Recap for the Bulk of May

As we head towards the end of another month, it’s interesting to look at how the month evolved and see what allowed the weather to change as it did. This particular analysis will include from May 1 up to May 26 with the main focus being on the Northeast.

Daily accumulation of may precipitation. Notice how the precipitation started above normal (red line )and shifted to below normal for the latter half of the month.
Daily accumulation of May precipitation. Notice how the precipitation started above normal (red line) and shifted to below normal for the latter half of the month.

Below normal precipitation shown in the Northeast US for the bulk of the month of May.

Below normal precipitation shown in the Northeast US for the bulk of the month of May.

Surface analysis of high pressure, low pressure, fronts and areas of precipitation for May 4th. Notice the position of the stationary front along the Eastern Seaboard.
Surface analysis for the morning of May 4th. Notice the position of the stationary front along the Eastern Seaboard.
Precipitation change from wet to dry in the Northeast for the first to last half of the month.
Precipitation change from wet to dry in the Northeast for the first to last half of the month.

 

This month featured a wet first half for Boston, MA as a general storm track (shown by the stationary front) went from the southwest to northeast along the Eastern Seaboard. This allowed for strong coastal storms to develop. This was particularly true on the 2nd and 4th when precipitation totals of nearly 0.40 inches and 0.50 inches respectively were observed in Boston. Thereafter, precipitation seemed to drop off with only 2 days of precipitation over 0.10 inch of rain (the 13th and 24th). The main reason appears to be that the general storm track was shifted offshore (shown by the midlevel lift or omega and a general suppressed pattern over the eastern half of the United States). Boston saw 1.70 inches during May whereas the normal rainfall from May 1 to May 26 is closer to 2.90 inches. Further addressing this point of the suppression of precipitation around the Boston area and north into Northern New England for that matter, the drought index disseminated by the US Drought Monitor shows an increase in the lowest category of drought from the beginning of May to the latter half of May.

Temperature for Boston for the month of May with normal temperatures and historical above and below normal temperature extremes shown.
Temperature for Boston for the month of May with normal temperatures and historical above and below normal temperature extremes shown.
normal to above normal temperatures shown in the Northeast US for the bulk of the month of May.
Normal to above normal temperatures shown in the Northeast US for the bulk of the month of May.
Shifts in the general area of low pressure systems from the first of the month to the latter half of the month. Notice the high pressure shift to the low pressure near the Northeast US.
Shifts in the general area of low pressure systems from the first of the month to the latter half of the month. Notice the high pressure shift to the low pressure near the Northeast US.

 

Shifts in the positions of lift from the first half of the month to the latter half of the month. Notice how it went from more lift to less lift over the Northeast US allowing for less precipitation over the latter half of the month.
Shifts in the positions of lift from the first half of the month to the latter half of the month. Notice how it went from more lift to less lift over the Northeast US allowing for less precipitation over the latter half of the month.

 

As far as temperature is concerned, many New Englanders had the general consensus that summer or even spring hasn’t gotten here yet and that it has been cooler than normal so far. Believe it or not, the temperature in Boston has been normal to above normal for the bulk of the month. Boston seems to have a lot more normal temperatures whereas a place further inland like Lawrence, MA seems to have a lot more above normal temperatures so sea breezes or onshore winds may have had a hand in keeping the those near the coast a little cooler for the latter half of the month.

Pacific North America teleconnection changed from a positive phase to a negative phase allowing a temperature and precipitation shift.
Pacific North America teleconnection changed from a positive phase to a negative phase allowing a temperature and precipitation shift.

 

The change during the month of May that may have allowed for the drastic change may have been the change to more of an offshore high pressure system that allows for warmer temperatures and suppressed large scale weather or just scattered convection in the form of thunderstorms. Dry air also seemed to win out in some situations allowing evaporation of rainfall before it touched the ground. This is actually shown in a teleconnection pattern that is encompassed with the Pacific North American so that the Northwestern US high pressure system and Southeastern US low pressure system give way to a Northwestern US low pressure system and general Southeastern US high pressure system. That’s a change from a positive phase to a negative phase.

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