textproduct: Pendleton

KEY MESSAGES

- Moderate snow accumulations across the WA Cascades through this morning, with intermittent light mountain snow/rain through the weekend

- Breezy south to southwest winds today

- Below normal temperatures through the week

DISCUSSION

Water vapor satellite imagery tonight shows an upper closed low over Vancouver Island and southwest BC, while the PacNW sits under a broad upper trough. Radar and nighttime satellite imagery show rain/snow showers rounding the southern flank of the upper closed low into the WA Cascades. Elsewhere, clear to partly cloudy conditions are present over the lower elevations, with low stratus over portions of the eastern mountains.

Today through Friday: The upper closed low just north of the Olympic Peninsula will continue to guide light to moderate rain/snow showers into the WA Cascades the remainder of today, with light rain/snow showers expanding across the OR Cascades and northern Blues later this morning into the evening hours. A cooler airmass associated with this low will continue to filter into the region this morning, with snow levels lowering to around 3.5-4.5kft by sunrise. The increased shower activity will bring snow accumulations of 3-7 inches above 4kft along the WA Cascades, with accumulations generally 1-3 inches above 5kft in the OR Cascades, Elkhorn Crest, Eagle Caps, and interior northern Blues(confidence 75-85%). A Winter Weather Advisory is currently in effect for the Upper East Slopes of the WA Cascades through this afternoon.

Shower activity is expected to weaken and become intermittent through Friday as the upper low remains parked offshore. Any additional snow accumulations along the Cascade crest will be between 1-2 inches through Friday night (confidence 65-75%), with very low chances (5-15%) of a dusting (~0.1 inches) across the remaining mountain zones.

Otherwise, a cooling trend associated with the upper closed low will result in afternoon temperatures lowering into the 50s today, then lower further into the mid 40s to low 50s tomorrow and Friday (confidence 75-90%). Chances will increase through the week for near to below freezing low temperatures in the lower elevations, with best chances (75-95%) arriving Saturday morning. Freeze warnings may be needed by Saturday morning for areas that have not received an area-wide season ending freeze. Breezy south to southwest winds (15-25mph with gusts to around 35mph) will continue to impact portions of central OR to the Columbia Basin today, with light winds returning later this evening.

Saturday through Tuesday: Ensemble guidance is in agreement of the upper closed low sliding inland Friday night into Saturday, with continued chances of light rain/snow in the mountains through at least Saturday afternoon(confidence 60-75%). Saturday night into Sunday, the upper trough will push into the northern Rockies, allowing an upper level ridge to slide over the region Sunday through Monday with dry conditions (confidence 55-65%). After Monday afternoon, ensemble guidance is split into three scenarios. The first scenario is the upper level ridge gradually pushing east, but the PacNW remains under dry/calm conditions with a slight warming trend(~50% of Ensemble cluster members). The second scenario is for a shortwave trough to flatten the upper ridge as it moves across southern BC and northern WA, with a return of light mountain rain/snow across the Cascades and northern Blues Monday night into Tuesday (~30% of members). The last scenario is for an upper trough in the eastern Pacific to slide into the northern CA and southern OR, resulting in light shower activity along the Cascades and eastern mountains. Forecast confidence for early next week is low (10-15%) given the spread in ensemble forecast scenarios. Lawhorn/82

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/

VFR conditions to prevail through the period. An upper weather system to the north will bring snow showers to the Cascades and interior Blues throughout today, with prob30 chances of rain showers at site PSC through 17Z, sites YKM/ALW through 21Z, and sites PDT/DLS through 01Z. Breezy winds 12-17kts with gusts to around 25kts will develop at sites PDT/BDN/PSC through this afternoon. Otherwise, winds will be around 12kts or less through the period. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 54 32 52 31 / 30 10 0 0 ALW 53 35 53 34 / 60 30 0 0 PSC 59 30 54 31 / 10 0 0 0 YKM 56 26 52 30 / 30 0 0 30 HRI 57 30 52 30 / 20 10 0 0 ELN 48 21 47 22 / 60 10 0 40 RDM 49 20 48 21 / 10 0 0 0 LGD 52 26 54 28 / 30 20 0 0 GCD 51 25 55 29 / 0 0 0 0 DLS 56 31 55 33 / 90 40 0 20

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 5 PM PDT this afternoon for WAZ522. OR...None.