textproduct: Pendleton

KEY MESSAGES

- Shower and Isolated Thunderstorm chances Tuesday.

- Breezy winds Tuesday and Friday.

- Widespread showers and isolated storms return late Thursday.

DISCUSSION

Current radar and visible satellite imagery shows dry conditions under most sunny skies as high level clouds begin to move in from the southwest. This is in response to the upper level ridge that kept conditions sunny and warm, continuing to depart to our east as a weak upper level shortwave moves onshore over Central British Columbia. Increasing cloud cover is expected through the remainder of the day, with high temperatures reaching into the mid-80s across the Columbia Basin, Gorge, Blue Mountain foothills, and Central Oregon. The passing of this system on Tuesday will provide a 10-15% chance of light showers and isolated thunderstorms, primarily across eastern Grant, southern Union, and southern Wallowa counties. Rain amounts will be minimal as both the HREF and NBM advertise only a 20-40% chance of measurable rainfall (0.01" or greater) over the aforementioned areas. The HREF and REFS also show the potential for isolated thunderstorms, primarily between the hours of 1PM and 7PM with surface CAPE of 200-300 J/kg and 20-25 knot shear present. Primary concerns related to any developing thunderstorms will be frequent lightning and breezy outflow winds.

The system will also lead to a pressure gradient forming along the Cascades, producing breezy winds along the Columbia Gorge, Blue Mountain foothills, and Kittitas Valley though much of the day - peaking in the early to mid-afternoon hours. The GFS and NAM forecast a pressure gradient of 7-8mb between Paine Field and Pasco, producing west-northwest winds of 20-25 mph with gusts of 30-35 mph. Confidence is high (70-90%) in these wind values as the NBM showcases a 45-55% chance of maximum wind gusts of 35 mph or greater over these areas. Winds will subside Tuesday night as the system exits to our east, but will return with a more robust system Thursday afternoon and extend through Friday. Thursday's wind concerns will be more focused across Central Oregon and the counties of Union and Wallowa as a pressure gradient is expected to develop along the Blue Mountains. South winds of 15-20 mph with gusts of around 30 mph will be possible. Friday's elevated winds will be more associated with the incoming upper level shortwave, providing breezy conditions across the Columbia Gorge/Basin, Blue Mountain foothills, and Kittitas Valley. West-northwest winds of 20-25 mph with gusts of 30-35 mph will again be possible and likely will include more areas of the Basin and foothills.

This second, more robust system late in the week will also bring with it ample moisture as it strengthens and digs south along the west coast. AI ECMWF and GFS ensembles suggest precipitable water of 0.8-0.9", which is 130-150% of normal. NBM guidance still struggles with timing and strength as indicated with 48 hour rain amounts of less than a wetting rain (0.10" or less) for the majority of the area, with the exception of the Cascades and higher terrain over the Blue Mountains. This mirrors the NBM of 48 hours ago, before it shifted to a much wetter scenario just 24 hours ago. LREF guidance has been more stable over the past 24 to 48 hours, with 91% of ensemble members highlighting widespread measurable rain and 47% of members suggesting widespread wetting rain across the area. Thus, the scenario of mountain zones acquiring a wetting rain and lower elevations of the Basin and foothills staying below 0.10" rain amounts Thursday and Friday is more probable. Embedded isolated thunderstorms may also be possible Friday afternoon, but current confidence is lacking due to guidance only suggesting CAPE values of 50-100 J/kg. This will need to be monitored as the event nears. 75

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

VFR conditions are forecasted through the TAF period at all sites. All sites will see relatively clear sky conditions throughout the period as well with a few/scattered/broken high clouds through this afternoon into the evening hours. Otherwise, winds will be generally light with sustained winds of less than 12 kts, with the exception of DLS and ALW experiencing gusts up to 15-20 kts this evening. Ames/01

Preliminary Point Temps/PoPs

PDT 52 76 46 74 / 0 0 0 0 ALW 58 77 52 75 / 0 0 0 0 PSC 55 80 49 77 / 0 0 0 0 YKM 54 80 48 74 / 0 0 0 0 HRI 56 79 49 77 / 0 0 0 0 ELN 50 71 44 69 / 0 0 0 0 RDM 45 75 38 73 / 0 0 0 0 LGD 52 79 45 77 / 0 0 0 0 GCD 51 80 45 79 / 0 0 0 0 DLS 56 75 51 77 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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