textproduct: Pendleton

KEY MESSAGES

- Scattered rain showers and Isolated thunderstorms developing Saturday evening and rain showers becoming more numerous though Sunday morning.

- Widespread wetting rain Saturday Night and Sunday

- Breezy to windy across the eastern Columbia River Gorge Saturday Night and Sunday Morning.

UPDATE

The incoming system continues to trend towards a wetter scenario, which is advertised by a 20-30% increase in precipitable water via the AI ECMWF ensembles over the last 24 hours. This is a direct result of increased resonance time near a closed low pressure system further offshore over the Eastern Pacific. The delay has also attributed to a stronger transient ridge of high pressure building ahead of this system, which will allow for thunderstorm development across the Blue Mountains and leading to increased potential for heavy downpours in and around fire scars of the Paradise and Hagen wildfires. The REFS suggests convection beginning around 6 PM and extending until shortly after midnight. As a result, a FLOOD WATCH has been issued across both burn scars from Saturday afternoon through Sunday evening as rain rates of around 0.5"/hour may be achieved within a developed rain core. 75

DISCUSSION

Regional radar was clear this afternoon with GOES satellite day cloud phase showing thin high level clouds spreading east across the Upper Columbia Basin. Eastern WA Eastern OR, while most of central Oregon and the southern Blue Mts were clear/sunny. The weak zonal flow aloft across the region Today will be disrupted as an upper low spreads onto the Oregon coast by late Saturday Afternoon and Evening, placing the Interior NW under a broad area of PVA advection/diffluence aloft. The showers that are already going on Today west of the Cascades will be diurnal in nature, so no rain is anticipated overnight. Increasingly broad scale lift and moisture transport will promote instability on Saturday, enough so for isolated to widely scattered thunderstorm and widespread wetting rains from few tenths of an inch to potential for over an inch in the Mountain zones though the day on Sunday, once the upper system exits the region. Initially a few showers are possible along the Cascade crest, but everywhere farther east will be dry through about the end the afternoon Saturday. HREF (ensembles) show a ramp up in the mean 1 hourly precipitation fields between about 5 and 9 pm, across the Eastern Slopes, the Columbia Basin and foothills of the Blue Mts. There is nothing all that anomalous with the PWATS for this system. About a 1 standard deviation in precipitable water shows up across the Cascades and the Eastern Mts Saturday afternoon/Evening and spreading north into eastern WA though Sunday morning as the cold conveyor belt lifts across the Palouse. Trends for thunder continue to be pushed back in time as the 12z HREF now shows 4hr calibrated thunder (10 to 30%) around 6z (nearing midnight Saturday) or so for the foothills of the Blues. The period of heaving rainfall will be between midnight Saturday and about noon Sunday with model spreads fairly large. Uncertainty at the Tri-Cities is a range of (using 25th to 75th percentile) anywhere form at least about a quarter of an inch to and inch and and quarter by late Sunday evening. That uncertainty/spread is compressed a bit over the mountain zones. For instance at La Grande where at least one half inch can be expected with a higher outcome around an inch an a quarter of rainfall. Rainfalls of around an inch are fairly likely (above about 60%) for the Paradise/Hagan fire areas. Westerly winds ramp up through the gaps on Saturday evening as well with the stronger period running from around midnight Saturday though late morning Sunday morning. The NBM mean shows potential for sustained winds in excess of 30 mph and gust in excess of 40 mph however these stronger winds are limited to the higher ridge-tops along the Gorge. Most other areas look breezy and falling under the general wind advisory criteria. The wet regime will hamper temperatures Sunday, where mid 60s will be the highest temps across the the Kittitas Valley, central Oregon and parts of the Basin. Dry weather and pleasant temperatures, with not a lot of wind will finish out the balance of the week before NBM trends toward a warmup by the next weekend.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

VFR conditions are forecast through the TAF period. Winds at all sites except RDM, BDN, and KDLS will be lower then 10 kts. BDN and RDM will experience gusting conditions of up to 20 kts from the 22Z to 00Z timeframe, with DLS reaching 15 kts. Low confidence (10-30%) in rain showers for YKM, DLS, BDN, and RDM starting around 22-00Z, with a PROB30 being issued for YKM.

Preliminary Point Temps/PoPs

PDT 47 76 45 61 / 0 0 90 80 ALW 52 79 49 60 / 0 0 90 90 PSC 50 80 47 63 / 0 0 90 90 YKM 54 76 50 63 / 0 10 90 100 HRI 49 79 46 63 / 0 0 100 80 ELN 48 72 44 59 / 0 0 100 100 RDM 40 71 36 60 / 0 10 80 10 LGD 47 81 44 59 / 0 0 90 80 GCD 46 81 43 59 / 0 0 100 40 DLS 54 77 51 66 / 0 20 90 60

PDT WATCHES/WARNINGS/ADVISORIES

WA...Flash Flood Watch from Saturday afternoon through Sunday evening for WAZ030. OR...Flash Flood Watch from Saturday afternoon through Sunday evening for ORZ502.