textproduct: Pendleton

KEY MESSAGES

- Widespread breezy to windy conditions Sunday.

- Active weather continues next week, but significant uncertainty in forecast details remains.

DISCUSSION

This afternoon through Saturday morning, another round of light precipitation is forecast as a surge of moisture from the Pacific wraps around the large offshore high and pushes inland. While 12Z CAMs were quite aggressive with precipitation for the mountains (Cascades and northern Blues), yielding totals of several tenths of an inch to over an inch along the highest terrain of the Washington Cascades and northern Blues, 18Z CAMs have trended significantly downward, in line with output from global ensemble NWP guidance and what would be expected in an environment void of any strong frontal forcing. Thus, confidence is high (80 percent) in lighter rain amounts of a trace to a couple tenths of an inch for the lowlands, with locally higher amounts for the mountains. Advection of warmer air into the region has raised snow levels such that mountain passes will likely (80 percent confidence) receive all rain.

Widespread breezy westerly winds are present this afternoon, but not strong enough to warrant any wind headlines. Breezy to windy westerly winds are forecast to persist through the majority of the next seven days, with strongest winds currently forecast Sunday and again mid/late next week.

Looking ahead, the 500-mb pattern is expected to turn zonal over the weekend into next week. Precipitation chances taper off Saturday east of the Cascade crest, but a moist zonal jet will keep at least a chance (30-50 percent) of precipitation along the Cascades through Sunday morning.

By late Sunday, clusters of like-solutioned ensemble members all indicate a return to a more active weather pattern to the forecast area. That said, there is still significant uncertainty in pattern details. We are currently keeping a watch on potential widespread windy conditions for the lowlands. The NBM indicates high (70-90 percent) chances of advisory-level wind gusts Sunday for the Kittitas Valley with medium-high (40-80 percent) chances elsewhere in the lower elevations. Will note there is some potential (40-60 percent) for reaching High Wind Warning thresholds (50 kt wind gusts) for the Kittitas Valley as well. After isentropic analysis, have held off on issuing any wind headlines for now due to uncertainty in timing/magnitude of winds mixing to the surface. That said, all guidance is showing a robust low-level jet of 35-55 kts around 850 mb through the Cascade gaps and over the Columbia Basin and adjacent foothills. Moreover, surface pressure differences of 10-15 mb between PDX and GEG are advertised by current 12Z/18Z operational NWP guidance, supportive of widespread advisory-level to perhaps localized warning-level winds.

Jumping forward in time to late Tuesday through late Thursday, the ECMWF EFI is signaling decent ensemble agreement (EFI values of 0.5-0.9) for anomalous precipitation and snowfall relative to model re-forecasts for the Cascades and their adjacent eastern slopes. Shift of Tails (SoT) values of 2 show some members are forecasting an extreme solution.

Taking a look at probabilities of snowfall amounts from the NBM for the 48-hr period ending 00Z Friday, there is a 10-40 percent chance of reaching or exceeding 1 inch of snowfall for the Kittitas and Grande Ronde valleys, with lower chances (up to 10 percent) for the Blue Mountain foothills and central/north- central Oregon. The Oregon Cascade passes have a medium-high (50-60 percent) chance of reaching or exceeding 6 inches of snowfall, while the Washington Cascades have a high (80-90 percent) chance and the northern Blue Mountains have a low- medium chance (10-50 percent). 86

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

VFR conditions prevail for all sites. Periods of light rain are forecast 06Z to 10Z late tonight for most sites (30% probability), except KYKM. Breezy winds of 20-30 kts may develop during the day Saturday for most sites, with an exception of KRDM/KBDN (40-50% confidence). Winds should start decreasing to 13 kts or below for KDLS, KPDT, KYKM, KALW, and KPSC around late Saturday afternoon into night. Feaster/97

HYDROLOGY

No flooding is currently forecast by the NWRFC for any of our mainstem rivers. Would like to note that uncertainty in snow levels, precipitation totals, and snow totals does indicate some potential (30 percent confidence or less) for flooding along rivers later next week if the right variables all align. This potential may not be adequately captured in GEFS-driven probabilistic output from the HEFS since the ECMWF ensemble is significantly wetter than the GEFS, though the Canadian ensemble is also significantly drier than the ECMWF ensemble.

Warmer, wetter solutions would favor increased rises along rivers, while cooler, snowier solutions and/or those with less precipitation would keep river levels lower. 12Z global ensemble clusters show a roughly 60-40 split among ensemble members in favor of cooler solutions, though there is more nuance to precipitation solutions. Roughly 30 percent of members show multiple inches of precipitation along the Washington Cascades with significant spillover (up to a couple inches) into the east side valleys, while roughly half of members keep more modest precipitation totals across the entire forecast area (still heaviest for the Cascades) and the remaining 20 percent keep drier conditions across most of eastern Oregon and eastern Washington with notable precipitation (more than a few tenths of an inch) confined to the Washington Cascades. 86

Preliminary Point Temps/PoPs

PDT 45 64 43 62 / 50 10 0 10 ALW 46 64 47 62 / 60 20 0 20 PSC 45 69 47 65 / 40 10 0 0 YKM 42 66 40 62 / 30 10 0 20 HRI 46 67 46 65 / 40 10 0 0 ELN 42 58 42 54 / 40 10 10 40 RDM 35 62 33 60 / 30 0 0 0 LGD 41 61 39 61 / 70 30 10 10 GCD 38 58 35 60 / 50 20 0 0 DLS 46 62 45 59 / 50 10 0 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.