textproduct: Portland

SYNOPSIS

Clear skies and calm winds may bring frost and/or fog development across much of NW OR and SW WA in the mornings Saturday through Monday. Mostly dry conditions return this weekend. More uncertainty for the return of precipitation early next week with limited weather impacts expected.

DISCUSSION...Now through Thursday

A northerly jet streak on the back side of the upper low will deepen overnight causing the upper level pattern to stretch into an elongated trough near the coast while the surface low drifts south and weakens offshore on Saturday. This will lead to a mostly dry day on Saturday, though as the trough finally moves inland Saturday, some wrap around moisture will bring 20-60% chance of rain showers over the OR Cascades and foothills late afternoon and evening with a 10-15% chance for Valley locations long and east of Interstate 5 south of Salem after 5 PM. Showers are expected to be light though on the chilly side as valley temperatures will quickly fall into the 40s around sunset. Completely dry conditions return Sunday as upper level ridging moves inland as the upper trough slides east of the Cascades on Sunday.

Temperatures tonight into Saturday morning are forecast to become much cooler than previous nights with widespread low temperatures in the 30s, though cloud cover will be an important factor in whether temperatures will fall to near or below freezing. Latest model guidance indicates skies will become mostly clear late this evening into the overnight hours as showers dissipate. Clear skies combined with calm winds and ground moisture from recent rain will allow for radiational cooling but also for fog formation. If temperatures fall quicker than forecast under clear skies before fog forms, then low temperatures near or below freezing are more possible. However, if fog ends up forming, this could limit how low temperatures can fall in areas with fog. Because of this uncertainty, have converted the Freeze Watch to a Frost Advisory, which is in effect for much of the area west of the Cascades, including the coast. Outside of the major metro areas and foggy locations, there are higher chances for minimum temperatures to fall below freezing (around 60-80%). Otherwise, probabilities are closer to 20-40%. Frosty conditions are likely again Sunday and Monday mornings. However, freeze and frost headlines are only issued during the growing season of April 1 through October 31. Therefore, no additional freeze or frost headlines will be issued after Saturday.

Ensemble guidance remains in relatively good agreement that upper level ridging and subsequent dry conditions remain over the region into early next week as another low pressure system develops in the eastern Pacific west of the OR/CA border and eventually moving inland over California midweek. However, there are a handful of ensemble members that bring the low pressure system inland over the PacNW sometime late Monday into Tuesday as a weak trough, allowing for a 10-20% chance of precipitation to return. If the drier solutions pan out, expect a slight warming trend with afternoon highs near 60 degrees for the interior lowlands through the middle of next week. Chances for rain increase slightly late week as some ensemble members indicate a frontal system approaching the area, but there is high uncertainty in specifics. At this point, no significant impacts are expected with the upcoming weather pattern. -03/DH

AVIATION...Through 06Z Saturday

Mostly clear skies have returned to the region late this evening with some lingering showers right along the far NW Oregon coast into southwest Washington. Expect this trend in the showers to prevail the remainder to the night with dry weather favored at almost all terminals outside of KAST.

The primary concern headed through tonight into Saturday morning will be fog formation. The moist ground, clearing skies, and calm winds have already begun to yield fog near KKLS and in some of the coast range valleys, and as the night goes on, confidence has increased in fog coverage rapidly expanding by 10-12z. The latest 00z REFS depicts a 50-70% chance for VIS to degrade below 1 mile at all inland terminals. Given the extremely favorable fog set-up and high resolution ensembles seemingly on-board, have added a period of LIFR/IFR conditions to most inland TAF sites Saturday morning. Confidence is low on exactly how quickly improvement may take place, or if fog/low clouds in the coast range will be able to extend to KONP and to a lesser extent KAST come the sunrise hours. Once fog/low clouds clear by 18-21z, VFR conditions return Saturday afternoon.

KPDX AND APPROACHES...Confidence has significantly increased for fog related impacts at KPDX by sunrise Saturday with satellite observations already showing fog/low clouds rapidly developing along the Columbia River to the NW of the the terminal - verified by 1/2SM VIS at KKLS. Given this trend (development faster than most models) there is uncertainty as to just how quickly conditions deteriorate overnight but confidence for LIFR VIS/CIGS peak in the 12-17z timeframe which is reflected in the TAF. Winds remain fairly light, less than 5-8 knots. -99

MARINE

Weakening low pressure offshore will maintain southerly winds across the waters through this weekend, generally remaining under 20 kt. Winds weaken over the weekend. Seas around 5-6 ft today gradually subside to 3-4 ft on Saturday. Offshore winds develop late Sunday and persist into early next week. A long period west-northwest swell also arrives Sunday night, building seas to around 8 to 10 ft at 15-18 seconds by Monday. Chances for seas over 10 ft on Monday remain at around 40-60% across the outer waters. This long period swell will result in a moderate threat for sneaker waves Monday and Tuesday. Caution should be used when in or near the water. The next frontal system likely approaches the waters mid to late next week, returning southerly winds. -10/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory until 9 AM PDT Saturday for ORZ101>103-105- 107>109-111>115-119-123>125. WA...Frost Advisory until 9 AM PDT Saturday for WAZ203>207. PZ...None.