Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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033 FXUS66 KPDT 021019 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 319 AM PDT Sun Jun 2 2024 .SHORT TERM...Today through Tuesday... Key Messages: 1. Windy conditions Monday. *Wind Advisories issued* 2. Significant widespread rainfall Sunday and Monday. 3. Rises on area rivers associated with enhanced rainfall. Current radar and infrared satellite imagery showing some light returns along the east slopes of the Cascades under partly cloudy skies. These light returns, which rainfall is not actually reaching the ground surface due to a present dry air at lower elevations, are a result of an approaching warm front that will bring widespread showers across our area later today. However, the main concern with this robust incoming system is associated windy conditions expected Monday as the cold front passes through the Pacific Northwest. This frontal system, coupled with the upper level shortwave trough that will be approaching the coast later this evening, will tighten isobars and develop a pressure gradient along the Cascades to produce windy conditions across the Gorge, Basin, Simcoe Highlands, northern Blue Mountain foothills, and the Kittitas/Yakima Valleys. Wind gusts of 45-55 mph will be possible, which has warranted a Wind Advisory on Monday between 8 AM and 11 PM over the aforementioned areas. Confidence in these wind values and advisory-level winds is high (80-90%) as the GFS and NAM both highlight a pressure gradient of 11.5-13 mb between Portland and Spokane, which is well above the normal advisory threshold of 11 mb. There is also good consistency with 850mb winds as both the ECMWF and GFS highlight a large swath of 35-45 mph winds over the Basin, foothills, Simcoe Highlands, and the Yakima/Kittitas Valleys. Further confidence in gleamed from the NBM, which suggests a 70-100% chance of earlier mentioned areas experiencing wind gusts of 48 mph or greater. When gusts are bumped up to 55 mph, a 70-90% chance still resides over the Simcoe Highlands with a 45-65% chance along the Lower Columbia Basin of Oregon and the northern Blue Mountain foothills. This will be further analyzed over the next 24 hours as these areas may need to be upgraded to to warnings. As mentioned earlier, widespread rainfall will occur by late morning today and extend through Monday morning as significant rain amounts are anticipated. This is a result of a moderate atmospheric river (AR) suppling the moisture to the passing upper level trough and associated frontal system. Both the ECMWF and GFS showcase a Integrated Vapor Transport (IVT) of between 600-800 kg/m*s as both ensemble products produce an 85-100% of IVT greater than 500 kg/m*s. These values translate to widespread precipitable water amounts of 1-1.5 inches, which is 175-225% above normal. Rain is forecast to peak between 11 AM Sunday through 11 AM Monday. Rain amounts of 1-2 inches over the Cascades and northern Blue Mountains, 0.50-1 inch along the northern Blue Mountain foothills of Washington and the Simcoe Highlands, 0.20-0.60 of an inch through the northern Blue Mountain foothills of Oregon and the John Day-Ochoco Basin, 0.10-0.30 of an inch over lower elevations of the Basin, and 0.01-0.10 for Central Oregon and the Yakima/Kittitas Valleys is anticipated during this timeframe. Confidence in these rain amounts is high (70-80%) as the NBM showcases a 70-100% chance of 1 inch or more over the Cascades and northern Blue Mountains, a 50-80% chance of 0.50 inch or more over the northern Blue Mountain foothills, and a 40-60% chance of 0.25 of an inch or more across lower elevations of the Basin, Central Oregon, John Day-Ochoco Basin, and the Yakima/Kittitas Valleys. There is also a slight chance (10-20%) of isolated thunderstorms developing late Monday morning and afternoon over eastern Grant, southern Union, and Wallowa county. The HREF does highlight low level shear of 45-55 knots, with most unstable CAPE (MUCAPE) values of 100-200 J/kg. Timing also seems to be a little earlier in the morning, which coincide with storms developing more to our north and