textproduct: Portland

SYNOPSIS

A series of complex frontal systems will move through the area this week each bringing chances for strong winds, heavy rain, and river flooding later in the week. Each system has the potential to be impactful in it's own way. In the short term, the main concerns lie with rain on Monday, and wind on Tuesday. There are increasing probabilities of some interior areas meeting Wind Advisory criteria on Tuesday. Heavy rain returns on Thursday but is shifting further south to areas that did not receive as much rain the last time around.

LONG TERM...Thursday through Sunday

Now as we transition into the later half of the week the weather appears to ramp back up with a trend towards another atmospheric river. On Thursday we will experience the highest precipitation of the long term forecast as it bleeds into Friday. The challenge with this specific system is that there is a lack of consistency with the synoptic flow. The atmospheric river has a high probability of occurring, but models are showing several shortwaves and lows trailing behind it in the flow. This would lead towards a more prolonged system vs a short burst of rain like the Monday/Tuesday systems. However, if you look at the guidance we typically use for atmospheric rivers, they show similar features to that of the earlier events. At this point, confidence is incredibly low but if we maintain the cold air with lowered snow levels, we could be looking at our first heavy snowfall of the season. In fact, the mean of the NBM 48-hr snowfall amounts shows as much as 35 inches of snow for Thursday & Friday combined at Mt Hood. There is a 10% chance of nearly 4 feet in those 48 hours. However, if the snow levels rise like they may do if the atmospheric river shifts further north, then we would see significantly less snow with a 10% chance of only 15 inches of snow. If you're planning on traveling over the passes this weekend, please continue to track the forecast shifts as this system develops further.

With the influx of additional heavy precipitation flooding again rises as a concern. While the extent of the flooding is not forecast to be as widespread at this point, we must still be diligent as there are still a number of rivers that are forecast to reach flood stage. Therefore, if you live near any rivers, please visit www.water.noaa.gov/wfo/pqr for localized forecasts.

Precipitation remains through the weekend. -27

AVIATION

Heavy rain expected over the next 12 hours as an atmospheric river moves over the region. Gusty winds will be associated with the passing front and therefore may see reduced VIS with falling CIGs and rain. The piece of the forecast that is a bit more unique is the widespread potential for LLWS. Between 2000-5000 ft AGL winds will be in excess of 40 kt with some areas reaching as high as 65 kt at 5000 ft. The surface flow will be more southerly while flow aloft southwesterly. Therefore, prepare for LLWS for the next several hours. As the front exits, post frontal showers will form. As the front passes, conditions will improve with low-end VFR CIGs after 00Z Tue. The wind threat remains through 06Z Tue for higher elevations and approaches.

KPDX AND APPROACHES...Conditions have remained VFR this morning, likely due to the lifting associated with the front. The lower atmosphere is well mixed which could combat the CIGs dropping significantly. There remains a trend towards MVFR though as the bulk of the front shifts over the terminal. Some hi-resolution guidance supports even stronger gusts exceeding 30 kt, most likely between until 20Z Mon. A strong low-level jet out of the south-southwest will also support low-level wind shear of 30-45 kt. This concern rapidly dissipates ~19-20z behind the frontal passage. -27/99

MARINE

A strong cold-frontal system is slated to pass through the waters today (Monday) leading to fairly impactful conditions across the waters. Gale Warnings are in effect for the inner and outer waters, including the Columbia River Bar, through the daytime hours with high confidence in sustained winds of 25-35 kt with gusts of 40-50 kt. At this point isolated storm-force gusts associated with a coastal jet accompanying the frontal boundary can't be ruled out Monday morning with a 35-45% chance of occurring somewhere along the southwest Washington or northwest Oregon coasts, and only 10-20% chances across the outer waters. While winds peak Monday morning, seas continue to build into Monday afternoon before slowly easing on Tuesday. Seas will most likely reach into the 15-18 ft range Monday afternoon, with a 10-35% chance of surpassing 18 ft beyond 30 NM north of Cape Falcon - 10% or less elsewhere. By Tuesday afternoon, significant wave heights are likely closer to 11-14 ft with a dominate period of 11-12 seconds.

Going forward a rather active weather pattern, even by December standards, continues through the work week bringing additional winds and wave hazards to the coastal waters. Seas are very likely to remain elevated above 10 ft (greater than 90% confidence) from Wednesday through Friday as additional weather systems move onshore. At least models are beginning to hint as some larger breaks between weather disturbances next weekend into the following week, but at this 7-8 day time-scale, the confidence in this slowdown materializing is only low to moderate at this time. -99/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Wind Advisory until 4 PM PST this afternoon for ORZ106-107. WA...Wind Advisory until 4 PM PST this afternoon for WAZ203. PZ...Gale Warning until 4 PM PST this afternoon for PZZ210-251>253- 271>273. Hazardous Seas Warning from 4 PM this afternoon to 4 AM PST Tuesday for PZZ210-251>253-271>273. Small Craft Advisory from 4 AM to 4 PM PST Tuesday for PZZ210- 251>253-271>273.