textproduct: Pendleton

DISCUSSION

Rest of Today through Saturday: A strong mid to upper level jet in zonal flow aloft will continue to sit over the region through the rest of today. The flow aloft will continue to facilitate the transport of an Atmospheric River, that has already brought several feet of snow along the WA Cascade crest and between 1 to 2 feet of snow to portions of the Northern Blues. In the lower elevations, precipitation amounts have not quite panned out to what hi-res short term model guidance has been suggesting the past day or two, but a few tenths have been observed along portions of the Blue Mountain foothills.

Through tonight, the AR will continue to be pointed at the PacNW, but will sag south as an upper level trough slides into central BC. An attendant cold front underneath the AR will also push south, with snow levels beginning to gradually lower. This will allow some areas of the WA Cascade crest and the northern Blues see a heavy, wet snow return this evening. By tomorrow morning, the upper trough will slide southeast into northern WA, accelerating a colder airmass into the region, and with it quickly lowering snow levels to 2kft or lower by mid Saturday morning. With the last push of AR moisture making its way across the forecast area, this will result in a period of light to moderate snow across the Cascades, with moderate to locally heavy snow across the Blue mountains. There is mod-high confidence (65-85%) that the WA Cascade crest will receive another 7 to 12 inches and the interior northern Blues another 3 to 6 inches through Saturday morning. As the trough axis passes east of the forecast area Saturday afternoon, a cool northwest flow will develop aloft. This will keep temperatures below normal Saturday, while light upslope snow showers will result in another 1 to 2 inches along the Cascade crest and the northern Blues.

The upper trough passage is also expected to bring breezy conditions across the forecast area Saturday. Winds will increase in the afternoon to 20 to 30 mph, while gusts will be 30 to 45 mph.

Due to warming conditions in the Northern Blues, snowmelt and light rain on snow has resulted in runoff into area rivers. Currently, only the Walla Walla river near Touchet and the Umatilla river near Gibbon are forecast to reach flood stage in the next 24 hours. These will continue to be monitored through next week.

Sunday through Thursday: Through most of next week, ensemble and deterministic guidance is in great agreement of an upper level ridge building over the western CONUS. The prominent high pressure over the region will result in a significant warming (for the time of year) across the region, with temperatures reaching the lower to mid 70s(10 to 15 degrees above normal) in the lower elevations by Tuesday, with moderate-high confidence (60-80%) that this will persist at least into Thursday.

While the lower elevations will remain dry under the upper ridge pattern, the WA Cascades, northern portions of the OR Cascades, and the Northern Blues will see persistent chances (40-70%) of light rain late Sunday through Thursday. The reason for this is a fairly large AR clipping the PacNW as it rounds the upper ridge. Both ECMWF and GFS ensemble suites show over 80% chances of IVT>250 kg/m/s clipping the Cascades and the northern Blues, which has resulted in these chances. The warm air advection underneath the upper ridge will also result in snow levels at least over 7kft across the forecast area through the duration of this AR, which will also mean most of this precipitation will fall as rain(confidence 60-70%).

Lastly, hydro concerns will arise not only from rivers coming from the Blues, but rivers along the WA Cascades will see rises next week. Snowmelt and the light rain on the existing snow pack will result in increased river levels, but confidence is low (15-25%) in any river segment reaching flood levels. Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

There is a mix of flight categories across the region, with most sites VFR. However, YKM/PSC has had persistent MVFR and ALW is LIFR due to rain, BR and low CIGs. These conditions are expected to linger. Rain is expected to continue through the evening and overnight hours before ending on Saturday morning. Conditions are expected to improve everywhere overnight and into Saturday morning. Winds will increase again and while some sites continue to gust around 25 kts, most sites will be in the 20 to 30 kts range through Saturday morning. 90

Preliminary Point Temps/PoPs

PDT 38 48 27 51 / 90 60 10 0 ALW 38 47 30 50 / 100 70 20 10 PSC 39 52 29 52 / 80 20 0 0 YKM 30 48 27 48 / 80 0 0 10 HRI 41 52 29 52 / 80 30 0 0 ELN 28 41 25 42 / 90 10 0 20 RDM 37 47 24 53 / 60 50 0 0 LGD 37 47 26 49 / 100 90 30 10 GCD 40 47 25 52 / 60 80 10 10 DLS 38 51 33 50 / 90 10 0 10

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning until 5 AM PDT Saturday for WAZ522. Winter Storm Warning until 11 PM PDT this evening for WAZ523. OR...None.