Several weeks of warm and dry mid-winter weather will come to a rather abrupt end later this week with the arrival of a deep and cold Gulf of Alaska trough on Thursday. In the meantime, however, much of the state will continue to see March highs in the 70s and overnight temperatures only locally dropping below 40. Snowmelt has occured over the past week in the Sierras and Cascades, which shouldn’t be too suprising in light of the almost uninterrupted sunshine and warm afternoon temperatures as of late. The arrival of the sharp and impressively cold trough on Thursday will be hearlded by a rapid drop in 850 mb temperatures (10-14 C over CA currently to -1 to -4 C by late Friday) and a corresponding drop in 500 mb thicknesses (falling to around 528 dm in the north). This system will be coming in over the ocean and will therefore have sufficient moisture to squeeze out some precipitation over the state, as well, in addition to the sharp drop in temperatures and the uptick in brisk north winds. Most precip with this system will probably be convective in nature, meaning in will be somewhat localized but possibly rather intense. Thunderstorm activity is a very real possibility, especially over the Central Valley late Friday and Saturday as the mid-March sun intensity interacts with very cold air aloft to create imprssive lapse rates. Snow levels will come down quite a bit, but with antecedent warm temperatures and little evaporational cooling to speak of they will not be exceedingly low. It’s a bit hard to tell at this point, but snow will likely fall below 3000 feet and possibly below 2500 feet if current model predictions pan out. In convective precipitation in the far north (mountains east of Eureka and the far northern Sacramento Valley), snow could accumulate at elevations as low as 2000 feet. Hail and sleet showers, as always, will be possible almost anywhere as cloud tops surpass the layer of glaciation. This will not be a record-setting cold event or low snow event by any means, but some impressive tstorm activity is possible over inland areas if things come together right. SoCal will see some precipitation out of this system, as the bottom of the tough pinches off and forms a small and brief cutoff over the CA Bight. Scattered showers and isolated thunderstorms are likely over the Southland by the weekend. By Sunday, most or all of the precipitation will have moved off to the east and skies will clear to make way for a couple of days of dry weather early next week. Indications are, however, that the cold and unsettled pattern will reestablish itself once again by day 9.
