The Washington Post

Long range look: Pattern to remain warmer than normal next two weeks

Jason recently wrote his thoughts about the first five days of the period. My forecast concentrates on the 6-14 day period which should average warmer than normal.

Yes, there may be a few cooler than normal days after a front passes sometime this week and next, but those days should be more than balanced out by the above average days that will have high temperatures in the 50s with the potential for a 60+ degree day again sometime early next week prior to the passage of the next front.

Beyond December 12, the pattern becomes trickier but I’m still leaning towards above normal temperatures. Precipitation is expected to be near to a little above normal in the December 8-12 period and above normal for December 13-16. The pattern continues to be below average for a significant snowstorm.

Technical discussion for December 8-12

The predicted 6-10 day mean pattern from the European and GFS models provides some insight into why temperatures probably will average above normal for period.

While both models show a pattern with above normal heights (or pressures) across Greenland and Iceland indicative of a negative NAO (which usually support cold weather in our area), they also are keeping enough ridging off the West Coast (the red area) to hold the mean trough well to our west so the coldest air would be dumped into the Plains rather than towards the East Coast. Even in the Plains, the air masses don’t look that cold.

More importantly, the high heights (warm shades) over the East suggests that any low coming east would track to our north putting us on the warm side of any storm. Both the European model and GFS and the bulk of the GEFS ensemble members have a storm tracking towards the Great Lakes sometime early next week. Ahead of that storm, we have our best chance of getting back into the 60s. That storm is also our best chance at getting decent rainfall through the 12th.

6-10 day heights (or pressure) at 18,000 feet (500 mb) and anomalies from last night’s European model (left) and GFS (right). Red areas indicate above normal heights (pressure) and blue areas below normal heights.

The ridge on the European 6-10 day mean extends father north towards Alaska than the GFS mean suggesting that it would deliver colder air into the Plains behind any front than the GFS. The disagreements in the models make it difficult to know how cold it might get towards day 10 (December 12-13).

European ensemble mean 850 mb (5,000 feet) temperature. Forecast valid, Wednesday, December 12.

Technical discussion for December 13-16

The forecast for this later period is largely predicated by the pattern on the Climate Prediction Center (CPC) super ensemble mean forecast of 500 mb heights (pressures at around 18,000 feet) averaged over a 5-day period centered on December 14th. At longer time ranges, most meteorologists rely on these type of maps rather than on individual deterministic daily forecasts of weather systems since there is little skill (actually no) skill in forecasting individual storms at these longer time ranges. Any skill is in predicting the larger scale features that stand out in a 5-day mean forecast.

Around December 14, we see a strong positive anomaly (red area) and associated ridge off the West Coast. The downstream response is a broad trough (blue area) to our west with cold air in the Rockies and Northern Plains. The Pacific ridge position is far enough off the coast that it forces the trough to develop far enough to our west to allow some ridging or a positive anomaly along the southeast coast. Such a pattern would favor storms tracking to our north rather than to our south which places the D.C. area on the warm side of the low track.

Another neat feature of the CPC product shown below is it offers the 10 dates since 1950 in which the 5-day mean patterns were the closest to the pattern shown below. A meteorologist can than utilize average temperatures from those 10 days to get a sense of how much the temperatures and precipitation during those dates compare to normal. In this case, such analog based composites of temperatures and precipitation fields from those dates indicate that the forecast pattern favors warmer than normal temperatures and above normal precipitation across our area. The warm, wet idea is shared by NOAA’s CPC forecast from yesterday.

CPC super ensemble mean analog composite centered on December 14, 2012.

The bottom line

The pattern still looks like one favoring above normal temperatures more days than below during the next two weeks despite the presence of a negative NAO. The problem with the NAO is it is not far enough to the west to keep our heights (pressures) below normal or strong enough to shove the storm track to our south. Therefore, storms track to our north and we stay on the warm side of the storm tracks. Instead, the position of the ridge in the Pacific and the trough to our west are the dominant players.


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