textproduct: NWS Phoenix

UPDATE

Updated 06Z Aviation Discussion.

KEY MESSAGES

- Rain chances return to portions of eastern Arizona with most activity focused over foothill and high terrain areas.

- Best chances for lower desert rainfall will occur Wednesday and Thursday, though most activity should remain to the north and east of the Phoenix metro.

- Near to slightly above normal temperatures will persist over the next several days.

SHORT TERM /Today through Thursday/

As one vorticity center lifts northeast through the Great Basin, a lingering, elongated trough remains across the SW Conus extending south of the Baja peninsula. Per WV imagery, several vorticity centers are embedded in this weak southerly flow and will stream north over the next 72 hours periodically enhancing ascent structure as well as importing marginally better moisture poleward. The first such feature currently over Sonora will progress through eastern Arizona later today while a more defined circulation center over the southern Baja ejects through central New Mexico Wednesday. A final piece of energy incorporating the entire trough base and full extent of cyclonic influence will surge through central Arizona Thursday afternoon/evening potentially bringing the greatest short term impacts to the local forecast area.

Objective analysis indicates a pool of 10 g/kg mixing ratios across SE Arizona early this afternoon with a marked gradient towards under 6 g/kg across the Phoenix metro. This has confined the area of instability largely outside the CWA though an increasing synoptic pressure gradient amplified by cool outflow will continue to enhance easterly boundary layer flow such that increasing shallow moisture should slosh westward over the next 24 hours. Forecast confidence is moderate regarding scattered showers and a couple embedded storms moving into southern Gila County this evening, albeit with low instability limiting the areal and temporal extent. Easterly winds should be invigorated by this activity which could conceivably result in some lofted dust into lower elevations, however even among the most aggressive high resolution models, a sudden surge and/or prolonged instance of gusty winds appears unlikely reducing the threat for dense dust.

Global and HREF members agree that a well defined circulation center will propagate from the northern Mexico through western/central New Mexico Wednesday. This will place the eastern portions of the CWA within a deformation zone west of the circulation center with the expectation of thicker mid/high clouds and persistent showers hampering insolation. However, with better boundary layer moisture in place (~10 g/kg mixing ratios), clearing over mountains west of the deformation band should help realize MLCape 1000-1500 J/kg. Northeast steering flow should direct storms and outflow along the Mogollon rim towards the Phoenix metro, however CinH around 50 J/kg will need to be breached. Multiple colliding outflow boundaries necessary to achieve lower elevation storms are certainly possible, however there is only marginal support for this outcome among the full model suite and have maintained relatively low POPs.

The main basal shortwave will pass directly into south-central Arizona Thursday with the full cyclonic curvature region and strong ascent signal juxtaposed with modest instability. Similar to Wednesday, lower elevation moisture levels may be too low to support more than isolated thunderstorm formation, however ascent structure will be far more conducive than Wednesday. Given this strong lift and DCape in excess of 1500 J/kg, models have been consistent in advertising at least 1 strong outflow boundary surging through the Arizona lower deserts. Colliding boundaries may be sufficient towards additional development in lower deserts, and pattern recognition and model evidence indicates this potential may skip over the Phoenix metro towards western Arizona, though have increased POPs marginally over the initialized NBM. Regardless, signals continue to show the possibility of an organized expansive and long duration blowing dust event.

LONG TERM /Friday through Tuesday/

Late in the week, shortwave energy will have ejected northeast and become absorbed into the stronger westerly flow over the plains. In its wake, deep quasi-zonal westerly flow will gradually scour away moisture such that PWATS fall below 1" and boundary layer mixing ratios under 9 g/kg by Friday afternoon. A period of shortwave ridging through the first half of the weekend will also impose expansive synoptic scale subsidence such that only the most susceptible high terrain locations would support brief afternoon deep convection. In fact, instability and ascent parameters are so poor Saturday that NBM POPs are essentially under 5% across the entire region.

Forecast confidence deteriorates significantly next week due uncertainties in a progressive westerly wave and potential interaction with tropical moisture. Ensemble members are in good agreement that a shortwave will deepen near the California coast early next week, then rapidly propagate through the intermountain West during the first half of the week. However, there is substantial differences in depth and timing of this feature, which will be crucial in determining the ability to tap deep, tropical moisture and resulting impacts across the SW Conus. In general, operational members are deeper and faster with this wave than the preponderance of NAEFS membership; and based on historical precedent, would favor a somewhat slower progression of the central cold core and trough axis though intensity towards the deeper solutions is preferred. While conceptually a slower, deeper outcome would favor drawing the full brunt of remnant tropical circulation and associated moisture into the Southwest (akin to the latest 12Z GFS), the wide uncertainty in the actual location of the tropical system combined with disparity in the location and timing of the trough passage yields a large variation of possible outcomes. Nevertheless at the very least, ensembles hint at jet energy and sheared shortwave drawing midlevel moisture inland Sunday/Monday yielding scattered showers and storms. Thereafter because of the vast ensemble spread, mandated NBM POPs are essentially modest values spread over a prolonged period and offer minimal actionable details at this time.

In term of temperatures, much like during the short term, not much fluctuation in day-to-day readings is expected to occur. Daily values will hover right around to just above normal, which translates to lower desert readings ranging between 94-105 degrees through the weekend. Overnight lows will be slightly more elevated compared to where they typically are for late September, likely an effect of lingering moisture and cloud cover (at least for some spots), with readings between 70-82 degrees. Meanwhile higher elevation conditions will be quite comfortable as most locations should be in the 80s, with perhaps a few spots touching the lower 90s. Lows for these spots in the upper 50s to lower 70s will be common.

AVIATION

Updated at 0606Z.

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

Potential RA/TS impacts and gusty outflow winds will be the primary forecast challenge and potential impact through Wednesday evening. Light E winds will persist through midnight before a more pronounced E/SE wind develops by 07-09Z and continues through Wednesday morning with speeds up to 8-12 kts and occasional gusts into the mid-teens. An isolated shower cannot be ruled out mid Wednesday morning and then there will be a slight chance (20%) for VCSH/VCTS at terminals in the late afternoon and evening. Storms are most likely to the N and NE Wednesday afternoon and will be capable of sending outflow winds down into the metro which may lead to additional activity. Timing currently looks to be around 01-02Z for the arrival of outflow winds. FEW to SCT clouds bases down to 8-10K ft AGL will be common through the TAF period with SCT to BKN mid and high clouds.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather impacts are anticipated through the next 24 hours with mostly clear skies. Winds will favor W/NW at both KIPL and KBLH through Wednesday morning and then a shift toward E/SE by the afternoon, with wind speeds mostly staying below 10 kts and periods of light and variable winds.

FIRE WEATHER

Near to slightly above normal temperatures will prevail over the next several days, with minimal day-to-day fluctuations. Daily rain chances return for the high terrain through the end of the workweek with lower desert chances focused on Wednesday. MinRHs between 15- 30% will be common across the region through the start of the weekend, with even higher readings (35-50%) for the far eastern districts. MaxRHs will range from fair to good over the western and central districts, to good and excellent for the eastern districts into the weekend. Drier air by the weekend will lower RHs slightly, but they should remain above critical fire weather thresholds. Outside of any thunderstorm related outflows, winds will follow familiar diurnal trends with typical terrain induced afternoon gusts around 15-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

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