Short Term /6 Pm This Evening Through 6 Pm Sunday/: Quite a bit of model variation in the short term. However, given the relatively fast sw flow aloft with very small embedded shortwaves that are right around the scale where the models will have some difficulty resolving this is not surprising. It does however make for a lower confidence than usual forecast for this time frame. The NAM/ECMWF and to a lesser extent the sref/cmc/UKMET want to bring some light precipitation across mainly the se 1/2 of the cwa late tonight in response to a weak 700 hpa shortwave passing just to the s/e. The GFS is dry, and does not really show this feature. The sref/ECMWF also suggest that pre-frontal precipitation could also reach far nw zones late tonight as well. Going to go with the preponderance of the models - and generally consistent with a blend of mav/met/NAM/sref pops and have a slight chance for light rain overnight, with pockets of freezing rain possible in the normally colder valleys of orange county, until temperatures there warm above freezing late tonight. In addition, both NAM and GFS time-height cross-sections suggest the pooling of low level moisture under a low level inversion centered around 950 hpa overnight, so expect a stratus Deck to develop. Also, the NAM and sref, more so than the GFS also suggest the likelihood of areas of fog developing as well. Given the abundant low level moisture, also have the potential for patchy drizzle as well, when it is not raining. For now it appears low clouds/fog/drizzle should lift by around mid-morning on Sunday per bufkit soundings, however would not be surprised if they lingered a bit longer. Regardless, will have a mid Deck overhead by late morning in response to the approaching cold front and weak passing 700-500 hpa shortwaves. So expect a mainly cloudy day as a result. Could also see some spotty showers in the afternoon ahead of and even behind the cold front, as it is forecast to become ana-frontal in nature in the afternoon. For lows tonight used a blend of two parts NAM 2-meter temperatures to 1 part each of mav and met guidance. This yields lows 10-15 degrees above normal. Expect a non-diurnal trend, with nearly steady temperatures in most areas, with the region being in between a warm front to the n and a weaker warm front to our south. With the cloud cover, do not expect the benefit of full mixing, with bufkit soundings showing mixing limited to around 975 hpa. For highs blended a mix down from there with NAM 2-meter temperatures and a blend of mav and met guidance. Expect readings mainly in the mid to upper 40s. However, if there is more sunshine in the afternoon than currently expected, could see some readings break the 50 degree mark - the probability of this though is low at this time.
Posted on: Sat, 01 Feb 2014 15:39:24 +0000