textproduct: Pendleton

KEY MESSAGES

- Cold morning temperatures continue through early week. *Freeze Warnings Issued*

- Warming trend through the week, mountain showers late in week.

DISCUSSION

Current radar and infrared satellite imagery shows very light returns across the northern Blue Mountains as low-to-mid-level clouds linger across mountain zones of the Cascade and Blue Mountains. This is in response to an upper level low pressure that is currently over the Columbia Basin, which originated over northern British Columbia and the Gulf of Alaska last week. The progression of this system over the past several days has led to a cooling trend, as it advects a cold airmass into the region. The result has been consecutive days of freeze products as a Freeze Warning is currently active across the Yakima and Kittitas Valleys until 9 AM today. Confidence in freezing temperatures being reached this morning is best across the Kittitas Valley, as the NBM advertises a 60-90% chance of freezing temperatures for Thorp and Ellensburg. The Yakima Valley has better chances to freeze east of Toppenish (35-55%) versus in and around Yakima (5-15%) via the NBM. This slowly dropping system will also attribute to light rain-snow showers across the Blue Mountains this morning. The NBM advertises a 40-60% chance of measurable snowfall before 11 AM, dropping to a 20-40% chance through the afternoon. Overall snow amounts are expected to stay below an inch, with only a 25% chance of an inch or more over the Elkhorns. Snow levels will be increasing through the day from 3500 feet this morning to 4500 feet by this afternoon.

The system will continue to slide south and into northern Nevada through the day, enhancing the cold airmass and allowing an incoming upper level ridge to begin to infiltrate from the coast. This ridge of high pressure will clear skies and shift flow aloft to more of a north-northeast direction. These two added factors will attribute to more widespread freezing concerns across the Columbia Basin, Blue Mountain foothills, and Union/Wallowa counties as a Freeze Watch has been issued Monday evening through Tuesday morning. Best chances for freezing temperatures Tuesday morning are highlighted across Union and Wallowa counties with a 90-95% chance via the NBM. The next highest percentages from the NBM are expected along the northern Blue Mountain foothills at a 65-85% chance. Chances are advertised as more sporadic across the Columbia Basin as the NBM highlights a 60-80% chance of freezing for Sunnyside and Hermiston, but only 10-20% chance for Yakima, Tri-Cities, and The Dalles.

The ECMWF and GFS AI ensembles pass a weak upper level shortwave through Central British Columbia and Alberta on Tuesday and Wednesday, shifting the departing upper level low pressure east into the Four Corners area. Flow aloft becomes more from the west due to the coastal ridge suppressing as the Canadian shortwave nears the northern British Columbia coast. As a result, high temperatures will increase 5 to 8 degrees on Tuesday. The Canadian shortwave moves quickly into Alberta Wednesday morning, which may allow for light showers over higher terrain of the Washington Cascades. 47% of ensemble members suggest only minimal amounts of any precipitation, as incumbent dry air, associated with the present ridge of high pressure, and snow levels around 8000-9000 feet will inhibit any rain or snow amounts apart from higher terrain.

Guidance is in agreement with the coastal ridge strengthening and pushing further inland Thursday and Friday, shifting flow aloft to southwest and returning high temperatures into the mid-to-upper-70s across the Columbia Basin and Central Oregon. The ridge axis shifts east Friday allowing an upper level low pressure to attempt to drop south from the Gulf of Alaska on Saturday. Ensemble members align with the majority of precipitation being confined to the Cascades, but there is uncertainty in how far moisture will extend into the east slopes and valleys. The two factors that would lead to showers pushing east into the Kittitas and Yakima valleys are timing and strength. The early and stronger dropping system is suggested to bring a better chance of precipitation into the east slopes of the Washington Cascades. However, this scenario only amounts to 20-30% of ensemble members. This system does look to set the stage for a second, more promising upper level trough to drop into the area from the Gulf of Alaska, but the same inconsistencies of strength and timing arise. Currently, 41% of ensemble members advertise widespread rainfall, with the majority (59%) keeping it along the Cascades and east slopes late Sunday into Monday. 75

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

VFR conditions and light winds of 10 kts or less are forecast at all sites through the period.

Preliminary Point Temps/PoPs

PDT 56 29 64 36 / 10 0 0 0 ALW 56 34 65 41 / 20 0 0 0 PSC 61 31 67 36 / 0 0 0 0 YKM 62 34 65 40 / 0 0 0 0 HRI 60 29 66 36 / 0 0 0 0 ELN 60 30 62 35 / 0 0 0 0 RDM 55 22 66 31 / 0 0 0 0 LGD 52 25 64 31 / 20 0 0 0 GCD 51 24 64 33 / 20 0 0 0 DLS 68 37 70 41 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Freeze Warning from 11 PM this evening to 9 AM PDT Tuesday for WAZ026>029-521. OR...Freeze Warning from 11 PM this evening to 9 AM PDT Tuesday for ORZ044-049-050-507-508-510.