textproduct: NWS Phoenix

UPDATE

00Z Aviation Discussion

KEY MESSAGES

- Slight chances for isolated showers and thunderstorms are possible across South Central Arizona through Thursday afternoon.

- Tropical moisture will arrive into the region late Thursday into Friday, further increasing rain chances across the Southeastern third of Arizona before chances shift into Southwest Arizona and Southeast California this weekend.

- Temperatures should remain in a near to below normal category for the first week of September, resulting in widespread Minor Heat Risk and localized areas of Moderate Heat Risk.

SHORT TERM /TODAY THROUGH FRIDAY/

A somewhat complex and rapidly shifting forecast continues to evolve due to several large-scale weather features evident in early afternoon water vapor imagery: 1) broad Pacific troughing positioned along the West Coast, 2) broad anticyclonic flow and mid/upper level high pressure downstream, centered over the Southeast US, and 3) Tropical Storm Marie that is currently well offshore, south of Baja California. Objective analysis shows the influence of the first two of these features extending into the Desert Southwest, resulting in confluent/convergent flow aloft. Additionally, a shortwave is apparent ejecting northeastward around the West Coast trough. Regardless of which is more responsible, the 12Z PSR sounding once again shows a fairly strong midlevel temperature inversion, indicative of continued subsidence over the forecast area. The sharp decline in moisture above the residual boundary layer is also apparent, which will once again begin to mix down and help further dry the mixed layer later this afternoon. Across the forecast area, yet another sharp dewpoint temperature gradient had formed this morning, this time focused closer to the N-S border between Maricopa and La Paz/Yuma Counties, indicating that better low level moisture still lies over the eastern CWA. This will be sufficient to support isolated showers and thunderstorms across South-Central AZ this afternoon, with better chances focused towards the south/east of Phoenix. With MLCAPE values up to around 500 J/kg and GFS bufr soundings indicating DCAPEs increasing to upwards of 1000-1500+ J/kg, updrafts will not be particularly robust, but any storms that do form could produce localized gusts of 40+ mph. With higher HREF neighborhood probabilities of 35+ mph gusts near storms (~30%) over the dust prone corridor in Pinal County, it would not be surprising to see these storms produce locally dense blowing dust.

Over the next few days, TS Marie will continue its (most likely) northwestward trajectory, eventually shifting further out into open waters. Global guidance still shows excellent low level moisture filtering up the Gulf of California and over Sonora thanks to the distant influence of TS Marie, then that better moisture pushes into Southeastern AZ Thursday night into Friday. However, ensembles now show a deeper West Coast trough and stronger southwesterly flow aloft, keeping moisture confined further south and east of the CWA during this timeframe. Higher terrain north, east, and southeast of the Greater Phoenix Area are still likely to see several rounds of showers and storms with some capable of producing strong wind gusts and locally heavy rainfall. But the latest WPC ERO updates for Day 3 (Friday) have shifted considerably, with areas under a slight risk (at least 15% probability of flash flood guidance being met within 25 miles of a point) now almost entirely to the south and east of the forecast area. Isolated instances of flash flooding across South-Central AZ remain possible, with some of the longer range CAMs revealing the potential for scattered showers and storms developing primarily during the overnight/morning hours Thursday night/Friday and possibly again for Friday night/Saturday.

Due to the presence of troughing to our west and a seasonable 500 mb hght field around 587-590 dam, temperatures will remain within a degree or two of daily normals through the end of this week. Lower desert highs will generally range from the upper 90s to around 105 degrees each afternoon, following morning lows from the upper 70s to middle 80s. The West Coast trough will also induce a compact 700-500 mb hght gradient over southeast CA and western AZ beginning today which will lead to breezy conditions across the Imperial and Lower Colorado River Valleys. Gusts could reach upwards of 25-30 mph in the aforementioned areas and potentially as high as 40 mph in far SW Imperial County.

LONG TERM /SATURDAY THROUGH TUESDAY/

The forecast becomes more uncertain heading into this weekend, but the general model consensus is showing the subtropical ridge over the southeast U.S. retrograding west into the Southern Plains. The western periphery of the ridge will likely nose into eastern AZ on Saturday. If this occurs, then the subtropical jet will likely shift farther west, setting up better moisture advection over SoCal and western AZ. This scenario is currently being reflected within the NBM showing a westward progression of PoPS from southcentral AZ early Saturday into southwest AZ and southeast CA late Saturday into Sunday. Due to an overall increase in moisture and extensive cloud cover this weekend, it is likely we will see lower instability which could limit convection and keep the threat of heavy rainfall at a minimum in our forecast region with better prospects of heavy rain and flash flooding over SoCal and S NV. There is still plenty of time for things to change and we will likely have to wait until we are within the high-res window for this weekend's forecast to become more clear.

Similar to rain chances, there is also a high uncertainty in the temperature forecast by this weekend as indicated by large (5-8 degree) spreads within the NBM inner quartile range, especially on Saturday. This is likely due to the strength of the ridge and how much cloud cover will occur over the region. It is likely that Saturday will be the coolest day with highs in the lower deserts mainly in the mid to upper 90s. However, as ridging aloft causes an overall increase in mid lvl hghts/thickness, temperatures may warm to near or even slightly above normal from Sunday into early next week.

AVIATION

Updated at 2315Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main forecast uncertainty will be the potential for shower and thunderstorm chances this evening. Most, if not all convective activity will be focused over the south and eastern portion of the airspace. A PROB30 has been maintained for KIWA while VCTS has been added to KPHX due to potential operational impacts beyond 10 miles of the terminal. Seeing activity within the vicinity of KSDL and KDVT cannot be completely ruled out, but chances at these sites remain too low to include mention of any degree of SHRA or TSRA. Anything that were to migrate near the terminals would likely result in little to no impacts, other than maybe a brief window of gusty and erratic winds. Outside of this, winds are expected to follow light and diurnal trends under a FEW low-level clouds decks.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected over the next 24 hours. Wind patterns will be similar to observations over the past few days with westerly flow favored at KIPL and a south- southwesterly/southwesterly component being common at KBLH, with breezy conditions during the afternoon and evening hours. MOstly clear skies will be present over the region through Thursday afternoon.

FIRE WEATHER

Drier conditions will prevail through Thursday with only isolated chances for showers and thunderstorms mostly relegated to the higher terrain of southcentral and southeastern AZ. Better chances for wetting rains return to the region late this week into this weekend. Afternoon minimum humidities will range from 7-15% across Southeast CA and between 15-35% for much of Southern AZ through the next few days and will likely increase above 25% areawide this weekend. Overnight recoveries will range from poor across the western deserts and good to excellent across the eastern districts. Daytime winds will remain elevated out of the south to southwest, with the strongest gusts in Southeast CA and along the Lower Colorado River Valley peaking between 20-30 mph during the afternoon/early evening.

PSR WATCHES/WARNINGS/ADVISORIES

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