textproduct: Pendleton

KEY MESSAGES

- Frigid morning temperatures Saturday *Freeze Warnings Issued*

- Light mountain showers Sunday and Monday

- Warming and dry through the week

DISCUSSION

Current radar and visible satellite imagery shows dry conditions under mostly clear skies. This is in response to local high pressure behind the cold front that has dropped south into northern California and Nevada, which will keep skies clear and flow aloft out of the north. These conditions will attribute to rapid cooling after sunset as overnight low temperatures may drop into the low 30s across the Lower Columbia Basin, northern Blue Mountain foothills, and the Kittitas Valley. Confidence is a bit lacking as temperatures were around 5 degrees warmer than expected Friday morning, and the NBMv5.0 suggests only a 10% chance of freezing temperatures over the aforementioned areas. However, the NBMv4.3 advertises a 40-50% chance of Saturday morning temperatures reaching 32 degrees or below. Expected light, north winds and mostly clear skies do support low temperatures near freezing, but the likelihood of values dipping much further is rather low. Due to the potential for freezing morning temperatures, a Freeze Warning has been issued across the aforementioned areas between midnight and 8 AM Saturday.

The upper level trough will continue to dig to our southwest Saturday as a reinforcing shortwave swings around the parent low Sunday, with another shortwave approaching from the northwest on Monday to provide just enough moisture to bring a return to mountain showers. Both the ECWMF AI Ensembles and GFS ensembles showcase precipitable water amounts of 0.25-0.40" both Sunday and Monday, which relate to 65-85% of normal. This suggests trace amounts of snow and rainfall along the Cascade crest Sunday and 0.03-0.07" along the Cascade crest, Blue Mountains, Elkhorns, and the John Day Highlands Monday as snow levels increase from 4000-5000 feet Sunday to 5000-5500 feet on Monday. Confidence in these rain amounts is moderate to high, with the NBM showing a 30-40% chance of measurable rainfall (0.01" or greater) extending into the Blue Mountain foothills and a 10-20% chance reaching lower elevations of the Kittitas, Yakima, Gorge, and Lower Columbia Basin regions.

Guidance is in good agreement with upper level ridging moving onshore and slowly passing through the Pacific Northwest through the remainder of the workweek. This pattern is expected to keep dry conditions across the area as temperatures slowly increase before peaking Friday and Saturday with highs potentially reaching into the low 80s across the Lower Columbia Basin. Ensemble members do struggle with how impactful and efficient an incoming upper level shortwave will be in breaking down the upper level ridge on Friday, directly relating to highs making it into the low 80s both days. Currently, only 35% of members show the ridge winning the battle and keeping Basin temperatures in the low 80s, with the remaining members suggesting highs in the low to mid-70s. Humidities will also be on the downward trend as afternoon values dip into the upper teens across portions of Central Oregon on Wednesday before expanding into the Hanford area Thursday and the John Day Basin on Friday. The 35% of ensembles related to 80 degree high temperatures Friday also align with afternoon humidities dipping into the upper teens across the Hanford area and through Central Oregon. However, an additional 27% of members, 52% in total, advertise afternoon humidities of 25% or below. Overnight humidity recoveries are still expected to be moderate to good (50-70%) over these areas on Friday, which is related to the relative zonal flow on Thursday and clear skies overnight allowing morning temperatures to cool to 40-45 degrees. 75

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

VFR conditions to prevail through the period. Winds will be light with diurnal wind shifts late in the evening. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 32 59 34 60 / 0 0 0 0 ALW 35 59 38 61 / 0 0 0 10 PSC 33 64 35 67 / 0 0 0 0 YKM 34 63 36 65 / 0 0 0 0 HRI 31 63 34 65 / 0 0 0 0 ELN 31 58 34 60 / 0 0 0 10 RDM 24 57 27 57 / 0 0 0 0 LGD 28 55 30 56 / 0 10 0 20 GCD 28 56 29 56 / 0 10 0 10 DLS 37 66 39 67 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Freeze Warning from midnight tonight to 8 AM PDT Saturday for WAZ026-028-029. OR...Freeze Warning from midnight tonight to 8 AM PDT Saturday for ORZ044-507.