Forecast update: What to expect Wednesday


PORTLAND, Ore. (KOIN) – We are now closing in on our Wednesday winter event – our last real chance for a low elevation winter wonderland. As always, we need the snowy ingredients in the bowl at just the right time.

In addition to weather model computations, we can take advantage of real-time observations and use satellite and radar for tracking. Now is a good time to think about what’s brewing over the Pacific a short distance away.

WHAT YOU NEED TO KNOW (GENERAL FORECAST): As of Tuesday, weather models are trying to work on a conclusion for the transition of a southerly wind (from a cold east wind) and the timing of the moisture. The second variable, moisture, as it relates to the position of the low pressure core offshore is still fluctuating – even in the short term.

So what does this mean for you? Snow chances south of Portland in the valley don’t look very likely. With the incoming low pressure, the southerly wind will pick up and warm air will take over and eliminate the snow threat. This is the typical outcome in our winter events that tend to be cold rain. Weather soundings are also supporting this south of Portland, showing the warmer air moving in from the top down.

You can see that by noon Wednesday, south of Portland will already be transitioning into that southerly flow. By 4 p.m., weather models have that south wind overtaking the east wind from the gorge, effectively shutting off the freezer door to the greater Portland metro area. Winds aloft at about 5,000 feet will transition more quickly in the day. Why does that matter? Any precipitation falling through the atmosphere may not fall as snow and that will make the actual transition to snow even more difficult for those to the south.

PORTLAND AREA UPDATE: Now focus your attention on the two graphics that I have in place (Futurecast at Noon and Futurecast at 4 p.m.). I have decided to use the EURO model to determine a chunk of this forecast, however, I have taken into consideration a blend of models.

Come mid-day, any moisture that can find it’s way to the northern section of the Willamette Valley should have the ability to fall as snow (light amounts). The cutoff will be very close to the Washington/Oregon border and the western edge of the Gorge by the evening. I am having a hard time buying the timing and amount of moisture the models are depicting. The heavier moisture should arrive near the passing of the front later in the day but we will look for some beforehand. I still would say that snow is possible down to the valley until that southerly wind kicks in around the evening commute. I believe Clark County and near the western edge of the Gorge still have that ability to hold on to that cold air for snow. This is where the greatest snow totals will likely be.

For you folks on the southern edge of the city, your threat for snow is not the same as those north and near the Gorge. Think the February event from last year. Right now snow totals are low, with most models keeping us at 0-2 inches by midnight. Be prepared for wet roads and a mix of snow for your evening commute at this time. Breezy conditions are also likely.

As always, a new weather model update will occur late Tuesday night and then again in the morning. We will have updates. We will make changes to the forecast and use the technology to its best ability.

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