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FXUS66 KPDT 200329 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 829 PM PDT Sun May 19 2024

EVENING UPDATE
Isolated showers are diminishing in coverage across south-central WA and north-central to northeast OR as instability wanes. Of note, the 00Z OTX sounding recorded over 600 J/kg of surface-based CAPE, though values were likely less over our forecast area based on observed vertical extent of the stronger cells and RAP-based mesoanalysis. Still, where convective activity was more robust over the east slopes of the Washington Cascades, some lightning registered on lightning products.
The evening update reflects minor changes to PoPs, removed thunder for Wallowa County and added it for the Wenatchee Mountains north of Ellensburg from 5PM-8PM. The threat of lightning is rapidly tapering off as sunset approaches so do not have it in the 8PM-11PM weather. Plunkett/86

PREV DISCUSSION
/issued 420 PM PDT Sun May 19 2024/

SHORT TERM
Tonight through Tuesday night
A positive tilt longwave trough extends across most of Canada and across the northwest interior of the US, and eastern WA/OR is on the backside of the trough and under a northerly flow aloft. The air mass is slightly unstable over WA and northeast OR, as evident by the diurnal showers increasing in coverage north and west of our forecast area. The trough is sagging southward, therefore showers will travel south with widely scattered showers by early this evening over the forecast area. The best chance of showers (40-60%) is over Wallowa County. The NBM places Wallowa County in a 20% chance of t-storms this evening as well, although the CAMS have limited instability and shear and there is a lot of cloud cover at this time to prevent surface heating. Will keep a slight chance there but confidence is low. Showers will end after sunset, and the rest of the night will be clear to partly cloudy and chilly. Subfreezing temperatures will once again be observed in the mountains and some of the adjacent valleys.

The instability associated with the upper trough is preventing surface winds from decoupling from winds aloft, and this has resulted in the relentless breezy winds. Since little change in the weather pattern is expected on Monday, the widespread gusts to 20-30 mph (locally higher) will continue. While a drier northerly flow will keep most of the forecast area precipitation free, instability and moisture remain east of the Blue Mtns where PoPs of 20-40% are forecast for Monday afternoon along with a slight chance of thunderstorms in Wallowa County.

A return to cool and wet conditions over the PacNW is set for Tuesday and Tuesday night. A deep closed low over B.C. will make its appearance and provide numerous showers. Precipitation chances increase in our area late Tuesday afternoon and evening to around 60-90%. The only exception will be over south central WA and along the Columbia Deschutes Plateau where rain shadowing will keep most locations dry or limit rainfall, and PoPs will show 20-40%. Although rainfall amounts will not be substantial, wetting rain will be observed over most forecast zones. NBM shows around a 40-60% chance of 0.5" or more rainfall in the Blues, Wallowas, and the eastern Cascade valleys (20% along the Blue Mtn foothills and Grande Ronde Valley) with this approaching system. Wister/85

LONG TERM
Wednesday through Sunday
Unsettled and cool weather is expected through the long term period as a couple of troughs move across the area Wednesday and Wednesday night and again during the Friday night to Sunday period. This will bring showery weather and a slight chance of mainly mountain thunderstorms. The Extreme Forecast Index highlights rain Wednesday along the Cascades, eastern mountains and Blue Mountain Foothills (which has a shift of tails raising a low possibility of an abnormally wet day there). It also highlights winds on Wednesday over most of the Oregon portion of the area and the Simcoe Highlands. On Thursday, while the EFI doesn't have significant snow, it does have a shift of tails as high as 1 over Wallowa county and with a slight chance of late afternoon thunderstorms there, that appears to be suggesting a low chance of heavy convective snow showers Thursday evening. In addition, the EFI has generally cool temperatures Wednesday in Washington and in the eastern mountains on Thursday.

