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FXUS66 KPDT 060317 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 817 PM PDT Sun May 5 2024

EVENING UPDATE
The main band of rain over the forecast area is beginning to dissipate and shift eastward into the mountains this evening as a deep low pressure system moves out of the region.
Orographic precip and windy conditions will persist as the forecast area remains under the influence of an elongated, negatively tilted trough centered over the Gulf of Alaska, but most of our population centers should expect dry conditions and gradually subsiding winds overnight. Broad NW flow aloft will then dominate overhead during the day Monday, allowing for more high mountain snow over the Blues and Oregon Cascades, where Winter Weather Advisories are in effect. Convective showers may also develop tomorrow afternoon across the Basin and foothills of the Blues with the trough axis overhead, but shower coverage is expected to be scattered in nature.

The ongoing Wind Advisory across the Oregon Basin and adjacent areas will be allowed to expire at 11 PM tonight, and will likely cancel the WWA for the Southern Blues early as precip is already subsiding and is expected to cease altogether before its expiration time at 2 AM. Otherwise all other headlines remain in effect. Changes to the forecast were minimal, and were focused mainly on updating winds and PoPs based on latest HRRR and ongoing trends. Evans/74

PREV DISCUSSION
/issued 436 PM PDT Sun May 5 2024/

SHORT TERM
Tonight through Tuesday night
Active weather across the forecast area continues this afternoon as the deformation band over the Basin and Blue Mountain foothills gradually weakens and shifts back east as the upper-level low over southeast Oregon begins to progress southeastward into the Great Basin. Flow aloft will remain westerly to northwesterly, supportive of upslope precipitation for much of the eastern mountains and Cascade crest. Have issued Winter Weather Advisories for the Cascade crest 09Z Monday until 06Z Tuesday for areas above 4500 feet. While afternoon temperatures will not be supportive of significant accumulation on Monday, overnight periods will see more in the way of accumulation with 4-8" of snow, locally higher, forecast for areas 4500-5000 ft. NBM probabilities are supportive of an advisory; 24-hr probabilities sit at 45-60% chance of 6" or greater, and 80-95% chance of 4" or greater through 12Z Tuesday. Over the southern Blues and Ochoco- John Day Highlands, precipitation associated with the deformation band will taper off this evening and overnight with existing Winter Weather Advisories slated to drop off by 06-09Z Monday.
Expect an additional 1-4" of snow, heaviest above 4500 feet. With regard to the breezy to windy westerly winds, 12Z HRRR guidance suggests winds will drop below advisory criteria later this evening into tonight so have opted to keep them as is.

Monday will see low chances (25% or less) of thunderstorms across the eastern mountains and foothills. Forecast mid-level lapse rates of 7-7.5 C/km and instability of less than 500 J/kg of surface-based CAPE will support shallow convection with a low- end lightning threat. Winds will remain breezy to windy, but confidence in widespread advisory-level winds or wind gusts was too low (generally <40% chance) to issue any new advisories.

Tuesday afternoon, flow aloft will turn northwesterly then northerly as the upper-level low over the High Plains retrogrades and an upper-level ridge builds offshore. Will see some showers over the eastern mountains, and perhaps along the Cascade crest, but confidence in any thunder is very low (<10% chance).
Plunkett/86

LONG TERM
Wednesday through Sunday
The long term period will see a ridge building out in the eastern Pacific early on and remaining in control of our weather through next weekend. This will give us a period of mostly sunny and dry weather with the main forecasting challenge being just how warm temperatures will get by next Saturday and Sunday. The Extreme Forecast Index indicates little in the way of unusual weather aside from above normal temperatures next weekend.

Models start out in good agreement Wednesday with a ridge along the west coast and a large upper low centered over South Dakota.
Circulation around the two features will send a north to northeasterly flow into our area. Skies will be mostly clear aside from some lingering clouds over the eastern mountains.
Temperatures will be warming 4 to 8 degrees from Tuesday with highs in the 60s with a few lower 70s in the Washington Columbia Basin and in the 50s in the mountains. Winds will be generally light though pressure gradients between the ridge and the low to our east will create 15 to 20 mph northwest winds in the Kittitas Valley.

On Thursday, models show the upper low splitting with most of the energy moving to the Great Lakes while a remnant low dives southwestward into Utah or eastern Nevada. This in turn allows the upper ridge to build into British Columbia and northern Washington.
Model ensemble members have some differences as to the exact locations of the low and the ridge axis, but they make little difference for our area. With the ridge overhead, dry and sunny weather continues with temperatures warming to the 70s in the Columbia Basin and in the 60s elsewhere. Pressure gradients between the ridge and the low will develop breezy northeast winds of 10 to 20 mph in the afternoon.

