textproduct: Pendleton

KEY MESSAGES

- Much cooler than average temperatures persist into the weekend as much as 8-12 degrees with a warming trend beginning Monday into mid-week.

- Unsettled weather with low chances of thunderstorms (15-25%) persisting into early next week.

- Little to low fire weather concerns owing to elevated afternoon humidity and showery weather.

DISCUSSION

Latest water vapor imagery shows a complex mid- level closed low over northwest OR with a shortwave trough moving into southeast BC. Additional embedded disturbances are rotating around its southern base with shortwave troughs presently moving into eastern OR and into northern CA. Latest radar display shows some newly developed convection near/off Rattlesnake Ridge and Indian Rock with some infrequent lightning in the last hour. Initial forecast concerns turn to thunderstorm chances across much of the area, albeit isolated coverage (15-25%), with the activity happening a few hours earlier than expected. Latest SPC mesoanalysis shows ~500 J/kg MUCAPE/SBCAPE being realized there west into the Cascades with the convection aided by orographic ascent in those aforementioned areas. Latest Day Cloud Phase RGB imagery shows the additional cu development is less impressive after the initial CI over Rattlesnake Ridge and Indian Rock so additional lightning may be hard to materialize.

Latest ensemble clustering and deterministic guidance show decent agreement increasing overall confidence in the large scale pattern evolution through Monday. Latest EPS means show scant movement east the next 48-hrs as it fills in with embedded shortwave troughs ejecting and elongate it to extend to offshore northern CA by Saturday. Lingering residual moisture with limited additional moisture being transported into the area will keep the weather unsettled and cloudy as this upper trough evolves. In general, the cooler air mass will remain in place with highs generally 8-12 F below average this weekend across much of the lower elevations. Of which, record cool highs will be approached for KHRI and KRDM at least Saturday and Sunday, respectively. While areas saw as much as a half inch to approaching 1 over the last 24-hrs this morning in Wallowa and parts of the extreme eastern mountains, lesser rainfall totals are expected this weekend in comparison over any 24-hr period. With the showery precip expected, chances for 0.5 or more largely peak only 30-50% in the Cascades and higher peaks of the eastern mountains.

As the upper trough starts to make progress east, 500 mb grand ensemble members are still agreeing on a weak neutral trough over Oregon and Washington though spread is increasing. Moving forward in time, the main low in the trough begins to split before it lifts and translates east. The southernmost low gets ejected and opens into a wave and, during this period of movement, the embedded short wave trough moves through the greater flow. Timing and the intensity of the low in the deterministic and ensembles then become more in disagreement.

Comparing ECMWF and GFS ensembles, both location and strength of CAA and WAA are in great disagreement in the beginning and this disagreement continues though the forecast. Deterministic cold air advection remains over the area for the entire forecast period.

Monday morning temperatures for the lower elevations in the foothills and Columbia Basin should be mid 50s, progressing to higher elevations around La Grande sub-50s is likely. Bend and the southern border of the CWA will see low to mid 40s. Monday afternoon we continue our unseasonable cool temps with highs in the mid to upper 70ds in the columbia basin and foothills, the Blue and Strawberry mountains will remain much cooler then its lower elevation neighbors with areas like Bend and La Grande being closer to the mid and upper 60s.

The WPC ERO Day 3 has the CWA in a marginal for excessive rainfall. WPC is discussing upgrading the area to a slight as model guidance becomes clearer. Higher amounts of precipitation on the east side of the Blues with a modest rain shadow to the west. NBM has chances peaking up to 67% for a 0.25" of precip or more mainly on or past the Blues on the east side. Holistically, the forecast area has a 24% chance or higher for at least a .01" that increases as you travel south on 84. We are in agreement with the WPC the amount of moisture that gets transported from Hurricane Marie will heavily influence what precip amounts will be.

Monday night into Tuesday shows the main trough traveling eastward into Montana and tracking into Alberta/Saskatchewan, while a secondary trough forms off the Oregon/California coast being influenced by Tropical Storm Eduardo. While the Ensembles agree on the timing of the movement of both 500mb troughs, there is uncertainty of the position and strength of the offshore secondary trough going into Tuesday afternoon. Ensemble members show showers from Monday afternoon into early Tuesday (73% of members) with the mid to late Tuesday showing no precipitation. Temperatures present should be near normal values, with drier conditions present as the trough moves NE.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

VFR conditions with on and off shower activity through tomorrow. Not expecting much heavy precip with this system through tomorrow, but there is a small chance (5 to 15 percent) that a moderate shower could pass through a TAF site and bring temporary MVFR/IFR conditions. Winds in general will be light in nature with most sustained winds less than 12 knots.

Preliminary Point Temps/PoPs

PDT 42 70 44 68 / 30 40 20 40 ALW 48 72 49 71 / 20 30 30 20 PSC 45 75 46 74 / 0 10 20 20 YKM 49 72 49 74 / 0 30 20 30 HRI 45 73 46 72 / 10 10 20 30 ELN 42 69 43 71 / 10 40 20 30 RDM 34 67 37 63 / 60 30 40 80 LGD 40 72 42 72 / 30 40 20 60 GCD 41 70 43 69 / 30 40 10 80 DLS 52 74 52 74 / 30 30 20 30

PDT WATCHES/WARNINGS/ADVISORIES

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