A slow start to the rainfall season – Reagan National was running a deficit of slightly more than 2.5” for the year before this morning’s quick 0.75″ or so of rain – has stoked some concern about a mini-drought for the area. These fears, while understandable at this premature stage of the growing season, may not find relevance by the end of spring. The evolving pattern does not offer much in the way of precipitation potential for the balance of April, but once to the late part of the month and beyond, a considerably wetter scene could develop.
La Nina and Nina-like years – particularly those that have occurred in the last 30 years – foster the optimism behind our wait-‘til-May statement. La Nina refers to cooler-than-average sea surface temperatures in the eastern equatorial Pacific Ocean. While water temperatures have not met the threshold for an official Nina event, they have been cold enough (0.5°C below normal in January and 0.7°C below in February) to suggest a Nina-like event is influencing the atmosphere.
Long-term forecasts are of course riddled with uncertainty, but analysis of past early-year Nina events can still help us assess the regional risk of drought and potential for rainfall.
Across all the early-year Nina events (19) since 1950 – an early-year event comprises those December-January-February periods which meet NOAA’s required conditions for an official La Nina designation – more than half, or nine, have featured above-normal rainfall amounts of over 3.99” (the 1981-2010 climatological average). Remarkably, two of the top three wettest Mays since 1948 occurred in 2008 and 2009 – both Nina years – when a total of 18.71” fell (10.66” in ’08 and 8.05” in ’09).
April is more of a dud, as only eight of the 19 early-year Nina events have posted rainfall amounts above the 1981-2010 mean of 3.06” with most of these events (10) failing to produce even 2.5” for the month. Some of the totals are infinitesimal, too: 1.86” in 1950; 1.17” in 1976; 0.03” in 1985; 1.68” in 2001; 1.92” in 2012. There is one positive statistic for April, namely that three of the top four wettest such months since 1983 took place in 2000 (5.13”), 2008 (4.92”) and 2009 (4.22”).
There are sound meteorological reasons that explain why rainfall is fairly light in April, but turns heavier in May. For one, a coast-to-coast ridge of high pressure tends to dominate the April pattern during early-year Nina events. This ridge helps blunt areas of more substantial rainfall to the west and north of D.C., and supports above-normal warmth across the mid-Atlantic.
Averaged across all of the early-year Nina events since 1985 (there are 11 such years), above-average rain amounts have focused on the eastern Midwest. The East Coast and Deep South have been the malefactors of below-normal precipitation.
The NCEP upper air analogs – a subset of periods that best represents the 500 mb pattern – present a similar look for the 8-14 day timeframe (centered on April 22), as a ridge sprawls across the southern tier of the U.S. and a large trough bellies up against the northern tier from its home base in Canada. This trough will occasionally buckle into the Lower 48, but progress quickly through. Most low pressure centers – and the heavier rains associated with them – will pass well to the north and west of D.C., leaving quick-moving frontal passages to account for much of our precipitation.
An important calculated variable – omega – helps forecasters determine precipitation potential. The omega calculation reveals which type of vertical motion will occur over a certain area. Positive values of omega indicate sinking motion (subsidence), while negative values indicate rising motion (uplift). Subsidence is unfavorable for rainfall as air warms and dries out on its way down; uplift, on the other hand, is much more favorable for rainfall as air cools, moistens and, most importantly, condenses. The condensation process leads to the development of clouds and precipitation.
Let’s bring the omega variable into the fold. Averaged over all of the Aprils that have occurred within early-year Nina events since 1950 (again, 19 in total), the Eastern U.S. has experienced a positive omega anomaly, or more than the normal amount of subsidence. This does not guarantee that drier-than-normal conditions will develop, though that has certainly been the case for the East on average.
May holds more promise for those closely watching Washington’s mini-drought potential. The 11-member set of early-year Nina analogs since 1985 favors ridging over the Southwest and New England, but a break in the ridge forms over the mid-Atlantic and Southeast. A storm track closer to the East Coast – along which individual systems can slow their forward speed as the New England high retreats off the coastline – transports moisture from the warming and increasingly juicier Gulf of Mexico and Atlantic Ocean into our region and the Northeast at large, while areas south of the Ohio River dry out.
The omega plot shows a sharp reversal in the relative vertical motion field over the East. A distinct area of above-average subsidence present in April gives way to a sizable negative omega anomaly. In fact, the more significant negative anomalies – representing the core of greater uplift compared to normal – appear over the mid-Atlantic.
More rainfall does not provide relief from the heat, however, as a secondary area of above-normal warmth develops over the East. A combination of stronger moisture flux into the I-95 corridor and southerly flow downstream from the mean storm track makes it warm and wet in Washington.
For all its promise, the month of May does have some bust potential, though certainly not to the extent of that observed for April. Less than 3” of rain fell in May of the following years: 1951, 1956, 1999, 2006 and 2011. And just last year, Reagan recorded only 3.28”, following a more daunting annual (through April 2012) rainfall deficit of 4.5”.
It doesn’t take a climatologist to claim that any rain would be welcome here. Climatology, however, helps evaluate the chance that enough precipitation will fall to prevent or alleviate drought conditions. The historical record (granted, it’s limited), pattern recognition and forecast trends all point to marginal rainfall for April, but also the opportunity for heavier rains in May that could eliminate and turn our year-to-date precipitation deficit into a surplus – and, therefore, quell fears for a developing D.C. drought.