Below normal temperatures through Memorial Day. Dry and breezy northerly conditions into Tuesday. Warming trend Tuesday and Wednesday followed by cooling the later part of the week and over the weekend with a chance of precipitation.
Discussion
Skies have cleared out across much of interior NorCal early this morning as surface flow transitions to northerly (offshore). Satellite shows some clouds lingering over the northern Sierra Nevada and northern mountains, but these have been decreasing. Northerly gusts of 15-30 mph have spread south through the northern half of the Sacramento Valley, and along the west side of the valley, as the northerly surface pressure gradient has tightened (MFR-SAC is up to about 9 mbs, and the RDD-SAC gradient has turned positive and is nearly 1.5 mbs). Further south, onshore gradient is gradually slackening. NorCal will be on the backside of the weekend trough today as it slowly shifts east.
A trailing short-wave will slide southeast into the northern Sierra later today resulting in more cloudiness, but any additional showers are likely to develop east of the crest ahead of the wave. Elsewhere, skies will continue to clear out and northerly flow will increase and spread south through the Central Valley.
This north wind event is expected to be similar in magnitude to last week’s. Gusts of around 30-35 mph will be possible in the Sacramento Valley to the north and west of Sacramento with not quite as strong gusts expected further south in the valley.
Combined with low RH (10-15%) and dry light fuels (grasses and brush), critical fire weather conditions are expected. Short-wave ridging builds in beginning Tuesday leading to warming and further drying. Surface pressure gradients will be slackening by the afternoon, but remaining breeziness and low RH will allow critical fire weather conditions to linger into the evening in the valley.
Temperatures are forecast to peak this week on Wednesday as the ridge moves overhead. Highs will be back into the mid 90s across the Central Valley resulting in moderate heat risk.
By Thursday, the ridge is forecast to give way to the large trough over the Gulf of Alaska as it edges closer allowing the return of light onshore flow and minor synoptic cooling.
Extended Discussion (Friday through Monday)
Ensembles and a majority of the clusters support the idea of the mid-week ridge shifting east late this week as the large upper low in the Gulf of Alaska shifts closer to the West Coast.
Cooler temperatures expected over the weekend (10-20 degrees lower compared to Wednesday) along with an increase in clouds, and even a chance for widespread light precipitation (mainly for the northern Sacramento Valley and over the mountains) as the GEFS IVT points toward a weak late-season AR dropping south into NorCal. Snow levels will be too high for any travel related impacts.