal shift back to an easterly direction overnight. Just a few higher clouds in overall clear skies.

Southeast California/Southwest Arizona including KIPL and KBLH: Winds will be the main impact through the period. Winds should maintain a westerly direction and KIPL and south to southwest at KBLH through the period. Confidence is high that gusts up to 20-30 kt will become common at both terminals this afternoon. There is even a moderate chance of gusts exceeding 35 kts at KIPL by this evening, mainly between 00Z-04Z.

FIRE WEATHER

A more unsettled weather pattern this weekend through early next week will result in near to below normal temperatures and slightly improved humidities. MinRHs this weekend will mostly average 15-25%, while overnight recoveries improve into a fair to good range (40-70%). Winds will somewhat follow diurnal patterns, but with more of a predominant southwesterly direction as winds increase areawide. Expect late afternoon/early evening enhanced gusts of 25-35 mph over much of the area today through Monday. A disturbance is then expected to move through the region late Monday into Tuesday bringing the potential for light showers focused mostly over the Arizona high terrain. Weak high pressure will then settle back over the region during the latter half of next week leading to drier conditions and temperatures at or just above normal.

PSR WATCHES/WARNINGS/ADVISORIES

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