textproduct: NWS Phoenix

KEY MESSAGES

- Temperatures around 3 to 8 degrees above daily normals will result in widespread Moderate Heat Risk, with isolated pockets of Major Heat Risk at times, through the middle of next week.

- Chances for showers and thunderstorms will mostly be focused over the Northern Arizona high terrain and portions of Southeast Arizona through the weekend and into early next week.

- Brief gusty winds from distant outflows and low probabilities for a thunderstorm or two over the northern and eastern foothills of Maricopa County cannot be ruled out today and Sunday in the late afternoon to evening time frame.

SHORT TERM /TODAY THROUGH SUNDAY/

The upper level pattern remains stagnant with broad ridging over the Eastern Pacific and troughing features straddling the ridge in the northern stream. Meanwhile, GOES WV imagery shows dry air beginning to infiltrate the region from the southwest, but models suggest the recent influx of moisture won't be completely displaced until mid to late next week. With that said you may have still felt the lingering moisture this morning as PWATs from the 12Z sounding were at 1.22" and dew points across the lower desserts this morning were in the upper 50s to lower 60s. This moisture has also been a key component in driving overnight lows to be in the upper 70s to mid 80s, or roughly 5F-10F degrees above normal for this time of year. Afternoon highs are in a similar boat at ~5F above normal as positive height anomalies are dominant across the Desert SW. This is in part of the broad ridging mentioned earlier but also a subtropical high to the region's SE has been building and slowly shifting westwards with H5 heights between 590-591dam. Unfortunately this subtropical high is positioned to far to our southeast, resulting in southwest flow aloft. This southwest flow will mean drier air continuing to be advected into the region over the next week, and afternoon highs will continue to be between 105F-110F. These temperatures will keep the region under Moderate to localized areas of Major HeatRisk.

With the subtropical high pushing further westwards the lifting mechanisms that came from the upper level low have also been displaced further westwards off the Baja coastline and subsidence will begin to take hold. Hope is not completely gone though, as storms can already be seen on satellite in the Northern AZ high terrain and far Southeast AZ. Though most of these storms are well outside our CWA an outflow could form and move into South-Central AZ by the late afternoon/evening. If this boundary does manifest it could be enough to trigger an isolated storm in the northern/ northeastern foothills as HREF MUCAPE in the afternoon ranges between 250-500 J/Kg, enough to sustain convective ascent. Conditions Sunday will be much of the same as CAMs suggest storm development mostly remaining in the Northern AZ high terrain and far Southeast AZ.

LONG TERM /MONDAY THROUGH FRIDAY/

Low level moisture will not be scoured out heading into next week, especially for the southeastern corner of the state, and so a continuation of diurnally driven, high terrain showers/storms looks likely. Activity will be reduced at times due to periods of stronger subsidence, one of which is captured as a stronger midlevel inversion in GFS bufr soundings during the Tuesday- Wednesday timeframe. This stronger anticyclonic subsidence is also expected to support temperatures a few degrees warmer, despite fairly stable H5 heights, resulting in a very modest expansion of Major HeatRisk across the western deserts. The pattern will change more noticeably by the end of the work week, as ensembles are pointing towards a stronger subtropical jet streak moving closer to and eventually onshore somewhere along California/Northern Baja (north/south discrepancies are apparent between the different ensemble suites). This will increase winds regionally as the subtropical jet imparts deep southwesterly flow, decrease heights aloft, and substantially scour out the low level moisture across the region. As a result, anticipate elevated fire weather concerns, especially for Southeast CA and Southwest AZ, temperatures moderating closer to daily normals, and the diurnally driven shower/storm chances over AZ high terrain to end.

AVIATION

Updated at 1110Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The potential for weak outflows this evening will be the main aviation question during the period. It appears now that the usual E'rly shift at KPHX has better potential of taking place, but a VRB group remains in the TAF to indicate lingering uncertainty. Outside of this window, W'rly flow will be common during the majority of the forecast. The only interruption to this pattern may be the presence of weak outflows, mentioned above, originating from distant convection. However, guidance continues to back away from this outcome, so NE'rly shifts at KDVT and KSDL have been replaced with typical evening VRB groups. The potential for an outflow is not zero, but they will highly dependent on convective location, therefore, amendments may be needed as we move through the day. KDVT and KSDL would be the most likely to observe any outflow switches. FEW-SCT mid-level clouds will be present but should gradually clear through today.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather concerns should exist through Saturday night under mostly clear skies. Confidence is good that a southerly wind component with occasional gustiness will be maintained at KBLH while directions oscillate between SSE and SW at KIPL. Some stronger 20-30kt sundowner gusts will be possible at KIPL Saturday evening, though confidence is only low to moderate.

FIRE WEATHER

Temperatures 3-8 degrees above normal will prevail with lower desert highs near 110F in some of the hotter spots through the middle of next week. Improved humidity levels will prevail through the weekend, with afternoon minimums generally between 15-20% and overnight recoveries as low as 25% in the far western districts and between 35-50% for most other areas. With the increasing moisture will also come slight chances for isolated shower and thunderstorm activity. Chances will become almost entirely confined to the high terrain of the eastern districts by this afternoon. Rainfall potential will be limited, so locally gusty outflow winds and dry lightning potentially leading to natural fire starts are the main concerns. Winds will generally remain terrain-driven outside the influence of thunderstorm outflows, with speeds at or below 15 mph and afternoon/early evening upslope and upvalley gusts up to 25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

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