textproduct: Pendleton

DISCUSSION

Key Messages:

1. Ongoing flooding and hydro concerns. *Flood Warnings & Watches Active*

2. Moderate atmospheric river event and windy early in the week.

3. Substantial mountain snowfall potential Tuesday onward.

Current radar and infrared satellite imagery shows light returns along the Washington Cascades under partly cloudy skies. This is in response to the stalled front over Central Washington that will slowly lift north through the day as an upper level ridge offshore builds and shifts onshore tonight into Saturday. These features will bring a much needed respite to the persistent rainfall we have experienced this week associated with a strong Atmospheric River (AR) event. Due to the significant rainfall earlier this week, the Naches and Yakima Rivers are currently in moderate flood stage. Currently, the Yakima River at Horlick and at Parker are in moderate flood stage, but are forecast to drop into minor flood stage this morning. The Yakima River at Easton and Umtanum and the Naches River at Cliffdell and Naches are in minor flood stage, and forecast to drop into action stage over the next 24 hours. However, not all river levels are on the decline, the Yakima River at Kiona is still rising and set to crest Saturday morning. It is currently in minor flood stage (13.66 feet as of 12:30AM) and is expected to reach moderate flood stage this morning before peaking just shy of Major flood stage at 15.61 feet. As a result of these elevated river levels and lingering mountain rain this morning, the Flood Watch has been extended through this afternoon, and all aforementioned river reaches have active Flood Warnings extending over the next 24 hours.

The upper level ridge shifts to our east late Saturday into Sunday as a strong upper level low develops in the Gulf of Alaska, enhancing southwest flow aloft through the Pacific and directing a shortwave into the Pacific Northwest on Monday. This shortwave is supplied with moisture originating near Hawaii and contains an Integrated Water Vapor Transport (IVT) of around 560 kg/m*s via the GEFS, which would related to a moderate AR event. The event is currently expected to peak between Monday and Tuesday, with rain amounts of 1-3 inches over the Cascade and northern Blue Mountains. Confidence in these rain amounts is low at this time (20-40%), due to uncertainty regarding the strength of the departing ridge and developing low which relates to the overall orientation and timing of the AR. 61% of ensemble members advertise less rainfall on Monday and 91% of members suggesting more precipitation on Tuesday in relation to the current forecast. This additional rainfall is reflected in river forecasts as both the Yakima and Naches are likely to bump back into minor flood stage by mid-to late week. In addition to another potential bout of excessive rainfall, a passing cold front will bring elevated winds across the Columbia Basin due to tightening pressure gradients Tuesday into Wednesday morning. The GFS highlights a pressure gradient of 13-14 mb between Portland (PDX) and Spokane (GEG), which relates to sustained southwest winds of 20-30 mph with gusts up to 50 mph across the Simcoe Highlands, North-Central Oregon, Blue Mountain foothills, and the Lower Columbia Basin. Confidence in these values is moderate as the NBM highlights a 50-80% chance of gusts reaching 45 mph or greater over the aforementioned areas.

The arrival of the cold front Tuesday into Wednesday will also usher in colder temperatures and lower snow levels. This may lead to winter arriving with some force, as these cooler temperatures interact with the ongoing AR event to bring substantial snowfall across the Cascade and Blue Mountains starting Tuesday. The current forecast has the Washington Cascades passes receiving 10-20 inches of snowfall Tuesday, 5-15 inches Wednesday, and another 10-20 inches Thursday. Amounts over the Oregon Cascades and the northern Blue Mountains of 5-10 inches daily Tuesday through Thursday as a secondary shortwave follows the earlier workweek upper level trough. These amounts would result in Winter Storm Warnings, but current confidence in these snow amounts is low to moderate (30-50%) as ensembles are in disagreement related to the timing and overall front/trough strength. 55% of ensembles are in alignment or expecting slightly more snowfall that mentioned in the forecast, with 45% of ensembles hinting at lower snow amounts. At this time, there is a 60-80% chance of a Winter Storm Warning (12 inches or greater) being issued across the Washington Cascades and a 50-70% chance over the Oregon Cascades (8 inches or greater). 75

AVIATION...12Z TAFs

KDLS might see some low clouds this early morning, which could make this site MVFR or lower. But, confidence is low on the extent (<30%). VFR conditions will return for KDLS around later this morning. Otherwise, the remaining sites will be VFR throughout this TAF period with winds less than 10kts. Feaster/97

PRELIMINARY POINT TEMPS/POPS

PDT 59 38 57 36 / 0 0 0 0 ALW 59 43 55 40 / 0 0 0 0 PSC 60 39 53 36 / 0 0 0 0 YKM 57 37 55 36 / 0 0 0 0 HRI 59 38 53 35 / 0 0 0 0 ELN 55 36 51 33 / 10 0 0 0 RDM 60 30 61 30 / 0 0 0 0 LGD 56 36 55 37 / 0 0 0 0 GCD 56 35 59 33 / 0 0 0 0 DLS 59 41 55 38 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

OR...None. WA...Flood Watch through this afternoon for WAZ026-027-521>523.