Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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165 FXUS66 KPDT 290300 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 800 PM PDT Sun Apr 28 2024 .UPDATE... Latest radar showed some precipitation over the Cascades and some isolated precipitation over the Blue Mountains. Based on radar trends and latest guidance, have decreased pops over the Blue Mountains tonight. Otherwise raised overnight lows in several locations due to the latest guidance. However, general forecast looks good with pops increasing over the entire area toward morning and into the day Monday. && .PREV DISCUSSION... /issued 447 PM PDT Sun Apr 28 2024/ Updated for Aviation... AVIATION... 00Z TAFS...VFR conditions are expected at all TAF sites through the period. Precipitation will move in toward Monday morning at DLS, BDN, RDm. DLS will see all -RA while BDN and RDM could see a mixture of -RASN at the onset. The other concern is gusty winds All sites are experiencing gusty winds, at least occasionally late this afternoon that are expected to continue into early evening. Winds should gust 20 to 30 kts. Winds should then decrease to around 10 kts after 02-04Z. Winds are expected to increase again on Monday morning through early afternoon and gust to around 25 kts. High confidence (90-100%). PREV DISCUSSION... /issued 329 PM PDT Sun Apr 28 2024/ SHORT TERM...Today through Tuesday night...Satellite imagery shows a broad area of low pressure with two distinct circulation centers off the British Columbia coast. Ensemble guidance is in excellent agreement (>99% chance) that this system will impact the PacNW tonight through Tuesday. The first shortwave and jet max will track into the PacNW tonight through Monday, facilitating mountain snowfall and widespread light precipitation for the lower elevations. Synoptic support from the jet max appears most robust for the central OR Cascades, and forecast precipitation and snowfall reflect this outcome. To touch on snowfall, NBM probabilities of 5 inches of snowfall for the Cascades range from 60-90% for the central OR Cascades, including over Santiam Pass, while probabilities are lower for the northern OR Cascades and the WA Cascades (20-50% chance of 6 inches). Winter Weather Advisories remain in effect 11PM this evening through 11AM Monday morning for the Upper Slopes of the Eastern Washington Cascades (above 3500 feet), and the East Slopes of the Oregon Cascades (above 4000 feet). The westerly flow aloft is supportive of a strong rain shadow to the lee of the crest so snowfall totals along US-97 south of Bend will be lower (<1"). Breezy to windy westerly winds will accompany the system on Monday, though forecast surface pressure gradients (6-9 mb, locally 10 mb PDX-GEG) do not appear supportive of widespread advisory-level winds Monday morning through evening. Moreover, 12Z HREF members are advertising widespread breezy to windy westerly winds, but ensemble maximum wind gusts fail to exceed 45 mph from any member. Hence, have held off on any wind highlights as confidence in sustained advisory-level winds is low (<50% chance). The second wave, a weakening closed upper low and attendant surface low, is forecast to swing into the PacNW Monday night through Tuesday. While confidence is very high (>95% chance) that we will see another round of precipitation, including light snowfall for the mountains and possibly a light snow shower (<50% chance) of an inch or less for the lower elevations (1500-2000 feet elevation), it is worth noting that there is still some ensemble spread regarding the track the low takes as it pushes into the PacNW. The most likely solution will once again favor the OR Cascade crest for heaviest precipitation from a synoptic standpoint. As the upper low moves overhead Tuesday afternoon, there is a low chance (10-15%) that there will be a thunderstorm across the region as steep lapse rates and daytime surface heating allow modest (<500 J/kg) SBCAPE. COnfidence on location is low given the aforementioned spread in the track of the low and the inherent randomness of convection. Plunkett/86 LONG TERM...Wednesday through Sunday...The long term continues to still be a bit out of sorts when it comes to the latter half of the period however, guidance has become a bit more in agreement regarding the first portion. With this development, confidence in the forecast for at least Wednesday through Friday has increased. Confidence steadily decreases through the period with moderate/high (70-80%) Wednesday and Thursday, moderate Friday (40-60%) and low over the weekend. Wednesday through Thursday the guidance is in somewhat of an agreement between the GFS, Euro and the NAM showing the upper level trough exiting the region and putting the region under primarily northerly flow. The next upper level low begins to slip down the Canadian coast Thursday morning flattening the flow to a more zonal flow before moving into the region. The main variances within the models is the timing and positioning of the next incoming upper level low. This will play a part in how much precipitation will be seen. NBM ensembles have the POPs mostly over the Cascades Wednesday before spreading to the Northern Blues with a minimal increase across the low elevations. Raw ensembles put 30-50% POPs over the Cascades and 30-40% over the Northern Blues with 30-50% probabilities of up to 0.05 inches of rain. Snow levels will be a above 4000 ft so very little snow accumulations expected with this round. Winds will be diurnally breezy NNW winds increasing slightly Thursday as the low crosses the Cascades. EFI has picked up on the cooler air associated with this low and shows a few areas in east central OR as well as the Columbia Basin having slightly below normal seasonal temperatures with the ensembles showing temperatures to be in the mid 60s. Ensembles show the mid elevations to be in the low to mid 50s while the upper elevations will be in the low to mid 40s. Friday models are still in some agreement with the upper level low mostly to the east and settled over the eastern mountains while the leading edge of the upper level ridge pushes into the PacNW. The ridge will bring with it drying conditions Friday. Main variances in the guidance is the amplitude of the ridge as it passes over. EFI shows that the temperatures will begin to moderate back towards near normal with ensembles showing the lower elevations back in the low 70s, mid elevations in the upper 60s and the higher elevations in the low 50s. Moving to the weekend, models and clusters become a bit skewed with the next incoming synoptic pattern. Some have the ridge remaining in place over the region while others have a tight closed low slipping into the PacNW with enhanced chances for precipitation. Forecast confidence over the weekend continues to be low (15-30%). Bennese/90 AVIATION...18Z TAFs...VFR conditions expected despite the increase in cloud cover incoming with the shortwave trough. Precipitation is expected to crest the Cascades and reach BDN/RDM/DLS between 12-15Z with 60-80% probabilities of rain in DLS and a 50-60% for RASN mix at BDN/RDM. CIGs will continue to to decrease through the period to 0VC050 and winds will continue to be breezy with sustained winds between 9-15 kts with gusts of 15-22 kts at all until this evening. Bennese/90 && .PRELIMINARY POINT TEMPS/POPS... PDT 39 54 33 55 / 10 70 10 40 ALW 42 56 37 57 / 20 80 30 50 PSC 45 62 40 62 / 10 40 10 20 YKM 38 58 33 60 / 0 40 0 20 HRI 42 60 37 61 / 10 50 10 20 ELN 39 54 32 57 / 20 30 0 20 RDM 31 49 27 51 / 10 50 0 20 LGD 36 49 30 49 / 20 80 30 60 GCD 34 49 28 50 / 10 70 10 40 DLS 43 57 38 57 / 30 60 10 40 && .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...77 LONG TERM....90 AVIATION...77