textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion

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/

In the northern stream, two features will be present and nearly stationary over the next week: upper level ridging off the West Coast and longwave troughing downstream over the North-Central CONUS. While these features will have some influence on the local conditions, the more relevant feature will be a subtropical high and associated anticyclonic flow to the southeast of the region. Abundant moisture has overspread the region under a period of southerly flow, with surface dewpoint temperatures ranging from the upper 40s to lower 60s and current RAP analyzed PWATs upwards of 1.0- 1.3" over the forecast area, representing values around the 99th percentile of climatology for mid June. However, midlevel water vapor imagery and 500 mb streamline analysis seem to indicate the flow aloft already shifting out of the southwest as an upper low off the Baja shifts further offshore and the subtropical high asserts itself further west. With this shift, the area of better synoptic ascent has also shifted away from the forecast area. Low and mid level moisture will remain in place across much of the state through the weekend and even linger into next week, but the lift necessary for showers and storms will mostly have to come from diurnal upslope convergence/orographic influences going forward, relegating the best chances for showers and storms to the Northern AZ high terrain and portions of far Southeast AZ near the international border.

Latest HREF membership supports the evolution described, with convective showers and thunderstorms developing over the Northern AZ high terrain by late this morning and far Southeast AZ this afternoon. Mean MUCAPEs upwards of 250 J/kg to as much as 750 J/kg and inverted-V soundings will mean that locally strong, gusty winds are a concern with any stronger storms that develop, however, they will be almost entirely outside the CWA. It is conceivable (and some CAMs do support this) that an organized outflow could form and accelerate downhill into South-Central AZ by the late afternoon/evening, providing enough lift to trigger a thunderstorm or two over the northern/northeastern foothills of Maricopa County. On Sunday, a similar scenario may play out, but with a few notable differences: higher instability (CAPE peaking between 500-1000 J/kg), but drier midlevels thanks to the sustained west southwest flow aloft through the weekend. This drier air aloft will entrain within updrafts, effectively reducing the intensity of convection from what measures of instability might suggest. There is some inherent uncertainty here to keep aware of through this weekend; enough moisture and instability will be present for showers and storms, but synoptic ascent mechanisms are largely absent. Subtle lifting mechanisms (e.g., a distant outflow from earlier Northern AZ convection, or gravity waves releasing elevated instability in the overnight/morning periods) and additional moisture influxes (strong, distant, moisture-laden outflows from vigorous convection in Sonora) will play a key role in what activity we actually see. We cannot rule out showers and storms, though current PoPs and thunder probabilities may be too low to appear in point and click forecasts.

Lower deserts highs will remain in a 103F-110F range, perhaps a degree or two warmer Sunday than today, as the subtropical high continues to inch closer to the region. Current H5 heights are sitting between 589-590 dam and expected to climb slightly by Sunday heading into next week to near 590-591 dam. Overnight low temperatures will also be above normal over the weekend, by 5F-8F, putting overnight lows in the upper 70s to mid 80s across the lower desert areas. This has lead to widespread Moderate to localized Major HeatRisk. With the absence of much overnight/ early morning relief from the heat ensure to stay hydrated and follow heat safety precautions.

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.