textproduct: NWS Phoenix

KEY MESSAGES

- Hot and muggy conditions will continue through mid week, particularly across southeast California and southwest Arizona where Extreme Heat Warnings are in effect through Thursday.

- Shower and thunderstorms chances will increase today across eastern and central Arizona with gusty winds and locally heavy rainfall possible.

- A disturbance will help to bring more widespread shower and thunderstorm activity on Thursday and Friday with the potential for heavy rainfall and localized flooding over eastern and central Arizona.

- Temperatures will cool later this week with below normal readings across much of Arizona to near normal for southeast California by Friday.

SHORT TERM /TODAY THROUGH FRIDAY/

Ample moisture over much of Arizona in combination with an increasingly favorable weather pattern is expected to drive active monsoon weather over the next few days. The subtropical ridge remains planted over the Northern Plains/Great Lakes extending westward into the Central Rockies. A broad area of negative height anomalies has also developed across the southern tier states and northern Mexico with multiple easterly waves from the Lower MS River Valley to just south of our region. This synoptic set up is a bit uncommon and doesn't happen every year, but it is likely to bring quite active monsoon weather into next week.

Current objective analysis is messy with a disturbance centered over Baja moving away from Arizona, a weak mid-level disturbance slowly moving southwestward through the Four Corners area, and a larger easterly wave over west Texas and eastern New Mexico. PWATs have improved to 1.5-1.75" over the lower deserts to 1.0-1.3" over the Arizona high terrain, or 125-150% of normal for mid July. As expected, yesterday was a quieter day after the large complex that upended the atmosphere profile on Monday. Weak, but persistent east northeasterly flow into Arizona will continue to advect additional moisture into our area over the next couple of days increasing PWATs to around 2" over south-central Arizona by Friday.

For the rest of this morning into the afternoon, modest destabilization is expected pushing MUCAPE to upwards of 750-1000 J/kg over the Arizona high terrain, but closer to 500 J/kg into the lower deserts. Hi-res guidance is fairly consistent showing widespread shower and thunderstorm development along the Mogollon Rim into the White Mtns and over southeast Arizona. A 5-10 kt northeasterly steering flow is likely to be enough to shift some of the storm activity toward the south-central Arizona lower deserts late afternoon/early evening but with overall low instability over the lower deserts the activity is likely to struggle. Outflow collisions driving additional storm development may occur with guidance pointing at this happening somewhere southwest of Phoenix, but in reality it could happen over Phoenix too. Forecast DCAPEs for the area have improved enough to worry about some gusty downdraft/outflow winds, but they should be sub- severe. Blowing dust should again be an issue for some areas with guidance favoring areas across southern Maricopa into Yuma/La Paz county later this evening. Storm movement looks to be fast enough to not have to worry about a flash flood threat much, but any training of storms could result in localized rainfall amounts of over an inch.

Overnight convection looks to be possible tonight, but it will most likely involve pockets of light to moderate shower activity before dissipating by around sunrise Thursday morning. The easterly wave over Texas and eastern New Mexico is forecast to shift farther westward Thursday into Friday, likely stalling out near El Paso. This disturbance is expected to help drive multiple rounds of showers and thunderstorms beginning Thursday afternoon and last through Friday. Guidance has backed off a bit on the strength of the upper level jet over New Mexico which will help to provide broad ascent over Arizona, but it should still be enough to bring more than the typical daily monsoon afternoon convection on Thursday and Friday. As moisture levels peak later Thursday into Friday, periods of moderate to heavy rainfall leading to localized flash flooding will become the primary weather concern. However, uncertainty is still high enough to hold off an any Flood Watches for the area. WPC currently shows a large Slight Risk area on their Day 2 and Day 3 Excessive Rainfall Outlooks, encompassing all but the southwestern quarter of Arizona. As PWATs push 2" starting Thursday, any deeper convection that may form will be capable of producing heavy rainfall rates of 2-3" per hour. This potential should be the highest Thursday afternoon through Thursday night, but it could linger through most of Friday if the weak to modest atmospheric instability can hold. We should have a better idea of details of this potential heavy rainfall over the next 12-24 hours as more hi-res model data becomes available.

