FXUS66 KPDT 181758 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 958 AM PST Mon Nov 18 2024 .MORNING UPDATE...Looking at ongoing precipitation, the current forecast seems to be doing a good job of conveying what we're already seeing. Meanwhile, made some minor changes to the temperatures to account for overnight lows. Main updates from this morning were to winds Tuesday evening through early Wednesday morning with our next significant system for the region. Models are coming in much higher on winds, particularly in the WA Cascades where we expect noteworthy snow to occur. To convey this, wanted to at least increase winds a bit, but expect that if models continue this trend that we will see another bump in forecast conditions with the afternoon update. Wanted to convey this now as we're less than 48 hours from onset and due to ongoing products from the current winter system, we do not have any watches out for this next system, so pushing out a forecast update will be the way to go. Goatley/87 .AVIATION...18Z TAFS...VFR conditions currently at all sites, but are expected to degrade to MVFR for KDLS/KRDM/KBDN due to light snowfall leading to reduced visibilities during the early morning hours on Tuesday. Ceilings will also drop late in the period as a result of the incoming system, with KDLS dropping the lowest to OVC025. Winds will be breezy this afternoon for KDLS/KPDT/KRDM/KBDN/KALW/KPSC with sustained winds of 10-15 kts and gusts of 20-25 kts. Winds will decrease this evening and continue to dissipate through the end of the period. 75 && .PREV DISCUSSION... /issued 316 AM PST Mon Nov 18 2024/ SHORT TERM...Today through Wednesday...Following yesterdays cold frontal passage the forecast area will be under a cool westerly flow today as the upper level trough passes. This westerly flow will continue to provide snow showers to the Cascade crest where Winter Storm Warnings remain in effect through this evening. An additional 4 to 8 inches of accumulations will be possible mainly along the Cascade crest at pass levels with lighter amounts along the lower eastern slopes. The northern Blue Mountains will see some lingering showers but with only additional accumulations of 2 to 5 inches so the warnings will be dropped with the morning forecast issuance. Overall, will see some lingering showers Monday night through Tuesday as the westerly flow begins to transition to a southwest flow ahead of the next system expected to arrive Tuesday night into Wednesday. This system will be supported by a deepening upper level trough over the eastern Pacific. A surge of moisture coming around the bottom of the trough will tap into some tropical moisture to form an Atmospheric River that moves into the Pacific NW coast late Tuesday into Wednesday. This in turn will push a warm front across the forecast area Tuesday night. Initial snow levels will start out between 1500 to 3000 feet then rise into Wednesday to 3000 to 4500 feet. The lowest snow levels will be focused along the eastern slopes of the Washington Cascades Tuesday night through Wednesday where a pool of cold air will trapped. Other than the lower elevations of the Lower Columbia Basin there will be some initial snow before turning to rain Tuesday night or early Wednesday. However, the eastern slopes of the Washington Cascades could see a prolonged snow event resulting in some advisory and possible warning type snow accumulations. The warm front will also be accompanied by some windy southerly winds mainly across eastern Oregon. High wind watches have been issued and are in effect for Tuesday night through Wednesday morning. LONG TERM...Thursday through Monday... Bottom Line Up Front 1. Rain, rain and more rain 2. Southwest flow will bring 'warmer' temperatures rising snow levels 3. Localized areas of flooding possible The long term will be a bit wet as as models show an upper level low will remain parked off the coast. Clusters show that the only variance in the long term is with the position of the upper level low until the later portion of the long term. Southwesterly flow into the region will keep a continuous flow moisture across the area with an embedded atmospheric river that will remain through at least Saturday. With that said, the long term will remain under wet conditions through the entire long term period with what appears to be little to no break in precipitation except on Saturday for mostly for the Basin, central OR and north central OR. NBM shows 70-100% probabilities of 0.40 inches of precipitation along the Cascades and Blues with higher amounts along the crests and 0.30-0.40 inches elsewhere through the CWA. Friday, probabilities drop slightly to 50- 80% for the same amount of precipitation as Thursday. Friday shows the first signs of a decrease with central OR seeing 0.08-0.20 inches with 50-70% probabilities, 30-60% for the Basin and foothills of the Blues to see 0.25-0.35 inches and 80-100% for above 0.50 inches along the mountains tops. Its going to be a very wet later part of the work week. Models show the southwest flow to continue through the period which will allow temperatures to remain seasonal through the period with the NBM showing temperatures to hover in the upper 40s to low 50s for much of the lower and mid elevations while the higher elevations will hover in the low to mid 30s. Even through Saturday elevations below 3000 ft will remain above freezing. The southwest flow will also aid in keeping the snow levels a bit higher with mostly the higher crests seeing any snowfall. However, models do show that beginning Friday night and Saturday morning, the upper level low will begin to makes its way onshore and make its way across the region. This will decrease the temperatures across the mid and higher terrains with snow expected to reach as low as 2500 ft with 15-20% probabilities of near 2 inches along the I-84 corridor of the Blue Mountains and 40-60% chance of near 3 inches along the I-80 corridor of the Cascades. However, clusters are bit out of sink with the timing of the upper level low moving inland. With all that said and the fact that the EFI is signaling this to be a well above normal rainfall event, along with the moderate to high confidence in the forecast, some localized flooding could occur due to the rain amounts coupled with some slight snow melt. While no widespread flooding is expected, be prepared as this will sure be a wet one. Bennese/90 && .PRELIMINARY POINT TEMPS/POPS... PDT 46 27 47 40 / 20 20 20 70 ALW 47 30 48 40 / 20 20 20 80 PSC 51 32 49 42 / 10 10 20 80 YKM 48 24 42 31 / 10 10 30 100 HRI 50 30 50 42 / 10 10 20 70 ELN 43 22 38 28 / 10 10 30 90 RDM 41 24 43 36 / 20 20 20 90 LGD 39 19 36 31 / 50 30 20 80 GCD 38 19 40 33 / 50 30 20 90 DLS 49 30 48 39 / 30 20 50 90 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...High Wind Watch from Tuesday afternoon through Wednesday morning for ORZ507-508-510-511. Winter Storm Warning until 10 PM PST this evening for ORZ509. WA...Winter Storm Warning until 10 PM PST this evening for WAZ522. && $$ SHORT TERM...91 LONG TERM....90 AVIATION...75