First half of January trending cold, any snow?
Watching two winter weather opportunities
A few people have been wondering about the Capital Gang Winter Outlook and whether January will closely follow the script. The original CWG call was for December to be cold and for January temperatures to average one or two degrees above normal January.
Now, it's looking increasingly like that at least the first half of January temperatures will average below normal and the 8-14 day forecast suggests that precipitation will be normal to a little below normal. The cold temperatures do raise the probability of us getting snow during any periods of storminess. However, in most La Nina winters such as this, moisture can be limited and snowstorms usually tend to be on the light to moderate side. As for the next week, models hint that there could be a little snow Friday from a clipper and another winter weather threat early next week.
Why does the first half of January look like it will be colder than normal despite the strong La Nina conditions in the tropical Pacific?
The answer lies in the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) which have been strongly negative through December. An earlier article discussed how these two oscillations could influence the temperature forecasts during a La Nina. For those who have forgotten, when the AO (or NAO) is in its negative mode, higher than normal pressures are located across the polar regions and lower than normal across the mid-latitudes. Since air moves from areas of higher pressure to lower, cold air feeds southward toward eastern North America when the AO is negative.
The impact of the AO this winter has been even stronger than normal because of the magnitude of areas of blocking high pressure that have been occurring at the higher latitudes. (Read Matt Rogers' article for discussion of a theory about what's causing the blocking)
Because of this blocking pattern, the AO is forecast to remain negative through the first half of January. NOAA's Climate Prediction Center (CPC) ensemble forecasts of the AO are shown to the right. Note that though the next 10 days the index is forecast to remain negative and only towards the end of the period is it forecast to rise to a little above zero. However, if you look at the 14-day forecast versus verification (bottom panel) you can also see that the AO has generally been running lower than forecast.
The left panel below shows the how much temperatures (in Celsius, a departure of 1C equals 1.7F) average below their normal values when the AO is negative during winter. The current CPC 8-14 day temperature forecast for January 11-17 is shown on the bottom right.
Not surprisingly, the pattern of below normal temperatures on the CPC forecast looks very similar to the typical pattern associated with a strongly negative AO. So far this year the state of the AO is trumping the current La Nina in terms of temperatures. Until the AO weakens or changes sign, our temperature are probably going to have more below normal temperature days than warmer than normal ones.
The CPC forecast also suggests that we have below normal precipitation during the period. The models are showing very cold air building in western Canada that looks like it will start plunging into the Plains next week. The Capital Weather Gang will be watching this arctic air mass to see how and when it might eventually affect our area.
What about Friday? I agree with Matt's assessment earlier today. Both the GFS and NAM models bring a clipper to the area giving the area a period of light snow. They are uniform in saying that the strongest low initially will be to our north but that a weaker low may pass to our south and then start to deepen as it scoots off the coast. The low center at around 5000 feet passes to our north with little or no warm air advection to transport much moisture into the region. Such storms typically produce a dusting to an inch at best. I'd place the odds of getting an inch at around 20% as the bulk of the 09Z SREF ensemble members give us no accumulating snow.
This morning's GFS and Canadian models have a storm early next week but they differ in the details. The GFS takes a low into the Ohio Valley by 7 a.m. Tuesday and then reforms it over eastern North Carolina by afternoon putting the D.C. area in that battle zone where precipitation type is up in the air. The Canadian model has the low going up towards the Great Lakes which would be a track favoring a rain storm. The GFS ensemble members are all over the place suggesting that either solution is possible or that the much weaker low could scoot harmlessly out to sea. The European model suppresses the storm early in the week to the south and southeast but holds back a second disturbance that later produces a little snow.
This pattern remains one in which the models are likely to struggle. The spread of the ensembles also suggests there will be little or no skill in model forecasts of any potential storm next week until we get much closer to any possible event. Later in the week, the CWG will take a closer look at the potential of any storm that might be on the horizon for next week and I'll offer my best guess for how the storm might evolve.
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