textproduct: NWS Phoenix

UPDATE

00Z Aviation Discussion

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Saturday with areas of Major to Extreme Heat Risk, despite temperatures slowly decreasing.

- Today and Friday will bring low end chances for showers and a few thunderstorms, focused more across southern and central Arizona.

- A pattern change this weekend will bring much better chances for rain, including including a chance of strong to severe storms across Southern Arizona mainly on Sunday or Monday.

SHORT TERM /TODAY THROUGH FRIDAY/

A broad area of anticyclonic flow aloft remains entrenched over the Desert Southwest, with current RAP analyzed H5 heights near 596-597 dam (values around the 99th percentile of climatology for this time of year) centered over eastern AZ/western NM. A gulf surge combined with upper level moisture advection from the remnants of a TC well offshore Northern Baja/SoCal has increased atmospheric moisture over the forecast area, but only modestly. The 12Z PSR sounding this morning indicated PWAT around 1.26", up from 1.07" at that same time yesterday. However, boundary layer- averaged mixing ratios remain unfavorable for any deep moist convection over the lower deserts today, with guidance keeping values around 8 g/kg at most through the remainder of the day and into this evening. Strong anticyclonic subsidence will also help to keep thunderstorms anchored mostly to terrain this afternoon into the early evening. Clusters of strong storms along the international border/far Southern AZ will be capable of sending a long lived outflow towards South-Central AZ lower deserts this evening, potentially strong enough to kick up dust in some of the more prone areas of Pinal County or Southern Maricopa County. This outflow, coupled with perhaps gravity waves from earlier, more intense convection over Sonora, may be able to trigger elevated showers late this evening into the overnight hours.

As we head into Friday, ensembles agree that the subtropical high and associated anticyclonic flow will begin to be displaced eastward as upper level troughing draws near/over the West Coast. During this time, the forecast area will be split between southeasterly flow aloft to its east and drier southwesterly flow to the west/northwest. If it weren't for this split flow with troughing impinging to the northwest, better quality moisture may be able to surge into the area for Friday. However, current models show only marginally better moisture in the low levels Friday (8-10 g/kg mixing ratios at most), resulting in continued low end storm chances focused mostly over higher terrain areas. A very low probability of strong to severe winds gusts from isolated convection over prominent terrain of Central to Northeastern La Paz County and from decaying storms moving off the Baja Spine into Southern Imperial County exists late Friday afternoon.

Aside from the low end storm chances through the end of the work week, the main story will be continued extreme heat conditions. NAEFS/ENS mean 850 mb temperatures above the 98th-99th percentile of climatology, peaking between 29C-33C, will support afternoon highs above 110F across the typically hotter lower desert locales. This translates to values between 5F-10F above daily normals. Increased cloud cover the next couple days will also prevent more efficient radiative cooling overnight, resulting in much above normal morning lows. In fact, the temperature at Phoenix Sky Harbor Airport never cooled below 95F this morning, a new record warm low if it holds through midnight and 4F above the previous record warm low.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

A significant pattern change is set to begin this weekend with a large Pacific trough slowly moving southeastward across the Western U.S., eventually pushing the subtropical high into the Southern Plains by Sunday. Our main forecast concern with this change will be the increasingly favorable conditions for thunderstorms, especially by Sunday and Monday. The influence of the trough may occur as early as late Friday night and early Saturday as a relatively strong jet streak moves through California into the Great Basin. This should result in increasing upper level support for showers and thunderstorms, but we will also have to have enough moisture. For now, guidance is unclear on the rain chances for early Saturday, but the NBM is trying to put a 30-40% chance in after midnight. The daytime hours Saturday are just as unclear as moisture is likely to continue to improve, but will it be enough to overcome a modest capping inversion. Guidance does show PoPs of 30-40% during the evening hours Saturday from Phoenix and areas to the north and east of Phoenix, but little chance to the west. Temperatures Saturday will cool a bit from Friday, but there should still be enough residual heat to push daytime highs to around 110 degrees across the western lower deserts to 107-109 degrees in the Phoenix area.

