Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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806 FXUS66 KPDT 152332 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 432 PM PDT Sun Sep 15 2024 .Updated for Aviation... SHORT TERM...This afternoon through Tuesday night...A band of precipitation remains set up across the Blue Mountains, aided by broad forcing for ascent beneath the right entrance region of a 250-mb jet. This jet is located on the east side of a southeastward-propagating upper-level closed low centered near the OR/CA border. Multiple distinct shortwaves rounding the base of the trough are evident on water vapor imagery. Ensemble guidance is in excellent agreement that the main low will continue to track southward into central CA overnight through Monday morning, which removes the jet support across our forecast area. However, some mid-level WAA and a weak shortwave impulse rounding the low over eastern OR may facilitate some showers overnight, though confidence in details remains low. Monday afternoon through Tuesday, the upper low will eject northeast from CA into the Great Basin and ultimately through the Intermountain West. Uncertainty remains medium (50% confidence in forecast PoPs/QPF) due to subtle differences in the low`s track. One scenario (~50% of ensemble members) favors a track that would result in a wetting rain for the northeast mountains while the other scenario (~50% of ensemble members) places drier conditions across the forecast area with the precipitation axis displaced to the south and east. Monday evening through Tuesday, breezy to windy westerly winds are forecast to push through the Cascade gaps as surface pressure gradients tighten in response to a second incoming low. This second trough will increase precipitation chances across the Cascades and into central Oregon Tuesday afternoon through Tuesday night as well. Plunkett/86 MORNING UPDATE...issued 1008 AM PDT Sun Sep 15 2024...A band of light, stratiform precipitation has set up across the Blue Mountains this morning, aided by broad forcing for ascent beneath the right entrance region of a 250-mb jet located on the east side of a southeastward-propagating upper-level closed low currently centered over the OR coast. Surface observations indicate the dry sub-cloud layer that was causing precipitation to evaporate prior to reaching the ground has finally saturated; recorded precipitation across regional gauges is generally less than 0.10", locally higher along preferential upslope terrain in the Blue Mountains. Through the day, precipitation is expected to remain focused across the Blue Mountains with the best chances of measurable precipitation in a narrow band stretching from eastern Deschutes County to western Wallowa County. Of note, 12Z CAMs continue to suggest weak instability (generally 250 J/kg or less) collocated with synoptic-scale forcing in southeast Deschutes/Wheeler, and southern Grant counties this afternoon and evening. Have opted to re-introduce a slight (15%) chance of thunder to the forecast, though confidence remains low given that the best forcing will be south of our CWA. Plunkett/86 LONG TERM...Wednesday through Sunday...The long term remains progressive in nature through the period. Ensembles indicate a deep low, following on the heels of this weekend`s system, will be moving southwards along the coastline of the PacNW and into Northern California through the day Wednesday. Unfortunately this track does prevent the system from producing noteworthy precipitation for our region, with any light showers that were expected from it moving out through the morning hours. This will keep conditions quiet Wednesday evening through Friday, with temperatures near normal (widespread NBM probabilities of highs 70+ degrees are 50-100%) and skies mostly clear until Friday evening as the next system begins moving into the region. Forecast uncertainty is high once we reach the weekend, as ensembles show multiple possible outcomes at that time. This includes a new trough approaching our area, high pressure ridging strengthening over the area and staving off this system, or even general zonal flow between ridging to the south and troughing to the north. It`s tough to believe the deep ridging outcome due to how progressive our pattern has been coupled with the strength of the system expected to come out of the Gulf of Alaska, and the NBM is advertising temperatures below normal with a shot at some rainfall over the weekend from a potential trough. Zonal flow may be the most likely outcome and we remain between systems, but until ensemble guidance comes into better agreement on next weekend, there is low confidence (20-30%) in the forecast beyond Friday. Goatley/87 AVIATION... 00Z TAFS...VFR conditions are expected through the TAF period. A passing shower can not be ruled out, especially for ALW, BDN or RDM this evening. Otherwise, winds should mainly be 10 kts or less. DLS, BDN and RDM will see some winds above 10 kts through early this evening then they will drop to 10 kts or less for the remainder of the period. && .PRELIMINARY POINT TEMPS/POPS... PDT 49 71 50 70 / 20 20 20 10 ALW 52 74 55 73 / 20 20 20 20 PSC 57 80 56 77 / 10 10 10 10 YKM 52 82 47 78 / 10 0 0 0 HRI 54 78 55 75 / 20 10 10 10 ELN 49 82 50 74 / 20 0 0 0 RDM 46 69 42 65 / 20 10 0 10 LGD 50 70 49 68 / 50 40 30 30 GCD 47 70 44 68 / 70 30 30 20 DLS 54 82 55 70 / 0 10 0 20 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...86 LONG TERM....87 AVIATION...77