textproduct: Pendleton

KEY MESSAGES

- Chance of thunderstorms this afternoon through overnight tonight.

- Breezy to locally windy conditions expected across the Columbia Basin/North Central Oregon through the Kittitas Valley.

- Moderate to heavy snow expected in elevated areas in the Northern Blue Mountains on Wednesday.

DISCUSSION

Current satellite imagery shows a closed off low pressure system just off-shore of Oregon and California bringing a maritime airmass into the PacNW region. High cloud cover tops are currently situated over Central Oregon that will continue to build into the area as the low moves to the east. Radar currently shows very light returns near the Cascade Mountains with a light thunderstorm developing in Umatilla County.

The synoptic pattern shows that the previous mentioned low pressure off-shore will continue pushing east, jumping rain chances up to 80-100 percent by the later afternoon hours today across the Blue Mountains and Foothills, with more moderate 40-60 percent chances across the rest of the CWA. Our greatest thunderstorm window will be this afternoon through the evening hours today as 72% of clusters develop 500-750 J/kg of CAPE through the Blue Mountains into the Foothills & Kittitas Valley. HREF ensembles have a cluster of >40 dBZ & greater than 50 MUCAPE paintballs rapidly developing in the Blue Mountains and Foothills by the 3-4 PM window that traject through the east going into the evening hours. By the late afternoon/early evening hours, the greatest thunderstorm chances will be in the Wallowas/Northern Blue Mountains as the low pressure system jogs across the east.

Guidance is in very good agreement that as the closed-low moves to the Idaho/Montana region, our weather will be more influenced by a secondary trough located in British Columbia jogging south, that will bring heavier precipitation rates on Wednesday morning through the evening hours. Guidance points that the 6-hour precipitation rate through Wednesday morning will be between 0.15 to 0.35 inches, with the more heavier rain situated along the Foothills and the Blue Mountains. NBM and cluster analysis are in good agreement that the heaviest rain will occur Wednesday morning through the late afternoon with the maxima QPF situated across the Northern Blue Mountains with at least 2 to 3 inches expected (70-80% confidence). The high confidence is backed by the EFI showing 0.9 to 1.0 values across the Northern Blues into the eastern most parts of the Columbia Basin with a shift of tails value of 2. One last thing to note about the precipitation values is that cluster analysis mostly show 0.3 to 0.5 inch 6-hour rain rates in the more impacted areas, but about 9% of members suggests stronger orographic lift from the north/northwest winds to bring up to 1.25 inches of rain in a 6-hour window along the Foothills and small parts of the Oregon Columbia Basin. Although chances are small, the possibility to see some localized flooding in exists. Tight pressure gradients from the second trough will enable breezy to locally windy conditions embedded with the rain showers across parts of the Columbia Basin through the Simcoe Highlands & Kittitas Valley. Wind gusts will be as high as 50 mph (60-80% confidence) that will bring reduced visibility along roads, especially for higher profile vehicles.

Cooler northern air from the second trough will bring snow levels down to about 4500-5500 feet across the CWA, bringing high level mountain snow into the region. Although the NBM wants to bring 20-25 inches in certain parts of the Northern Blue Mountains (mainly above 4500 feet), there's great uncertainty that will manifest with the 10 to 1 snow ratio and being in a late season system. Temperatures are just below freezing Wednesday with the highs around 30-35 that could regress some of the snow accumulation. Given that, we don't have much confidence to warrant a warning for this system, however the range indicated in the advisory depicts that of a high-end advisory.

Thursday onwards will bring a drier and warmer period thanks to a rex-style high pressure pattern. Mostly dry conditions with a few showers developing in the Wallowas and Northern Blues but no major impacts expected through the weekend.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

VFR conditions will prevail through the evening. Rain begins to move in this evening and tonight. Rain will become steadier and possibly heavier overnight which will lead to MVFR CIGS (and possibly VSBYS) by morning.

Winds will increase and become gusty with winds 25 kts and will increase further on Wednesday, with gusts possibly as high as 30 to 35 kts.

HYDROLOGY

Moderate to heavy rain over the next 36 hours is expected to lead to rising levels on area streams and rivers. Currently, several forecast points along the Naches, Walla Walla and Grande Ronde Rivers are expected to rise above action stage as early as tomorrow morning. Currently, the Umatilla at Gibbon and Pendleton are forecast to reach minor flood stage. The biggest concern would be along small streams and creeks flowing off the Blue Mountains where intense periods of rain could lead to rapid rises and minor localized flooding. However, the National Water Model is not indicating any significant flooding along these small streams and creeks. 83

Preliminary Point Temps/PoPs

PDT 43 50 39 57 / 90 100 50 40 ALW 44 51 41 57 / 90 100 70 50 PSC 48 59 42 65 / 90 90 30 30 YKM 48 63 41 66 / 60 70 10 10 HRI 45 55 40 62 / 90 90 30 20 ELN 45 57 39 59 / 60 70 10 10 RDM 36 52 27 59 / 50 60 0 0 LGD 40 46 37 51 / 100 100 90 50 GCD 38 46 33 52 / 100 90 60 20 DLS 48 58 43 64 / 50 70 10 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory from 5 AM to 11 PM PDT Wednesday for WAZ024-029- 521. Flood Watch from 11 PM PDT this evening through Thursday evening for WAZ029. Winter Weather Advisory from 5 AM to 5 PM PDT Wednesday for WAZ030. Wind Advisory from 8 PM this evening to 11 PM PDT Wednesday for WAZ026. OR...Wind Advisory from 5 AM to 11 PM PDT Wednesday for ORZ041-044- 507-508-510. Flood Watch from 11 PM PDT this evening through Thursday evening for ORZ502-507-508. Winter Weather Advisory from 5 AM to 5 PM PDT Wednesday for ORZ502.