Multiple low pressure systems resulted in widespread precipitation (0.5 to 3 inches, or more) from the Mississippi Valley to the East Coast during late March. Heavy to excessive rainfall (more than 5 inches) soaked southeast Louisiana and triggered flooding across Tennessee this past week. Along with the flooding, a severe weather outbreak affected Alabama, Georgia, Tennessee, and Kentucky on March 25. Mostly dry weather persisted farther to the south, including the Florida Peninsula and south Texas. A cold front brought strong winds to the northern Plains on March 29 but little or no precipitation. Rain and high-elevation snow was limited to coastal Washington and the northern Cascades across the Pacific Northwest, while mostly dry weather prevailed throughout California. 7-day temperatures from March 23-29 averaged below (above) normal across the western (eastern) U.S.
According to AHPS, 7-day precipitation amounts across Puerto Rico averaged at or above normal from March 23 to 29. Due to heavier amounts across inland areas, a slight decrease in D0 (abnormal dryness) and D1 (moderate drought) was made to parts of Puerto Rico.
The weather conditions across the U.S. Virgin Islands (USVI) during this USDM week (03/24/21-03/30/21) reflected contrasting forces. An upper-level trough moved through the territory during part of the week, but high pressure ridging, dry air, and stable conditions dominated the upper-level atmosphere for the most part. Patches of low-level moisture moved across the islands at times in the trade-wind flow. The result was scattered showers with limited precipitation totals.
Radar-based estimates of rainfall (QPE) for the 7 days ending at 12z on Tuesday showed areas with half an inch or more of precipitation over parts of the islands, with other parts receiving less than a tenth of an inch. USGS well measurements revealed a continuing steady decline in groundwater levels on all three islands. Due to the dry weather conditions, the Virgin Islands Fire Service has issued a ban on burning across the territory, effective March 15.
Weekly rainfall reports on St. Croix ranged from 0.05 inch at the CoCoRaHS station at Christiansted 4.1 ESE (based on 3 days of data) to 0.45 inch at Christiansted 1.6 E. The East Hill station recorded 0.06 inch and 0.30 inch fell at the Rohlsen AP. Monthly totals for March (through the 30th) ranged from 0.53 inch at Rohlsen AP to 1.69 inches at Frederiksted 0.5 N. For those stations with normals, the month was drier than normal at Rohlsen AP (35% of normal) and East Hill (56%). With Standardized Precipitation Index (SPI) values at Rohlsen AP and East Hill in the D0-D1 range for the last 1 to 12 months, D1-SL continued on St. Croix.
On St. Thomas, weekly rainfall totals ranged from 0.02 inch at Charlotte Amalie 1.4 NE (based on 4 days of data) to 0.30 inch at King AP. March monthly totals (through the 30th) ranged from 0.32 inch at Anna's Retreat 2.5 ESE and Charlotte Amalie 5.1 E to 0.83 inch at Charlotte Amalie 1.4 NE. King AP recorded 0.61 inch for the month so far, which is 45% of normal. Reports of cracked ground have been received from the Bordeaux area. Severe drought continued on St. Thomas, but the status was changed from D2-S to D2-SL to reflect mounting dryness at the 12-month SPI time scale.
On St. John, the station at Windswept Beach reported 0.38 inch of rain this week, bringing the March total (through the 29th) to 0.94 inch. The long-term average for March is 1.73 inches, which means March 2021 is about 56% of average. The CoCoRaHS station at Cruz Bay 1.6 E reported 0.46 inch for the week and 0.90 inch for the month. The CoCoRaHS station at Cruz Bay 0.8 NE reported 0.35 inch for the week (with 6 days missing) and 1.14 inches for the month. With moderate and severe drought indicated by the Windswept Beach SPI at 1- to 3-month time scales, D2-S continued on St. John.
Another dry week along with strong winds on March 29 resulted in a slight expansion of extreme drought (D3) across North Dakota and northern South Dakota. This expanding D3 area was based mostly on SPI at EDDI at various time scales. Soil moisture remains below the 5th percentile for much of North Dakota. Based on snow water content running near average and 6 to 12 month SPIs, D3 was improved by one category across north-central Wyoming. Following multiple changes during the previous two weeks across Colorado, only minor improvements were needed this week. Localized improvements were made to small areas of southern Colorado, based in part on WYTD (since Oct 1, 2020) precipitation.