Sunday into possibly Monday very well could be quite active as guidance is pointing at very good moisture advection into Arizona, somewhat directed by a passing tropical system to the southwest of southern Baja. Atmospheric parameters improve on Sunday with MUCAPEs anywhere from 1000-2000 J/kg and little if any CIN during the afternoon hours. The presence of the trough over or just to the northwest of our area should also provide some modest shear and upper level support for storm potential on Sunday and/or Monday. Depending on the evolution of the trough, heating, and how much moisture gets pulled into our area, we may need to worry about a strong to severe storm potential as well as a heavy rainfall threat as PWATs may reach near 1.75". Drier air will also be trying to work into western portions of our area by Sunday, leaving little chance for rain over southeast California and potentially southwest Arizona. NBM PoPs reflect this with at most 10-20% chances over the western deserts, but PoPs rise to 40-60% from Phoenix and areas to the north and east of Phoenix on Sunday. There is much higher model uncertainty going into the mid to latter half of next week as GEFS favors moisture staying mostly in place while the EPS favors some more significant drying. Part of this uncertainty is due to a highly active tropic region to our southwest which very well could influence our region at times.

Temperatures are at least slated to drop a good amount over the weekend into early next week with forecast highs falling back to near normal on Sunday and then at or just below the normal range early next week. We can't even rule out having highs in the 90s for a day across south-central Arizona if the rain activity is widespread enough on Sunday and/or Monday.

AVIATION

Updated at 0030Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

The main aviation concern during the TAF period will be gusty outflow winds this evening. Timing of the boundary looks to be between 004-06Z with a S/SE'rly component being the most favorable outcome at this time. With this orientation favored, blowing dust will also be an operational concern, but forecast uncertainty regarding the strength of the outflow remains high and therefore no significant reductions in VIS have been included in most TAFs, with the exception being KIWA. However, if near-future forecast trends favor stronger winds, amendments to VIS at the other terminals may be needed. VCSH, with perhaps isolated TSRA may develop tonight/early Friday morning, but direct impacts at the terminals will be minimal outside of some potentially erratic winds at times if activity is close enough to the sites. Outside of all this, winds will favor light and diurnal trends with some periods of VRB conditions.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected through the next 24 hours. Winds at KIPL will favor an E/SE component throughout much of the period, while winds at KBLH favor the S to SSW. VRB conditions should also be anticipated, especially during the overnight hours. SCT-BKN mid/high clouds will persist throughout the period.

FIRE WEATHER

Hot and relatively dry conditions will continue into this weekend, with low end chances for isolated to widely scattered showers and thunderstorms. Chances then increase Friday night into Saturday, especially across Southern and Central Arizona. Afternoon minimum humidities will improve slightly to 15-20% Friday and then 17-25% Saturday further over the weekend as rain chances increase over Arizona. Outside the influence of distant thunderstorm outflows, winds will be relatively light and follow diurnal tendencies through Friday, with modest upslope/upvalley gusts in the afternoon. Southerly to southwesterly winds then increase Saturday, particularly across the western deserts, where afternoon gusts increase up to 20-30 mph.

CLIMATE

Record highs:

Date Phoenix Yuma El Centro ---- ------- ---- --------- Aug 27 114 in 2023 120 in 1981 117 in 1981 Aug 28 117 in 2023 118 in 2023 117 in 2009 Aug 29 116 in 2023 117 in 1954 117 in 2005

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Saturday for AZZ530>555-559- 560.

Extreme Heat Warning until 8 PM MST Friday for AZZ556-561-562.

CA...Extreme Heat Warning until 8 PM PDT Saturday for CAZ562-565>567- 569-570.

Extreme Heat Warning until 8 PM PDT Friday for CAZ561-563-564- 568.