textproduct: NWS Phoenix

KEY MESSAGES

- A weak weather disturbance will move across the region late this week resulting in a few higher terrain showers and temperatures settling closer to the seasonal normal.

- Dry weather with locally breezy conditions will return during the first half of next week before rain chances return by next Wednesday and Thursday.

SHORT TERM /Today through Sunday/

Elongated troughing along the west coast of North America will gradually progress inland, while significantly weakening and obtaining a positive tilt orientation over the next 48 hours. Shallow moisture over south-central Arizona remains abundant with sfc-H8 mixing ratios near 9 g/kg, albeit with a substantial dry layer around H7. Modeling continues in good agreement bringing a decaying front inland ahead of this trough and cold core later tonight, albeit accompanied with limited ascent structure, and per forecast soundings incomplete top down saturation. However, deep SW flow should promote a period of moist, orographic lift with minor rainfall accumulations primarily focused over foothills and mountains north and east of Phoenix. While not precluding a brief shower across lower elevation communities through Saturday morning, the preponderance of guidance suggests at most 10-15% chance of accumulating rainfall outside of preferred upslope areas.

As the trough axis shifts through the forecast area over the weekend and H5 heights descend into a 564-568dm range, temperatures throughout much of the lower/middle troposphere will cool markedly with sfc readings falling closer to the daily normals. Ensemble guidance spread is somewhat larger than would be anticipated with official NBM output falling towards the lower end (25th percentile) of the guidance envelop. With subsidence and a direr airmass filtering into the region, this make sense with respect to low temperatures, however forecast BUFR soundings suggest any prolonged period of afternoon insolation could force highs a few degrees above the mandated NBM forecast. Otherwise, building high pressure north of the CWA will promote increasing northerly winds Sunday afternoon with the initial push of stronger gusts funneling down the lower Colorado river valley (typical for this time of year), but just short of advisory criteria.

LONG TERM /Monday through Friday/

Once the trough exits into New Mexico Sunday, sharp upstream ridging will continue to promote strong sfc pressure rises over the Great Basin resulting in very dry northerly flow sweeping through the region. Surface dew points plunging into the teens and 20s (and possibly lower) will be common by Sunday night with a strong pressure gradient translating eastward into Arizona. Breezy conditions should expand across ridge tops north and east of Phoenix Monday morning with sheltered valley locations potentially experiencing readings flirting with freezing. This outcome would be most likely in rural, lower desert areas of La Paz and Maricopa counties (i.e. Salome, Bouse, etc.) where lows falling below freezing could occur for the first time this winter. Uncertainty with respect to impacts of winds mixing the boundary layer even in valleys, along with forecast readings only marginally below freezing per NBM output precludes a watch issuance at this time, however at least local minor freeze issues will need to be monitored.

The weather pattern is likely to get complicated mid/late next week as guidance is still depicting a cutoff low lurking well west of the Baja peninsula. An elongated ridge is also expected to set up across California through the Great Basin and Southwest by Tuesday, forming a temporary East Pacific Rex block. This ridge should boost temperatures slightly midweek, but eventually guidance shows a strong moisture push ahead of the cutoff low by late Wednesday into Thursday. Depending on the amount of moisture, upper level jet forcing, and the positioning of the cutoff low, we may see rain chances enter the picture as early as Wednesday late afternoon, but more likely Thursday into Friday. Uncertainty remains very high with the evolution of this next weather system and overall flow pattern given massive North Atlantic blocking and model differences in rectifying the upstream response.

AVIATION

Updated at 1740Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation concerns are expected through the TAF period. Winds will be aob 5-10kts and will maintain an easterly component through the late morning hours before resuming typical diurnal trends this afternoon. However, before the westerly shift this afternoon fully develops a southerly component can be expected during the transition period. Expect mostly SCT to BKN mid to high level clouds this afternoon and evening which will progressively thickening to OVC with ceilings dropping to aob 7-10 kft by the end of the TAF period.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concern are expected through the TAF period. Mostly high level clouds with some mid level clouds expected to enter the area by this afternoon. Winds at KIPL will maintain a westerly direction with speeds around 5-10 kts and KBLH will favor a southerly to southwesterly direction with similar speeds.

FIRE WEATHER

A weakening weather system will gradually shift through the region through Saturday. This will bring cooler temperatures, elevated humidities, and additional chances for light rain mainly over the high terrain of the eastern districts tonight into Saturday. MinRHs will remain around 40-60% areawide through Saturday before lowering to close to 20% starting Sunday. Anticipate winds to remain fairly light areawide, generally 15 mph or less, then breezy conditions arrive Sunday into early next week as a dry cold front is likely to sweep the area from the north.

PSR WATCHES/WARNINGS/ADVISORIES

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