textproduct: Pendleton

KEY MESSAGES

- Cooler, below normal temperatures early in the week

- Lingering showers today across Central Oregon

- Mountain snow, widespread showers, and breezy winds midweek

DISCUSSION

Current radar and infrared satellite imagery shows light returns across the Oregon Cascades under partly to mostly cloudy skies. This is in response to the slowly departing cold front associated with a passing upper level shortwave trough located along the Canadian border. The cooler air behind the cold front will drop high temperatures 5-10 degrees from Sunday. Skies will clear through the morning and early afternoon, setting up a rather chilly Monday night/Tuesday morning as efficient radiational cooling will bring morning temperatures into the upper 20s across the Lower Columbia Basin, Blue Mountain foothills, and Central Oregon. High temperatures will rebound slightly Tuesday into the low to mid-50s, but still will be 5-7 degrees below normal.

ECMWF and GFS AI ensembles are in agreement with a strengthening upper level trough offshore as a surface low pressure develops over Northern California later today into Tuesday. These features will keep light shower chances over Central Oregon and the John Day Basin today (45-55%) and Tuesday (30-60%), with snow levels hovering between 3500-4500 feet both days. Minimal snow amounts are expected, with a 60-65% chance of 48 hour snow totals of 1 inch or more across Santiam Pass through Tuesday via the NBM. The better chances for rain and snow will come on Tuesday as mid-to upper level support is present due to the offshore upper trough passing through the area. As a result, a slight chance (~15%) of isolated thunderstorms will be possible across eastern Deschutes, southern Crook, and southern Grant Counties Tuesday afternoon. The NAMNest, HRRR, and NSSL all indicate the potential for isolated, discrete thunderstorms Tuesday between 2 PM and 8 PM, with rain amounts at or exceeding a wetting rain (0.10" or greater).

Ensembles are in good agreement related to an incoming upper level low pressure strengthening over the Gulf of Alaska Tuesday, passing through the Pacific Northwest Wednesday and Thursday. This system will bring a return to substantial snow amounts across the Cascades through the period, with the potential of 48 hour snow totals of 10-15 inches over Santiam Pass and 5-8 inches over Snoqualmie and White Passes. The Blue Mountains are also expected to pick up 4-8 inches during this timeframe. However, uncertainty in the upper low's path and strength is present, which directly relates to snow amounts and moisture efficiency and transport over the Cascade crest. Currently, 30-45% of members suggest Winter Storm Warning snow amounts being met across the Oregon Cascades (8 inches or more), with the NBM advertising an 80-90% chance. It is worth noting that the NBM also incurs an overall spread of 7 inches related to 48 snow totals Wednesday through Thursday. Will hold off on issuing any winter watch products due to this uncertainty, but a winter product (warning or advisory) is very likely (>80%) across the Oregon Cascades during this timeframe. Winter Weather Advisories are also expected along the Washington Cascades and the northern Blue Mountains midweek, as the NBM shows a 55-70% of 6 inches of snowfall Wednesday through Thursday.

A vorticity maximum will also be associated with the passing upper level low pressure Wednesday afternoon and evening, which may lead to a slight chance (10-20%) of isolated thunderstorms. Both the NAM and GFS showcase MUCAPE of 200-300J/kg and 30-40kt shear across Central Oregon, North-Central Oregon, John Day Basin, Grande Ronde Valley, and Blue Mountains/foothills. 74% of LREF ensembles indicate these convective parameters, which align with the potential for isolated, discrete thunderstorm development primarily between 2 PM and 8 PM Wednesday. Rain amounts of 0.15-0.35" are expected in these areas as a 70-90% chance of a wetting rain (0.10" or more) is indicated by the NBM.

Upper level ridging returns late Thursday through early Saturday, bringing a return to warmer and drier conditions. Ensembles then hint at another upper level shortwave passing Sunday as a broad trough approaches the British Columbia and Washington coasts early next week. 75

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

The RDM and BDN terminals are in a stratus and light -SN/-RA environment as moisture is lifted across the Oregon Cascades Today. Ceiling improvement from this mornings LIFR conditions is likely to be maintained (70%) to at least above the IFR level as the probability of ceilings less than 1 kft is 30% at Bend and 10% at Redmond by 21z based on HREF (ensembles). Elsewhere expect VFR conditions and light winds though this TAF period.

Preliminary Point Temps/PoPs

PDT 50 29 50 38 / 10 10 20 20 ALW 50 32 52 40 / 10 0 20 20 PSC 55 29 55 38 / 0 0 10 10 YKM 53 28 54 35 / 0 0 0 10 HRI 54 29 54 38 / 0 10 10 10 ELN 49 26 50 32 / 0 0 0 0 RDM 49 29 53 35 / 40 40 30 10 LGD 48 28 53 38 / 50 20 50 60 GCD 48 34 55 38 / 70 60 80 70 DLS 57 34 55 41 / 0 10 20 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Winter Storm Watch from late Tuesday night through Thursday afternoon for ORZ509.