Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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522 FXUS66 KPDT 280947 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 247 AM PDT Sun Apr 28 2024 .SHORT TERM...Today through Tuesday...Satellite imagery tonight shows areas of low to mid level cloud cover scattered across the mountain areas with high level cloud cover pushing east of the Cascades. Meanwhile, radar imagery shows scattered showers developing along the windward side of the Cascades and across portions of the interior norther Blues. Today, a weak frontal system west of the Cascades will push east, while under a westerly flow aloft. Precipitation will mainly be confined to the Cascades as this system moves across the intermountain PacNW, with rain and snow showers above 4kft-4.5kft along the Cascades, and rain showers across the eastern mountains. In the lower elevations, the frontal system passage will tighten surface pressure gradients, resulting in breezy west to southwest winds across the lower elevations. The strongest winds are expected along the southern Blue mountain foothills, Simcoe Highlands, and the the exposed higher terrain throughout the Columbia Basin; NBM probabilities show at least 60-75% of gusts reaching 40 mph in the Columbia Basin and adjacent foothills, while the Simcoe highlands are closer to a 90% chance. Winds are expected to decrease overnight, though locally breezy conditions will still be possible as pressure gradients are slow to weaken. By late tonight into Monday morning, an upper low offshore northern BC will be pulled south towards the PacNW by a strengthening shortwave rounding it`s western periphery. The shortwave trough is progged to reach the PacNW early Monday morning, then exiting into ID and western MT late in the evening. The shortwave arrival will result in snow levels dipping to between 2.5kft to 3.5kt across the forecast area, allowing snow showers to develop across most of the mountain areas. The heaviest snow accumulations are expected to occur Monday morning for the Cascades, and by the early afternoon for the northern Blues. Snow amounts will generally be 5-8 inches along the Cascade crest, where winter weather advisories have been issued between 11pm tonight through 11am Monday morning; NBM probabilities do show a 60-70% chance of at least 5 inches of snow accumulations by 11am in along the Cascade crest. In the northern Blues, snow accumulations will be just shy of advisory criteria with 4-6 inches of snow expected, and up to 3 inches across the remainder of the eastern mountains. Light rain showers putting down a couple hundreths of an inch can be expected across the lower elevations Monday, though rain-shadowing on the leeward side of the Cascades will result in only a trace of precip in the Kittitas/Yakima valleys and central OR. Weak instability and a cold core aloft associated with the shortwave passage will also result in isolated chances of thunderstorms developing across the Cascade crest, Blues, and Wallowa county in the afternoon. The shortwave trough passage overhead will also result in a strengthening of surface pressure gradients, with westerly winds between 20-30mph developing during the day Monday. However, current deterministic guidance and hires short- term guidance show a lack of any jet support overhead with winds only between 30-35kts at 850mb and 700mb, resulting in a low confidence (30%) for strong gusts to develop. That said, NBM guidance is depicting 60-80% chances of wind gusts meeting or exceeding 45mph in portions of the Simcoe Highlands, the OR Columbia Basin, and the Kittitas valley. By Monday evening, the shortwave trough will have exit to the east while the remnants of the parent low swings into the PacNW by Tuesday morning. In the Monday overnight hours, winds will taper off in the lower elevations, while light precipitation continues across the mountains. The first half of Tuesday, the remnants of the parent low will move across the forecast area, resulting in a brief increase in rain/snow chances across the mountains, with light rain showers across portions of the Blue mountain foothills, central OR, and north central OR. Isolated thunderstorms will once again be possible over the Blues and Wallowa county thanks to the cold core aloft associated with the upper low. Locally breezy west to southwest winds up to 15-20mph will redevelop across the lower elevations Tuesday afternoon, but quickly diminish into the evening hours. A cooling trend Monday and Tuesday will result in afternoon temperatures in the 50s to lower 60s both days. As for morning lows, expect a return to the mid 30s to lower 40s across many lower elevation locations, except upper 20s to mid 30s across central OR. Lawhorn/82 .LONG TERM...Wednesday through Sunday...While ensembles still exhibit a bit of disagreement with regards to the synoptic pattern during the latter half of the work week onward, deterministic models are in better agreement, increasing forecast confidence for at least the first half of the period. The general picture painted by the deterministic GFS and ECMWF is this: A broad Arctic low departs to our southeast Wednesday, with northwesterly flow aloft ushering in a chance for some light mountain precip, followed by a weak embedded shortwave riding the NW flow on Thursday. Transitory ridging then builds in on Friday, before models hint at a period of amplified SW to S flow heading into next weekend. The GFS hints more at a closed low centered offshore, while the ECMWF suggests more of an atmospheric river pattern. Ensemble guidance is in good agreement on the midweek Arctic low moving out of our region, but discrepancies arise on what exactly occurs behind the low. Clustering solutions are more or less in agreement on the NW flow behind the departing low, but after that, Friday onward, some ensemble members lean toward a more stubborn transitory ridge, while others suggest the embedded shortwave in the NW flow on Thursday may deepen out to a broader trough. Ridging appears to be the more favorable solution, however with some ensembles keeping the ridge lingering about through the weekend, this runs into conflict with what deterministic guidance depicts. This is at the tail end of the forecast period, where uncertainty is to be expected, but for now will say forecast confidence is high (70%) Wednesday and Thursday, moderate for Friday (50-60%, as ridging does appear to materialize across the majority of guidance), and low Saturday onward (20-30%). So what does the overall weather look like for the period? Expect cool conditions lingering about under NW flow Wednesday into Thursday, with overnight lows potentially dropping into the 30s even in the lower Basin, while light PoPs of about 15-30% occur across our mountain and foothill zones with snow levels in the 4000-5000 ft range. Friday looks to be the most pleasant day of the week with dry conditions and seasonable temps of highs in the 60s and 70s, while next weekend is shrouded in uncertainty. Perhaps ridging persists, and nice weather prevails, or deterministic guidance verifies and we see area-wide shower chances and potentially storms should strong SW/S flow aloft occur. Need better consensus across guidance to determine for sure. Evans/74 && .AVIATION...12z TAFs...VFR conditions expected, with main concern for the period being gusty conditions across all sites. An oncoming low pressure system will invite widespread sct-bkn high clouds, as well as gusty winds out of the W/NW. Gusts could approach 30 kts at times this afternoon, especially for PSC and DLS. Precipitation is largely expected to remain confined to the mountains, however lower cigs may develop along nearby sites as a result of this oncoming system. These low clouds are expected to be around 5-8 kft sct-bkn, but cannot rule out an isolated light rain shower causing cigs to briefly drop to MVFR at times in the afternoon. These low clouds are expected to lift during the nighttime hours Sunday night, with winds backing off as well, albeit only temporarily. Evans/74 && .PRELIMINARY POINT TEMPS/POPS... PDT 61 38 54 32 / 10 10 50 20 ALW 64 42 56 37 / 20 20 80 20 PSC 66 45 61 41 / 0 10 30 0 YKM 62 38 57 34 / 10 20 20 0 HRI 66 41 60 37 / 0 10 30 10 ELN 57 35 51 32 / 10 40 20 0 RDM 57 33 49 27 / 0 10 30 0 LGD 58 37 47 30 / 30 30 70 20 GCD 60 36 49 29 / 10 10 50 10 DLS 63 42 57 38 / 10 50 50 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory from 11 PM this evening to 11 AM PDT Monday for ORZ509. WA...Winter Weather Advisory from 11 PM this evening to 11 AM PDT Monday for WAZ522. && $$ SHORT TERM...82 LONG TERM....74 AVIATION...74