The Winter Transition is Imminent

It is like falling off a meteorological cliff.  And it happens every year.   The transition to winter weather in the Pacific.

It is a real meteorological curiosity.   Our Septembers are marvelous, and the first two weeks of October are often splendid, with low clouds in the morning and lots of sun in the afternoon.  The trees turn flaming colors and our sunsets are often magnificent.    A view above the fog yesterday, supplied by Greg Johnson of Skunk Bay Weather takes your breath away (below).  We are living in Paradise.

But then, generally in the third week of October, a meteorological switch is pulled and the weather changes.  The storm track, which had been happy to savage southern Alaska, moves southward into our region, shifted by the development of a broad area of low pressure over the Gulf of Alaska.   Rain and clouds move into the Pacific Northwest, 60s and 70s become fond memories, and Northwest residents hunker down for roughly four months of moisture and inadequate sun.

It is the price we pay for the bounty of water that gives us massive hydropower, great agricultural productivity, our fisheries, and more.  It is also protection from a major northward onslaught of Californians.

This morning is a typical mid-October day, with low clouds over the lower elevations but bright sunshine aloft (see the  9 AM visible satellite image Saturday morning).  If you want sun now, get above roughly 1500 ft or head eastward.  Sun should reach the lowlands later today.

Our nice fall weather will end later on Tuesday, as the circulation over the Pacific moves towards its winter arrangement.  One way to see this transition is from a very nice graphic found in the Seattle RainWatch website that shows the precipitation forecasts from the  NAEFS ensemble prediction system for Seattle.

Remember that an ensemble systems runs many forecasts, each slightly different, that helps one get a measure of the uncertainty of the forecast.  In this graphic, the median of all the forecasts (of 6-h precipitation) is shown by the horizontal red line and 50% of the forecasts are within the boxes.  The extremes are indicated by thus plus signs.

The message is stunningly clear.  Dry until Tuesday,  followed by two wet events, and then we settle into a period of light precipitation.   Very typical for this time of the year.

Looking at the NOAA/NWS GEFS ensemble, but this time viewing surface air temperature at Seattle, we can expect warming temperatures the next few days, including perhaps our last 70F day this year, followed by a decline into highs in the 50s (the black line is the average of the ensemble forecasts....a very good forecast in general).  Not much uncertainty for the first few days, but a lot at the end (the various forecasts in gray have differences).

Finally, let me show you how the atmospheric circulation will change by presenting the current and future winds near jet stream level--the 300 hPa pressure level or around 30,000 ft.  The colors indicate the wind speeds and the solid lines are the heights of the pressure surface (equivalent to pressures on a constant height surface).

Today at 2 PM, there is a low-amplitude ridge of high pressure over us and a weak jet stream is across southern Alaska and Canada.

But by Wednesday at 11 AM the situation is radically different.  A tough of low pressure has pushed south of the Aleutians and a MUCH strong jet stream has set up due west of us.   The meteorological hose will be directed right at us.   Be ready.

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