textproduct: Pendleton

DISCUSSION

Through this Evening: A weak frontal boundary is starting to stall against the Cascade crest this afternoon, with light rain and high peak snow showers as a result. Otherwise, decks of cirrus have spread across most of the forecast area, while isolated areas of low cumulus clouds are also apparent across the inland PacNW.

Tonight through Thursday morning: An upper trough passage tonight through Wednesday night will bring widespread precipitation chances to the PacNW. Snow levels will start off just above 5kft with the approach of the trough, but drop to 4kft to 5kft as the trough moves inland early Wednesday morning. The heaviest of the precipitation will align with snow levels just falling to or below the surface along the Cascade crest early tomorrow morning, resulting in wet snow accumulations of 2 to 5 inches by Wednesday afternoon(confidence 75-85%). Through the rest of Wednesday, snow levels will continue to lower to 3kft to 4kft, resulting in more widespread snow in the mountains. Scattered snow showers will develop as the trough axis and an attendant cold front move through the region. Post frontal passage, lapse rates are expected to steepen across the OR portion of the forecast area and along the northern Blue mountains and adjacent foothills, resulting in a marginal convective potential in these areas Wednesday afternoon into the early evening. As the trough axis pushes further east, snow rates will become very light across the forecast area. Through Thursday morning, Santiam Pass will see a 80-90% chance for snow accumulations greater than 6 inches. As a result, a winter weather advisory has been issued for the Oregon Cascade East Slopes above 4,500ft for Wednesday. At Snoqualmie pass, snow levels will generally be above pass level through Wednesday, resulting in only a 35-50% chance of an inch of snow accumulating through Thursday morning. That said, a few hires models (i.e. UW-WRF) are indicating the potential for a Puget Sound Convergence Zone to setup along the Cascade crest Wednesday night, which could result in greater snow accumulations (confidence 20-25%). In the interior northern Blues and in White Pass, there is a 70-85% chance of 3 inches of snow through Thursday morning; there remains a 65-80% chance of 4 inches of snow through Thursday morning along the Elkhorn crest.

Below the mountains, most lower elevation locations will see a 45-80% chance of rain accumulations greater than 0.05 inches develop as the trough axis and an attendant cold front sweep through the area. Heaviest rain amounts will be along the northern Blue mountain foothills, where there is a 50-80% chance of accumulations greater than 0.2 inches through Thursday morning. Otherwise, ensemble guidance is depicting breezy winds (gusts 35-50mph) developing with the aforementioned cold front passage Wednesday (confidence 70-85%), with strongest winds along the Cascade peaks/ridges and ridges in the Columbia Basin.

Thursday through Sunday Afternoon: There is good agreement in the latest ensemble cluster guidance that northwest flow aloft will develop initially, leaving light rain/snow chances along the Cascade crest and Northern Blue mountains Thursday. Weak northerly flow will then develop over the PacNW Thursday night into Friday as a ridge narrows offshore and a trough digs equatorward in the inland western CONUS. Weak mountain showers along the Cascade crest will continue under this flow regime throughout Thursday, with dry conditions elsewhere. Ensembles remain in good agreement Friday through the Sunday afternoon that northerly flow will shift to the northwest as the ridge flattens offshore, resulting in light showers across the Cascades and Northern Blues (confidence 55-75%). Snow levels will also rise Friday night into Saturday, limiting snow to mountain peaks, with rain or rain/snow mix mainly below 6kft to 6.5kft (confidence 50-65%). The flattened ridge and a closed low off the Baja California will also result in warming temperatures across the PacNW over the weekend, with moderate confidence (55-65%) that afternoon temperatures in the lower elevations will warm into the mid 60s to low 70s. Otherwise, locally breezy west winds (25-40mph gusts) will redevelop in the daylight hours each day through Sunday (confidence 60-70%).

Sunday Evening through Monday: There is growing consensus amongst ensemble cluster members (ECMWF, Canadian, GFS) that upper level troughing will develop over the PacNW Sunday night into early next week(confidence 45-60%), though there is timing and strength discrepancies amongst the cluster solutions. Regardless, the general trend is for cooler temperatures to return to the PacNW with increasing mountain snow chances and breezy to windy conditions by Monday, though confidence is low (25-30%) in specifics at this time. Lawhorn/82

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

VFR conditions will prevail through the forecast period. Rain is expected to reach TAF sites after 13Z for YKM/ALW/PSC and 19Z for DLS/RDM/BDN/PDT. CIGs will will remain VFR at 5kft. Winds will also increase to 15-20g25-30kts after 19Z across DLS/RDM/BDN/PDT as a front sweeps across form the Northwest.

Preliminary Point Temps/PoPs

PDT 44 52 36 50 / 50 90 60 30 ALW 45 52 38 51 / 80 100 80 50 PSC 44 59 37 58 / 40 80 20 10 YKM 39 56 34 54 / 40 60 10 0 HRI 44 57 38 56 / 40 90 30 10 ELN 37 50 35 48 / 50 70 20 10 RDM 38 48 30 45 / 20 90 30 0 LGD 41 48 35 45 / 60 100 90 70 GCD 39 46 33 42 / 30 100 80 60 DLS 44 55 41 53 / 70 90 50 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Winter Weather Advisory from 4 AM to 10 PM PST Wednesday for ORZ509.