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/

Southwesterly flow aloft is still apparent in midlevel water vapor imagery, as the region is sandwiched between troughing over a good chunk of the Western US and expansive ridging centered over the Southern Plains and Mississippi River Valley region. Mid and high level clouds continue to stream northeastward over the area, with some areas of embedded virga showers/sprinkles registering on early morning MRMS composite reflectivity. These will likely continue through the early morning hours until the influence of the southwesterly flow aloft diminishes. The forecast remains largely on track, with a pattern shift occurring by the middle of the week. Troughing over the Western US is shown retreating to the north, allowing the subtropical high to retrograde westward and settle almost directly overhead Wednesday. During this time, the subtropical high will quickly strengthen with H5 heights aloft briefly peaking near climatological maximum values for this time of year, around 596-597 dam. NAEFS/ENS mean 850 mb temperatures between the 80th-95th percentiles of climatology, around 26C-30C over the forecast area, will support afternoon highs around 5F above daily normals. Periods of mid to high clouds have been maintained overnight due to the continued southwesterly flow and are expected the next few nights due to decaying or distant upstream convective activity over Sonora/Southeastern AZ, which will prevent more efficient radiative cooling. Thus, morning lows are forecast to attain a 7F-10F above normal range, offering little relief from the daytime heat. These temperatures will translate to widespread Moderate HeatRisk with areas of Major HeatRisk across the typically hotter lower desert locales. As a result, Extreme Heat Warnings go into effect later this morning 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.

With the pattern change occurring by the middle of the week, flow aloft will shift southeasterly, and daily gulf surges are expected to reestablish. These factors will combine with vigorous upstream convection over Sonora to help push abundant low level moisture into the area, especially by Wednesday night. Mixing ratios will surge back above 10-12 g/kg across South-Central and even Southwest AZ, supportive of lower desert storms if there is a sufficient lifting mechanism. However, anticyclonic subsidence centered nearly directly overhead will act to suppress deep moist convection except over prominent terrain features of Southeastern and Eastern AZ through Wednesday. Latest HREF does indicate the potential for spotty elevated showers to develop tonight across portions of the forecast area, but this is likely conditional on sufficiently strong gravity waves propagating north and west from earlier strong storm clusters over Sonora.

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 0510Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No significant weather issues will exist through Tuesday night under periodic mid/high cigs. Winds will behave nearly identical to the past 24 hours including a later than usual overnight switch to easterly, as well as limited afternoon gusts.

Southeast California/Southwest Arizona including KIPL and KBLH: No weather issues will exist through Tuesday night under occasional mid/high cigs. Winds will generally favor a W/NW heading through the afternoon before switching to a southerly direction. Periods of nearly calm conditions will also be common.

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 from 10 AM this morning to 8 PM MST Thursday for AZZ531-532-535>544-546-548>551-559.

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

CA...Extreme Heat Warning from 10 AM this morning to 8 PM PDT Friday for CAZ562-566-567.

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