textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Hot and muggy conditions will continue today, however temperatures will cool into a near to below normal range by the end of the week.

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

- Thunderstorms may become more widespread Thursday and Friday with the potential for heavy rainfall and flash flooding.

SHORT TERM /Today through Friday/

Expansive ridging aloft covers much of the Conus this afternoon under a quasi-rex block pattern featuring a distinct easterly cutoff low over central Texas bounded by high pressure arcing from the mid- Atlantic to the northern plains, then into the Southwest. This places the forecast area under deep easterly flow subject to shortwave energy passing from the aforementioned Texas cutoff towards the southern periphery of the high pressure ridge situated over Arizona. This general pattern is typical for mid/late July, albeit more amplified with stronger anomaly centers than is typical, resulting in more impressive forcing mechanisms around the region. Meanwhile, this synoptic pattern has allowed ample moisture to stream into much of the CWA with boundary layer mixing ratios in a favorable 10-12 g/kg range while total column PWATs exceed 1.50" setting the stage for heavy rainfall potential with any slow moving or training thunderstorms.

With the local environment recovering from convection Sunday and Monday, the forecast area is situated in a fairly favorable location for sustained thunderstorms with modest stretching and divergence at jet level between well defined easterly waves/cutoffs over Texas and western Mexico and a SW jet over the Great Basin. Forecast BUFR soundings for this afternoon/evening suggest MLCape 500-750 J/kg materializing after daytime heating and coincident with minimal CinH. While the moistening of the boundary layer has reduced DCape somewhat, values in excess of 1200 J/kg still appear probable indicating robust outflows likely producing very gusty winds (albeit largely sub-severe), blowing dust, and sufficient lift capable of initiating new storms. The minimal inhibition may allow new storms to form along a single outflow, however colliding outflows will be more beneficial, and the preponderance of model output indicates these collisions focused over western Maricopa County this evening. Forward storm motion and influence from rapidly propagating outflows should restrict the flash flood threat this evening, though congealing storms could produce a localized threat.

The synoptic pattern will become even more conducive for robust thunderstorms Thursday as at least one shortwave shed from the aforementioned Texas cutoff progresses towards south-central Arizona during peak heating. With convective debris and outflows pumping additional moisture into the middle and lower troposphere, total column PWATs 1.75-2.00" should be common while boundary layer mixing ratios settle around 12-13 g/kg by Thursday afternoon. This moisture profile should result in a near equal MLCape/DCape balance in a "sweet spot" of 1000-1500 J/kg historically present during big monsoon thunderstorm events. Convective inhibition should be nearly absent late afternoon/early evening, and with underlying deep layer ascent, it should only take weak outflows to initiate additional storms. Intersecting outflow boundaries are also likely with consolidating storms over parts of the CWA resulting in heavy rainfall and flash flooding. Rainfall rates at least 2-3"/hr appear likely given the moisture profiles, and a Flash Flood Watch has been issued for south- central Arizona counties. At this point, HREF and global model output suggests northern Pinal and SW Maricopa counties as the focus for the heaviest rainfall.

Considerable forecast uncertainty exists Friday contingent upon the strength and expanse of thunderstorms Thursday night. A large subset of modeling indicates the atmosphere becoming completely overturned by storms and outflows Friday morning with forecast soundings depicting a classic, post-event moist adiabatic profile. Should this come to fruition (better than a 50% chance), convective potential the remainder of Friday would become minimal, at best, given the combined extensive cloud cover and poor lapse rates resulting in essentially no instability. A slight alternative outcome is hinted at by recent operational ECMWF and NAMNest output which suggests a well defined MCV juxtaposed with a pool of higher theta-e advecting north through the Phoenix metro Friday morning. While these type model forecasts with morning storms forced by an MCV and copious deep moisture are common during the monsoon, they only come to fruition occasionally every summer, but when this scenario unfolds, very efficient rain rates and flooding ensues. Regardless, the strongest model evidence indicates only small pockets of instability remaining Friday afternoon, and have cut mandated NBM POPs substantially as even the most aggressive modeling that always forecasts storms has little convective response Friday afternoon/evening.

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 0015Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty outflow winds are expected to be the primary aviation impact this evening, with low potential (20-30%) for VCTS/TSRA. An outflow from the north and from the east may move through, with the easterly outflow expected to be the stronger of the outflows. Most probable timing for outflows and VCTS/TSRA is between 01-05Z, with diminishing activity and wind magnitudes after 05Z. Some high- based showers may move into the Phoenix area early Thursday morning, but probability is too low to include in the TAF. Odds of VCTS/TS are higher Thursday evening/night, with potential for multiple outflow passages and wind shifts, as well as very heavy rainfall. Confidence in wind directions this evening and tonight is low to moderate, with an initial N and/or E shift initially. E winds may then prevail through the night, with some light variability, but a SW wind may also develop, dependent on rain activity southwest of the Phoenix area. Cloud bases will mostly stay at or above 8-10K ft AGL. The dust potential is low.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. SE winds will be common at KIPL, though a brief switch of the W/SW this evening can't be ruled out. S/SW'rly winds will prevail at KBLH with a brief window of gusts around 20-25 kt overnight and again Thursday afternoon. Mostly clear skies will give way to increasing mid-level clouds tonight into Thursday morning.

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.

Flood Watch from Thursday afternoon through late Thursday night for AZZ534-537>563.

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