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FXUS66 KPDT 032334 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 434 PM PDT Fri May 3 2024

Updated for Aviation


AVIATION
00Z TAFS...VFR conditions are generally expected through the period, except for BDN and RDM, and possibly DLS. An upper low will move toward the coast tonight and inland over northern California Saturday night. Rain will develop in advance of this low as early as tonight for locations such as DLS, RDM and BDN and continue through much of the day on Saturday. RA is expected to develop later Saturday elsewhere. Latest guidance suggests likely MVFR conditions at BDN and RDM on Saturday (60-80% confidence)
and a lower chance at DLS (20-30%) confidence. At this time, will keep DLS TAF VFR, but may need to add some MVFR conditions in later issuances.

Winds will also increase at most sites on Saturday and could gust as high as 25 kts.

PREV DISCUSSION
/issued 329 PM PDT Fri May 3 2024/

SHORT TERM
Today through Sunday Night

Key Messages:

1. Significant rain and mountain snow through the weekend.

2. Some area rivers nearing or exceeding action (bankfull) stage.

3. Breezy winds late Saturday through Sunday evening.

Current radar and visible satellite imagery showing light to moderate returns beginning to approach the west slopes of the Oregon Cascades as mid-to high level clouds extend into the Basin. This is in response to a robust upper level trough that is dropping from the the Gulf of Alaska and closing in on the Pacific Northwest coast. The low and associated cold front will pass through our area on Saturday as wrap-around moisture will keep widespread precipitation chances in the forecast through Sunday afternoon. This cold front will allow for snow levels to quickly drop 6000-7000 feet early Saturday morning to 2500-4000 feet early Sunday morning across Central Oregon. These dropping snow levels coupled with ample moisture will lead to significant, advisory level snowfall to occur across the east slopes of the Oregon Cascades and through the John Day-Ochoco Highlands. This has warranted to issuance of Winter Weather Advisories beginning at 5 AM Saturday across the east slopes of the Oregon Cascades and starting at 5 PM along the John Day-Ochoco Highlands above 4000 feet. 4 to 10 inches of snow is expected across the east slopes above 4000 feet and 3 to 7 inches of snow is likely across the John Day-Ochoco Highlands above 4000 feet. There is also a potential for significant snowfall across the southern Blue Mountains with 2-5 inches possible, mainly above 4000 feet. These amounts across the southern Blue Mountains are just below advisory criteria, but will need to be closely monitored.
Confidence in these snow amounts are moderate to high (70-80%) as the NBM suggests a 65-85% chance of 6 inches or more snowfall across the east slopes of the Oregon Cascades Saturday afternoon through Sunday afternoon, a 70-90% chance of advisory level snow amounts (3 inches or more) across the John Day-Ochoco Highlands Saturday evening through Sunday evening, and a 40-60% chance of advisory level snow amounts (5 inches or more) along the southern Blue Mountains Saturday evening through Sunday evening.

Substantial low and mid-level moisture is associated with this system as rain accumulations of 0.50 to 1.25 inches are expected across lower elevations of the Basin Saturday morning through Sunday night, with 1.5-2 inches likely across the Blue Mountains and Cascades. The only areas that are expected to see 0.25-0.50 of an inch include the Bend-Redmond area and the Kittitas Valley.
Confidence in these rain amounts are moderate (50-65%) as the NBM suggests a 40-65% chance of 0.50 of an inch of rain across lower elevations, with higher chances residing along the northern Blue Mountain foothills and the John Day Basin. The NBM also highlights a 50-60% chance of rainfall of 1 inch or greater over the northern Blue Mountains and Elkhorns, with only a 10-30% chance over the Oregon Cascades. This moisture will also translate to rises across area rivers, with only the Naches River near Cliffdell currently expected to reach action (bankfull) stage briefly Saturday evening through Sunday afternoon. The John Day River at Service Creek looks to stay just shy of action (bankfull)
stage Monday afternoon, but will continue to monitor as the weekend event unfolds.

