| Day | Date | High/Low | Forecast |
| Fri | 31st | 59/49 | Muted sunshine with considerable high clouds. Variable evening high clouds. |
| Sat | 1st February | 62/52 | Partly cloudy with high clouds through the evening. |
| Sun | 2nd | 67/53 | Mostly sunny day with scattered high clouds. Scattered evening high clouds. |
| Mon | 3rd | 67/52 | Mostly sunny day with scattered high clouds. Scattered evening high clouds. |
| Tue | 4th | 61/54 | Mostly cloudy through the evening. Slight chance of morning light rain. Increasing chance of rain in the afternoon. Rain becoming likely as evening progresses. |
Synopsis
An upper level, split flow, wind pattern exists over the northeastern Pacific. Currently, its configuration along the West Coast supports a trough (low pressure) just west of the Pacific Northwest and a ridge (high pressure) over much of the Southwest (including southern California). The magnitude of the current ridge would typically produce warmer than normal weather. However, in this case, no off-shore flow exists (just a very weak, northerly, surface pressure gradient). So, the sea breeze, along with considerable high clouds (more on that in a bit), is keeping daytime temperatures from rising a whole lot (had expected better warming earlier in the week when the numerical models were predicting a weak but full fledged, off-shore flow). A little further warming is anticipated over the weekend (leveling off Monday) as high pressure aloft strengthens some more. However, at most, inland areas may return to seasonable temperatures.
Although dry weather exists in the Southland today, much of northern California is experiencing wet weather (southern end of the aforementioned, Pacific Northwest trough). Over the next several days (lasting into early next week), the Pacific Northwest trough will strengthen and tap into a moisture rich, atmospheric river (part of that tap is also responsible for sending high clouds into the Southwest). Periodic surges of heavy precipitation are forecast up north (particularly from north of the Bay area and inland to the northern Sierras…high snow levels due to “warm” atmospheric river). Some model were predicting minor rains getting as far as western Santa Barbara County early this weekend, but now, it’s not likely to get south of northwest San Luis Obispo (high pressure aloft should dissipate rain clouds further to the south).
The majority of model forecasts do show wet weather reaching southern California by late Tuesday. Uncertainty remains (for now) on how widespread precipitation will get and where the most precipitation will fall (aside from mountains versus the coast). There’s even some uncertainty about how long lingering showers may last next week (one model solution keeps some showers going well into Thursday). Planned daily reconnaissance flights (Air Force and NOAA research planes) into the region of the atmospheric river will hopefully bring clarity with the numerical models this weekend.
For now, I’m sticking with today’s model consensus. Some areas of light rain could begin Tuesday morning. Chances should increase in the afternoon (wet weather slowly migrating from west to east). Unless the storm slows down its forward progress (a nagging possibility), I’m expecting most of Tuesday evening will be a wet one in L.A. County. Depending on which model forecast is right, there could be a period of heavy intensity rain late Tuesday evening into early Wednesday (enough for localized, flooding concerns). Since the atmospheric river will sag south with the accompanying frontal band, snow levels should stay above 8000 feet for most of precipitation event. Snow levels may ultimately fall to 6500 feet, but only scattered, light showers should remain by then (beginning Wednesday late morning). Even if the scenario for lingering showers on Thursday verifies, they should be scattered in areal coverage and light in intensity.
Storm totals will depend on whether a period of heavy precipitation falls. However, unlike the last storm, totals shouldn’t vary as greatly by County. Assuming the heavy rains occur, storm total in the lowlands (away from coastal facing mountains) should range in the half inch to inch range (0.75 – 1.50 not out of the question though). Otherwise, totals may be half that. Snow above 8000 feet could reach one to 1.5 feet (most models weaken the storm considerably upon reaching the Inland Empire, however). One scenario that could upend storm projections would be if the storm stalls over some part of the Southland. Totals could then be much higher there and much lower elsewhere.
One way or another, dry weather should return for late next week (current projections favor a dry, subsequent weekend). However, a number of model forecasts show more wet weather returning sometime during the 11-13 February period. Model forecasts vary greatly from a minor storm to a moderately strong one. Most favor a more typical, cold storm (snow levels around 5000 feet). Under most scenarios, cooler than normal weather would be the rule for most of the next couple of weeks.
Next issued forecast/synopsis should be on Monday, 3 February.