Warming trend could raise temperatures 5 to 15 degree F above normal
Published on Dec 13, 2013 - 7:30:12 AM
December 13, 2013 - After the very cold air dropped out of Canada last week, the region had very cold overnight temperatures. The coldest, densest air was at higher elevations and gradually sank into the Central Valley. As the air sank into the Valley, compressional warming occurred in the foothills above the inversion. As time went on and the cold air mass modified, overall nocturnal (overnight) temperatures gradually warmed, especially in the foothills and west slopes of the Sierra Nevada.
Ridging dominates the West Coast at least into early next week, as temperatures will continue to warm. The potential exists for more cold frontal impacts over northern California Wed/Thu with fairly stout northerly winds behind the system next Fri/Sat.
Short Term Discussion (Today through Monday)
A weak system is now exiting northern California early this morning. The system really only brought some quick moving cloud cover across the area, and a few spits of precipitation to Shasta County.
A ridge will build over the region behind this system today and Saturday to keep the region dry with cool nights and mild afternoon temperatures. The warming trend this weekend into early next week could could raise temperatures 5 to 15 degree F above normal. The warming will be aided by some adiabatic warming from the mild northerly to northeasterly winds. Therefore, fog is not a big concern, but some patchy shallow ground fog may start to develop over the weekend near the waterways and flooded rice fields from around the Sutter Buttes/Butte Sink southward. The fields around Sacramento International Airport, including adjacent I-5, are usually a magnet for this type of patchy ground fog.
Extended Discussion (Tuesday through Friday)
First part of extended period Tuesday looks dry under upper level ridging. Surface high pressure over the Great Basin combined with an upper low off the central California coast will bring breezy east winds down some Sierra canyons during the morning hours. The upper ridge is expected to flatten just a bit on Tuesday but daytime highs are still forecast to come in as much as 10 degrees above normal for the time of year. A Gulf of Alaska frontal system is forecast to kick the weak offshore low inland on Wednesday. Although no precipitation is expected from this low, most areas will see increased cloudiness and slightly cooler temperatures.
The frontal system is forecast to drop into the Pacific Northwest on Wednesday but models vary on timing. ECMWF and GEM would bring precip threat into far NORCAL by Wednesday afternoon. Much slower GFS holds off until late Wednesday night and latest 06z GFS run holds off until Thursday morning before bringing precip threat to NORCAL. No matter what the timing, this inside slider type system looks to be quite dry with precipitable water values forecast at well under 1/2 inch. Therefore, any precip amount would be small. Trough axis passage is currently forecast for Thursday, either early or late, depending on the model and this will be the time any light precip will be most likely. Although again, any precip amounts would be very limited. Dry northerly flow develops behind this system on Friday ending any precip threat. Current model projections would indicate a fairly breezy Friday going into the weekend with daytime highs by the end of next week closer to normal for mid December.
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