textproduct: Pendleton

KEY MESSAGES

- Breezy to windy today.

- Winter weather and wind returns late Tuesday. Winter weather headlines have been issued for the Cascades.

- Near- to sub-freezing morning lows Thursday and Friday for the lowlands.

DISCUSSION

A deformation band brought copious precipitation to northeast Oregon and southeast Washington late yesterday into this morning. Satellite shows the upper low responsible for the wet weather has tracked east into the Rockies. In its wake, skies have partially cleared, and strong cross-Cascade pressure gradients (~9-11 mb difference between PDX and GEG) in tandem with strong cold air advection (CAA) have driven widespread breezy to windy westerly winds through the Cascade gaps and into the Columbia Basin. Strongest winds have been noted through the exit region of the eastern Columbia River Gorge, lower Columbia Basin of Oregon, Kittitas Valley, and foothills of the northern Blue Mountains of Oregon. Previously, anticipated the winds to slacken by late morning to early afternoon, so opted to highlight the winds with Special Weather Statements. In hindsight, Wind Advisories would have been justified as numerous sites such as PDT, HRI, and ELN have either flirted with or maintained advisory-level sustained winds (30 mph) or gusts (45 mph) into the afternoon hours. Either way, winds should weaken by tonight as the shortwave that is tracking overhead pushes east and pressure gradients slacken.

The break in significant weather will be short-lived as another strong weather system is expected (95 percent confidence) to arrive late Tuesday morning into Tuesday afternoon. Ensemble and deterministic NWP guidance are in excellent agreement that this system, the shortwave trough currently present in the Gulf of Alaska, will drop southeast into the Pacific Northwest with its attendant cold front expected (90 percent confidence) to lower snow levels below mountain pass levels by later Tuesday morning (Washington Cascades) through Tuesday afternoon (Oregon Cascades). Winter weather headlines have been issued for moderate to heavy snowfall for the Cascades. Confidence is high (80 percent) in 4-8 inches of snowfall for the Washington Cascades with 6-12 inches for the Oregon Cascades, locally higher along the crests. The bulk of the snowfall will likely (80 percent confidence) end through Wednesday morning or early afternoon as the front sags southeast, but upslope snow showers will persist through Wednesday night or Thursday morning as the upper low moves overhead.

Precipitation for non-mountain areas will be mostly rain below 1500-2000 feet, though uncertainty exists in snowfall between 2000 and 4000 feet. Much of this uncertainty is driven by how long the frontal band lingers over the lowlands, and also in post-frontal snow showers.

Widespread breezy to windy southwest winds are expected Tuesday afternoon and night in advance of the cold frontal passage. By Wednesday afternoon, winds will become westerly to northwesterly in the wake of the front and remain diurnally breezy to windy through Thursday. Probabilities of exceeding advisory levels is widely varying (30-80 percent), highest for wind-prone locations of the foothills of the Blue Mountains and north-central Oregon Tuesday then the Kittitas Valley for Wednesday and Thursday.

Near- to below-freezing morning temperatures are forecast for many lowland areas Thursday and Friday, just in time for growing season. NBM calibrated probabilities suggest medium-high chances (60-90 percent) of freezing temperatures for the Yakima and Kittitas valleys, medium probabilities (30-70 percent) along the foothills of the Blue Mountains, and low-medium (20-40 percent) for the lowest elevations including the eastern Columbia River Gorge and lower Columbia Basin.

An upper-level ridge will likely (80 percent confidence) build into the Pacific Northwest Friday into the weekend, yielding lighter winds and mostly dry conditions. By Sunday, uncertainty in the 500-hPa pattern grows, but some flavor of a 500-hPa closed low or trough is advertised by ensemble clusters.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

VFR conditions continue for the TAF sites. Will see precipitation occur for all TAF sites by the afternoon hours along with some breezy to locally windy conditions. Winds could gust up to 35 mph, especially in the Columbia Basin sites including the DLS. Temporary sub-VFR conditions are possible to develop (5-15 percent chance) if more moderate showers develop over TAF areas, lowering VIS and/or CIGs.

Preliminary Point Temps/PoPs

PDT 57 36 58 41 / 50 0 10 60 ALW 58 40 58 42 / 80 10 20 80 PSC 67 40 61 44 / 20 0 20 40 YKM 63 35 58 39 / 10 0 20 60 HRI 62 38 60 42 / 10 0 10 40 ELN 56 35 51 32 / 10 0 40 70 RDM 54 26 56 35 / 20 0 10 50 LGD 53 32 56 39 / 90 20 10 90 GCD 52 30 58 38 / 80 30 10 70 DLS 60 41 58 41 / 10 10 40 80

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 5 PM Tuesday to 11 PM PDT Wednesday for WAZ030. Winter Weather Advisory from 11 AM Tuesday to 11 PM PDT Wednesday for WAZ522. OR...Winter Weather Advisory from 5 PM Tuesday to 11 PM PDT Wednesday for ORZ502. Winter Storm Warning from 5 PM Tuesday to 5 AM PDT Thursday for ORZ509.