textproduct: NWS Phoenix

KEY MESSAGES

- Above normal temperatures will prevail through the rest of the work week, resulting in widespread Moderate Heat Risk and areas of Major Heat Risk.

- Extreme Heat Warnings will be in effect for portions of the lower deserts through the end of the work week.

- Thunderstorm coverage is expected to increase during the latter half of the week across Eastern and Central Arizona, with slight chances becoming introduced for the South Central Arizona lower deserts by Thursday.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Not much has changed in the current upper atmosphere conditions over the past 24 hours with the region remaining between a large troughing feature over much of Western CONUS, and an expansive ridging over the Southern Plains and Mississippi River Valley region. Mid to high level clouds continue to stream northeastwards through the late morning hours due to southwesterly flow aloft. However the flow will begin shifting out of the southeast as the subtropical high moves westwards towards the Four Corners region starting this afternoon. The subtropical high is expected to peak Wednesday, with H5 heights quickly building to 596-597 dam, near to at NAEFS climatological max for this time of year. This will help drive afternoon high temperatures across the lower deserts around 5F above normal for this time of year. Additionally, while southwesterly flow usually advects drier air into the region, Pacific tropical moisture was able to be pulled in keeping good moisture in the mid to upper atmosphere. This has lead to overnight cloud coverage hindering radiative cooling and PWATs have remained between 130-150% of normal, with morning dew points consistently reading in the mid 50s to low 60s. Due to these factors the overnight low temperatures are in the mid to upper 80s, or near 7F- 10F above normal, and providing very little relief in the heat throughout this week. Major HeatRisk has resulted across a large portion of the lower deserts and an Extreme Heat Warning is in effect for the Phoenix Metro, portions of South-Central and Southwest AZ including the Yuma area, and portions of Imperial County including the Imperial Valley. Extreme Heat Warnings expand slightly further north Wednesday along the Lower CO River Valley and surrounding communities.

As mentioned earlier the subtropical high will begin shifting winds aloft out of the southeast starting today and will continue through the week, with daily gulf surges likely. A notable increase to 1000- 700 mb mixing ratios will occur Wednesday night quickly increasing to 10-12 g/kg across SC AZ and even into the lower western deserts starting wednesday. Unfortunately, with the subtropical high expected to be directly over head, strong anticyclonic subsidence storms will struggle to develop in the low deserts, although not impossible. HREF models continue to show between a 30-50% chance of storms developing in southeastern AZ, and chances expand northwards into SC AZ, even into southern Pinal County by Late Wednesday, but with the ridge overhead it will remain conditional.

LONG TERM /THURSDAY THROUGH MONDAY/

Late this week is shaping up to be a more active period for monsoon thunderstorms across at least South-Central AZ. By Thursday, anticyclonic subsidence will begin to diminish as the subtropical high weakens overhead, and abundant low level moisture will remain in place through the rest of the week and linger into the weekend. Latest NBM guidance seems to favor Friday with the best chances for lower desert thunderstorms, which makes sense given the weaker subsidence aloft and plenty of moisture and instability to support numerous thunderstorms over the high terrain by the afternoon, sending outflows into the lower deserts and triggering additional thunderstorms by the evening. GFS bufr soundings indicate a good balance between CAPE and DCAPE Thursday evening, with values between 1000-1500 J/kg, which is typically a good indicator for severe weather potential over the Valley. However, CIN may be a bit stronger than preferred, and the development of thunderstorms that can utilize this environment will be entirely dependent on the presence and timing of a sufficient lifting mechanism (i.e., thunderstorm outflow boundary). This conditional threat continues Friday, but with perhaps lower DCAPE and shallower inverted-V soundings. Strong thunderstorm winds, dense blowing dust, locally heavy rainfall resulting in instances of flash flooding, and frequent lightning will all be on the table for thunderstorm hazards later this week.

The forecast for this weekend into early next week is much less certain, however, a few more details are coming into focus tonight compared to the guidance available in previous forecast updates. Ensembles are in better agreement that mean troughing will establish over the Northwest US, pushing the northern stream and likely a series of upper lows and their attendant jet streaks as far south as the Great Basin. WPC cluster analysis reveals discrepancies on how far south the mean troughing will establish and how strongly negative the midlevel height anomalies will become over the Northwest US. The current consensus is for flow aloft to shift south to southwesterly as a result, meaning the more typical monsoonal pattern which briefly sets up this week will be short-lived, becoming something more akin to a transition season setup. This does not, however, mean that thunderstorm chances leave the forecast. Drying will begin to occur from west to east, but low level moisture will not be scoured out from Southern AZ. Thunderstorms will likely continue to favor development over prominent terrain features, and lower desert storm chances will be dependent on outflows or a nearly uncapped environment. As for now, NBM/WPC PoPs have come in lower this weekend compared to Thu/Fri, and timing of the better chances seems a bit unrealistic, reflecting the high uncertainty and nebulous forcing mechanisms. Regardless, the main message for this weekend into early next week will be continued thunderstorm chances over at least South-Central AZ higher terrain areas, but likely a decrease in activity from Thu/Fri due to the pattern shift and until southeasterly flow returns.

With the subtropical high diminished and then displaced by Northwest US troughing this weekend, temperatures will follow a downward trend, attaining near normal values likely by Saturday or Sunday, ending the extreme heat conditions.

AVIATION

Updated at 1819Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No significant weather impacts expected through the TAF period under periodic mid/high cigs. Light E/SE winds will veer to the W/SW by around 20-21Z at all terminals. A gulf surge tonight around midnight may briefly increase W/SW winds to 10-14 kts. Diurnal E winds will redevelop toward 10-12Z Wednesday followed by an early W/SW shift around 16Z. A few high-based showers cannot be ruled out around midnight tonight.

Southeast California/Southwest Arizona including KIPL and KBLH: No significant weather impacts are expected through the TAF period at both terminals under occasional mid/high clouds this afternoon and evening and then SCT to BKN mid level clouds tonight after midnight. A few high-based showers and virga may also pass through the area after midnight through Wednesday morning, but confidence is too low to put any mention in the TAFs. Winds will be light through the TAF period, mostly below 8 kts, and KBLH will favor a N/NW direction through this afternoon and then slowly shift CCW to a S direction late tonight. Wind directions at KIPL will favor an E/SE component through this afternoon, followed by a period of W/SW this evening - early tonight. There will be storms along the mountains west of KIPL this afternoon, which may push an outflow boundary through the terminal, but the storms are expected to stay in the mountains.

FIRE WEATHER

Above normal temperatures will prevail through the end of the work week, then temperatures are likely to decrease closer to normal over the weekend. Afternoon minimum humidities will generally range between 18-30% areawide through much of the week following fair to good overnight recoveries between 35-70%. Fairly typical diurnal winds with occasional afternoon upslope breeziness can be expected through Thursday, followed by an increase in southerly to southwesterly winds Friday into this weekend. Rain and thunderstorm chances will remain low and mostly limited to the eastern Arizona high terrain through midweek before slight chances become introduced to the lower deserts of at least the eastern districts during the latter half of the week.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ532.

Extreme Heat Warning until 8 PM MST Thursday for AZZ531-535>544- 546-548>551-559.

Extreme Heat Warning from 10 AM Wednesday to 8 PM MST Friday for AZZ530-533.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ562-566-567.

Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Friday for CAZ569-570.