textproduct: Pendleton

KEY MESSAGES

- Long-duration atmospheric river event will bring heavy snow to the WA Cascades tonight-Friday, and high winds to the interior Northwest late tomorrow.

- As much as 1-3 feet of snow tonight to tomorrow night in the WA Cascades with a lull in winter weather at pass-level Wednesday afternoon-Wednesday night before resuming Thursday.

- Strong winds over 50 mph anticipated Wednesday evening to Thursday morning across the lower elevations and foothills.

- OR Cascades and northern Blues will see moderate to locally heavy snow of 6 inches and more rest of today through tomorrow.

- Increased confidence in a warming trend next week with highs as much as 10 to 20 degrees above average by Tuesday.

DISCUSSION

An active 72-hrs sensible weather-wise as an long duration atmospheric river event sets up. Latest water vapor imagery shows broad moisture curving anticyclonic in the eastern North Pacific offshore CA and OR that will be reinforced with the AR making landfall tonight. Latest ensemble guidance shows excellent agreement in its timing and direction with probability of AR conditions greater than 250 kg/ms penetrating inland over 90% tonight with high confidence in at least a 48-hr window of this IVT being realized into early Friday. Meantime, near zonal orientation expected with its axis and is roughly aligned with the OR/WA border Wednesday-Friday.

Late tonight into tomorrow, the primary mid-level shortwave will lead to a developing surface low with its attendant warm front crossing into the Lower Columbia Basin mid-late afternoon tomorrow. This low and its frontal region will coincide with a strong surge of IVT aimed at the WA Cascades that will also occur in tandem with strong winds aloft spilling east and rising snow levels. With this IVT surge, guidance supports high chance (>80%) of IVT > 500 kg/ms making landfall centered around tomorrow afternoon. Also, strong 850 mb winds of 50 kts will spread into the area east of the Cascades after 00Z Wed/5 pm Thu.

Heavy snow is expected to pick up this afternoon in the WA Cascades but will then be followed by a lull. Snow is expected to ease early afternoon Wednesday to 2 am Thursday as changeover to a mix or rain occurs stemming from snow levels rising above 4,500 ft during the day before falling below 3,000 ft by 5 am. After the initial window of heavy snow at pass-level in the WA Cascades, concerns will pick up again Thursday and continue into early Friday morning. Of which, Snoqualmie Pass has a 80-90% chance of 12 inches or more tonight-tomorrow with another chance for 1-2 foot Thursday morning-early Friday morning. The high confidence in heavy snow in the WA Cascades stems in part from the orientation of the AR and also the strong synoptic forcing. Meantime, sufficient AR penetration inland will support moderate snow inland with the northeast Blues chance of 8 inches or more around 80% late tonight through late tomorrow before rising snow levels limit snow. Southward, the OR Cascades will see locally moderate snow at the upper slopes, including Santiam pass. However, lower confidence in snow impacts stemming from the warmer environment there. Overall, a strong signal is in place with EPS ensemble members in agreement on high QPF and snowfall with extreme forecast index values approaching and exceeding 0.9 Wednesday night-Thursday. Further, the high-end potential is quite high for QPF and snow with a shift of tails greater than 2 for QPF Wednesday night- Thursday, and again for QPF Thursday night-Friday.

While winds pick up tomorrow afternoon onward, the warm environment and high snow levels will promote wet, dense snow that will limit blowing snow concerns. As such, limited blow snow is expected along the upper slopes of the eastern WA Cascades with the initial round. This is further supported when looking at HREF and West-WRF joint probabilities. Elsewhere, these winds will translate eastward with high potential to mix down to the surface late tomorrow afternoon. The windiest period is forecast 8 pm to 2 am, and the highest winds expected over exposed ridges and summits. Confidence further bolstered by Extreme Forecast Index of 0.7-0.9 Wednesday night-Thursday across south central to eastern WA.

Current thinking is the northern Blues has some potential for another winter headline with that area having a greater lull in winter weather Thursday-early Friday. Meantime, areas could see snow linger as the AR drifts south and weakens over time late Friday-Saturday. Of which, snowfall amounts appear not as impactful Friday night-Saturday.

This weekend and beyond, ensemble guidance in good agreement with a building upper-level ridge over the West Coast. Latest EPS forecast show 500 mb mean heights exceeding the 90th percentile offshore coast of CA that will then shift Monday with the ridge axis extending then over the northwest. This will start a warming trend with highs forecast 5 to 15 degrees above normal Monday rising to 10 to 20 degrees above normal Tuesday (upper 60s to lower 70s in the lower elevations.

AVIATION /06 TAFS THROUGH 06Z THURSDAY

VFR conditions prevail for all sites. The embedded disturbance in mid-level zonal flow will continue bringing snow showers across the East Slopes of WA Cascades. In addition, light rain showers remain forecasted for most sites around tonight and during the day Wednesday (30% probability).

Winds at all sites have decreased to 12 kts or less from this evening to early night. However, KBDN/KPSC may see gusty winds of 20- 25 kts late tonight with KPDT, KYKM, and KALW gusting at 25-35 kts during daytime hours Wednesday (>50% confidence). Guidance do show KDLS potentially having gusts of 20-30 kts Wednesday evening, but confidence is 15-30%. Feaster/97

Preliminary Point Temps/PoPs

PDT 35 56 45 56 / 30 50 50 70 ALW 37 55 46 55 / 50 70 70 80 PSC 37 59 47 60 / 10 30 30 40 YKM 29 55 40 55 / 40 70 80 60 HRI 37 59 47 59 / 20 30 30 50 ELN 27 46 36 46 / 40 80 80 70 RDM 29 54 40 55 / 20 30 30 40 LGD 29 48 42 50 / 60 80 80 90 GCD 28 53 43 55 / 20 40 20 60 DLS 37 54 45 55 / 80 80 90 80

PDT WATCHES/WARNINGS/ADVISORIES

WA...High Wind Warning from 8 PM Wednesday to 11 AM PDT Thursday for WAZ026-028-029-521. Winter Weather Advisory from 8 PM this evening to 8 PM PDT Wednesday for WAZ030. Winter Storm Warning from 5 PM this afternoon to 5 AM PDT Friday for WAZ522. OR...High Wind Warning from 8 PM Wednesday to 11 AM PDT Thursday for ORZ044-507-508-510. Winter Weather Advisory until 5 PM PDT Wednesday for ORZ509.