textproduct: Pendleton

KEY MESSAGES

- Region-wide rainfall expected today, beginning in the late morning.

- Dry weather and warmer than average temperatures Wednesday and Thursday.

- Widespread rain and breezy winds possible Friday through the weekend, especially along Cascades and Blue Mountains.

DISCUSSION

GOES-West satellite imagery continues to show a frontal low offshore of the coast progressing towards the northeast. Likewise, radar imagery shows localized bands of light returns around the Columbia Basin, however most of the precipitation is not reaching the ground. As the cold front progresses onshore, significant rainfall is likely to occur along the Cascades and northern Blue Mountains beginning late this morning/early afternoon. Given the terrain-driven nature of this rainfall, there is a wide spread in potential rainfall totals from guidance. Areas in the Cascades may see as little as 0.05 to 0.25 inches (10% chance of being lower) or as much as 1.5-2 inches (10% chance of being higher). Lowland areas are expected to see significantly lower amounts, with similar best/worst case scenarios of 0 to 0.6 inches for the central Columbia Basin, 0 to 0.8 inches for the SW Columbia Basin, and 0 to 0.5 inches for the Blue Mountains.

By late Tuesday into Wednesday, the synoptic 500 hPa setup is expected to transition from the current, predominantly zonal flow pattern, into a much more pronounced trough/ridge pattern. Current ensemble guidance positions the trough portion of this pattern offshore, digging southernly around Tuesday afternoon into Thursday morning. The associated ridge portion of the pattern is expected to deepen simultaneously, with the ridge axis positioned east of the OR/ID border. This is expected to bring temperatures around 5-15 degrees above normal throughout the region, with cooler temperatures late into the week as the trough progresses eastward.

While there is high confidence (greater than 90% chance) in the occurrence of this trough/ridge pattern mid-week, uncertainty exists in regards to the exact timing/shape of the trough, particularly with its arrival onshore Friday/Saturday. Ensemble clusters appear to be centered around two main scenarios, one being a deeper, more unified trough (around 30% of members) and the other being a weaker main trough alongside a cutoff low to the south (around 70% of members). Scenarios that favor a deeper main trough support higher precipitation odds, whereas the cutoff low scenarios, especially ones with more zonal flow for the main trough axis, more limited precipitation. Still, regardless of exact scenario, a moderate chance exists for at least wetting rains (at least 0.1 inches within 24 hours) for much of the region Saturday and Sunday (40-50% for the Columbia Basin, 50-75% chance for surrounding regions). Breezy winds are also likely to be expected with this system, with 50-75% chance of gusts exceeding 40mph on Friday/Saturday night along the John Day Highlands and southern Blue Mountains. A 30-50% chance of 40mph or greater gusts also is present for the northern Blue Mountains and Wallowa Mountains at this time.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

VFR conditions are likely (90% confidence) to persist through most of the period. There is a high probability (70-90% chance) of light rain across the region (between 09/21Z-10/07Z depending on location) which could temporarily cause ceilings to drop into MVFR conditions (20-30% confidence). The best chance of MVFR conditions is for YKM. Winds are going to remain below 10KTS for DLS,PDT,YKM,ALW, and PSC (90% confidence). BDN has the best chance of seeing winds in the low teens with gusts into the low 20s possible (Confidence 70-80%). Garty/26

Preliminary Point Temps/PoPs

PDT 60 43 65 43 / 20 70 20 20 ALW 58 47 62 48 / 20 90 30 20 PSC 58 38 59 37 / 30 70 40 20 YKM 56 35 55 38 / 40 80 50 30 HRI 59 39 60 39 / 20 70 20 20 ELN 51 30 49 35 / 40 90 60 40 RDM 60 37 64 38 / 50 40 0 0 LGD 58 39 59 44 / 0 60 10 10 GCD 60 39 63 38 / 10 50 10 0 DLS 57 43 58 44 / 80 90 40 10

PDT WATCHES/WARNINGS/ADVISORIES

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