textproduct: Pendleton

KEY MESSAGES

- Warm, dry conditions through the weekend

- Breezy to locally windy through the Cascade gaps today, then widespread breezy to windy Monday

- A low-pressure system and cold front are likely (80 percent confidence) to arrive by Monday afternoon and evening, but details are still uncertain

DISCUSSION

Water vapor imagery reveals a couple features of interest upstream in the Pacific Ocean. The first feature is a shortwave approaching the British Columbia coast, and the second is a more pronounced shortwave located just south of the Aleutian Islands. Overhead, a west-northwest flow associated with a flattening ridge is present.

Ensemble NWP guidance is in excellent agreement that the first shortwave will track inland across British Columbia through the day, remaining well north of the forecast area. However, it will keep quasi-zonal to zonal flow over the Pacific Northwest and prevent the ridge from amplifying. A cross-Cascade temperature gradient during the afternoon (warmer on the east side) will aid in producing modest pressure gradients (5-9 mb between PDX and GEG) and drive breezy to locally windy westerly winds through the Cascade gaps, especially the eastern Columbia River Gorge, Kittitas Valley, and wind-prone portions of north-central Oregon. Not anticipating winds to reach advisory criteria -- at least not over a widespread area -- based on the lack of a strong jet aloft (peaking at 25-35 kts between 925 mb and 850 mb), and NBM probabilities of exceedance support a low-medium chance (30-60 percent) for the aforementioned areas. Still expecting pressure gradients to slacken by a few millibars on Sunday, resulting in breezy, but not windy, westerly winds through the Cascade gaps. Otherwise, zonal flow aloft will keep dry conditions in place.

The shortwave south of the Aleutian Islands is passing over a baroclinic zone. Ensemble NWP guidance is unanimous in suggesting it will induce surface cyclogenesis through the day, and deepen into a closed upper-level low through Sunday as it digs southeast just off the coast of southeast Alaska and British Columbia.

While there are slight differences among ensemble systems, they are all in agreement that the low will continue to dig southeast through Monday, with the low center remaining offshore. This will result in another warm day, especially for eastern Oregon and eastern Washington which will be under southwesterly flow aloft and away from the Cascade gaps and influence of cooler marine air.

Breezy to locally windy southwesterly winds are forecast prior to the arrival of widespread breezy to windy westerly winds by Monday afternoon and evening as a cold front sweeps across the region. Winds may approach advisory criteria across the region, but confidence is just medium (50 percent) in details of the wind forecast given ensemble spread in the timing/location of the low as it tracks into the Pacific Northwest by late Monday.

In addition to likely (75 percent or higher) chances of precipitation across the Cascade crest, precipitation chances (50-80 percent) spread eastward across the Blue Mountains and their foothills by afternoon and especially evening. Chances of wetting rain (0.10" or more of precipitation) through Tuesday morning are modest (10-50 percent) for the southern Blue Mountains region, but medium-high (40-80 percent) for the northern Blue Mountains and Wallowa County, and even higher for the Cascade crest (70-95 percent chance). Will note some CAMs (notably the RRFS) are suggesting some convection across the northern Blue Mountains and foothills Monday afternoon ahead of or along the cold frontal boundary. Confidence in this solution is currently low, and do not have a mention of thunder in the forecast at this time.

By Tuesday, and especially Wednesday, ensemble systems begin to diverge in their representation of the 500-mb pattern. Per ensemble cluster analysis of latest 00Z runs, roughly 60 percent of members favor a deeper low centered over NE California and NW Nevada while the remaining ~40 percent show a slightly shallower low centered over the Great Basin (27 percent of members) or southeast Oregon (12 percent of members). A slight chance to chance (15-50 percent) of showers is forecast both days, but confidence in precipitation amounts and temperatures is lower than average given the sensitivity of the forecast to the location and magnitude of the low.

Ensemble solutions continue to diverge Thursday and Friday, but over 60 percent of ensemble members show the low over central California or the Great Basin and favor continued showery weather.

Thunder chances through the week will be tied to the location of the low, and currently have a 10-25 percent chance each day Monday through Friday.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

VFR conditions are forecast for the next 24 hours at all sites. Skies will remain clear to partly cloudy, predominantly thin cirrus. Breezy winds of 10-20 kts with gusts of 20-30 kts will persist at DLS, and periods of breezy winds are forecast to develop this afternoon and evening at all other sites.

Preliminary Point Temps/PoPs

PDT 83 50 82 50 / 0 0 0 0 ALW 84 55 82 56 / 0 0 0 0 PSC 89 54 87 54 / 0 0 0 0 YKM 88 52 85 53 / 0 0 0 0 HRI 86 54 85 53 / 0 0 0 0 ELN 78 49 76 48 / 0 0 0 0 RDM 83 43 84 45 / 0 0 0 0 LGD 82 47 83 48 / 0 0 0 0 GCD 84 45 86 46 / 0 0 0 0 DLS 82 54 83 54 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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