textproduct: Pendleton

KEY MESSAGES

- Scattered thunderstorms capable of strong/damaging gusts and isolated large hail will impact the forecast area this afternoon and evening

- Heavy downpours embedded within thunderstorms will bring an excessive rainfall threat to the Cascade east slopes today

- Breezy to locally gusty winds through the Cascade gaps and Lower Columbia Basin Friday

DISCUSSION

Today through Friday: Satellite imagery tonight shows a closed low over the CA/NV border, with mid to upper level cloud cover swinging across the forecast area. Otherwise, winds are light with dry conditions across the area.

Today will see increased convective activity across the PacNW as the needed ingredients for convection will combine with a mid-level wave rounding the CA/NV closed low into the PacNW. The deep closed low to the south will continue to place a south to southeast flow into the forecast area. This will lead to increased surface based instability area-wide, with mean surface based CAPE values from the HREF and REFS between 800-1500 J/kg, with locally higher values (~2000 J/kg) across the mountainous terrain. Low level lapse rates are forecast between 8-9.5 C/km with 0-6km shear between 30-45 kts through this afternoon and evening. This combined with the mid-level wave will result in scattered thunderstorm activity across the forecast area, with thunderstorms developing across central OR as early as 12pm, and across portions of northeast OR and southeast WA between 2pm-3pm. CAMs indicate that the shear and mid-level wave will continue to support thunderstorm activity into the late evening hours as the activity advects northward into south central and southeastern WA. The convective parameters in place this afternoon would also suggest the potential for thunderstorms to produce damaging winds and large hail. The Storm Prediction Center has issued a slight risk for strong thunderstorms across much of the forecast area.

Thunderstorms this afternoon will also tap into PWATs around 0.9-1.3 inches, resulting in the potential for heavy downpours to accompany any storm. Both the HREF and REFS show a 45-65% chance for rain rates to exceed 0.25" per hour along the Cascade east slopes this afternoon, with a 40-60% chance of 1" between 5PM-11PM as well. This will lead to an increased risk of flash flooding along the OR Cascade east slopes. A marginal excessive rainfall risk has also been issued along the Cascade crest and east slopes in OR and in WA.

Otherwise, temperatures will warm across the lower elevations into the 80s, except upper 80s to lower 90s in the Lower Columbia Basin this afternoon.

Friday through Wednesday: Friday, the upper low to the south will swing across the Great Basin and become an open wave trough in response to an upper trough swinging into BC. The trough axis and difluent flow aloft will result in scattered showers across the Cascades, central OR, and the eastern mountains throughout the day, with isolated thunderstorms possible across the eastern mountains. An increasing pressure gradient across the Cascade crest will result in breezy to locally gusty winds (sustained winds 20-30 mph with gusts 30-45 mph) developing through the Cascade gaps and into portions of the Columbia Basin Friday afternoon.

Saturday through Sunday, ensemble guidance is in good agreement that the upper trough will slowly swing north in the northern Rockies, with isolated light shower activity limited to the higher terrain of the northern Blues and the Eagle Caps during this period. The remainder of the forecast area will stay dry with breezy winds (sustained 15-25mph with gusts 25-40mph) through the Cascade gaps.

Monday into Wednesday, weak high pressure aloft will build into the region behind the trough exit to the north, resulting in widespread dry conditions and light winds. Temperatures will also be on a warming trend, with widespread 80s returning by Tuesday (confidence 55-65%). Lawhorn/82

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

VFR conditions to prevail through this morning, with MVFR or lower conditions possible this afternoon and evening. Thunderstorm activity is expected to develop this afternoon and evening across the region, with a 30-50% chance of thunderstorms impacting sites. Thunderstorms will be capable of heavy downpours that may bring MVFR or lower vsby/CIGs, strong outflow winds (variable directions with gusts 50kts or greater), hail, and frequent lightning. Most sites will see storm activity cease through the evening, however model guidance indicates light rain showers continuing at sites RDM/BDN through period. Prevailing winds will mostly be light overnight, except winds at site DLS will be 12-15kts with gusts to around 22kts. Lawhorn/82

HYDROLOGY

The Weather Prediction Center has issued a marginal risk of excessive rainfall along the Cascade crest and east slopes through late tonight. PWATs will increase to between 0.9-1.3 inches (between 75th to 99th percentile for May) by this afternoon and evening along the Cascades, allowing heavy downpours to develop. Training storms and/or exceptionally heavy rain rates (0.3 to 0.5 inches/hr) in this area could lead to flooding and flash flooding.

Preliminary Point Temps/PoPs

PDT 85 51 69 43 / 10 60 10 0 ALW 86 57 71 48 / 10 60 10 0 PSC 93 55 75 46 / 0 60 10 0 YKM 93 57 73 43 / 0 80 40 0 HRI 89 54 70 44 / 0 60 10 0 ELN 89 54 65 40 / 0 80 50 0 RDM 80 42 61 32 / 80 90 50 0 LGD 84 49 71 42 / 40 60 50 10 GCD 81 43 68 38 / 60 40 60 10 DLS 91 56 67 45 / 10 70 40 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.