Sunrise 5:45 AM   Sunset 8:12 PM Moonrise 1:30 AM   Moonset 9:51 AM

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FXUS66 KPDT 302316 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 416 PM PDT Tue Apr 30 2024

UPDATED AVIATION
00Z TAFs...Main interest through the forecast period will be the breezy conditions across RDM/BDN through 04-08Z. Elsewhere winds will continue to be elevated between 06-12kts. Most TAF sites will see light shower activity on and off through the period as well. Even with the showers, VIS will remain P6SM, CIGs will be above OVC/BKN050 so conditions will remain VFR.
Bennese/90



PREV DISCUSSION
/issued 257 PM PDT Tue Apr 30 2024/

SHORT TERM
Today through Thursday night
Satellite and radar reveal scattered showers over the mountains with more isolated activity across the lower elevations. From a synoptic perspective, mid-level water vapor imagery shows an upper-level trough over the PacNW, accompanied by a circulation center over northern OR and southern WA. This trough will track east-southeast across the PacNW through the evening and is providing synoptic-scale lift across the region. Moreover, cool temperatures aloft coupled with daytime heating have produced steep low- and mid-level lapse rates. At the surface, observations indicate mostly light snow or rain/snow mix over the mountains due to cool temperatures and a dry boundary layer; elsewhere, rain or graupel showers are forecast.

While most convective activity has been shallow, one cell in the Columbia Basin has managed to produce a brief signal on GLM products. The expectation is that with modest surface-based CAPE (100-350 J/kg HRRR-advertised), and steep low- and mid-level lapse rates (9-10 C/km, and 7-8.5 C/km, respectively) the convection will be sustained through the afternoon before waning this evening as instability diminishes and the trough exits to our east. Most shower activity will be shallow, but there is a low (10-20%) chance of cloud-to-ground lightning, highest chance over the Blue Mountains.

Overnight, NBM probabilities paint a low to medium chance (25-60%)
of sub-freezing minimum temperatures for portions of the Yakima and Kittitas valleys, the lower Columbia Basin, and the foothills of the northern Blue Mountains. Confidence in widespread sub- freezing temperatures is too low to issue a Freeze Warning, especially with westerly winds expected to keep the boundary layer mixed through much of the night.

Wednesday night and early Thursday morning, the next system from the Pacific is progged to arrive from the Gulf of Alaska. Guidance is in good agreement that the open wave or weak closed low will track southeast across OR mainly producing precipitation for the Oregon half of our forecast area. Snow levels will be low enough that the mountains will see another round of light snow while lower elevations will see light rain with perhaps a rain/snow mix for portions of central OR.

The system is anticipated to exit by Thursday night with dry conditions forecast. Plunkett/86

LONG TERM
Friday through Tuesday
Run-to-run discontinuity of the medium range models continues, leading to uncertainties in the forecast for this coming weekend into early next week. Overall, there will be spring-like conditions, but forecasting details have been challenging the past several days.

Confidence is moderate to high that most of Friday will be dry with near to slightly above seasonal temperatures. A weak transitory ridge will provide a brief break in precipitation and winds. Meanwhile, an upper low will deepen offshore and will increase the southwest flow aloft west of the Cascades.
Precipitation will spill over the Cascade Range Friday evening, and the rest of the forecast area will have an increasing chance of precipitation Friday night and Saturday. Current forecast will show 80-90% PoPs along the Cascades, 60-80% from the Cascade east slopes to the Blue Mtns, and 40-60% east of the Blues on Saturday.
The ECMWF and a majority of its ensembles advertise more QPF compared to the GFS/GEFS based on the position of the low that is farther north along the OR coast compared to northern CA. The ECMWF EFI from the 00Z runs paints an area of 0.6-0.8 of the QPF over portions of eastern OR, indicating there is a chance of moderate to heavy showers with this system. There are also hints of a late snow event for the Blue Mountains this weekend on the EFI (shift of tails around 2.0), but the snow-to-liquid ratios should be very low this time of year (5:1) that any chance of appreciable accumulations over the passes is low.

The models are out of sync Saturday night through Tuesday, therefore it's difficult to describe the forecast with certainty. PoPs decrease Saturday night and Sunday, but there still is around a 40-60% chance of mountain showers. There will be an upper low and large trough over the western U.S., but the question will be the position. A majority of the ensembles favor the center of the low over the Great Basin, leaving a cyclonic flow or split flow aloft over WA/OR. The area most likely to receive precipitation during this time will be from the Blue Mtns eastward. The low is progged to weaken and track eastward early next week, but now the question is another trough off the coast.
The Canadian and GEPS are the strongest with the offshore trough while the others show a more westerly flow across WA/OR or a weak shortwave trough. Since it will be a WSW flow with variable amounts of moisture, will forecast a chance of mountain rain/snow showers and a slight chance of showers elsewhere. NBM has breezy conditions early next week which looks reasonable based on the westerly flow aloft. Wister/85

PRELIMINARY POINT TEMPS/POPS
PDT 34 59 34 61 / 60 0 10 30 ALW 37 62 38 65 / 50 10 10 30 PSC 38 67 40 68 / 20 0 10 20 YKM 32 65 35 67 / 20 0 10 10 HRI 36 65 39 66 / 40 0 10 20 ELN 34 60 33 66 / 30 0 10 10 RDM 27 56 35 55 / 20 0 40 30 LGD 33 55 30 57 / 70 10 10 40 GCD 31 54 34 54 / 70 10 30 50 DLS 39 61 43 65 / 30 0 30 20

PDT WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.

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