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Posted at 03:17 PM ET, 11/27/2012

Long range look: Warm weather pattern to start December, then turning volatile


The first four days of December will at times feel more like fall than winter with above normal temperatures expected to hold sway across the region. High temperatures will probably be in the 60s from Sunday through Wednesday with one of those days possibly approaching 70 degrees. Those first days look to be on the dry side.

Beyond the 4th, how temperatures and precipitation shape up is much less certain as a strongly negative Arctic Oscillation (AO) - which may allow colder air to filter south - starts to exert its influence on the pattern.

From December 6 through 11, the weather is expected to be more variable with some cooler than normal days interspersed with warmer ones leading to temperatures probably averaging near the normal highs of around 50 and lows in the mid-30s. However, there could be significant day-to-day differences.

Precipitation is expected to be near normal (with below normal precipitation expected across the Southeast and above normal amounts just to our north) over this stretch.

Overview

In longer range forecasting, meteorologists often use the location of pressure or “height” patterns and their difference from average to help forecast whether temperatures and precipitation will be above or below normal across a region. These patterns are often intricately connected around the globe and are often referred to as “teleconnections.” Certain patterns typically favor either above or below normal temperatures. Complicating the forecast into December, two different teleconnection patterns will be warring with each other for dominance.

Our old friend the negative phase of the Arctic Oscillation (AO) with high pressures across the latitudes to our north - which favors colder than normal temperatures across our region - will be battling a negative Pacific North American pattern which usually brings us warmer than normal temperatures.

The indices measuring these teleconnections have tanked and are forecast to stay negative through the next couple of weeks (see below).


Observed and forecast levels of the Arctic Oscillation (top) and Pacific North American (PNA) pattern (bottom). (NOAA Climate Prediction Center)

December 2-6 forecast reasoning discussion

Through December 4, the negative PNA pattern - characterized by a dip in the jet stream over the West Coast (and stormy weather) and warmth in the East - will hold dominion over the weather across the country.

The Day 8 superensemble model forecast of mean 500 mb (aboit 18,000 feet) heights and anomalies (differences from normal) centered on December 4 is shown below along with the latest NOAA Climate Prediction Center 6-10 day temperature forecast ending December 6.


Day 8 super ensemble model mean 500 mb heights and anomaly chart centered on December 4 (left) and the CPC temperature forecast (right) for the December 2-6 period (warm colors represent warmer than normal temperatures, blue shades show colder where colder than normal temperatures are forecast). (NOAA)

The blue lines on the 500mb mean forecast indicate the area where the heights are below normal. Note that the anomaly is centered near the Pacific Northwest coast but extends to the west, which is a negative PNA look. The location of the trough off the coast and the very close spacing of the height lines (black lines) as they cross the West Coast signals that a potent jet stream will be impinging upon the coast and will be accompanied by loads of moisture.

The pattern over the Pacific is one that typically brings very heavy precipitation to the coast of California and the Sierra range where there are now 5-day forecasts of over 11” of rain (which translates to obscene snow amounts at high elevations). The heavy precipitation could lead to run-off problems in parts of California. This may become the big weather story during the next couple of weeks.

Also, the strong onshore flow and firehose into the West Coast floods the country with warm air. The negative PNA pattern with above normal heights still hanging tough across the East should also keep any storms to our north putting us in the warm sector. The CPC temperature forecast for the 6-10 day period keeps almost the entire U.S. warmer than normal except right along the Southeast coast. While the D.C. area is forecast to experience above normal temperatures during the period, the warmest temperatures relative to normal will probably remain across the Intermountain West and Southern Plains.

December 6-11 forecast reasoning discussion

There are signs that somewhere in the December 6-8 time range that the negative AO may start exerting influence on the pattern. The Day 11 super ensemble model mean pattern starts showing a negative anomaly over the Southeast. The above normal heights and pressures to our north and below normal heights and pressure to our south usually help bring cooler air from the north into our region.

The pattern shown below usually leads to below normal temperatures across the Southeast and they often extend as far north as our area. When I composited (averaged) the 10 analog dates (historically similar patterns) supplied by the 11-day model forecast, the end result showed our temperatures averaging below normal with the coldest temperatures relative to normal extending into the Southeast.

However, several reasons crop up for not buying into the colder than normal scenario so quickly. The CPC experimental temperature forecast map (below right) based on the forecasts of a number of ensemble members give us more than a 50% chance of having above normal temperatures. The flow pattern shown in the Day 11 model forecast still shows very strong flow into California which suggests that most of the country will still be flooded with air emanating from the Pacific. So even behind cold fronts, the air may only be slightly cooler than normal while ahead of fronts the temperatures probably will be above normal. Finally, the CFS2 climate model forecast for December 5-11 is forecasting above normal temperatures for the period. Therefore, with the conflicting guidance, I’m forecasting temperatures to be near normal for the Dec 5-11 time range. This seems like the safest course. The same holds for precipitation.


Day 11 super ensemble model mean 500 mb heights and anomaly chart centered on December 7 (left), and the CPC experimental temperature forecast (right) for the December 6-11 period (warm colors represent warmer than normal temperatures, blue shades show colder where colder than normal temperatures are forecast).

Beyond mid-December: A little more about the AO and what it might mean for this year’s winter

Yesterday morning the AO fell to -1.95 and most ensembles are predicting it to continue to fall with it likely dropping below -3.

Don Sutherland - a weather hobbyist who posts on the American Weather forums - has looked at eleven similar cases when the AO dropped that low during late November or early December. The mean duration for the eleven different events was 30.5 days with a median of duration of 36 days. He notes that the 4 cases that followed a strongly negative AO during October were tenacious with the negative AO lasting between 30 and 48 days.

While the sample size is small, Sutherland’s analysis suggests the current negative AO could last through December and that the AO during the month will average in the negative range. If so, the latter part of December could average colder than average.

Don also noted that the average seasonal snowfall for Reagan National during those 11 seasons was 21.8 inches and the median was 17.6 inches, both around 5 inches more than for all seasons.

However, those high numbers may be misleading. Bob Hill, another hobbyist, looked at the 18 Decembers when the AO index measured below -1.0. The AO remained negative in 15 of those 18 seasons during January. 17 of the 18 seasons had a negative AO average for the December-February period.

What does the prevalance of a negative AO in those cases say about snow? The average snowfall for the 18 seasons was 19.8 inches. However, only 8 of the 18 had normal or above normal snowfall proving again how much luck plays in the D.C area having a big winter. The snowfall during 30 percent of those years exceeded 20 inches suggesting that the chance of having a big snow year would not be negligible and is probably more likely than during most seasons (especially those in which a positive AO is the dominant mode).

In summary, odds favor the AO being negative more than positive this winter season suggesting that temperatures will probably not be as warm relative to normal as last winter and could even end up averaging below normal. Yes, we are more likely to have a snowy winter than during years when the AO is positive or when you look at all years. But a strongly negative AO does not always translate into a snowy winter.

By  |  03:17 PM ET, 11/27/2012

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