Hot and humid conditions will continue to prevail over the western deserts through the rest of the workweek with the most oppressive conditions today and Thursday. Extreme Heat Warnings remain in effect for much of southeast California and the Lower CO River Valley with forecast highs 110-113 degrees today and 107-111 degrees Thursday. Adding in the high surface dew points in the 60s, afternoon heat indices of 113-118 degrees are expected both days. Temperatures will not be quite as bad across south-central Arizona with forecast highs of 104-108 degrees today in Phoenix before quickly dipping to below normal starting Thursday. Due to the periods of showers and thunderstorms in the area lasting through Friday, daytime highs Friday are only expected be in the 90s.

LONG TERM /SATURDAY THROUGH TUESDAY/

Moderate forecast uncertainty is still an issue for this weekend into next week as day-to-day shower and thunderstorm chances are likely to persist. Some slight drying over the weekend as the easterly wave to our east southeast gradually weakens is likely to bring lower rain chances compared to Thursday and Friday, but that does not mean it will not be active in some areas. As of right now, guidance is favoring the Arizona high terrain this weekend with daily PoPs of 50-70%. An easterly steering flow is also likely to persist which will enable at least some chances (20-30%) of showers and storms into the lower deserts this weekend. Lingering ascent also looks possible this weekend from the easterly wave, although this is more likely to benefit far southeast Arizona and northern Mexico.

Model uncertainty increases further during the first half of next week as guidance is shifting where the subtropical high center may be and whether or not another easterly wave will impact our region. There seems to at least be some agreement in gradually shifting the high center southwestward to somewhere over the Central Rockies or Central Plains Sunday into Monday, but where it goes thereafter is much more uncertain. The GEFS favors the high center settling anywhere from the Four Corners area to into central New Mexico, while the EPS shows it more over New Mexico into western Texas. The first half of next week is still likely to be fairly active as far as storm potential, and could even be very active if the easterly wave becomes a big factor. However, the latter half of next week may involve a decrease in monsoon activity if the high center truly moves closer to or into Arizona.

NBM temperature trends show below normal temperatures lasting through the weekend for much of Arizona and likely persisting but gradually warming through the first half of next week. Locations across southeast California and southwest Arizona should see temperatures right around normal with most days seeing little monsoon convection.

AVIATION

Updated at 1000Z.

Gusty outflow winds from distant thunderstorms, abrupt wind shifts, and the potential for direct TS impacts late this afternoon/early evening will be the main aviation weather concerns through the TAF period under FEW-SCT and at times BKN mid and high cloud decks. There remains a low chance of temporary VCSH conditions for the next few hours with minimal impact expected. Winds will follow fairly typical diurnal tendencies through early this afternoon with speeds under 10 kts. Thereafter, thunderstorms developing over the higher terrain areas to the north and east of Phoenix will likely migrate southwestward along with a surging outflow boundary late in the afternoon. Gusts with this initial boundary may reach upwards of 20- 25 kts. Coverage of thunderstorms within the vicinity of the terminals is still highly uncertain and thus have maintained PROB30 groups. Any thunderstorms that do form near the terminals could cause gusty and erratic winds, with 50% probabilities of gusts over 30 kts. Additional outflows from the south may then reach the terminals during the evening, causing an abrupt wind shift and bringing another period of gusty winds.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation weather concerns are expected through at least Wednesday afternoon, with the potential of an outflow moving in Wednesday evening. At KIPL, winds will primarily stay southeasterly with the possibility of a temporary SW switch during the early evening. At KBLH, winds will generally fluctuate between SW and SSE. Overall, wind speeds should generally remain aob 12 kts. There are signals that a potential southerly outflow producing some gusty winds from remnant convective activity across AZ may move in later this evening.FEW-SCT mid to high clouds will prevail throughout the period.

FIRE WEATHER

Active monsoon weather with elevated moisture levels will continue through the rest of the week. Shower and thunderstorm chances will increase over the eastern districts today through Thursday with only isolated chances over the western districts. Heavy rainfall will also be possible across the eastern districts by Thursday afternoon. MinRHs will improve from east to west over the next few days, increasing from 20-35% today to 30-50% by Friday. Outside of potential thunderstorm outflows, winds for the eastern districts will tend to be light and somewhat favor diurnal trends. Winds will predominately favor the south while becoming breezy starting Thursday across the western districts.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Thursday for AZZ530-532.

CA...Extreme Heat Warning until 8 PM PDT Thursday for CAZ562-563- 565>567-569-570.