textproduct: Portland

SYNOPSIS

A cold front moving through the area will bring widespread rain early this afternoon. Rain transitions to showers late this afternoon/evening with a 10-20% chance for thunderstorms. Chances for showers and thunderstorms continue on Tuesday. Warmer and drier conditions return Wednesday to Thursday, but wrap-around moisture will return chances for showers across the Cascades on Thursday and Friday. Cooler temperatures return at the end of the week.

DISCUSSION...Today through Sunday

Radar, satellite, and surface observations as of early Monday afternoon depicts widespread rain along the coast and Coast Range as a cold front associated with an upper-level trough pushes through the area. Between now and 3 PM, this band of rain should spread into the I-5 corridor as the front pushes further inland. Rainfall amounts will be light and non-impactful, while southerly to southwesterly winds will be breezy as the front pushes through. Surface observations were showing wind gusts up to 30 mph along the coast. As this front moves inland, expect wind gusts to increase up to 20-25 mph along the I-5 corridor, and stronger gusts up to 30-40 mph through the central Columbia River Gorge, Upper Hood River Valley, and higher terrain across the Cascades.

After 2-4 PM today, rain transitions to post-frontal showers as the front exits eastward. Cooler air filtering in aloft with the incoming trough will lead to increased instability across the area. Based on REFS guidance and sounding profiles, CAPE appears limited (50-110 J/kg) this evening with the highest CAPE values along the coast. Therefore, there is a 10-20% chance for thunderstorms through this evening, with the highest chances along the north OR/south WA coast, inland southwest Washington, and the Portland/Vancouver Metro Area. Any passing thunderstorms may produce lightning, brief heavy downpours, erratic winds, and/or small hail.

Cool and showery conditions prevail on Tuesday as the upper- level trough turns into a closed low over Oregon and gradually shifts southward toward California/Nevada. Will see another 10-20% chance for thunderstorms on Tuesday, mainly between 11 AM-5 PM. Chances are better along the I-5 corridor for thunderstorms Tuesday afternoon compared to this afternoon as CAPE values are forecast higher around 100-200 J/kg.

Temperatures rebound Wednesday and Thursday as high pressure re- builds over the area and the aforementioned low drops down into the Great Basin. High temperatures are forecast near or slightly above normal along the I-5 corridor for this time of year, warmest from the Portland/Vancouver Metro northward. Chances for exceeding 80 degrees across the Portland/Vancouver Metro on Wednesday are around 15-25%, and 30-50% on Thursday. The rest of the interior valleys have a 20-30% chance for exceeding 80 degrees on Thursday. While Wednesday remains predominately dry and warm, will note that wrap-around moisture will result in a 30-50% chance for showers and a 10-20% chance for thunderstorms across the Cascades on Thursday. Areas west of the Cascades still trend dry on Thursday, but can't rule out a stray shower or two moving into the I-5 corridor. If shower or thunderstorm coverage is more widespread, cloud cover could result in slightly cooler temperatures (lower 70s as opposed to upper 70s and low 80s across interior valleys).

Friday into the weekend, temperatures cool down as ensemble guidance suggests another trough moving in from the northeast Pacific and lowering 500 mb heights. Chances for showers remain on Friday due to lingering wrap-around moisture from the Great Basin low, however, conditions dry out over the weekend due to limited moisture despite the troughing. -10

AVIATION

Showery activity throughout the region has lead to a mixed bag of MVFR and VFR CIGs/VIS this evening. That will continue to be the trend over at least the next 6 hours, though could last as late as 18Z Tue. There are chances for gusty outflow winds from any passing showers as well as the slight chance (less than 15%) for thunderstorms. No thunder has been observed and rather just seeing rain and gusty winds. With the rain and wind, some areas of reduced visibility has been detected. MVFR and even some areas of IFR VIS has been reported along coastal terminals due to increasing onshore flow and marine stratus. Not expecting this to filter too far inland, but the effects may be observed in some inland runways near the coast. Partially clearing skies with the showery activity will also aid in radiational cooling and thus there are increasing chances for MVFR CIGs throughout the Willamette Valley. Confidence is low in the length of time they will last though as the overall environment is still quite well mixed which will counter act some of the settling.

Through Tuesday, winds will shift to the north as the low pressure system that brought rain today moves southward. Areas within the Willamette Valley and along the coast could see some elevated winds with sustained speeds greater than 10 kt after 18Z Tue. There remains a slight risk (15% chance) for thunderstorms on Tue, but with the drier air within the vicinity, it is looking less likely.

KPDX AND APPROACHES...Generally VFR with showers. Showers will reduce CIGs to MVFR temporarily and may produce isolated gusts up to 20 kt. There is around a 30-50% chance of MVFR CIGs after 10Z Tue as conditions begin to settle. If there are more cloud breaks prior to that time, then those chances will increase. Not expecting fog at this time. Winds shift northerly in the afternoon after 18Z Tue which may produce gusts as high as 20 kt. Speeds are not nearly enough for any kind of LLWS, though those landing on E-W runways may experience localized weak LLWS of +/- 5 kt. -27

MARINE

A westerly swell moves into the waters tonight, pushing seas in the mid teens around 14-16 seconds. A Hazardous Seas Warning continues for all the waters until 11 AM Tuesday, with the Columbia River Bar waters being added as buoy 46029 continues to observe seas increasing above 15 ft. Seas subside to 10-12 ft by Tuesday evening and gradually subside into Wednesday. Seas will remain around 8-11 feet through Thursday, then dropping well below 10 ft Friday into the weekend. ~12/19

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Warning until 11 AM PDT Tuesday for PZZ210- 251>253-271>273.