textproduct: Pendleton

KEY MESSAGES

- Warming and drying trend peaks today

- Breezy to windy conditions Thursday

- Isolated thunderstorm chances and widespread rain Saturday

DISCUSSION

Current radar and infrared satellite imagery shows dry conditions as a couple high level clouds stream into the Columbia Basin from the north. This is in response to a weak, quickly passing shortwave that will lead to a transient upper level ridge moving into the Pacific Northwest this afternoon. As the axis of the ridge positions itself directly over the area by mid-afternoon, flow aloft shifts from northwest to more from the west to allow for a slightly warmer day today under clear skies. High temperatures will increase about 5 degrees from Tuesday, reaching into the mid-to upper 80s across lower elevations of Central Oregon and the Columbia Basin, which is about 2-4 degrees above normal for this time of year. The present high pressure ridge will also promote dry conditions as afternoon humidities drop into the mid-to upper teens across Central Oregon and the John Day Basin, with values of 20-25% extending into the Columbia Basin and the east slopes of the Cascades. Moderate overnight humidity recovery is also anticipated across Central Oregon, John Day Basin, and the Blue Mountains with max values Thursday morning hovering between 40-50%.

Warm conditions will continue into Thursday as highs reach back into the mid-80s across the Columbia Basin and Central Oregon. However, an incoming upper level shortwave will approach from the north to bring a 10-20% chance of rain across the northern Blue Mountains and foothills after 5PM associated with a surface cold front. Minimal rain amounts are likely due to the incumbent dry airmass. Winds will also become breezy due to the frontal boundary, with winds further enhancing due to surface high pressure infiltrating along the Washington/Oregon coasts. West- northwest winds of 20-30 mph with gusts up to 40 mph will be possible across the Columbia Gorge, Simcoe Highlands, and the Kittitas Valley, with 15-25 mph and gusts up to 35 mph possible along the Blue Mountain foothills Thursday afternoon - peaking between noon and 6PM. Guidance has decreased the strength of the gradient over the Cascades in the last 24 hours, with the GFS and NAM both dropping the gradient expected between Paine Field (PAE) and Pasco (PSC) by 1-2 mb. The NBM has also decreased the chance of advisory-level wind gusts (45 mph or greater) from 22% to 9% at Ellensburg. This decrease in wind severity is a result of a weak shortwave, spinning off a closed low pressure located in the Eastern Pacific, getting caught in the flow of the dropping trough and moving onshore along the California/Oregon border. This feature helps to suppress the upper level ridge over Southern California and Arizona and relaxes the pressure gradient. The shortwave is a direct result of the overall strength of the closed low pressure offshore, which aligns with 59% of ensemble members.

The shortwave ejects to our east Friday morning as a strong upper level system drops from Eastern Alaska and moves offshore north of Vancouver Island and quickly strengthens as it tracks just offshore of the Washington and Oregon coasts Saturday morning. During its descent on Friday, a weak ridge of high pressure builds over Central and Eastern Oregon to provide mostly clear skies through the early afternoon. Clouds will increase through the evening as the system approaches the coast, pushing inland Saturday afternoon. The presence of the weak high pressure ahead of the incoming system will lead to a slight chance (10-15%) of isolated afternoon thunderstorms across the east slopes of the Cascades and isolated evening thunderstorms along the northern Blue Mountains. Primary concerns will include frequent lightning, breezy winds, and brief downpours associated with the significantly moist airmass. The AI ECMWF and GFS ensembles highlight precipitable water amounts of 0.8-0.9", which is 120-150% of normal. This will lead to expected rain amounts of 0.25-0.50" along the east slopes of the Washington Cascades and the Blue Mountains/foothills, 0.10-0.20" over Tri-Cities/Hermiston and the John Day Basin, and 0.02-0.10" across Central Oregon. The NBM advertises a 50% or greater chance of a wetting rain (0.10" or greater) across the entire area, with 60-70% chances through the Blue Mountain foothills and east slopes of the Washington Cascades, and 70-80% chances through the Eastern Mountains. Even though ensembles struggle with the timing of the system, the strength and efficiency in tapping into the moisture associated with the offshore closed low is present in 94% of members - with 64% of members suggesting an even wetter scenario than the current forecast. Moisture lingers over the area on Sunday, with an additional 0.10-0.20" across the Northern Blue Mountains and 0.01-0.10" over the Columbia Basin and Blue Mountain foothills. 75

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

VFR conditions prevail through the period. Most winds will be sustained less than 12 knots throughout the period. SCT to BKN clouds will move in the area through the latter part of the period, but currently expecting cloud heights at 10000 to 25000 feet.

Preliminary Point Temps/PoPs

PDT 84 53 83 48 / 0 0 0 10 ALW 84 60 83 53 / 0 0 0 10 PSC 86 55 87 51 / 0 0 0 0 YKM 84 58 81 50 / 0 0 0 0 HRI 86 56 86 52 / 0 0 0 0 ELN 82 57 72 46 / 0 0 0 0 RDM 87 47 84 44 / 0 0 0 0 LGD 84 51 85 46 / 0 0 0 20 GCD 87 52 85 48 / 0 0 0 0 DLS 88 60 82 54 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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