textproduct: NWS Phoenix

UPDATE

Updated Aviation

KEY MESSAGES

- High pressure will center itself over Arizona and strengthen late this week resulting in hotter temperatures and widespread major to extreme Heat Risk.

- Extreme Heat Warnings have been expanded to cover the entire forecast area starting Thursday, and continuing through the weekend.

- Daily chances for showers and thunderstorms will continue over eastern Arizona high terrain, but will decrease considerably and become almost entirely confined to the most prominent terrain features the remainder of the week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

A couple MCVs tracked northward through Gila/Pinal/Maricopa counties this morning providing forcing for showers and releasing elevated instability of up to 1000-1500 J/kg (backed up by earlier RAP analysis and the 12Z PSR sounding) to produce isolated thunderstorms and locally heavy rainfall for some. While not all saw rainfall, portions of the East Valley received anywhere from a few hundredths of an inch up to around 0.3" from this morning's activity. Further east in Southern Gila County, automated gauges that were lucky enough to have convective cells track overhead recorded upwards of 0.25" to as much as 1.0" in some spots. Rain-cooled, humid air overspread much of South-Central AZ this morning, dropping morning lows to a few degrees below the daily normals, ranging from the upper 70s to middle 80s and increasing dewpoint temperatures to between the middle 60s and lower 70s. With cooler temperatures to start the day and delayed surface heating, expect daytime highs several degrees cooler than originally forecast, likely reaching near normal values (105F-107F in the Phoenix Metro) across South- Central AZ and a portion of Southwest AZ, whereas temperatures will remain several degrees above normal for the western deserts.

The forecast area remains under easterly to southeasterly flow aloft on the western periphery of a now weaker subtropical high centered approximately over Northern NM at this hour. Current 500 mb RAP analysis and midlevel water vapor imagery suggests a subtle easterly wave and yet another vort max or MCV feature just east of the northern tip of the Gulf of California. These will pass into Western AZ and be pulled northward today thanks to the anticyclonic flow about the subtropical high. As we are now on the backside of a few MCVs and the aforementioned easterly wave, subsidence seems to be favored over the region and at the very least over South- Central AZ. Latest GFS bufr soundings for KPHX display a midlevel inversion this afternoon, indicative of this subsidence, and poor 850-700 mb lapse rates can be seen in early afternoon PHX ACARS soundings, putting a considerable cap on the local environment. However, this cap will erode through the remainder of the afternoon, and CAMs do indicate the potential for thunderstorms to form over some of the typically favored terrain such as the Tabletop Mountain Wilderness and the Rim/far eastern Gila County. These could produce locally gusty winds and heavy rainfall. Wind shear is also still present, with bulk effective shear upwards of 25-35 kts over the southeastern third of the state. While storm coverage is very uncertain and likely to be quite low based on latest CAMs, any that do form could develop rotating updrafts and be capable of severe wind gusts. HREF consensus is that the threat is greater towards the AZ/NM border in the southeastern corner of the state, where the cap will erode quicker and allow surface based convection to develop late this afternoon. A few severe- warned cells have already developed over the White Mountains.

This evening/overnight tonight, less evidence exists that an upstream MCV will form and track northward over the forecast area, however, the possibility cannot be entirely ruled out. It is highly dependent on thunderstorm coverage and intensity upstream over Sonora or the southeastern corner of the state. CAMs do seem to be picking up on the influence of another axis of vorticity being pulled northward around the anticyclone and providing enough lift ahead of it to produce showers or even an isolated storm over Southwest AZ tonight. On Wednesday, ensembles remain in good agreement that flow aloft will shift southerly to even a slightly southwesterly orientation, starting a W-E drying trend. By late Wednesday, lower desert near-surface mixing ratios drop below 10 g/kg, essentially ending lower desert storm chances. Daily thunderstorm activity across the region will diminish and remain diminished heading through the rest of the work week. Ensemble mean H5 heights will continue in a 594-596 dam range, between the 90th and 95th percentile of CFSR climatology, supporting above normal highs around 109F-113F for the typically hotter spots Wednesday.

