Sunrise 5:59 AM   Sunset 7:51 PM Moonrise 3:43 PM   Moonset 4:40 AM

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FXUS66 KPDT 191643 AAA AFDPDT

Area Forecast Discussion...Updated National Weather Service Pendleton OR 943 AM PDT Fri Apr 19 2024

UPDATE
Current radar and visible satellite imagery showing dry conditions and clear skies across the region. This is in response to the northwest flow aloft advecting cooler and drier air into the area as an upper level ridge slowly moves onshore through the day today. The two concerns in the short term consist of the potential for freezing temperatures across the Kittitas Valley tonight into Saturday morning, and breezy winds nearing Advisory criteria overnight Saturday into Sunday morning as an upper level trough and associated cold front passes through the Pacific Northwest. Winds will again be light tonight as the upper level trough axis passes through the area, but southwest flow aloft and a shift from offshore to onshore winds will slightly increase cloud cover - which may be enough to inhibit the efficiency of radiational cooling overnight. Confidence in freezing temperatures across the Kittitas Valley tonight is currently low to moderate (30-50%), as the NBM suggests a 45% chance of Saturday morning low temperatures dropping to 32 degrees or below at Ellensburg.

The other concern resides with the potential for Advisory level winds across the Basin and east slopes of the Washington Cascades Saturday night into Sunday morning associated with the passing trough and associated cold front. Pressure gradients are expected to tighten along the Cascades as a result, with the GFS, NAM, and SREF all highlighting a 10-12.5mb difference between Portland and Spokane. However, the GFS is alone in anticipating the gradient will exceed 11.5 mb, which is below the normal advisory threshold of 12 mb. The NBM aligns with this uncertainty, showing a 20-50% chance of wind gusts of 47 mph or greater across the Blue Mountain foothills and through north-central Oregon. A better chance resides in the Simcoe Highlands, as the NBM presents a 50-70% chance of gusts of 47 mph or greater. These two weather concerns will be analyzed further as updated guidance is released over the next several hours. 75

AVIATION
18Z TAFs...VFR conditions will prevail through the period. Some mid/high clouds will stream into the area overnight.
Winds will be 8-14 kts with higher afternoon/early evening gusts.
Winds will decrease overnight to 8 kts or less. Earle/81

PREV DISCUSSION
/issued 457 AM PDT Fri Apr 19 2024/

SHORT TERM
Today through Sunday night
Nighttime microphysics RGB imagery reveals mostly clear skies across the entire forecast area this morning. On the synoptic scale, the forecast area is between an offshore upper-level ridge and a trough downstream over the Northern Plains. This is translating to a dry northwesterly flow aloft with a well-defined West Coast thermal trough (WCTT)
along the OR coast resulting in low-level east-northeasterly winds. These winds are keeping breezy conditions along ridges and preventing a widespread freeze for the lower elevations. That said, have opted to issue Freeze Warnings for the foothills of the northern Blue Mountains of OR/WA due to near- to sub-freezing temperatures that have developed in areas that have decoupled.
Sub-freezing temperatures for these zones are not expected to be ubiquitous (elevated ridges that do not decouple will stay warmer), but enough of our population centers are being impacted to warrant the highlights. Elsewhere, have also opted to issue a Freeze Warning for the lower Columbia Basin of OR where temperatures are hovering near freezing. The Yakima Valley is observing a wide range of temperatures - lower 30s to lower 40s depending on the influence of the previously mentioned northeast winds. Have opted to not issue a Freeze Warning here as coverage of sub-freezing temperatures this morning is expected to remain patchy.

Offshore winds will persist through the day as very little changes on the synoptic scale. Expect locally breezy east-northeast winds, especially for ridges and elevated plateaus with low afternoon RH.

Tonight into Saturday morning, the offshore upper-level ridge is expected to push onshore, followed by a trough passage Saturday afternoon and evening. This will lead to a reversal in wind direction. Ensemble guidance is still exhibiting some uncertainty with regard to the exact track of the trough, and sustained winds and gusts associated with it. Confidence is low-medium (20-50% chance) in reaching Wind Advisory criteria Saturday afternoon and evening across the Columbia Plateau region. Otherwise, some light precipitation is forecast for the mountains, primarily the Cascades, with mostly dry conditions for the lower elevations.

Sunday, precipitation chances taper off as the trough exits to the east and an upper-level ridge moves along the coastline. Breezy west winds are forecast through Sunday, but are not expected to reach advisory levels (<20% chance). Plunkett/86

LONG TERM
Monday through Friday
Starting Monday morning to Tuesday night, an upper-level ridge moves over the PacNW as the previous trough passes through North Rockies. Late Tuesday night, another trough will occur onshore that will dominate over PacNW as it deepens throughout Thursday. Dry conditions for Monday and Tuesday will stick around due to the ridge building over the region. Rain showers are expected in the low elevations Wednesday into Thursday night with low possible chance (<30%) of snow within the mountain areas. Snow levels will be around 2000-4000ft Monday but increases up to 4000-6000ft Tuesday into Thursday night. As the deepening trough continues over PacNW, chances of precipitation increases while snow levels remain high (>70% confidence). Breezy conditions will occur with wind gusts up to about 20-30 kts across the Simcoe Highlands and Kittitas Valley Wednesday but might come and go until the end of these next few days (50-70% confidence). Temperatures will moderate between the upper 60s to low 70s through this period (50-70% confidence).
Feaster/97

PRELIMINARY POINT TEMPS/POPS
PDT 63 35 66 38 / 0 0 0 20 ALW 67 37 69 42 / 0 0 0 40 PSC 68 40 71 44 / 0 0 0 10 YKM 65 36 67 36 / 0 0 0 10 HRI 67 37 70 42 / 0 0 0 20 ELN 61 33 64 37 / 0 0 0 20 RDM 61 33 70 30 / 0 0 0 0 LGD 60 34 69 36 / 0 0 0 20 GCD 63 36 73 35 / 0 0 0 10 DLS 68 42 68 43 / 0 0 0 10

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

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