textproduct: Pendleton

KEY MESSAGES

- Additional mountain rainfall with amounts in of 1 to 1.5 inches over the next 18-hrs promoting a flash flood and debris flow risk for burn scars in the northern Blue mountains.

- General drying and modest warming trend this upcoming work week.

- Next chances for rain confined mainly to the mountains mid- week, late Tuesday/Tuesday night and late Thursday/Thursday night.

DISCUSSION

Latest water vapor imagery shows the upper-level low over eastern WA and complex troughing over the West Coast. Meantime, ample moisture is seen on the western side of this low that is steadily progressing eastward into the interior with the main shortwave trough rotating east into southeast OR. Latest radar display shows a swath of light rain starting to cross the western WA Lower Basin towards the eastern mountains. 1-hr rainfall totals have been light, a few hundredths to a tenth of an inch at lower elevations, while 6-hr totals have been reaching 0.1-0.2 inches. Elsewhere, a quarter inch to a half inch has been seen along the eastern slopes of the WA Cascades as of 1 PM PDT.

The aforementioned area of light precip will continue eastward this afternoon across the rest of the Lower Basin with orographic enhancement anticipated across the northern Blue mountains. Appreciable moisture is being tapped into this system for this time of year with IVT forecasts showing 200-300 kg/ms penetrating into the Basin and the eastern mountains; EPS mean precipitable water forecasts approaching/reaching 150% of normal across the area. This increases confidence in the next round of moderate to locally heavy rain in the northern Blues and Wallowas. Latest 24-hr LPMM by HREF show a reasonable 1-1.5 inches ending 12 Z tomorrow morning/Monday, elsewhere a tenth to a quarter inch in the lower Basin and foothills with less than a tenth across north central and central OR. Confidence is bolstered in this outcome with NBM probs showing 50-70% chance for over 1 inch across the northern Blues. Otherwise, the combination of residual moisture and recent rainfall could promote a minor risk for patch fog, mainly along valleys and drainages of the Lower Basin and in areas of the eastern mountains. Confidence in the fog threat isnt high (30-50%) at this time when it comes to occurrence.

After this upper-level system departs east across the Northern Plains and Canadian prairies, large scale guidance is consistent in the development of another upper-level trough midweek overhead with additional amplification seen upstream of the upper-level ridge across the Gulf of Alaska into AK. While ensemble guidance shows some spread, the upper trough and its closed low digs farther south into CA by mid-week with clustering guidance tilting moderately towards this outcome by Thursday (~60%) with then increasing odds in general mean troughing persisting to Friday (~50%). Over this time period, a short drying trend will be seen after this current system exits the area before precip chances increase late Tuesday-Tuesday night (20-50%, highest in the mountains) followed by an uptick again in precipitation chances over the eastern mountains Thursday/Thursday night. Meanwhile, a warming trend is expected with highs near average by Thursday with readings slightly above average Friday.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

MVFR to VFR conditions anticipated across the forecast area today. The development of additional rainfall may create brief periods of MVFR conditions at terminals, with otherwise VFR conditions expected. Gusty wind of 15g25kt remains in the forecast for most terminals through the early evening, with gusty winds persisting at KPSC through the overnight hours. Branham/76

Preliminary Point Temps/PoPs

PDT 45 70 44 68 / 60 0 0 0 ALW 50 71 50 70 / 80 0 0 0 PSC 46 74 45 72 / 40 0 0 0 YKM 44 69 47 67 / 20 0 0 10 HRI 46 73 44 71 / 50 0 0 0 ELN 41 65 42 62 / 30 0 0 30 RDM 33 71 36 71 / 10 0 0 0 LGD 43 70 42 72 / 90 0 0 0 GCD 41 70 42 74 / 20 0 0 0 DLS 51 74 50 74 / 10 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Flash Flood Watch until 8 PM PDT this evening for WAZ030. OR...Flash Flood Watch until 8 PM PDT this evening for ORZ502.