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Northern CA Forecast Discussion
FXUS66 KSTO 172117 AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
117 PM PST Mon Feb 17 2025

SYNOPSIS...
Patchy morning fog development possible again into Tuesday morning, with light precipitation still expected on Wednesday. A trend toward above normal temperatures and dry weather is then anticipated late week into the weekend.


.DISCUSSION

... Latest GOES-West satellite imagery shows mostly cloudy skies across interior NorCal as the region remains loosely influenced by broad troughing aloft. Partly to mostly cloudy skies are generally expected to prevail the remainder of the day, gradually clearing into Tuesday morning as ridging aloft begins to build in. As the ridge does build in, a period of breezy northerly winds is expected on Tuesday, with some gusts to 15 mph possible at times. The timing of decreasing cloud cover and increasing winds will have a strong bearing on potential for Valley/adjacent foothills fog development into Tuesday morning. Still, some 40 to 70 percent HREF probabilities of dense fog development exist from Marysville southward, although the eastern Valley and adjacent foothills are the currently favored locations for this. After the potentially soupy start to Tuesday, partly to mostly sunny skies in conjunction with additional warming from the northerly breeze, near to slightly above normal high temperatures are expected by the afternoon. Readings in the low to mid 60s are anticipated across the Delta, Valley, and foothills, with even 40s to mid 50s at higher elevations. This brief dry and warm period then looks to become abruptly interrupted by a transient shortwave trough moving into Wednesday. Ahead of this system, breezy southerly winds gusting 20 to 25 mph are expected, before shifting to a northerly direction late Wednesday into Thursday. Additionally, some precipitation impacts look to accompany this midweek system. The quick moving nature of the trough will limit overall precipitation totals, but the comparatively further southward trajectory is expected to yield more widespread precipitation potential. All in all, precipitation totals of 0.25 to 0.5 inches are expected from the Interstate 80 corridor northward, with a trace to a few hundredths possible further southward in any isolated showers that are able to sustain themselves further. With fairly persistent snow levels around 5000 to 6000 feet, most accumulating snowfall is expected above 5500 feet, with appreciable accumulations of 1 to 3 inches possible above 6000 feet. While snowfall accumulations are expected to remain light, current guidance indicates much of the snowfall occurring during the afternoon and early evening hours, which may result in some mountain travel impacts during the Wednesday evening commute. While some ensemble discrepancies exist regarding the shortwave ejection into Thursday, ridging aloft is expected to build in behind it. Main differences between ensembles at this time pertain to the magnitude of northerly wind gusts on Thursday, with some members indicating gusts . Otherwise, dry and warming weather begins to take hold as ridging builds in, with similarly near to slightly above normal high temperatures as Tuesday expected by Thursday afternoon.


.EXTENDED DISCUSSION

(Friday THROUGH Monday)... Ensemble guidance remains largely in agreement on ridging aloft amplifying through the end of the week and into the weekend. This will usher in a more pronounced period of persistent warm and overall dry weather into next weekend. As a result, above normal temperatures are favored across the extended forecast period. Generally 20 to 40 percent probabilities of reaching 70F throughout the Delta, Valley, and foothills expand to 40 to 70 percent probabilities for Sunday and next Monday. While no strong signals for wind impacts are anticipated with this pattern shift, some breezy north to east winds driven by diurnal trends would generally be expected.






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