D.C. area snow chances decreasing, not done
In the middle of a milder period with more warmth to come and no real snow threats on the horizon, all while leaving the "peak" of winter behind, local snow lovers are probably feeling a bit depressed. Perhaps day after day of a Snow Potential Index of 0 has not helped matters much.
Following above average snowfall in December and January, much of the area has managed 8 to 18 inches of snow so far this season, including 9.5" at Washington and 11.9" at both Dulles and Baltimore. A vast majority of the snow came during Commutageddon. February has been quiet so far.
While near average seasonal snowfall is still within reach in many spots, we may need some help getting there as we chew away at available winter days. However, it may be that a year such as this (moderate to strong La Nina) offers more tail-end hope than others.
Before getting started, let's put to rest the idea that a warm-up in mid-to-late February is the end of winter. Plenty of snow has fallen after that period (see: 2/22-23/87, 3/13/93, 3/9/99, 3/1-2/09), and after a significant warm-up (see: 2/22/87, 3/8/93, 3/3/99, 2/27/09). It is also true that factors like increasing temperatures and sun angle play prominent roles in diminishing snow potential, but late-season storms can be quite explosive thanks to growing temperature contrasts and plenty of cold air to the north still to be tapped.
When examining the last 30-season average ending 2009-10, the final accumulating (.1"+) snowfall at Washington came during March or April exactly 50 percent of the time. The final two weeks of February account for an additional 20 percent of final snowfalls. So in 70 percent of years, there has been measurable snow following mid-February.
If you live away from National Airport (most people), your odds of getting more snow are even better. A brief comparison to other local NOAA observation locations shows 70 percent of final snowfalls have happened in March or later at Dulles compared to 67 percent at Baltimore. Add in another roughly 10 percent at Dulles and 17 percent at Baltimore for the second half of February.
Though I painted a positive picture (for snow lovers) above with high percentages of additional accumulating (.1"+) snowfalls to come, when looking at events that dropped snow greater than 1" at National, the 30-season (1980-81 thru 2009-10) average for each major snowfall month is 18.7 percent in December, 39.2 percent in January, 31.4 percent for February, and a measly 8.8 percent in March. November barely scraped by with 2 percent, and April was a no show.
So we are, of course, running out of time with every day that passes. The raw trend line from February through March is decidedly downhill [DCA | BWI], as the peak [DCA | IAD | BWI] of snow generally occurs from late January through mid-February at all area climate locations though there may be a secondary peak heading into March.
Loyal readers of CWG are aware that La Nina has been underway this winter. Some folks find it helpful to base future projections on similar ENSO (i.e. either El Nino or La Nina) phases of the past. Just beware the sample of La Ninas is small* (also, for some years prior to 1950, whether it was a La Nina and the strength are subject to debate). It also leaves out some years "close" to Nina but not quite there. Still, there is some semblance of an image in the fog. The chance of late season snow during La Nina, particularly into March, is seemingly higher than during a "typical" winter around here.
The full La Nina sample -- 14 weak seasons / 14 moderate to strong seasons -- for Washington in the list at the bottom produces a similar snow peak result when compared to "normal," with 1"+ events most likely in January. However, the sample features a more even distribution through the end of the season with 23 percent of such events occurring in February and 17.7 percent in March. In moderate-to-strong La Ninas, the events are even more "tail heavy" with the February-April period picking up a total of 46 percent of the 1"+ snows.
Examining 3"+ and 5"+ inch events, the idea that La Nina March may contain greater-than-normal snow opportunities appears again, though the overall sample of events is increasingly small, and many of the bigger snows were long ago (pre-1950). Not to mention that strong Ninas appear to have less "big" snow overall than other types.
During the 30-season period ending 2009-10, 3"+ snowfalls during March made up 9.6 percent of all snowfalls for any month and 5"+ March snowfalls made up 12 percent of all 5"+ snowfalls. Moderate-to-strong La Nina March runs 31.6 percent (tied with January for most) on 3"+ events and 28.6 percent (tied with Jan. and Feb.) for 5"+ events.
Recent moderate and strong La Ninas have proven either futile for late season snow or abundant. In the moderate La Nina of 2007-08, D.C. recorded its final snow on February 20 with 1" at DCA. The strong La Nina of 1999-2000 concluded with .4" on February 18. Another moderate La Nina, this time in 1998-99, brought 8.9" to D.C. during 3 events from late February through mid-March with the majority (8.4") falling on March 9.
If there are any immediate conclusions to be drawn, it seems the odds are historically quite good to see the ground whitened again, especially if you live at an elevation greater than National (which is pretty much everybody!). How white the ground ends up is another matter entirely.
What do you think? Does the recent trend of less snow at the end of the season hold or does La Nina try to send some late-season magic our way?
* La Nina events in the overall sample: 28, 14 of which are moderate or strong. Weak: 1903-1904, 1908-1909, 1910-1911, 1924-1925, 1938-1939, 1950-1951, 1956-1957, 1962-1963, 1964-1965, 1967-1968, 1971-1972, 1974-1975, 1995-1996, 2000-2001; Moderate: 1909-1910, 1933-1934, 1942-1943, 1954-1955, 1970-1971, 1998-1999, 2007-2008; Strong: 1916-1917, 1949-1950, 1955-1956, 1973-1974, 1975-1976, 1988-1989, 1999-2000.
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