Friday sees the upper low weakening further and model clusters show that most of the ECMWF and Canadian members favor a more southerly position over southern California and Arizona while GFS favors the low being over northern Nevada and California. While we continue to have sunny and dry weather, the GFS solution is several degrees cooler than the other two models. The NBM favors the warmer temperatures and have temperatures in the mid 70s to lower 80s with mid 60s to lower 70s in the mountains.

Saturday and Sunday sees the upper low dissipating and moving east though 64 percent of the ECMWF and 20 percent of the other models keep a lingering circulation over northern Arizona and central California on Saturday. All of the models keep a broadened ridge over the west with the axis overhead on Saturday. On Sunday, just a broad flatter ridge remains with 77 percent keeping the ridge axis directly over Oregon and Washington while the other 23 percent have the ridge centered to our east over Montana. Dry and fair weather is expected with temperatures continuing to warm though with more differences between the model ensemble members.
The best compromise yields temperatures 10 to 15 degrees above normal both days with highs in the 80s in the lower elevations and in the 70s in the mountains. However, ensemble probabilities show a 25 to 50 percent chance of hitting 90 in the Columbia Basin and Yakima Valley while central and north central Oregon have a 40 to 70 percent chance of hitting 80 degrees each day. Perry/83

AVIATION
00Z TAFs
MVFR to VFR conditions to prevail through the period. An upper low pressure system continues to bring rain impacts to sites PDT/ALW/PSC this afternoon, and is expected to continue up to 05Z-06Z this evening for PDT/ALW and until 03Z for site PSC. Rain impacts will redevelop tomorrow at sites DLS/PDT/RDM/BDN/ALW, with western sites seeing precip develop after 14Z, and eastern sites after 18Z. CIGS of bkn-ovc at sites DLS/RDM/BDN/YKM/PSC will be AOA 5kft to 8kft AGL through the period. Site ALW will see CIGS 1kft to 3kft improve to 3kft to 5kft overnight and through tomorrow. Site PDT will see periodic CIGS down to 2kft, but will mostly be 3kft to 5kft through the period. Winds of 12-20kts with gusts 25-30kts will continue at sites YKM/ALW/PSC through the period. Winds of 20-25kts with gusts 30-35kts will continue at sites DLS/RDM/PDT, with winds decreasing to 12-18kts after 3Z at sites DLS/RDM and after 9Z at site PDT.
Lawhorn/82

PRELIMINARY POINT TEMPS/POPS
PDT 38 56 38 55 / 90 80 40 20 ALW 43 59 40 58 / 100 80 40 30 PSC 46 64 44 65 / 60 40 10 10 YKM 37 62 37 63 / 10 30 0 0 HRI 42 62 42 62 / 60 50 10 10 ELN 39 58 37 57 / 20 30 10 0 RDM 32 52 30 50 / 20 70 40 0 LGD 37 54 35 49 / 100 90 70 50 GCD 33 51 32 46 / 90 90 90 50 DLS 45 58 42 59 / 50 70 20 0

PDT WATCHES/WARNINGS/ADVISORIES
OR...Wind Advisory until 11 PM PDT this evening for ORZ041-044-507- 508-510.

Winter Weather Advisory until 5 AM PDT Tuesday for ORZ502.

Winter Weather Advisory until 2 AM PDT Monday for ORZ503.

Winter Weather Advisory until 11 PM PDT this evening for ORZ506.

Winter Weather Advisory from 2 AM to 11 PM PDT Monday for ORZ509.

WA...Wind Advisory until 11 PM PDT this evening for WAZ024-026-521.

Winter Weather Advisory until 5 AM PDT Tuesday for WAZ030.

Vancouver
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Sun -- 12:46 AM PDT     0.07 feet Low Tide
Sun -- 04:38 AM PDT     Moonrise
Sun -- 05:30 AM PDT     1.71 feet High Tide
Sun -- 05:51 AM PDT     Sunrise
Sun -- 01:22 PM PDT     -0.01 feet Low Tide
Sun -- 05:43 PM PDT     Moonset
Sun -- 05:55 PM PDT     1.49 feet High Tide
Sun -- 08:23 PM PDT     Sunset
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