California has once again entered one of the extended mid-winter dry periods that characterize climate here during the predominantly “wet” season. A strong ridge of high pressure offshore is deflecting all storm activity north into Alaska and Canada, only to swoop back equatorward on the far east side of the blocking pattern over the central and eastern U.S. Heavy snowfall and cold temperatures will be prevalent across the eastern 2/3 of the U.S. for the next 3-5 days. For CA: expect continued dry weather this week. Seasonable temperatures are expected in NorCal, while it will be somewhat warmer than average in the south. Highs and lows will be generally 50-60/30-40 in NorCal and 60-70/35-45 in SoCal. Valley fog is likely under this stable pattern, esp. with wet soils from recent precipitation. Some strong offshore winds are possible at times this week, especially in SoCal, as a result of cold air damming on the east side of the Sierras (850 mb temps of 5 to 6 C over coastal CA and -4 to -5 C over W. Nev.). The powerful ridge currently over or just offshore CA will very slowly retrograde back towards the west over the next 7 days. The models always have extreme difficulty with this particular pattern for some reason, so explicit model forecasts are not really useful more than 3-5 days out at this point. Given the model tendencies and the current moderate-strong La Nina base state, I would expect that there is a decent chance of some very cold weather over the state of CA before the end of the month. It will almost certainly be bitterly cold over 80-90 percent of the Continental U.S.; the question is whether not not (or to what degree) this unmodified Arctic air may make it west of the Rockies. The GFS and ECMWF have been flip-flopping over the past few days, but each model has had its fair share of weird and unbelievable prognostications (heavy snowfall in San Francisco, deep freeze in San Diego (and I mean on the beaches!)). Needless to say, the circulation downstream of a shifting or developing Rex-blocking pattern is hard to model (at best), so we need to keep an eye on the potential for extremely cold weather in the not-so-distant future. A similar pattern developed last winter and did eventually result in very cold temperatures and some low-elevation snowfall over the state. This is generally not a pattern conducive to major precipitation events anywhere in the state (dry and cold airmass) but it can certainly result in interesting or dramatic conditions. The only exception is if the pattern becomes very retrogressive and pinches off a strong cutoff low offshore of CA, as the GFS has indicated a couple of times in the past 24 hours. It’s also possible that we have 2-3 weeks of dry weather with chilly mornings but nothing really spectacular as the Arctic airmass misses us 500-1000 miles to the east. We’ll see.