After an extraordinarily wet period, the big question is whether April will be more typical. Average April rainfall at Seattle-Tacoma Airport is 2.59 inches, a far cry from the 9.44 inches we endured in March.
So let's take a look at some of our most powerful prediction tools, whose skill degrade rapidly past a week or so into the future. First, there is the output of the North American Ensemble Forecasting System (NAEFS) shown below. By looking at many numerical simulations encompassing NAEFS we can get an idea of uncertainties. The second panel is precipitation and the median values (half the ensembles are more, half less) are shown by the horizontal line. 75% of the ensembles are within the yellow boxes. Fairly dry the next few days and then wetter Friday to Sunday. But next week looks fairly dry with a warm spell on the 9th and 10th. Not too bad really.
The UW WRF model is consistent with this. Here is the 72 h precipitation forecasts precipitation totals ending 4 AM on Friday. Not too bad...less than a third of an inch over the lowlands.
But early next week it all changes, with a large ridge developing over the eastern Pacific (see upper level map for Monday at 10 AM). Dry and warm.
For us, this implies that we might get some weak systems, but no downpours, later next week.
Forecast skill drops rapidly after that, but we do have one tool to use: the NWS Climate Forecast System (CFS) that goes out 9 months! Her is the anomaly (difference) from climatology (average April precipitation) for April. Modestly above-normal precipitation over the western side of Washington (green color). Normal east of the Cascades.
Professor John Delaney will be giving several talks on the topic: 20,000 Gigabytes under the Sea.
More information here.
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