textproduct: NWS Phoenix

UPDATE

Updated Aviation 00Z Discussion.

KEY MESSAGES

- Rain chances diminish through the rest of the day, with chances for isolated light showers lingering mostly over the high terrain north and east of Phoenix.

- Another disturbance will bring chances for light rain mainly focused over the Arizona high terrain Friday night into Saturday.

- Temperatures will cool to near normal over the weekend then return to slightly above normal during the first half of next week.

SHORT TERM /TODAY THROUGH SATURDAY/

Rain totals over the last 48 hours have ended up quite similar to the forecast amounts from earlier this week, with lower deserts generally ranging from a trace to a few tenths of an inch and foothills/higher terrain areas north of Phoenix on the order of 0.25- 0.50". Unsurprisingly, rainfall over western JTNP underperformed, but the Lost Horse RAWS received a respectable 0.66". An exception to the aforementioned lower desert totals would be portions of far southeast CA and western La Paz County along and near the Colorado River, where a final, moderate intensity rain band tracked as the IVT plume progressed last night, bringing their totals up to 0.40- 0.70". The combination of wetting rains, clearing skies early in the morning and calm winds led to the development of dense fog within the lower Colorado River Valley and a few adjacent valleys in SE CA.

The forecast area is not completely done with rain chances in the short term. The upper level pattern currently paints a similar picture to that of yesterday, with a large trough centered off the NorCal Coast, broad downstream ridging centered over the TX/LA Gulf Coast, and deep southwesterly flow over the Desert Southwest. The shortwave disturbance responsible for near record levels of IVT penetrating the Interior West is seen in current water vapor imagery ejecting eastward over the Rockies this afternoon. With its departure, the midlevels of the atmosphere have rapidly dried, evident in PHX ACARS soundings early this afternoon, particularly above ~725 mb. However, low level moisture remains, and so isolated light showers or weak convective showers (though profiles are not ideal, 50-100 J/kg MUCAPE exists) cannot be completely ruled out over South-Central AZ, mostly favoring areas where upslope ascent is occurring under southwesterly flow.

The upper level pattern will shift over the next 48-72 hours, overall progressing eastward, but ensembles are in decent agreement that a PV anomaly will detach from the base of the trough currently off the west coast and retrograde offshore, well to the southwest of the region by Sunday. As this transition occurs, the Pacific trough will move onshore and acquire a positive tilt as it approaches the forecast area Friday night into Saturday. The passage of the trough will promote another round of rain chances, though moisture will be more scarce with this system and higher to the east/south of the area, and so the high terrain east of Phoenix will be most favored for any rain chances across South-Central AZ. The passage of this trough will also usher in noticeably cooler air this weekend with lower desert highs mostly dropping into the upper 60s, or within a couple degrees of normal.

LONG TERM /SUNDAY THROUGH NEXT FRIDAY/

Model guidance has shifted away from any rain chances early next week as much drier air is likely to filter into the region from the north Sunday into Monday. We should also see some ridging over the region early next week, but guidance continues to change the positioning of a potential cut- off low off the coast of Baja. If the ridge fully takes over across our region, we are likely to see a boost in temperatures by around Tuesday with highs potentially pushing near 75 degrees by Wednesday. Although guidance continues to overall suggest the cut- off low will become a factor for our region again at some point later next, forecast confidence remains quite low. NBM PoPs seem too high for next Wednesday and Thursday given the uncertainty, so we have trimmed PoPs by 30% across the entire area. If the cut- off low does move back close to our region or even into our region later next week, it at least looks promising for some meaningful moisture return with PWATs again forecast to reach near 1", or around 250% of normal.

AVIATION

Updated at 1823Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Light winds, aob 10 kts, and VFR conditions will prevail through the TAF period. SCT to BKN clouds around 5-6k ft will persist and will become FEW this evening. A few lower clouds, down to 2k ft, may develop Friday morning in the dawn hours, but are not expected to pose any impacts to terminals. E'rly to NE'rly winds will persist through the evening, then followed by more typical diurnal trends by early tomorrow afternoon. A few spotty weak showers cannot be ruled out through the afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

No significant weather impacts are expected through the next 24 hours. Mostly clear skies currently will give way to SCT to BKN high clouds by this evening. 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. Localized fog along the Colorado River near KBLH is less likely Friday morning, but cannot be ruled out.

FIRE WEATHER

An unsettled weather pattern will result in temperatures cooling to within a few degrees of normal by Saturday, elevated humidities, and additional chances for light rain over the high terrain of the eastern districts Friday night into Saturday. MinRHs will remain around 40-60% areawide through Saturday. A dry cold front will likely pass over the region from the north Sunday into early next week, causing breezy north winds across the AZ high terrain and in the vicinity of the lower Colorado River Valley. Until then, anticipate winds to remain fairly light areawide, generally under 15 mph. The front will bring a drier air mass over the region, decreasing humidities Sunday into early next week.

PSR WATCHES/WARNINGS/ADVISORIES

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