east through the afternoon. Heavy rain and gusty winds will be the primary concern associated with any developing storm cells, with a slight chance (<15%) of hail. At this time, there is a low chance (<10%) of any storm cells exhibiting severe characteristics. The final concern through Tuesday resides with this ample moisture and rainfall attributing to river rises on some area reaches. This has resulted in the issuance of a Hydrologic Outlook as periods of moderate to heavy rain are anticipated to occur - especially over mountainous terrain. At this time, only the Naches River near Naches and near Cliffdell is forecast to exceed action (bankfull) stage, with the reach near Cliffdell reaching action stage Monday morning and near Naches Monday afternoon. Confidence in reaching action stage near Cliffdell is 49%, and near Naches is 12%. However, with the significant moisture arriving with this system and enhanced precipitation expected over our mountain zones, localized flooding and flashy rivers may be a result. Make sure to have multiple ways of receiving pertinent weather and hydrologic information over the next 48-60 hours. 75 .LONG TERM...Wednesday through Sunday...Models continue to struggle with the strength and position of a developing upper level ridge over the western US. WPC cluster analysis does show a westward trend compared to yesterday with only 20% of members showing an axis near the Continental Divide with 80% hovering around the OR/ID border. This would certainly increase confidence in a warmer pattern for the PacNW. However, models have decreased/slowed its amplification keeping it more of a flat ridge through Thursday or Friday then amplifying it quickly over the weekend. Overall, confidence has increased (70-80%) for a more ridgy pattern, but confidence in temperatures remains low to moderate (25-35%) as raw temperature values range between 10-15 degrees. This range would be the difference between daytime highs around 5 degrees above normal to 10- 15 degrees above normal. The ECMWF continues to be the warmer of the models with temperatures 10-15 degrees above normal by the weekend. Ensembles mimic the WPC cluster analysis, but GFS pushes the ridge axis east quicker allowing for a better chance of afternoon/evening convection over the mountains Sunday. Both models keep general S-SW flow over the area from Thursday onward which will continue the threat of afternoon/evening convection over the mountains. Given consistency, confidence is moderate (40-50%) that convection will develop in the afternoon/evening from Thursday onward, but will certainly depend on where the ridge axis ends up. In summary, daytime highs will be near normal Wednesday and above normal through the rest of the period. Afternoon/evening showers/thunderstorms should develop over the mountains beginning Thursday and continuing through the weekend. Earle/81 && .AVIATION...06Z TAFS...VFR conditions and dry weather are expected overnight, then -RA will overspread the area on Sunday. DLS will likely see some MVFR conditions later Sunday afternoon/Sunday evening, and MVFR coan`t be ruled out in other locations, especially if there are periods of moderate or heavy rain, but confidence is low (<30%) in the lower conditions for the other sites at this time. Winds have increased at PDT and ALW to around 30 kts at PDT and 20 to 25 kts at ALW, but these winds should decrease overnight. BDN and RDM will gust between 25 and 30 kts on Sunday. The other TAF sites should be 10 kts or less though some local gustiness can`t be ruled out, especially around heavier rain. && .PRELIMINARY POINT TEMPS/POPS... PDT 67 53 66 46 / 60 80 70 10 ALW 70 55 67 49 / 80 90 90 20 PSC 70 58 72 54 / 80 70 60 10 YKM 66 50 67 45 / 70 60 30 20 HRI 70 56 70 50 / 60 70 50 10 ELN 62 48 63 45 / 80 60 50 20 RDM 66 52 63 44 / 40 50 30 10 LGD 67 53 63 43 / 70 90 80 10 GCD 68 53 66 44 / 70 80 70 10 DLS 67 56 66 52 / 80 80 40 20 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Wind Advisory from 8 AM to 11 PM PDT Monday for ORZ041-044-507- 508. WA...Wind Advisory from 8 AM to 11 PM PDT Monday for WAZ024-026>029- 521. && $$ SHORT TERM...75 LONG TERM....81 AVIATION...77