MUWIC Newsletter - December 2006

(Issued: December 1, 2006)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

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LEAD STORY: “Developing El Nino to Influence Winter Season”

A developing El Nino (warming in the equatorial Pacific Ocean) will be a frequent newsmaker this winter as it helps direct this winter’s outcome across the US. The current El Nino episode is near “moderate” strength (relative to past episodes) and it is expected to last into Spring 2007. November’s unusual weather—above normal temperatures and precipitation across our region—is somewhat the result of this developing El Nino. Does this mean that a warm, wet winter is on tap? Perhaps. However, there are a couple of other factors to consider and, thus, lovers of winter weather should not give up hope.

For a more-detailed look at what the winter may hold for our region, scroll down to the Winter Outlook. Wishing you a wonderfully holiday season and happy New Year!

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EVENT OF THE MONTH

November 16^th brought an extraordinary weather event as a slow-moving front produced a three-hour deluge. Between 3:00pm and 6:00pm, the MU Weather Center measured 2.8 inches of rain; other parts of the county received between one and three inches of rainfall. Localized flash flooding made for a memorable evening commute for many folks.

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STAT OF THE MONTH

Rainfall the last six months has totaled nearly 33 inches, or more than 10 inches above normal. Sure makes the Drought Watch of Spring 2006 a distant memory…

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NOVEMBER CLIMATOLOGY (MU Weather Station)

Average High Temp: 56.5 F

Average Low Temp: 39.8 F

Average Monthly Temp: 48.2 F

(Departure from Normal: +5.0 F)

Precipitation: 6.46 inches

(Departure from Normal: +3.31)

Annual Precipitation: 46.15 inches

(Departure from Normal: +8.07)

Snowfall: Trace

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DECEMBER NORMALS, RECORDS & FACTS

December is the third coldest month of the year with an average monthly temperature of 34F. Normal high/low temperatures drop from 46/28 on the first of the month to 39/23 on New Year's Eve. The record high of 78F was set on the 5th (2001), and the record low of -7F was recorded on the 18th (1916). Average liquid precipitation (including melted snowfall) is 2.98 inches, and the long-term average snowfall is 4.6 inches. Winter officially arrives at 7:22 p.m. on December 21st.

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WINTER OUTLOOK 2006 - 2007

Winter outlooks are ubiquitous these days with just about every TV meteorologist, private forecasting firm, and weather weenie blogger opining on what they think the winter has in store. The National Weather Service is the most recognize source of seasonal outlooks, and chances are you’ve seen their forecast for a slight warm bias and “equal chances” precipitation. (Overall, a “fuzzy” forecast that offers little guidance for southeastern PA.) Several private meteorologists are enamored to the idea of a colder- and snowier-than-normal winter. I don’t side with any of the above-mentioned forecasts. Here’s why.

Climate “Signals”

A wealth of research has revealed the usefulness of numerous global indices in determining climate trends. The best known—and most reliable—is the ENSO cycle in the equatorial Pacific. Depending on the ENSO phase, there are different influences on winter weather in our area. A weak to moderate El Nino is strongly correlated to above normal snowfall in our region, whereas a strong El Nino typically signals a mild, wet winter (with rain more common than snow). The opposite mode, La Nina, overwhelmingly results in a warmer and drier winter in the Eastern U.S. The bottom line: Recent trends and model forecasts suggest a strengthening El Nino into mid-winter, so the currently board line “moderate” El Nino may become marginally “strong” in early 2007.

There is also an alphabet soap of other sometimes helpful indices (EPO, PDO, AO, QBO, et al.) to consider. In an ENSO “neutral” or “weak” episode (like last winter), I would consider some of the above indices as providing important clues. However, this season’s moderate to moderate-strong El Nino will overwhelm these signals and remain the primary influence on the US weather pattern. The one exception is the North Atlantic Oscillation (NAO) index, which can remain relevant in the face of a building El Nino. In particular, a strong “negative” NAO can overwhelm the El Nino influence over the Northeastern US.

Unfortunately, the NAO trend for the coming months is not clear cut as the ENSO trend. Unlike ENSO, which relates to sea-surface temperature (and changes very slowly over several months or more), the NAO is an index of atmospheric pressure that frequently changes over the course of several days or weeks. A “negative” NAO indicates high-latitude blocking near Greenland and a high likelihood of below normal temperatures (and potential storminess) in the Northeast. Conversely, a “positive” NAO often signals ridging along the East coast, a storm track west of the Appalachians, and above normal temperatures in the Eastern U.S. Research has shown that warm water near the southern coast of Greenland and cooler water near the Canadian Maritimes and Eastern US coast favor a bias toward negative NAO. The bottom line: Although complex and ever-changing, current sea-surface temperature anomalies in the Atlantic resemble the configuration that should favor a negative or neutral NAO during the first half of the winter. Beyond that, it’s anyone’s guess.

Local Outlook

As explained above, I believe the NAO and El Nino signals are the primary influences of the winter ahead, while the many other indices will play a weak secondary roll, if any. The NAO is the more important signal for the Northeastern US, but it’s also more transient and hard to predict. It’s my sense that periods of “negative” NAO are more likely early in the winter and thus December into January may be the coldest period of the winter relative to normal. What’s more, if El Nino strengthens to a marginally strong episode, the effects of the NAO index may be overwhelmed during the second half of the winter. I do not have a high confidence in the outcome of the late-winter period, but it’s my hunch that a warm bias will prevail. So, expect a winter season with above average variability and a better chance for cold and snow early on and less frequent visits late in the season.

The big story this winter will be an active southern storm track fueled by the building El Nino. Such storms will be significant precipitation producers along the California coast, across the Gulf Coast and southeast, and occasionally up along the mid-Atlantic coast. Of course, the big question always relates to precipitation type. At times of negative NAO, these storms will more likely take a favorable track toward the New England coast and produce significant snowfall in PA. However, during periods of the neutral or positive NAO, these storms may track to our west (leading to rain or snow changing to rain) or track off the southeast US coast (un-phased) leaving PA mild and dry. In the end, total precipitation should be 110 to 125% of normal.

Seasonal snowfall will be directly related to the prevalence of negative NAO episodes. Assuming a few periods of negative NAO—most likely early, but perhaps also a couple weeks of negative NAO late in the season—chances are good that we will see at least near-average range snowfall (20 to 30 inches). The wild card is always whether or not there will be a large snowstorm (10 inches or more) that makes or breaks the seasonal snowfall total. While no one can say for sure if such a storm will affect southeastern PA this winter, chances are good that one will (given a moderate El Nino and periods of negative NAO). In the end, seasonal snowfall in Lancaster will likely total near the high end of the normal range (around 30 inches). The three-month temperature anomaly (December through February) may be slightly above normal, with an early-season cold bias and late-season warm bias.