textproduct: Pendleton

KEY MESSAGES

- Widespread precipitation today with light to moderate mountain snow and breezy to windy winds. *Winter Weather Advisory in effect*

-Persistent precip Thursday into next week with breezy to gusty winds.

DISCUSSION

Current conditions: Water vapor and radar imagery shows the transient ridge breaking down as the upper trough offshore pushes light rain showers across the Blue mountains, portions of the Columbia Basin of WA/OR and North Central OR. Surface observations reveal precip amounts up to 0.10 inch within those areas from the past hour.

Today through tonight: The upper trough passage will make its way across the PacNW with the southerly flow through later this morning. By afternoon into late evening, the trough axis and frontal system will then push further east into our region before moving out of our area tonight through Thursday morning. Precip chances will continue to increase, resulting in widespread rain showers and light to moderate mountain snow or rain/snow mix. WA/OR Cascades crest and Blue mountains will see precip totals exceeding 0.50 inch (60-90% prob from NBM 24-hr precip) through this afternoon. However, there is a 20-40% prob of 1 inch of greater for the Cascades crest and 40-50% prob for the Blues. This is due to the moisture support weakening over the Cascades crest while increasing for the Blues and eastern OR mountains with the trough axis moving eastward tonight through Thursday morning. The lowlands may see about 0.10 inch or less (50-80% prob). In addition to the frontal system, steeper lapse rates are expected across portions of our forecast area and along the northern Blue mountains and adjacent foothills, resulting in a general risk for thunderstorms in these areas this afternoon into the early evening (10-20% chance).

Light to moderate mountain snow will develop across the Cascades, Blue mountains and portions of eastern OR. As the upper trough makes its way to the PacNW this morning, snow levels will start off above 5kft at pass levels for wet snow to accumulate between 2 and 5 inches over the portions of eastern OR mountains, and the crest of WA Cascades (30-50% confidence). By the afternoon through Thursday morning, the upper trough will then push further east across the PacNW along with the frontal system. This will bring widespread snow over the mountain areas with snow levels of 3-4kft during that time. The Blue mountains and wallowa mountains may see snow totals exceeding 5 inches this afternoon through Thursday morning as the trough axis passes (50-90% confidence). Confidence is 75-95% for Santiam Pass to get snow accumulations more than 6 inches, resulting in keeping the winter weather advisory in effect for the East Slopes of OR Cascades for Wednesday from 4am-10pm PST. Snoqualmie Pass may have snow accumulations of 1 inch or less with higher snow levels (above 2.5kft). Model guidance (RAP and NAMNAST) are in good agreement with these snow amounts for those mountain areas. The models continue to show tight surface pressure gradients forecast over Kittitas Valley to the Columbia Basin. Breezy winds (gusts 35- 50 mph) are expected with the frontal passage today as well (70-90% confidence). The strongest winds will mainly develop over the Cascade peaks/ridges and ridges in the Columbia Basin.

Thursday into next week: Models and ensembles will remain in great agreement with that northwest flow aloft Thursday before becoming more northerly Thursday night into Friday. Persistent light snow and rain/snow mix are forecast along the mountain areas (WA/OR Cascade crests, Blue mountains, and eastern OR mountains) through Friday. Precip chances will gradually decrease to 20-40% for the Blues and eastern OR Friday afternoon through late Saturday night. Lowlands will enter a dry period starting later Friday afternoon into late Saturday night, thanks to that broad high pressure offshore. Sunday onward, we will enter into a troughing pattern where multiple of frontal systems may return. There is timing and strength discrepancies amongst the cluster members with the upper trough next week, but seemed to be favorable of the associated precip.

Thanks to these systems, gusty winds (35-50 mph) will be expected over the ridges at the WA Cascades and Kittitas Valley (60-80% confidence). The timing for those strong winds would be Thursday, Sunday, and Monday during afternoon and evening hours. The remaining locations will have breezy winds (20-30 mph) (60% confidence). Feaster/97

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/

VFR conditions prevail for all sites with light rain showers. Latest guidance are depicting that KDLS, KPDT, KRDM, KBDN, and KALW could see low clouds at 2.5kft or lower between 12Z and 18Z this morning (30% probability). All sites will have winds gusting at 20-30kts with sustained winds of 12- 20kts late this morning through evening hours. While the winds decrease for KRDM/KBDN/KYKM after 03Z this evening, breezy winds (12-15kts) may linger for KDLS/KALW/KPSC around that time. Feaster/97

Preliminary Point Temps/PoPs

PDT 52 37 51 35 / 90 60 30 30 ALW 53 38 52 37 / 100 70 50 40 PSC 58 37 58 36 / 80 30 0 20 YKM 56 34 53 33 / 70 10 0 20 HRI 56 38 56 36 / 90 40 10 20 ELN 50 35 48 33 / 80 20 10 20 RDM 47 29 44 28 / 90 30 0 10 LGD 49 35 44 31 / 100 90 70 60 GCD 46 32 41 30 / 100 80 60 40 DLS 54 41 53 41 / 100 50 30 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Winter Weather Advisory until 10 PM PST this evening for ORZ509.