textproduct: NWS Phoenix

KEY MESSAGES

- Temperatures should remain in a near to below normal category for the first week of September, resulting in widespread Minor Heat Risk and localized areas of Moderate Heat Risk.

- Drier conditions will prevail through the middle of the work week before potentially another period of wet weather materializes late week into the weekend.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Another strong W-E gradient in surface dewpoints was noted this morning in the vicinity of the CO River Valley, with dewpoint temperatures in the 40s across much of Southeast CA and 60s to lower 70s across Southwest AZ. The higher dewpoints across our eastern CWA are indicative of the better low level moisture that remains in place, however, the 12Z PSR sounding reveals a very dry layer around/just above 500 mb, thanks to the sustained southwesterly to westerly flow aloft. With daytime heating, anticipate further drying of the low levels as the drier air aloft mixes down into the boundary layer. However, there will be enough moisture to support isolated deep moist convection over prominent terrain features of Southwest AZ (primarily Central/Northeastern La Paz County) and perhaps the Tabletop Mountains of Southwestern Pinal/Southern Maricopa Counties. The intensity of convection will be reduced compared to yesterday, with a reduction in instability and the addition of subsidence, which is evidenced by a strong midlevel inversion present in GFS bufr soundings and the 12Z PSR sounding. Differential heating across the quasi dry-line in Southwest AZ may provide a focus for convective initiation as it progresses eastward this afternoon. This, coupled with lingering effective bulk shear around 25-30 kts presents the possibility for some of the La Paz County storms to organize and become strong. A very isolated risk for a severe thunderstorm or two cannot be entirely ruled out. Fields of cumulus are already visible on satellite imagery early this afternoon, stretching from far Southeast CA through the CO River Valley and into La Paz County. We will be keeping a close eye on this area for further development.

Ensembles remain in excellent agreement that southwesterly flow aloft with deep Pacific troughing along the West Coast will be maintained through at least the middle of the work week, keeping the subtropical high suppressed and displaced well east of the region. In fact, a compact PV anomaly/area of strongly negative midlevel height anomalies has dived southward and can be seen in midlevel wv imagery early this afternoon starting to move onshore along the NorCal Coast, resulting in a restrengthening of southwesterly flow aloft and further drying through Wednesday. This will lead to another quiet day across our forecast area with any isolated convection relegated to the higher terrain of northern and southeastern AZ. This will also lead to an uptick in daytime winds, particularly across Southeast CA/Southwest AZ. Ensemble mean H5 heights will remain in a seasonable range, around 587-590 dam. Mean 850 mb temperatures between 22C-26C (between the 20th-60th percentiles of climatology) will support surface temperatures within a few degrees of the daily normals through Thursday, with lower desert highs generally ranging from the upper 90s to around 105F, following morning lows from the upper 70s to middle 80s.

LONG TERM /THURSDAY THROUGH SUNDAY/

The forecast becomes a bit more muddy heading into the end of this week and through the weekend. The NHC shows Tropical Storm Marie has formed well of the coast of Southwestern Mexico. Most forecast models show this storm curving out into open waters by this weekend, however there is good evidence that at least some tropical moisture will get pulled up into Mexico and making it as far north as southern AZ as early as Thursday/Thursday night. Roughly 90% of the GEFS and EPS members show PWATs increasing from 1.5" on Thursday to around 1.8-2.0" over portions of southcentral and southwest AZ by Friday or Saturday. Given this likely increase in moisture, NBM PoPs have increased to around 40-50% with the initial moisture push late Thursday into Friday. Thereafter, PoPs remain quite low in a 20-30% range through the weekend which is most likely due to uncertainty in timing and coverage of rainfall within the deterministic model output.

Although there will be adequate moisture return, the positioning of the ridge/trough features over the west will play a key role in how much rainfall may actually materialize. If the ridge strengthens too much and retrogrades wwd, then the focus for heavier rainfall may be much further west into SoCal and north into UT and CO as indicated by the latest ECMWF and GFS runs. It will likely be at least 2 or 3 more days until we have a good idea on what will happen late this week, and any forecast that does not show a range of possible outcomes should be disregarded at this time.

Uncertainty in the temperature forecast by this weekend is also apparent as indicated by large (8-10 degree) spreads within the NBM inner quartile range. Once our region does begin to draw in the Pacific moisture, temperatures will likely dip down to around 3-5 degrees below normal by the end of this week. However, as ridging aloft attempts to build over the area this weekend, temperatures may warm closer to normal by Sunday or Monday.

AVIATION

Updated at 2350Z.

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

No major aviation concerns are expected during the TAF period. The typical light and diurnal wind this evening will not be as clear cut with directions 170V250 before an earlier-than-usual switch to the east take place. However, forecast confidence is low and amendments may need over the next few hours. Regardless, familiar diurnal trends will resume by tonight. Some regional convection may be present through sundown, but chances of impacts at the terminals is less than 10%.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Winds will favor a westerly and south-southwesterly/southwesterly component at KIPL and KBLH respectively. Regional convection will be present around the region through around sunset, but impacts at the airport will be minimal at best perhaps a decaying outflow being all that comes to fruition.

FIRE WEATHER

Drier conditions will prevail through Thursday with only isolated chances for showers and thunderstorms mostly relegated to prominent terrain features across AZ. Better chances for wetting rains area-wide return late this week into this weekend. Afternoon minimum humidities will range from 7-15% across Southeast CA and between 15-35% for much of Southern AZ through the next few days. This will follow poor to fair overnight recoveries across the western deserts and good to excellent overnight recoveries across the eastern districts. Daytime winds will remain elevated out of the south to southwest, with the strongest gusts in Southeast CA and along the Lower Colorado River Valley peaking between 20-30 mph during the afternoon/early evening.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.