textproduct: NWS Phoenix

KEY MESSAGES

- Slightly above normal temperatures through early next will cool to near or slightly below normal levels during the middle of the week.

- Dry conditions with typical afternoon breeziness will prevail through the beginning of next week with wind speeds increasing during the middle of the week.

- A slight chance of thunderstorms will exist over the eastern Arizona high terrain Monday, albeit with limited rainfall potential.

SHORT TERM /Today through Monday/

Early morning objective analysis depicts remnant weak troughing over northern Mexico, however an enhanced subtropical jet and approaching upstream East Pacific energy will displace this feature into the southern plains over the next 24 hours. In its place, transient shortwave ridging will build into the SW Conus with H5 heights increasing into a 580-584dm range through the weekend while also forcing deep southerly flow on the western periphery of the anti- cyclone. As ensemble numerical spread remains very narrow, forecast confidence is excellent showing dry conditions and temperatures 2F- 4F above normal with the warmest lower desert locations just breaching the 100F threshold.

The aforementioned southerly flow will briefly import marginally better moisture into southern and eastern Arizona, though with mixing ratios no more than about 5-6 g/kg, and only a narrow depth of potential saturation above the H7 layer. However, the forecast area will align nearly perfectly in the left front jet quadrant with impressive divergence and strong ascent. This pattern of incoming cooling aloft and steepening lapse rates juxtaposed with a narrow plume of saturated lift suggests scattered showers and a few storms during the day Monday over higher terrain east of Phoenix. Limited instability with large T/Td spreads and a dry sub-cloud layer would favor dry lightning strikes and gusty outflow winds versus accumulating rainfall. Have cut NBM POPs modestly as they appear a bit aggressive given the thermodynamic profiles and presence of enhanced evaporation even over mountainous areas.

LONG TERM /Tuesday through Friday/

An overwhelming preponderance of ensemble members are now advertising a western Conus pattern previously preferred by human forecasters with developing blocking, and deepening slow moving features affecting the western Conus. As a result, confidence has improved that pronounced negative height anomalies entering the Pacific NW will descend into the Great Basin, then stall in the form of a large cutoff low, perhaps as far south as southern UT/northern AZ. Given this evolution, temperatures will retreat into a below normal category during the middle of the week, and possibly persisting through the remainder of the week. Automated NBM/WPC output has started reflecting this trend, though it would not be unexpected that future forecasts trim a couple more degrees closer to the lower quartile of the numerical distribution during the latter half of the week.

Otherwise, the greatest impact with this system should be increased wind speeds with a seasonally strong jet and aggressive height falls entering the region. The initial surge will enter SE California late Tuesday, then sweep across the entire CWA on Wednesday. Deep mechanical mixing should tap higher momentum 20-30kt winds through the boundary layer with the most intense speeds in downsloping regions of SE California and around terrain features of the eastern CWA. Recent mandated NBM forecasts have started catching up on this increased wind speed paradigm, however some of the deficiencies in the ensembling process are likely still restricting the full extent of wind speeds. Regardless, advisories in the most wind prone areas of SE California are looking more likely, while low relative humidity and dry fuels may foster an enhanced fire danger across much of the region. Depending on the positioning and progression of the attendant closed low, these impacts may persist into Thursday before relaxing late in the week.

AVIATION

Updated at 0435Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns under mostly clear skies can be expected throughout the TAF period. The overall wind pattern will follow the typical diurnal tendencies with speeds generally aob 10 kts, with some occasional afternoon/early evening gustiness in the mid to upper teens.

Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns under clear skies can be expected throughout the TAF period. At KIPL, winds will generally be out of the southeast while at KBLH, winds will generally be out of the south. Overall wind speeds will range between 5-13 kts, with some elevated gusts Saturday afternoon, especially at KBLH.

FIRE WEATHER

Temperatures will remain in a slightly above normal category through early next week before modest cooling arrives during the middle of the week. Minimum afternoon humidity levels will fall into a 10-20% range with poor to fair overnight recovery of 20-40% improving closer to a fair to good range next week. A marginal increase in moisture will result in a slight chance for afternoon thunderstorms and a few lightning strikes over high terrain of eastern districts Monday. Little to no accumulating rainfall should be anticipated, and the threat for new wildfire starts due to lightning strikes will be heightened. Typical afternoon upslope gustiness of 15-25 mph will be common through early next week with speeds increasing markedly during the middle of the week. Stronger gusts over eastern district high terrain and through the lower Colorado River valley Wednesday combined with low RH and dry fuels may result in near critical conditions.

PSR WATCHES/WARNINGS/ADVISORIES

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