textproduct: Portland

SYNOPSIS

A weak front will move over the area today bringing light rain. A pair of quick moving frontal systems associated with subtropical moisture will bring additional rounds of heavier rainfall and river rises on Monday and Tuesday. The probability for exceeding minor flood stage at the majority of rivers remains around 20-25% or less. Breezy to gusty southerly winds are also expected both days with the strongest winds likely Tuesday night. Onshore flow continues with showers on Wednesday. Lowering snow levels will allow for snow to fall at pass level in the Cascades. Precipitation continues through late next week, with potential for another strong system on Thursday.

LONG TERM...Wednesday through Saturday

Snow levels are expected to drop significantly by Wednesday morning with the passage of the upper trough. This will allow for snow to begin accumulating at pass level on Wednesday. The weather setup on Wednesday will feature onshore flow with zonal flow aloft, likely maintaining showers enhanced by orographic effects. There remains quite a large spread of uncertainty for snowfall amounts through Wednesday night. Areas with the probability for the highest snow amounts are within the south Washington Cascades where there is a 50-60% chance of receiving 6+ inches and a 20-30% chance of 12+ inches in 24 hours. To describe that uncertainty even further, the NBM's 10-90th percentile (essentially the driest and wettest scenario) gives a range of 1"-13" at Santiam Pass. The upper end range has decreased from last afternoon which shows the potential for the range to narrow. With that said though, for those preparing to travel, it is critical that you keep an eye on the forecast.

As the synoptic pattern progresses into late next week there is a trend for yet another atmospheric river on Thursday into Friday. At this point confidence is quite low as there remains significant inconsistencies between ensembles, individual models, and run-to-run outputs. The Warning Prediction Center (WPC) has increased the risk of excessive rain to a "slight risk" which means there is increasing probability of exceeding flash flooding guidance. Now, that comes down to rain rates and total rain accumulation. As previously stated there remains significant uncertainty. However, it's never to early to be thinking about the impacts of heavier rain. With our soils very saturated and rivers full, the Monday/Tuesday rain will add even more water to the situation. Then we add even more on Thursday/Friday and we could see potential flooding - especially on rivers that remain high. One positive is that the track is shifting a bit further south with this latest forecast which will give a little bit of a break to southwest Washington. The snow levels also remain low so the precipitation will be snow in the Cascades.

On Saturday we have seen a slight shift in the forecast. If you look on our website and you live at around 1300 ft you may notice a rain/snow mix in the forecast. You are seeing that due to what we call the "minimum slow level". This is essentially the lowest snow level that models are suggesting (around a 10% chance of seeing snow at this level). In order to communicate that in our forecast it will show as a rain/snow mix. However, snow levels are closer to 2500 ft so there is much higher confidence in higher elevation snow. -27

AVIATION

A weak frontal boundary traversing the region is bringing mixed flying conditions with scattered light rain showers this morning. Showers are numerous enough that periods of lowered cigs or restricted vis are likely through the next 2-4 hours at area terminals, most likely along the coast and north of KSLE. Otherwise, south to southeast winds increase to 5-10 kt inland and 10-15 kt with gusts to 20 kt along the coast through today.

In the wake of this weak system, another more potent one will approach the region this evening and through tonight bringing strong winds and widespread rain after 03-09z Mon. At this point, largely MVFR cigs are favored during steady rain with generally 10-20% chances of IFR cigs after 12z Mon. Surface winds will also continue to increase as latest high-resolution guidance favors wind gusts reaching at least 30-35 kt along the coast and 20-25 kt inland. A strong low-level jet reaching 50-60 kt out of the south-southwest at only a few thousand feet off the surface will additionally yield increasing low-level wind shear of 35-55 kt through Monday morning at all terminals. This shear will be unidirectional at most terminals, however surface winds remaining southeasterly later into the period at Portland-area terminals will introduce a directional shear component as well.

KPDX AND APPROACHES...VFR conditions expected to prevail with scattered light rain showers during a weak frontal passage this morning. Any shower moving over the terminal may yield lowered cigs and restricted vis, most likely to MVFR levels, through the next 2-4 hours. High confidence that conditions return to VFR as showers exit after 21-22z Sun. Winds remain out of the southeast at around 5 kt into this evening. Another, more potent system arrives by 06z Mon bringing steady rain and increasing winds. Cigs/vis expected to trend to MVFR by 10-12z Mon while winds turn increasingly out of the south and build to around 15 kt with gusts to 25 kt tonight and through Monday morning. Some high-resolution guidance supports even stronger gusts exceeding 30 kt, most likely after 12z Mon, but confidence is too low to include in TAFs at this lead time. A strong low-level jet out of the south-southwest will also support low-level wind shear beginning overnight. -36

MARINE

A weak frontal system moves through the coastal waters today. This front has begun to amplify winds this morning with gusts of 25 kt being reported at area buoys. This front will bring an end to the calmer sea conditions we have been seeing and set the stage for days worth of strong winds and hazardous seas. Small Craft Advisory remains in effect for all of the waters and the Columbia River Bar due to wind speeds of 15-20 kt with gusts up to 25 kt. Through the afternoon those winds will increase further. By late tonight into Monday the winds will transition to gale force wind speeds. These gales will be strongest within the inner waters due to the strong southerly flow. There are some models suggesting up to a 50% chance of storm force winds in some locations of the inner waters. However, there is a much higher probability for widespread gales. The seas at this time will build to around 16 ft due to the addition of a high wind wave.

A series of frontal systems and a low pressure system in the Gulf of Alaska will enhance the westerly flow and thus westerly swell. The compounded features will cause seas to increase considerably. Current forecast shows seas of around 18 ft on Tuesday (combination of wind wave and the fresh westerly swell). There is a 10% chance of seas of 20 ft and less than a 5% chance of seas exceeding 22 ft. Hazardous seas expected starting Wednesday. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory until 1 AM PST Monday for PZZ210-253-273.

Gale Warning from 1 AM to 4 PM PST Monday for PZZ210-253-273.

Small Craft Advisory until 10 PM PST this evening for PZZ251- 252-271-272.

Gale Warning from 10 PM this evening to 4 PM PST Monday for PZZ251-252-271-272.