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Enhanced infrared satellite image of the west coast shows clear skies over Northern California as of 5:15 a.m. with nothing in the chute upstream. The far northern Sacramento Valley's heat signature shows up nicely in this image. The gray shades indicate cooler surfaces in the mountains surrounding the valley.

Unisys

Ridge-trough pattern forecast for 5 p.m. today. The orange color contours over Northern California suggest rather high 500 millibar heights and warm temperatures aloft (since warmer air expands, literally pushing higher into space). The widely-spaced black isobars, or lines of equal surface pressure, indicate weak gradients and light winds.

Unisys

Ridge-trough pattern forecast for 5 p.m. Wednesday shows cooler, windy weather on the way. The green color contours over the Pacific Northwest indicate lower 500 millibar heights, or cooler air moving in. The black isobars or surface pressure lines show a rather steep gradient across Northern California, suggesting gusty northwest to north winds. These winds will probably feel rather cool even though they're blowing down slope and warming, since the air above is forecast to be so cool to begin with.

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Afternoon temperatures on the far northern Sacramento Valley floor should pop back up toward the mid-80s today under bright blue skies. The normal top temp at Redding Municipal Airport on this date is 83 degrees.

High pressure – essentially an area of warming, sinking, drying air aloft – is strengthening over Northern California this morning, driving out the weak frontal band that gave us Sunday’s cloud cover and mild temperatures.

Redding Muni reported clear, calm skies and a coolish 52 degrees just before 5 a.m. The dew point temp was a rather moist 50 degrees.

The narrow gap between the air and dew point temps again indicates the air this morning is close to saturation, so don’t be surprised to see a few fog patches in favored spots near creeks.

The essentially flat weather pattern means there are no steep temperature or pressure gradients nearby in any of the four cardinal directions, so winds should stay light.

Wind is nothing more than air rushing from higher to lower pressure, like air escaping from a balloon. Forecasters look at pressure gradients – or how steep the slope between higher and lower pressure will be – for indications of how hard the wind will blow.

Forecasts do show a 1.4 millibar (mb) gradient developing between Medford and Redding by 11 a.m. for light down slope winds. Even that small gradient should ease into afternoon. The light north wind should keep visibility sharp and nicely show off the fresh snow atop Mt. Shasta and Lassen Peak.

When skies are clear and winds are light, it’s possible to estimate top afternoon temperatures using forecasts for temperatures at the 850 mb level in the atmosphere, or roughly 4,500 feet overhead. Forecast 850 mb temperatures of 18 degrees C would support a top temp at the surface of 89 degrees today.

That reading seems high, since the air is still so moist. Some of the sun’s energy will go to evaporating that moisture. A high in the lower-mid 80s, or up to 85 degrees with a bit of down slope warming, seems more likely. But an update could be necessary.

Windy and cooler later this week

Forecasts show strong surface high pressure building off the Pacific Northwest, behind a low that’s supposed to swing through western British Columbia today.

High pressure to our north is a classic recipe for gusty down slope or north winds in the Sacramento Valley, and forecast Medford-Redding pressure gradients jump to 8.1 mb by 5 p.m. Tuesday.

The recent soaking rains were not nearly enough to get us out of fire danger. Strong dry, gusty north winds will quickly ramp up the threat, should they materialize.

These gusty winds could persist into Saturday as a deep low pressure – essentially a vast pool of cooler air aloft – slides across the Pacific Northwest and dives into Nevada.

This low could cool our top afternoon temps back into the 70s by Thursday. Temps in the 80s could return by Sunday.

Long-range forecasts suggest dry and mild weather through Oct. 20.

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