LONG TERM /THURSDAY THROUGH MONDAY/

Ensembles remain in good agreement that the subtropical high over NM will retrograde into AZ and strengthen, especially by Friday- Saturday, with mean H5 heights building to a 599-601 dam range, near or just above CFSR climatological maximum values. NAEFS/ENS mean 850 mb temperatures will also approach climatological maximum values around 32C-35C. Under drier conditions, much clearer skies thanks to reduced thunderstorm activity, and strong anticyclonic subsidence, heights/temperatures aloft in this range will support afternoon surface temperatures peaking between 115F-120F, with values on the higher end of that range favored in Southeast CA. Saturday is shaping up to be the hottest day, with NBM advertising 25-40% probabilities of reaching 120+F in some of the hottest spots across Southeast CA and along the lower Colorado River Valley. Overnight lows are also forecast to warm into the upper 80s to lower 90s, providing very little relief from the heat. Coverage of Major HeatRisk builds starting Thursday and is widespread Friday-Sunday, with areas of Extreme HeatRisk indicated in those areas where overnight lows do not cool below 90F. As such, widespread Extreme Heat Warnings go into effect for the entire forecast area Thursday and will continue through the weekend.

Long range global guidance continues to indicate moisture increasing across the area as the subtropical high weakens late this weekend into the following week. However, details are very uncertain this far out. Current consensus of the GEFS and ENS is for the northern stream to impinge further south, flattening the subtropical ridge. meanwhile, flow aloft on the southern periphery of the subtropical anticyclone strengthens and turns easterly to northeasterly, exposing the region to the possibility of easterly waves and better shear/steering flow to carry storms off Eastern/Southeastern AZ terrain features. Regardless of the eventual increase in moisture and thunderstorm activity, this will also help end the extended period of extreme heat conditions by early next week.

AVIATION

Updated at 2300Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major weather issues will exist through Wednesday evening under a few passing mid/high cloud decks. There is uncertainty in timing and duration of the typical easterly wind shift late tonight, with the majority of guidance suggesting a very late switch and only a few hours in duration. There is a low probability that winds never make the easterly switch at KPHX. Otherwise, confidence is good that TS will remain relegated to mountains with very low chances of distant outflows surviving into lower elevations.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues will exist through Wednesday evening under occasional passing mid/high clouds. Trends in winds will be similar to the past 24 hours favoring a southerly component at KBLH, and varying between SE and SW at KIPL. A few gusts 15-20kt will be common late afternoon/early evening. While a few SHRA may pass through the area overnight, chances for direct impact to any terminal is too low to mention in this TAF package.

FIRE WEATHER

Fairly typical monsoon conditions remain in place through tonight before more notable drying occurs from west to east on Wednesday. Low chances (10-20% at most) for scattered showers and even some isolated storms remain possible this evening and overnight for portions of Southwest and South-Central AZ, with gusty, erratic winds and locally heavy rainfall the main concerns. Near to slightly above normal temperatures will continue through mid week before even hotter temperatures arrive late week into the weekend. Humidities will decrease through the rest of the week, with afternoon minimums dropping into a 7-15% range across the western districts and 12-20% range across the eastern districts, following poor to fair overnight recoveries starting Wednesday night. Winds outside of thunderstorm outflows will follow fairly typical trends with upslope/upvalley afternoon and early evening gusts into a 20-25 mph range common. As the drying occurs during the latter half of the week, locally elevated fire weather conditions are likely to develop, with low afternoon humidities and the typical upslope/upvalley breeziness.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530-532-537- 540-542>544-546-548-550-551.

Extreme Heat Warning from 10 AM Thursday to 8 PM MST Sunday for AZZ531-533>536-538-539-541-545-547-549-552>563.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ567-569-570.

Extreme Heat Warning from 10 AM Thursday to 8 PM PDT Sunday for CAZ560>566-568.