The incoming system will also attribute to breezy winds that will begin tonight ahead of the approaching cold front. Winds are expected to increase to 30-40 mph across the base of the northern Blue Mountain foothills beginning shortly after midnight tonight and extend through much of the morning on Saturday. The cold front will pass late Saturday morning to allow these winds to slacken through the afternoon as a pressure gradient begins to set up along the Cascades to increase winds through the Simcoe Highlands, eastern Gorge, and the northern Blue Mountain foothills beginning Saturday evening and extending through the day on Sunday. Winds are expected to peak between midnight on Sunday through noon Sunday as advertised by the GFS, NAM, and SREF indicating a 11-12.5mb pressure difference between Portland and Spokane. These values are just shy of advisory criteria as gusts of 35-45 mph will be possible. Winds will be slowly decreasing through the late afternoon and evening, but gusts of 30-40 mph will still be possible until early Monday morning. Confidence in these wind gusts are high (80-90%) as the NBM suggests an 85-95% chance of wind gusts of 47 mph or greater across the northern Blue Mountain foothills and the Kittitas Valley. This may warrant the need for wind highlights to be issued, so stay tuned. 75

LONG TERM
Monday through Friday
Elongated troughing will bring about a cool weather pattern for much of the work week, before a high pressure ridge moves in and warms up temps toward seasonal averages and perhaps even above normal values by the start of next weekend.

Ensembles are in pretty good agreement on the synoptic pattern next week, starting with a deep low centered over the Gulf of Alaska that partially envelops the PacNW with cold air. Guidance suggests that this low will essentially split into two systems: a low that remains over northern Canada and another low that gradually shifts eastward into the northern Plains. During this splitting process, the parent Alaskan low will advect in moisture into the forecast area on Monday, however QPF guidance early on indicates that this will primarily be a mountain/high elevation event. NBM probabilistic guidance depicts only about a 20% chance of much of the Basin seeing at least 0.05 inches, with chances increasing across our foothill zones (40-60%) and central Oregon (45-55%). The warm air advection from this weekend's system might shield the forecast area from lower snow levels, with Monday's levels in the 4000-5000 ft range, but cannot completely rule out our Cascade Passes from seeing some light accumulating snow. That being said, confidence is on the lower end (30-40%) due to how borderline snow levels are in the NBM for Monday.

Once the low splits, our forecast area will find itself on the western flank of the departing low to the east, resulting in some cold air advection via northerly flow aloft. Temps look to remain below average Tuesday into Wednesday by as much as 15 degrees, however temperature ensembles do not currently suggest a freeze risk for the Basin. PoPs fall off on Tuesday as drier air prevails overhead with PWATs in the 0.2-0.3 range, leaving primarily light, orographically driven precip over the mountains thanks to N/NW flow aloft.

For Wednesday onward, the forecast looks to be dry and warmer.
Ensembles are in pretty strong agreement that a ridge will build in by the end of next work week, allowing temps to soar well into the 70s across our population centers. Should high pressure persist, as 8-10 day ensemble clustering suggests, highs in the lower Basin could even reach into the low 80s by next weekend. Confidence is high (70%) in this warm and dry forecast panning out based on what ensembles are showing. Evans/74

AVIATION
18Z TAFS
VFR conditions for today with currently high clouds at most sites, except for KPDT/KALW having clear skies. Cloud coverage will increase starting this afternoon into tonight/Saturday morning. KPDT might have winds gusting up to 24kts tonight as the low off OR coast pushes a cold front over PacNW. This will also bring rain showers into KDLS/KRDM/KBDN/KYKM Saturday morning as well. Winds will be less than 10kts, but will increase for KPDT/KALW this evening and overnight. Feaster/97

PRELIMINARY POINT TEMPS/POPS
PDT 51 68 37 47 / 20 40 90 80 ALW 55 72 41 50 / 10 30 90 80 PSC 56 72 46 56 / 10 30 90 70 YKM 49 63 44 62 / 30 80 70 40 HRI 54 70 41 54 / 20 40 90 80 ELN 44 61 42 57 / 30 80 50 40 RDM 40 49 31 47 / 70 80 50 50 LGD 47 65 37 47 / 10 20 90 80 GCD 46 61 34 44 / 10 80 90 90 DLS 49 56 45 55 / 50 80 70 50

PDT WATCHES/WARNINGS/ADVISORIES
OR...Winter Weather Advisory from 5 PM Saturday to 11 PM PDT Sunday for ORZ506.

Winter Weather Advisory from 5 AM Saturday to 11 AM PDT Sunday for ORZ509.

WA...None.

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