textproduct: Pendleton

KEY MESSAGES

- Widespread precipitation through early Wednesday.

- Dry and warming conditions through remainder of week.

- Weak weekend system returns rain chances to area.

DISCUSSION

Current radar and visible satellite imagery shows widespread light showers across the Cascades, Blue Mountains, and the Columbia Basin. This is a result from continued southwest flow aloft coupled with a stationary front that is draped across the Pacific Northwest. In addition, an elongated upper level trough, located off the British Columbia coast, is beginning to break off and become a closed low that will separate from the overall flow. This trough has helped to pull moisture in from the southwest, but will lose this efficiency as it separates from the upper level flow and push further out into the Pacific Wednesday morning. Until then, these features will keep light showers in the forecast today before slowly tapering off tonight into Wednesday morning.

The presence of the stationary front will help to enhance shower potential by adding lift, but also will provide a wide range of snow levels across our area. Snow levels across the Washington Cascades will be lowest at 3500-4500 feet, with the Oregon Cascades, Blue Mountains, and Wallowas at 6000-7000 feet. As a result, snowfall amounts of 2-5 inches are expected at pass- level and above across the Washington Cascades, with 1-4 inches likely over the Oregon Cascades, northern Blue Mountains, and the Wallowas above 4500 feet. Rain amounts at lower elevations will range from 0.45"-0.55" across the Blue Mountains, foothills, and the Simcoe Highlands to 0.20-0.35" over Central Oregon, Lower Columbia Basin, and the Yakima/Kittitas Valleys through early Wednesday morning.

High temperatures will be slightly below normal for much of the region today, as temperatures peak in the low to mid-40s across the Lower Columbia Basin, Blue Mountain foothills, Wallowa county, and the Yakima/Kittitas Valleys. Highs will be slightly warmer across Central Oregon and further south of the stationary front, with afternoon temperatures in the low to mid-50s. These highs will swap on Wednesday due the timing of a weak passing cold front and incoming surface high pressure, which will initiate a warming trend that will extend through the remainder of the workweek as surface high pressure strengthens. High temperatures will approach the low 60s across Central Oregon, Lower Columbia Basin, and the northern Blue Mountains on Friday as the trend peaks. Conditions will also stay dry, with the exception of high, mountain terrain across the Cascades and Blue Mountains that may accumulate up to an inch of daily snowfall - Wednesday being the best chance.

The weak passing cold front Wednesday and the subsequent high pressure sliding in behind will also bring with it breezy conditions across the east slopes of the Washington Cascades, Lower Columbia Basin, and Blue Mountain foothills. A passing upper level trough will further tighten the pressure gradient along the Cascades to promote these elevated, west-southwest afternoon winds, as gusts of up to 35 mph will be possible both days. Confidence is high in these gusts as the NBM advertises a 60-90% chance of 35 mph gusts or greater over the aforementioned areas. These winds are expected to stay below advisory criteria (45 mph gusts or greater) both days, with only a 20-40% chance of gusts exceeding 44 mph - best chances across the Blue Mountain foothills on Wednesday.

Ensembles are in the good agreement that the cutoff low that will separate from the overall upper level flow midweek will rejoin the flow and approach the Washington/Oregon coasts over the weekend. In addition, both GFS and ECMWF AI models reintroduce southwest flow aloft and light showers across the Oregon Cascades and Central Oregon early Saturday morning. 56% of ensemble members suggest showers as early as 4 AM Saturday, with 87% aligning with measurable rainfall (0.01" or greater) extending across our Oregon zones and staying south of the Washington border on Saturday. However, rain amounts are lacking as the NBM shows only a 20-45% chance of measurable rainfall over our Oregon zones. Snow amounts will also be minimal with up to an inch at higher terrain as snow levels hover between 3500-4500 feet.

Inconsistencies with ensembles and deterministic guidance stems from system strength associated with the high pressure midweek and the reintroduction of the upper level trough/low feature into the weekend. Guidance seems to align more with the incoming trough feature weakening substantially due to the incumbent high pressure. This seems to 'eat away' at the low as it approaches, which is especially evident in the ECMWF AI and ensembles. The GFS AI/ensembles and the CAN also depict this, but not to the extent which is related to the ECWMF allowing the earlier ridge and high pressure to be slightly stronger. Needless to say, no substantial impacts are expected from this weekend system, which is likely to bring only light precipitation before drier conditions return early next week. Guidance is in good agreement with upper level ridging and high pressure returning in the wake of the passing weekend system. 75

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Variable conditions across all sites, with VFR at KALW/KRDM, MVFR at KPSC/KBDN, and IFR at KYKM/KDLS due to reduced ceilings of 600-800 feet. Conditions are expected to degrade for all sites as ceilings drop to between 400-800 feet through Tuesday morning. Fog may also develop and decrease visibilities between 1/2SM and 2SM at KYKM/KALW/KPDT through the morning hours. Widespread light to moderate rain is also anticipated to persist through much of the period, slowly tapering off Tuesday evening. The only terminals expected to stay MVFR are KRDM/KBDN as ceilings drop to around 2000 feet mid-morning. 75

Preliminary Point Temps/PoPs

PDT 34 49 31 54 / 80 20 0 0 ALW 36 49 34 53 / 80 30 0 0 PSC 33 54 32 55 / 50 10 0 0 YKM 28 49 27 51 / 30 0 0 0 HRI 34 52 31 55 / 60 10 0 0 ELN 27 44 29 47 / 30 10 10 10 RDM 31 47 21 52 / 50 10 0 0 LGD 36 46 26 51 / 90 50 0 0 GCD 36 45 24 50 / 70 40 0 0 DLS 36 51 32 53 / 60 20 10 10

PDT WATCHES/WARNINGS/ADVISORIES

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