MUWIC Newsletter - December 2003

(Issued: December 2, 2003)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

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LEAD STORY: “Third Warmest November on Record”

If you thought November felt unusually mild, you are right. The average temperature of 48.6 degrees is 5.4 degrees above normal and just shy of the warmest November on record (+5.55 in 2001).

Interestingly, November was a rare month that posted both a record high and record low temperature. The high of 82 degrees on the 3^rd was a record, as was the low of 19 degrees on the 10^th. Still, warmer-than-normal conditions ruled for most of the month with 23 days coming in above normal. Keeping with the recent trend, precipitation was also greater-than-normal with 4.39 inches falling. Normal precipitation for November is 3.15 inches.

Does the mild, damp November weather tell us anything about the winter ahead? I’ll delve into this and much more in the Winter Outlook (below).

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EVENT OF THE MONTH

A sharp cold front crossed the area around 6:30 p.m. on November 24^th, dropping the temperature 16 degrees—61 degrees to 45 degrees—in just over 15 minutes. Rain showers accompanying the front mixed with sleet and wet snow in some locations, producing the season’s first trace of wintry precipitation.

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STAT OF THE MONTH

Five of the last six months have featured above normal rainfall, with 31.03 inches of total precipitation from June 1^st to November 30^th. This amount is almost 9 inches above the long-term average for the period and nearly 20 inches more than what fell during the same period during the drought of 2001.

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NOVEMBER CLIMATOLOGY (MU Weather Station)

Average High Temp: 58.7 F

Average Low Temp: 38.4 F

Average Monthly Temp: 48.6 F

(Departure from Normal: +5.40 F)

Precipitation: 4.49 inches

(Departure from Normal: +1.34)

Annual Precipitation: 45.62 inches

(Departure from Normal: +7.54)

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 2:04 a.m. on December 22nd.

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WINTER OUTLOOK

One technique for long-range forecasting involves the use of analog months or seasons—that is, identify past months or years with similar weather (to the recent past) and glean from that what might occur down the road. Given the extraordinary warmth of the last few weeks, it makes sense to begin the Winter Outlook with a search for analog Novembers, and then see what followed in December.

In our 90-year climatological database there are eight Novembers with similar warmth: 2001 (+5.55), 1975 (+5.44), 1964 (+5.15), 1918 (+5.02), 1999 (+4.77), 1948 (+4.75), 1979 (+4.6), and 1963 (+4.5). Following these above normal Novembers was an above normal December in five of the eight analogs. In two analog cases December was near normal and in only one case was December below normal. Consequently, it seems that a simple temperature analog favors an above normal December. When analoging temperature AND precipitation, however, you get a different result. Of the eight analog Novembers listed above, five featured below normal precipitation, two had above normal precipitation and one near normal. Interestingly, the five “warm, dry” Novembers are the same five cases that were followed by an above normal December. The three remaining Novembers (with near-normal or above-normal precipitation) were in fact not followed by a warm December. Thus, the best analog Novembers are 1948, 1963, and 1975—all were wet and mild like this November—and the Decembers of these years averaged about 2 degrees below normal with an average of 7 inches of snow.

Medium-range computer guidance supports this analog through at least December 10^th or 15^th. Increased blocking over Greenland and ridging in western Canada favor below normal conditions, while an active Pacific jet will bring a few storms into the East. Snow lovers should recognize that the ingredients are in place for some early season snow, say, sometime before December 15^th. Furthermore, should we establish some snow cover in early to mid-December, this would help maintain low-level cold air and increase the odds of snow, not rain, from storms developing later in the month, and it would make a below normal December almost a “lock.”

The outlook for January and February is more uncertain. First, there is no analog signal for these months—that is, there is no correlation between a warm, wet November and late winter weather. So, let’s look to the central Pacific for a clue as to what we might expect. The neutral ENSO of recent months is showing signs of shifting into a weak El Nino phase in the coming months. This warming of the central Pacific usually enhances a low latitude storm track into the southwest US and can lead to above normal precipitation across the Gulf coast and sometimes the East coast. A weak El Nino provides no signal for temperature trends here in PA, so we’ll have to keep an eye on cold air production in Siberia and Northern Canadian as well as the sign of the North Atlantic Oscillation (NAO). Unfortunately, we cannot forecast the NAO more than a week or two into the future, so we are left with little information to leverage in making a late winter forecast.

Given the current state of the global jet stream patterns, however, it would seem safe to rule out a “dry” or “persistently warm” winter. Instead, we may see a rather active (read “stormy” and “windy”) pattern with frequent large temperature swings. This, of course, is not uncommon in our region and therefore this winter might end up in the general category of an “average” winter for the Lancaster area. For reference, the long-term average snowfall in Lancaster is 26 inches—though, a single storm (or miss) can go a long way to making or breaking a winter in terms of snowfall.

In summation, there are no solid signals as to the direction of this winter’s weather, so I’m left to leverage analog months, the current global weather patterns, and intuition (based on 20 years as a meteorologist in this region) in making this year’s outlook. The highlights of this year’s outlook:

Ø In the wake of an extremely warm November, December should average about two degrees below normal.

Ø There will be a few opportunities for early season snowfall. Lancaster could receive above-normal snowfall for December (normal is 4.6 inches).

Ø January and February will continue to feature frequent storminess, although, the storm track may vary greatly (at times to our west and at time to our east). Total precipitation (rain and melted snow) will be above normal, while aggregate December through February temperatures average out “near normal” (+/- 2 degrees from normal).

Ø There will be less snowfall than last winter (63 inches), but likely a bit more than the long-term average of 26 inches. Let’s say between 30 and 40 inches of total snowfall.