The weather conditions across the U.S. Virgin Islands (USVI) during this USDM week (5/20/20-5/26/20) were dominated by a mid- to upper-level ridge which brought stable and dry air and warm temperatures. Rohlsen AP on St. Croix had maximum temperatures in the 90s F for most of the week and tied a record with 91 on May 20. High pressure, which persisted over the central to eastern Atlantic, fed easterly trade winds across the USVI. Pockets of moisture were embedded within the trade-wind flow, but the upper-level ridge inhibited convection. A weak layer of Saharan dust helped keep the weather dry.
Radar-based estimates of rainfall for the 7 days ending at 12z on Tuesday depicted little to no precipitation across the islands. Up to a fourth of an inch was indicated for parts of St. Thomas, with a tenth of an inch to no rainfall indicated for the rest of the islands.
Due to the very dry weather conditions, the Virgin Islands Fire Service (VIFS) issued a ban on burning. Effective Friday, May 22, all outdoor burning was temporarily prohibited in the territory.
On St. Croix, 0.02 inch of rain was reported for the week at Rohlsen AP, East Hill, and the CoCoRaHS stations at Christiansted 1.8 ESE and 4.1 ESE, although the CoCoRaHS station at Christiansted 1.8 ESE was missing several days. The station at the UVI Sheep Farm reported 0.06 inch for the week. Rohlsen AP has measured 0.54 inch of rain so far this month, giving May 2020 a rank of ninth driest. The normal May precipitation at Rohlsen AP is 4.15 inches. April 1-May 26 has had 1.15 inches, ranking fourth driest out of 61 years, equating to a percentile of 0.07 which is at the D2 USDM level. Year-to-date precipitation is at 78% of normal. East Hill has reported 1.01 inches of rain for the month so far, which is about 31% of normal, and 2.08 inches for April 1-May 26, which is about 38% of normal. May monthly totals (based on data received so far) included 0.34 inch at Christiansted 4.1 ESE, 0.56 inch at Christiansted 1.8 ESE, and 0.68 inch at the UVI Sheep Farm. The Standardized Precipitation Index (SPI) values for the Rohlsen/Hamilton Airport and East Hill stations ranged from D0 to D3 levels. East Hill SPI is at D0 levels for the last 1, 9, and 12 months, but the Rohlsen AP SPI values are at the D1 level for the last 1 and 12 months, the D0 level at the 6-month time scale, and D3 level for the last 9 months. Current groundwater data is not available for the USGS Adventure 28 well; the latest data is through April 25, 2020, but it was in steady decline for a month to that point. The USDM status was worsened to D2-SL for St. Croix.
On St. Thomas, King AP had no measurable rain while the CoCoRaHS station at Anna’s Retreat reported 0.02 inch. The May monthly totals are a trace for King AP and 0.63 inch for Anna’s Retreat. The May monthly normal for King AP is 3.06 inches. With 0.92 inch in April, this ranks April 1-May 26, 2020 at the airport as the third driest such period out of 57 years of data. This translates to the 5th percentile in the historical record, which is at the D3 USDM level. The year-to-date precipitation at King AP was about 80% of normal. The SPI at King AP was at the D0 level for the last 9 months, but dropped to the D1 level for the last 3 months and continued at the D3 level for the last month. The USGS Grade School 3 well level has been steadily declining since the end of March, with the decline accelerating precipitously in the last week, and is now the lowest it's been in the last 12 months. The USDM status was worsened to D2-S for St. Thomas.
On St. John, the station at Windswept Beach reported 0.06 inch of rain this week through May 25, bringing the May total to 1.20 inches. The long-term average for May is 4.54 inches. The SPI continued at the D1 level for the last 3 and 9 months and worsened to the D1 level for the last month. The USGS Susannaberg well level has been steadily declining since the end of January, and is now the lowest it’s been since August 2019. We have no USDM impacts table for the USVI (https://droughtmonitor.unl.edu/Data/StateImpacts.aspx), but nearby Puerto Rico is included in the impacts table, and D2 includes “Trees and plants show stress”. Photos provided by the Windswept Beach observer showed widespread stress of trees across St. John (Mary’s Point), including a stressed Maran bush which is a very drought-tolerant species. As a consequence of the impacts, precipitation, and groundwater data, the USDM status for St. John was worsened to D2-S.