MUWIC Newsletter - February 2003

(Issued: February 3, 2002)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

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LEAD STORY: "7th Coldest January on Record"

This is a heck of a way to run a warmer-than-normal winter!

Last October, November and December, the National Weather Service released a series of long-range outlooks that forecasted a better-than-normal chance of a mild winter across the northern half of the US. In reality, the first two-thirds of the winter (December and January) have been significantly colder than normal most places east of the Mississippi River, and the Northeast US is experiencing the coldest winter in almost a decade. Locally, December was about 3 degrees below normal and January concluded over 5.5 degrees below normal--the 7th coldest January on record (dating back to 1914).

Don't blame El Nino; blame a "negative NAO" (i.e. North Atlantic Oscillation). This lesser-known index relates to changing oscillations in atmospheric pressure over the northern Atlantic Ocean. For most of the last two months, the NAO index has been "negative" and, during the winter months, a negative NAO signals jet stream blocking over the North Atlantic and a better than normal chance of displacing Arctic air southward into Eastern Canada and the Northeast US. Bottomline: so far this winter, the NAO has played a greater role in dictating our weather pattern than the much-hyped El Nino (warming in the equatorial Pacific Ocean).

Unfortunately, the NAO is not as good of a "long-range trend" forecasting tool as El Nino. Where as El Nino comes and goes over the term of years--changing sea surface temperatures evolve over many months--the atmospheric pressure oscillations over the North Atlantic vary greatly over the course of a few days or weeks. Consequently, use of the NAO is limited to signaling trends in the upcoming week or two, whereas El Nino remains useful in speculating about climatological trends up to a year in the future.

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EVENT OF THE MONTH

This month's "event" lasted 12 days--that is, a streak of 12 consecutive days with temperatures holding at or below freezing. This is the longest such streak since December of 1989 when we recorded 13 consecutive days without temperatures nudging above freezing. The record longest such streak is 15 days, set in January 1961 and February 1979.

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JANUARY CLIMATOLOGY (MU Weather Station)

Average High Temp: 31.2 F

Average Low Temp: 18.0 F

Average Monthly Temp: 24.6 F

(Departure from Normal: -5.5 F)

Snowfall: 7.4 inches

(Departure from Normal: -2.0)

Total Precipitation (liquid equivalent): 1.71 inches

(Departure from Normal: -1.30)

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FEBRUARY NORMALS, RECORDS & FACTS

Normal high/low temperatures range from 39/21 during the first week of the month to 45/26 by month's end. The record high for February is 78 degrees (25th in 1930) and the record low is -17 F (7th in 1935). Normal liquid precipitation (rain and melted snow) is 2.47 inches. Average snowfall for the month is about 7.5 inches.

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FEBRUARY OUTLOOK

While the first 4 days of the month will average above normal, I am not optimistic that this trend will continue. Currently the major global jet stream features are realigning in a way that looks very similar to what we saw in mid-January. Therefore, it seems likely that unseasonably cold air will return for at least the next week or two. The leading edge of the arctic charge will push through today (2/4), and a second and third charge may arrive on/around Friday (2/7) and Monday (2/10). At this time it looks like each shot will be progressively colder.

Initially it looks like deep northwest flow will keep us mainly dry through the first few days of the turn to colder. A period of light snow or flurries is possible late Thursday (2/6) and again late in the weekend (2/9-10) as successive shots of arctic air move in. However, it's the future interaction between the northern jet stream and the increasingly active southern jet stream branch that is most intriguing. Absent from the pattern for most of January, the southern branch jet stream flow is coming alive and it will undoubtedly send several disturbances across the southern states in the coming weeks.

If the two branches of the jet stream can "phase" and work together we just might see a significant Nor'easter sometime this month. Right now, it looks like these features will not come together in the near future, so the storm track will likely remain suppressed to our south through February 10th. However, climatology favors a farther and farther northward track as February wears on, and I would not rule out one or two phasing events during the middle or later part of the month. In the end, February may end up a couple degrees below normal with near normal precipitation.