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May 2011
North Country Monthly Climate Summary

TEMPERATURE SUMMARY...

Temperatures across Vermont and northern New York were near to above average. Monthly average temperature anomalies were +1 to +2 degrees above average. More specifically, high temperatures were above to well above average over the latter third of the month (between the 18th and the 31st). The warmest temperatures would be observed on May 31st, with most locations reaching the mid to upper 80s. The coolest high temperatures would be observed during mid-May, due to extensive cloudiness owning to a slow-moving low pressure system. Overnight low temperatures would dip below freezing on May 1st, but would generally stay above freezing (in the 40s in the 50s) through the rest of May.

TEMPERATURE STATISTICS ACROSS THE NORTH COUNTRY FOR THE MONTH OF MAY...

  Burlington Montpelier Massena Saint Johnsbury
Avg. Temp 59.2 57.8 57.9 58.9
Departure +2.7 +3.4 +1.4 +2.6
Highest 86 on 26th, 31st 85 on 31st 87 on 31st 83 on 26th, 31st
Lowest 37 on 1st 31 on 1st 32 on 6th 31 on 1st

Below are daily temperature graphs for the month of May for Burlington, Montpelier, Massena, and Saint Johnsbury.

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PRECIPITATION SUMMARY...

May followed April as being one of the wettest months in 2011 for Vermont and northern New York. As was the case in April, the heaviest May precipitation totals would be located across the northern Adirondacks of New York and the northern and eastern Green Mountains, where amounts between 7 and 11 inches would be observed. The driest May precipitation totals were along the St. Lawrence River Valley of New York and along Connecticut River Valley in Vermont. The most substantial precipitation event was across central and eastern Vermont on May 26th and 27th, when slow-moving strong to severe thunderstorms would drop a significant amount of rain, resulting in flash flooding in parts of Barre and Montpelier, VT. In addition, St. Johnsbury recorded over 5 inches during this event.

PRECIPITATION STATISTICS ACROSS THE NORTH COUNTRY FOR THE MONTH OF MAY...

  Burlington Montpelier Massena Saint Johnsbury
Monthly Total " 8.67 9.90 5.32 10.15
Departure +5.35 +6.58 +2.51 +7.15
Greatest 24hr 1.80 on 3rd-4th 4.51 on 26th-27th 1.19 on 14th-15th 5.04 on 26th-27th
SNOW/SLEET
Monthly Total "
Greatest 24hr


BURLINGTON FORECAST AREA ACCUMULATED MONTHLY PRECIPITATION (IN INCHES) FOR THE MONTH OF MAY

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MONTHLY WEATHER PATTERNS AND GLOBAL CLIMATE DRIVERS...

The 500 mb geopotential height anomaly map for April showed a large upper-level trough over the western half of the United States (note the blue colors indicating below-average geopotential height anomalies in Fig 5). Also of interest is the zone of positive geopotential height anomalies over the Canadian Maritimes and the area of negative geopotential height anomales just to the east of Bermuda. This anomaly configuration, where a region of below-normal geopotential height anomalies is located south of a zone of above-normal geopotential height anomalies, results in a blocked weather pattern known as a "Rex block". This pattern often results in a slower eastward progression of weather systems, which may be partly responsible for the very wet May experienced across our region.

Click to enlarge

The ongoing weak La Nina would continue to weaken through May. Below-average sea surface temperatures (SST's) across the central equatorial Pacific Ocean would weaken further, with a continued presence of warmer than average SST anomalies over the western South America coastline. By the end of May, La Nina had weakened significantly enough that ENSO-neutral conditions had begun to set in across the equatorial Pacific Ocean. The Climate Prediction Center called for ENSO-neutral conditions to continue through the 2011 Northern Hemisphere summer months.

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Figure 1: Burlington Monthly Temperature Trend
Figure 2: Montpelier Monthly Temperature Trend
Figure 3: Massena Monthly Temperature Trend
Figure 4: Saint Johnsbury Temperature Trend
Figure 5: Monthly Precipitation Map
Figure 6: 500MB Geopotential Height Anomalies