The precipitation event of April 15-17 brought streamflow across Connecticut to significant levels, with high flow rates not seen in a month. 9 of the 61 USGS stream gages in the state reported flows above the two-year recurrence interval for that stream on April 16 (click the gage number to see that stream's current status):
Before the storm, baseflow across the state had been trending lower since mid-March, with around half the stream gages in the state reporting low flows just before the storm (see table below). Earlier in the year, from mid-January to mid-March, above-freezing temperatures melted the snowpack, and storms tracked more consistently over the state than they had at the very beginning of the year. Before mid January, then again from mid-March to mid-April, storms were diverted to our northwest or southeast by persistent blocking patterns. A look at all USGS Connecticut streamflow graphs for the past five months clearly shows how flows were consistently near or below normal until mid-January, then suddenly went to higher levels for about two months, then returned to lower flows (big download, takes a while).
If the rain predicted for April 18-20 does not get diverted, streamflow should continue at relatively high levels for a few days. After that, long-range models show another blocking pattern for a couple weeks, so streamflow may again trend lower. See the discussion of Precipitation Patterns below for more information.
On April 12, 27 of 61 USGS stream gages in CT were reporting low flows*. See table below. By April 15, (just before the heavy precipitation of April 16) that number had risen to 36.
Here are the numbers from previous weeks:
Mar 29: 30 of 61 USGS stream gages in CT were reporting low flows*.
Why do we count? The flow between storms is an indicator of stream conditions. When water levels are low, portions of the streambed become too dry to support the aquatic life that usually colonizes those areas. Groundwater seeping from the stream banks can help organisms survive for short times; therefore, well measurements can also be used to identify areas where aquatic life might be in jeopardy.
A count of the number of USGS stream gages reporting low flow at their lowest point between storms is valuable as an indicator of conditions statewide. If the number of streams that reach low levels between storms is increasing over time, it raises concern for the recovery of stream ecosystems from dry conditions. If the count decreases, we can look for recovery. If most of the low-flow gages are in the same area of the state, it could indicate a regional problem. We also note any record low flows for the date (see Sep 14 discussion).
Brief high water, even flooding, caused by sporadic heavy rain does little to alleviate long-term effects of dry conditions, because it takes time for ecosystems to recolonize streambeds after a drought has killed off aquatic organisms. Direct observation of any particular segment of stream is necessary to determine actual streambed conditions, because there are only 61 stream gages in Connecticut and the conditions they report may not represent the conditions in the hundreds of streams with no gages.
Your water utility and town are the best sources of information on the status of your water supply. A few systems post their current capacities regularly on their websites. For example:
Aquarion maintains on its website a weekly updated water usage report that includes water demand graphs, system transfers, system capacities, days usable storage remaining, and precipitation. As of Mar 13, their Greenwich system was at 88% of total capacity, up from 49% in January. Aquarion's Stamford system was at 93%, up from 64% in January. Their Bridgeport system was at 100%.
MDC reports its reservoirs were at 100% of capacity on Mar 1. The South Central Regional Water Authority reports that three of its reservoirs were at 100% on Feb 28 and one was at 90%.They reported their total capacity is at 93%. Their long-term average for the date is 82%.
The most recent Monthly Reservoir Status Summary available as of Mar 19 was the January report issued Feb 26. In it, CT DPH reported that the reservoirs of 34 large water systems averaged 93% Usable Storage, which is 101% of where they usually are in January.
In Connecticut, the USGS has 10 real-time groundwater monitoring wells. 6 of these compare their current measurements to multiyear records. On Apr 17, all monitoring wells showed a jump in the water table due to the long duration of the rain. Click here to see their current status.
The USGS Groundwater Watch website displays data from 60 other wells that are measured once a month, comparing the most recent measurements to each well's measurements taken in that month in previous years. At the end of March, most wells were near or above normal. Click here to see their current data.
Monitoring the water table can provide clues to future conditions. There was an apparent pattern in 2013-2016 in which normal groundwater levels in winter and spring alternated with increasingly well-below-average levels in summer and fall. Although 2017 groundwater levels did get as far below average as they did in 2014 or 2015, they did not drop as fast as in 2016. Click here to see all the USGS graphs for their real-time groundwater measurements that compare levels from the last four years to the median level for each day of the year (lots of data, so it takes a long time to load).
Groundwater levels influence streamflow. The water level in streams and rivers between storms depends on the flow of water out of the ground into the stream channel. The higher the water table, the more water is available to keep streams flowing. During a drought, smaller upland streams dry up faster than the larger rivers (click here for details from a previous Know Your Flow! webpage). An occasional brief, powerful rainstorm does not do much to help groundwater levels rise. The rain falls faster than the ground can absorb, and thus much of it becomes stormwater runoff. In contrast, public water supply systems that depend on reservoirs do benefit from heavy rains because the reservoirs collect the runoff. The runoff, however, can wash pollutants into the reservoirs, lowering water quality.
What a difference a day makes. All of Connecticut had a 30-day rainfall total well below normal. The April 16 storms brought 2 to 4 inches of rain from southwest to northeast across the state, bringing 30-day totals ranging from normal to 25% above normal. The northwest and southeast corners received 1 or 2 inches, which brought 30-day totals in those areas up to 75% to 90% of normal.
Back in mid-January, a significant change in storm tracks shifted our weather from drier than normal to wet. The National Oceanic and Atmospheric Administration (NOAA) January 2018 Synoptic Discussion explained how waves in the jet stream and associated ridge and trough surface patterns had caused generally dry conditions in the eastern United States for most of the time in January. Then, when the jet stream briefly switched to a straighter pattern, it allowed significant precipitation and snow-melting temperatures to reach us until mid-March. Another report described the persistent pattern of storms steered around a “Terribly Tenacious Trough” over the eastern United States. NOAA's March 2018 Synoptic Discussion explains our mid-March shift to drier conditions: "With the ... storm track locked in place, much of the central Plains and Southeast to coastal Northeast were left drier than normal."
On Tuesday, Apr 17, the National Weather Service 7-day forecasts for Connecticut were predicting some rain Apr 18-20, then dry conditions return. A long-range (16-day) computer model was predicting another blocking pattern for a couple weeks keeping precipitation south of us.
Each month, the NWS Climate Prediction Center issues analyses of global atmospheric conditions that could influence general temperature and precipitation patterns. Their March 15, 2018, prognostications for the next year show increased chances for above-normal rainfall through the summer. This is very different from their analyses over the past year or so in which they gave equal chances for above, for normal, and for below normal precipitation, but we actually got below normal precipitation. The Mar 15 release of the monthly U.S. Seasonal Drought Outlook from the National Weather Service Climate Prediction Center indicated no drought likely for Connecticut through June. These seasonal trends are based on a continued weakening of La Nina.
The April 12 U.S. Drought Monitor rated all of Connecticut "No Drought". There was some expansion of abnormally dry conditions in the northeast. Click here for their complete discussion of April 10 data.
The lack of any drought or abnormally dry areas in Connecticut does not signal a return to normal weather this year. Below is a comparison of mid-March this year with three previous years and the driest months later in those years. In March 2017, the drought of 2016 was still affecting Connecticut. Conditions improved after that before becoming dry again. The other years show there were small scattered abnormally dry areas in the northeast, similar to this year. Low rainfall produced dry conditions later in all those years.