textproduct: Pendleton

KEY MESSAGES

- Warming trend continues, peaking Tuesday.

- Isolated thunderstorms possible through Tuesday.

- Dry and windy conditions Tuesday. *Fire Weather Watch Issued*

DISCUSSION

Current radar and visible satellite imagery shows dry conditions under mostly clear skies. This is a result of a weak and dry cold front that has stalled over Southeast Oregon, bringing slightly cooler high temperatures across the region. The upper level ridge will rebuild Monday and Tuesday, enhancing southwest flow aloft and increasing high temperatures back into the mid-90s Monday and nearing the triple digits for areas across the Lower Columbia Basin on Tuesday. The NBM suggests a 30-40% chance of reaching 100 degrees across Tri-Cities and Walla Walla. These high temperatures, coupled with minor overnight relief due to morning lows still around 60 degrees, will lead to areas of moderate (2 out of 4) HeatRisk Monday and Tuesday. These areas will be more confined to the Eastern Gorge, Hanford area, and the Yakima/Kittitas Valleys on Monday, being more widespread across the Lower Columbia Basin and Blue Mountain foothills on Tuesday.

An upper level shortwave will approach from the south this evening into Monday, allowing for a slight chance (10%) of developing thunderstorms across Central Oregon this evening. Chances will materialize around 1600 and peak after 2000 for southern Deschutes, Crook, and Grant counties. Better thunderstorm chances (20-30%) are expected on Monday as two weak shortwaves converge and a surface low develops over the Columbia Basin. CAPE values of 400-500 J/kg and low level lapse rates of 7-7.4/km would provide enough instability for storms to initiate, which aligns with the Storm Prediction Center including our southern and eastern zones in their General Thunderstorms (0 out of 5) category. These areas would primarily include Deschutes, Grant, Crook, Union, and Wallowa counties. The surface low again develops over the Columbia Basin Tuesday as an incoming upper level trough moves inland along the British Columbia coast. This will bring a more widespread threat of isolated thunderstorms across the Blue Mountains and extending through Central and Eastern Oregon, albeit low (10-20%) chances at this time. Wind shear is between 15-25 kts each day, which relates to slow moving storm cells that should not develop enough for severe thunderstorm conditions. Thus, gusty winds, lightning, and small hail are the main concerns.

The incoming upper level trough on Tuesday will not only promote thunderstorm potential across our eastern mountain and Central Oregon zones, but will also increase winds across the Columbia Basin and Kittitas Valley. This is due to the pressure gradient between the incoming trough and departing upper level ridge, which enhances into Montana and southern Saskatchewan. West winds of 10-20 with gusts up to 30 mph coupled with afternoon humidities in the low to mid-teens will lead to elevated fire weather concerns. This has warranted the issuance of a Fire Weather Watch across these areas through the afternoon and evening on Tuesday.

Near normal temperatures are expected Wednesday onward as flow aloft becomes more zonal. Breezy diurnal winds will still occur across the Eastern Gorge through the week, extending across the Lower Columbia Basin Friday into Saturday as another upper level trough approaches the area. 75

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

VFR conditions are forecast at all sites through the period. Winds will be diurnally and terrain-driven, predominantly sustained at 10 kts or less with periodic higher gusts during the afternoon and evening. There is a very low chance (5-10 percent) of showers and thunderstorms in the vicinity of BDN/RDM later this afternoon, with those low chances of showers continuing through the night; most likely, activity will remain to the south and east. There is a low (10-15%) chance of showers and thunderstorms in the vicinity of BDN/RDM again Monday afternoon.

FIRE WEATHER

The present upper level ridge to our southeast and the incoming shortwaves this evening and again on Monday will keep the isolated thunderstorm threat across Central and Eastern Oregon, with the best chances expected on Monday. Precipitable Water of around 0.70" will lead to the potential for a brief downpour, but it is more likely that the majority of developed thunderstorms may say dry. Any cells developing today should stay to our south, but Monday's extent moves further north into zones: OR696, OR700, and OR705. Thunderstorms will also be possible for zones: OR693, OR694, OR695, OR697. OR698, and OR699, but these zones have yet to be deemed Red Flag Ready related to current fuel status. Thunderstorms again develop on Tuesday associated with a larger trough passage across Central Canada, but increased precipitable water of around 0.90" should inhibit the occurrence of dry lightning.

Tuesday's incoming trough will also lead to critical fire weather concerns across the Lower Columbia Basin and the Kittitas Valley associated with breezy winds and low humidities. Thus, a Fire Weather Watch has been issued for OR/WA691 and WA 690 between 11 AM and 10 PM Tuesday. Sustained west winds of 10-20 mph with gusts up to 30 mph and humidities as low as 13% will be possible. The NBM advertises a 50-80% chance of Red Flag Conditions being met for all areas included in the current watch. 75

Preliminary Point Temps/PoPs

PDT 55 90 57 94 / 0 0 0 0 ALW 58 91 62 96 / 0 0 0 0 PSC 56 94 58 98 / 0 0 0 0 YKM 58 95 61 98 / 0 0 0 0 HRI 56 93 60 97 / 0 0 0 0 ELN 54 92 59 91 / 0 0 0 0 RDM 53 90 53 91 / 0 10 10 0 LGD 54 88 56 93 / 0 10 0 10 GCD 55 89 54 95 / 10 30 20 10 DLS 60 96 62 91 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Fire Weather Watch from Tuesday morning through Tuesday evening for WAZ690-691. OR...Fire Weather Watch from Tuesday morning through Tuesday evening for ORZ691.