The Southern New England Drought Situation

(Last July, I posted on the ongoing Southern New England drought.  Please see the post "Severe Drought Expands in Southern New England" for a refresher on the causes of drought in our region and how we got to the dry conditions. - ZG)

With all the recent rains and the turn of the seasonal calendar to Spring, I think it's a good time to revisit our drought situation of 2016.  Winter and early spring are good times to add to the water table thanks to low evaporation rates.  Where do we stand as we prepare to enter the warm season?

Recent Rainfall
Over the past 14 days, 3-5" of precipitation have fallen across Southern New England.  There have been all kinds of precipitation from snow to sleet to plain rain.
NWS 14-day precipitation analysis (image Weatherbell)
The rivers are as full as they have been in years and with the additional rainfall in the forecast this week some minor flooding is possible.  To put into context just how long it has been since most of us have had to worry about river flooding, the Blackstone River at Northbridge hasn't been at 8 feet (Action Stage) since 2011.  Right now the river is forecast to rise to 6-7 feet by Wednesday afternoon.  This is okay, minor flood stage is 9 feet.  The total rainfall on Thursday and Friday might push the river close to 8-9 if enough rain is able to fall.    The official forecast is 6 feet, but experience tells me to take the over in regards to these forecasts.
NWS/NOAA Blackstone River @ Northbridge official forecast
While it is certainly a big change from the past few years, is this just a short-term pattern change or is this a signal that the prolonged dry spells are coming to an end?  Let's look at precipitation trends over the past several years.

Precipitation/Drought Analysis
Let's look at how the drought has evolved since late March 2015.  This is the Drought Monitor in 3 months increments ending last week.
US drought monitor March 2015-March 2017
Drought conditions have been improving so far this winter, but they also improved last winter.  As a matter of fact, January, February, and March (JFM) are not driving drought conditions.  First quarter precipitation is contributing more than expected to the overall "precipitation budget" over the past 27 months.  JFM is 33% of the time period (9 months out of 27).

Over a 3 year period, one would expect 32.52" of precipitation in JFM.  In the past 3 JFM's (2015, 2016, 2017), 32.24" of precipitation have fallen in Boston.  If you take the average precipitation over 824 days starting on Jan 1 and ending on Apr 4 two years later, you come up with 95.04".  The 34.2% is found by dividing 32.52/95.04.  The actual precipitation from Jan 1, 2015, to April 4, 2017, is 81.06".  So we take the observed 32.24" and divide that by the observed 81.06" to determine that 39.8% of Boston's precipitation has fallen in JFM since the beginning of 2015.

(How to read this chart- the departure from normal is represented by the function  

  depature since 2015= (p(15)+p(16)+p(17))-(Annual Average*y))

where p is precipitation and y is the number of years being averaged.  Annual departure uses y=2 since we are only using data from 2015 and 2016.  So that function for Boston would be

  annual departure since 2015= (34.80+33.05)-(41.78*2)= -15.71)

Coming into 2017 Boston, Worcester and Northbridge are all at least 8" below normal over the past two years.  Precipitation during the first quarter of 2017 is up in Boston, average in Worcester and down along the Blackstone River.  Note that this data does not take into account the rains so far in April.  So let's add those total to the data and compare 2015, 2016, and 2017 through April 4th.

Even though Northbridge and Worcester are 15 miles apart there is a difference of over 1.5" so far in 2017.  So far this year a lot of heavy precipitation has been south of the MA Pike and along the coast.  While we certainly need a lot more (and we will take a chunk out this week) and we aren't gaining or losing much ground either way.   Given the deficit, you might think that this is bad news, but it's not. The amount of moisture in the soil is much higher than at this time last year and 2015.
2015, 2016, 2017 soil moisture percentile (image Univ. of Washington Atmosphere Science Department)
 The soil moisture improvement is a bigger deal than one may think.  When the soil is dry like it was last year precipitation that falls just runs off to the rivers/lakes without being absorbed by the ground.

 Water Supply

That is a lot of numbers and information.   This is my way of saying that yes, there is an improvement.   But the path to recovery is narrow and a dry spring will put us in big trouble.  The Quabbin reservoir is at 82.1% of capacity.  It dropped to 79% in November, but 82.1% is still below normal for this time of year.  The Whitinsville Water Company continues a *Stage 1 Water Restriction*.  Still, as of March 1, Central Massachusetts was downgraded from a *Drought Watch* to a *Drought Advisory*.
March 1, 2017, Drought Status
Here is how a time series of the Quabbin water levels since the 1940's.  The 1960's should stick out!
MWRA Quabbin Water Levels 1948-pres
The Wachusett Reservoir dropped to 90.9% capacity on March 1 but was at 94.4% on April 1.  This isn't abnormal as the Wachusett's levels are generally lowest in March by design.  The water supply in Central/Eastern Massachusetts is in good shape at the moment.

Looking Ahead
NWS PM Headlines Tuesday, April 4, 2017 
A flood watch is now posted for Thursday-Friday.  While April showers bring May flowers, April floods bring May buds and summer suds.  But if April doesn't flood, there will be no buds as the gypsie moths will not die.  The gypsie moths are a post for another day.  They will return again in 2017 and I will explain why (if you don't already know).

Thank you for reading.  Questions, comments, complaints all welcome

-Zack Green


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