Tuesday, August 8, 2017

Fog and Smoke Epilogue... the next day

The tone of yesterday's note may be a bit too glib (ie "not to worry").  Now Tuesday and we did see light fog on the Sound, but not near yesterday's coverage, even though the WRF meteogram from yesterday would seem to imply even more today.The forecast this morning, however, did include "patchy fog," so the computer models may have looked out the window yesterday.

Synopsis: PZZ100-081900- 254 AM PDT Tue Aug 8 2017 .Synopsis for the northern and central Washington coastal and inland waters...High pres offshore with lower pres inland will maintain onshore flow of varying strength through Thursday. A layer of smoke from wildfires in British Columbia will continue to drift across the area today which may impact visibilities this morning before improving during the day today. $$
Light wind becoming N to 10 kt in the afternoon. Wind waves 1 ft or less. Smoke and patchy fog in the morning then patchy smoke in the afternoon.
N wind to 10 kt in the evening becoming light. Wind waves 1 ft or less. Patchy smoke in the evening then smoke after midnight.

The air temp at WestPoint Lighthouse did not get close to the dew point as early, but still within 1º and maybe getting closer even at 7 am. Generally we would think that reduced visibility is one of the easier things to forecast since it is determined by broad air mass properties (temp and dew point)... in contrast to wind, which in Puget Sound is extremely sensitive to just a slight rotation of the isobars.  But it seems that adding a lot of smoke to the equation complicates the results, which makes it more difficult to predict temperatures as precisely as we need for this. Recall we were warned of record breaking temp records, that did not quite materialize, in large part because of the smoke.

But despite the wording of the visibility notices in the forecasts, the UW WRF model did do a good job on forecasting (T-TD) temperature difference, ie suppression of the wet bulb temp—a term that originates from the way dew point is measured using two thermometers, with one covered by a wet wick so it gets cooled by evaporation by an amount that depends on the relative humidity of the air.

UW forecast from 5pm last night shows lowest suppression coming at 8 am today, which is about what we saw. The one anomalous low point right at 8 am is likely a coincidence.

These are the actual measurements from West Point Lighthouse (WPOW1). Vertical scale is T - Td (ºF) (suppression of the wet bulb) and the data points are an hour apart with the scale indicated in red. There is a link at WPOPW1 for past hourly data, which can be copied and pasted into a spreadsheet to compute and plot the results.

So it seems the message of this amateur analysis is that looking at T dropping to TD is a dependable way to predict the onset of fog, but figuring the thickness of the fog does not come out of that simple analysis; that will take further analysis.  

...standing by for meteorologists to tell us how we do that.

See related note on UW Sounding Generator, which shows the temperature inversion we are having.

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