textproduct: Pendleton

KEY MESSAGES

- Light mountain precipitation continues.

- Hydro concerns persist through much of the day. *River Flood Warnings and Advisories Active*

- Warming trend through week as mountain precipitation lingers.

DISCUSSION

Current radar and infrared satellite imagery shows light returns still occurring over the northern Blue Mountains under partly cloudy skies. These light snow showers will continue to taper off through the early morning hours as an upper level ridge slowly builds offshore, which is ahead of an approaching weak warm front that will bring a return to light showers over the Washington Cascades this afternoon. Snow levels will be steadily increasing from 1000 feet this morning to 4000 feet later this evening. As a result, light snowfall over the Washington Cascades is expected to transition to rain around 5 PM. Before that time, 1-2 inches of snowfall is expected at pass-level across the Washington Cascades.

Limited precipitation of less than 0.05" of rain is expected across the northern Blue Mountains associated with the incoming warm front later today. This is welcome news and will allow swollen and flooded river reaches to subside and drop below flood stage within the next 24 hours. The Walla Walla River near Touchet is currently cresting in moderate flood stage at 15.2 feet, and forecast to drop into minor flood stage this morning before dropping below flood stage late this evening. Thus, a River Flood Warning is in effect for the Walla Walla River near Touchet until 9 PM tonight, and is forecast to drop below action stage early Tuesday morning. The only other river currently in flood stage is the Umatilla River near Gibbon, which is in minor flood stage and expected to drop below flood stage later this morning. As a result, the River Flood Warning for the Umatilla River near Gibbon extends until 7 AM, and forecast to drop below action stage late tonight. Other river reaches that are currently in action stage include the Grande Ronde River at Troy and the Naches River near Naches, but both are forecast to drop below action stage by mid-morning. Hydro concerns may return later in the week for the Naches, Yakima, and Grande Ronde Rivers associated with the incoming atmospheric river event.

The upper level ridge axis moves over the area on Monday along with an embedded shortwave and surface warm front passage that will bring light showers to portions of the Columbia Basin and Blue Mountain foothills through the afternoon. The frontal boundary will also allow for a pressure gradient to form across the Washington Cascades that will enhance winds along the east slopes and potentially lead to higher winds aloft mixing down to the surface, likely focused through the Yakima Valley during the late afternoon and evening timeframe. Confidence is lacking that wind gusts will reach advisory level (45 mph or greater), but gusts of 20-30 mph will be possible.

Snow levels will soar to around 9000 feet by late Monday morning, keeping mountain showers mostly in liquid form as accumulations of up to 0.50" along the Washington Cascade crest and up to 0.10" over higher elevations of the northern Blue Mountains are likely. Snow levels are expected to stay elevated and above 7500 feet through the remainder of the week as the ridge axis slides to our east and southwest flow aloft is enhanced by a developing upper level low pressure off the Gulf of Alaska. Ensembles are in good agreement with these upper level features advecting warm air and moisture from the subtropics in the form of an extended weak to moderate atmospheric river. Ensemble members suggest that flow aloft should have enough of a southerly component which will lead to a low chance (<20% chance) of any precipitation reaching lower elevations of the Columbia Basin Monday onward, staying confined to the Washington Cascades and east slopes. However, 60% of ensemble members do hint at measurable rainfall (0.01") reaching lower elevations of the Columbia Basin on Wednesday. Less than 20% of members suggest this occurrence any other day.

Moisture may have a tough time breaking through the Cascades and into the Basin, but temperatures will not. High temperatures will break into the mid-to upper 60s across the Columbia Basin and into the low 70s over Central Oregon Monday, and only increase as the week progresses. By Thursday and Friday, high temperatures will reach into the mid-to upper 70s across the Columbia Basin and Central Oregon. In fact, the NBM advertises a 55% chance of Redmond reaching a high of 80 degrees on Tuesday, which increases to a 70% chance by Friday. There is even a 40-60% chance of 80 degree high temperatures across the Blue Mountain foothills and the Tri-Cities area on Friday. However, confidence in these probabilities this far in advance is low (<20%) as only 25% of LREF ensemble members suggest even a low chance (10%) of 80 degree temperatures in the Tri-Cities on Friday. However, this scenario is worth noting in relation to the unseasonably strong upper level ridge influencing the overall pattern across the western CONUS. 75

AVIATION /06Z TAFS THROUGH 06Z MONDAY/

VFR conditions currently across all sites, which will stay the course through the period. Ceilings are expected to drop to between 5-8kft this afternoon in response to a weak passing warm front, but should slowly improve through the early morning hours on Monday. Winds will stay light and below 10 kts for all sites. 75

Preliminary Point Temps/PoPs

PDT 51 40 65 46 / 10 20 10 10 ALW 49 42 62 49 / 10 40 30 20 PSC 52 40 65 48 / 10 10 10 10 YKM 49 33 64 42 / 10 30 10 10 HRI 52 40 66 47 / 10 20 10 10 ELN 41 31 56 39 / 20 50 30 10 RDM 54 37 72 39 / 10 20 0 0 LGD 49 37 62 43 / 10 50 20 10 GCD 52 37 67 42 / 10 30 10 10 DLS 51 40 67 46 / 10 30 10 10

PDT WATCHES/WARNINGS/ADVISORIES

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