Cluster analysis shows model ensemble members starting out in good agreement on Wednesday in having an upper low and trough over the area though the GFS deterministic run ejects it into Idaho faster than the ECMWF and Canadian. Agreement remains good on Thursday in having the trough over Idaho though there are differences as to whether the flow over our area will be north, northeast of northwest. On Friday about 80 percent of the models have a shallow weaker trough over the Rockies while the other 20 percent have a stronger trough still centered over Idaho and down into California. On Saturday, models have 55 percent of the ensemble members with a closed low and trough over our area while the rest have a much shallower and weaker trough. On Sunday, 66 percent of the models have a trough over our area or just to the east in Idaho, 22 percent have a very shallow trough over Idaho and a weak ridge along the PacNW coast and the rest have an approaching trough offshore with a southwesterly flow over our area.

Wednesday will have a chance of rain over most of the area with the heaviest rain over the mountains with up to a half inch while the Blue Mountain Foothills will get one to two tenths of an inch and the rest of the area just a few hundredths of an inch.
Instability from the low overhead will give the eastern mountains a slight chance of thunderstorms in the afternoon and expanding into the Blue Mountain Foothills and north central Oregon in the late afternoon. Snow levels will be around 4500-5000 feet, so up to an inch or two of snow will be possible on the higher mountain peaks. Tight pressure gradients will create west to northwest 20 to 30 mph winds in most of the lower elevations with the Kittitas Valley, Simcoe Highlands and the southern Blue Mountain Foothills having a 45 percent chance of gusts to 40 mph. Temperatures will be in the mid 50s to mid 60s with 40s and lower 50s in the mountains.

Thursday, with the trough and low moving east, will have a chance of mountain rain with snow above 6000 feet. Showers will taper off in the evening and the lower elevations will be mainly dry.
Being closer to the low, Wallowa county will have enough instability in the afternoon for a slight chance of thunderstorms.
Rain amounts in the mountains will be up to a tenth of an inch.
Temperatures will warm 6-8 degrees from Wednesday.

On Friday, the NBM compromises between the model solutions and has a slight chance of rain in the mountains becoming a chance in the mountains and Blue Mountain Foothills Friday night. The eastern mountains will have a slight chance of thunderstorms in the late afternoon. Rain amounts are less than a tenth of an inch.
Temperatures will be near normal in the upper 60s to mid 70s with 60s in the mountains.

Next weekend, with wide differences between the models, have a chance of rain in the mountains and the Blue Mountain foothills with the rest of the area mainly dry. Rain amounts are up to a tenth of an inch Saturday and just a few hundredths of an inch on Sunday. Thunderstorm chances will be limited to a slight chance each day over the higher terrain of the eastern mountains.
Temperatures are 3-5 degrees cooler Saturday and then warm back up 3-5 degrees on Sunday. Perry/83

AVIATION
00Z TAFs
Overall, VFR conditions are expected through the next 24 hours. Expect some SCT-BKN cigs at 7K-10K feet AGL through the evening with some isolated showers then clearing overnight. KDLS will have west winds of 10-20 kts with gusts to 30 kts through 04-06Z then start decreasing and drop below 10 kts.
Other sites will see 5-15kt winds through evening then decrease.

PRELIMINARY POINT TEMPS/POPS
PDT 39 68 42 65 / 10 0 0 10 ALW 43 71 47 68 / 10 10 0 10 PSC 46 76 50 70 / 10 0 0 10 YKM 39 75 45 67 / 20 0 0 20 HRI 42 75 48 69 / 20 0 0 10 ELN 41 69 44 63 / 10 0 0 20 RDM 31 65 36 62 / 10 0 0 10 LGD 36 62 38 65 / 10 10 0 10 GCD 34 63 37 65 / 10 10 0 10 DLS 44 71 48 64 / 10 0 0 30

PDT WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.

Vancouver
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Sun -- 12:20 AM PDT     0.27 feet Low Tide
Sun -- 03:46 AM PDT     Moonset
Sun -- 04:50 AM PDT     1.54 feet High Tide
Sun -- 05:34 AM PDT     Sunrise
Sun -- 12:59 PM PDT     0.10 feet Low Tide
Sun -- 04:58 PM PDT     Moonrise
Sun -- 05:17 PM PDT     1.21 feet High Tide
Sun -- 08:17 PM PDT     0.96 feet Low Tide
Sun -- 08:40 PM PDT     Sunset
Sun -- 09:19 PM PDT     0.97 feet High Tide
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Vancouver, Columbia River, Washington (dubious accuracy), Tide feet

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