Widespread precipitation (1 to 2.5 inches) fell throughout much of the central and eastern Corn Belt. Abnormal dryness (D0) and moderate drought (D1) were decreased in spatial coverage across Illinois and Indiana. Despite this rainfall during late March, soil moisture (below the 20th percentile) and SPIs at various time scales support maintaining D1 across parts of northern Indiana, northern Ohio, and lower Michigan. Northeast Illinois and eastern Indiana missed out on the heavier rainfall during late March, prompting a slight expansion of D0 for these areas. Also, D1 was added to northeast Ohio based on increasing 90-day precipitation deficits, 90-day SPIs, and soil moisture below the 20th percentile. A 1-category improvement was made to eastern Minnesota and western Wisconsin where more than 1.5 inches of rainfall occurred this past week. 1 to 2 inches of rainfall resulted in a reduction in D0-D2 across central Iowa. However, SPIs dating back 9 months continue to support D2-D3 long-term drought for northwest Iowa.
Mostly dry weather (less than 0.5 inch) prevailed across northwest Pennsylvania where abnormal dryness (D0) was expanded. Based on 60 to 90-day SPIs, low 28-day average streamflows, and soil moisture below the 20th percentile, moderate drought (D1) was added to parts of northwest Pennsylvania and adjacent areas of western New York. Increasing short-term precipitation deficits along with a number of low 28-day average streamflows support the addition of D0 across western Maryland and parts of West Virginia. Despite the 1 to 1.5 inches of rainfall this past week across southern New England, lower streamflows supported an expansion of D0 across parts of Massachusetts and southern New Hampshire.
Abnormal dryness (D0) continues for northern and eastern Mainland Alaska with no changes this week. The depicted D0 will be reassessed next week as current snowfall data becomes updated.
Based on rainfall during mid to late March, a 1-category improvement was made on Maui, Molokai, Lanai, and Kahoolawe. Therefore, Hawaii is currently drought free with only lingering abnormal dryness (D0). Reports from Hawaii indicate a greening of vegetation on Maui but abnormal dryness continues on the northwest side of the Big Island.
The weather features across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (03/24/21-03/30/21) included a persistent near-equatorial trough across Palau and western to southern Yap State, accompanied at times by weak circulations; converging trade winds that formed an Inter-Tropical Convergence Zone (ITCZ) over eastern Micronesia with embedded surface troughs; and seasonally dry trade winds over northern portions of Micronesia, especially over the Marianas. The surface convergence was coupled at times with upper-level divergence. South of the equator, instability in the upper levels of the atmosphere combined with moist surface air to generate showers over American Samoa.
Satellite-based estimates of 7-day precipitation (QPE) showed a band of precipitation stretching eastward across Micronesia (reflecting the surface troughs and ITCZ) and patches of precipitation over the Samoan Islands. Rainfall totals of 2+ inches were widespread from the equator to 10 degrees North latitude over western Micronesia and between 3 N and 8 N over eastern Micronesia, with embedded 4+ inches of rain. Little to no rain was indicated across the Marianas. (It should be noted that the satellite QPE is designed to detect precipitation from showers produced by cold high-topped clouds, whereas showers from diurnal island convection and weak systems are typically produced by warmer clouds with limited vertical development that go undetected by this satellite tool.) The satellite QPE detected a patchwork of 1+ inches of rain across the Samoan Island region with lesser amounts in between the 1+-inch patches.
This week continued the dry (below the 1-inch weekly minimum needed to meet most water needs) streak on Guam (0.18 inch of precipitation recorded) and the Commonwealth of the Northern Mariana Islands. Less than half an inch of rain was reported at Rota (0.02), Saipan NPS (0.34), Saipan IAP (0.49), and Saipan ASOS (0.85). Likewise, with the exception of Dededo (where 4.08 inches was recorded), March has been dry at Guam, Rota, and the stations on Saipan. D0-S continued for Guam and Rota and D1-s continued for Saipan.
In the Federated States of Micronesia (FSM), it was a dry week (less than the 2-inch weekly minimum needed to meet most water needs) at Lukunor (1.66 inches with 2 days missing), but wet at the rest of the analyzed stations. Rainfall totals for the wet stations ranged from 2.33 inches at Yap to 10.77 inches at Pohnpei. Fananu received 2.64 inches of rain this week (with 3 days missing) and 11.90 inches for March so far (both of these values are above the weekly and monthly minimums). This is the fourth consecutive wet week, and the island has recovered from the earlier drought impacts, so the status was improved from D0-S to No Drought or Abnormal Dryness.
At Kapingamarangi, 2.50 inches of rain was recorded this week and 8.91 inches for March through the 30th (likewise, both of these values are above the weekly and monthly minimums). This is the third consecutive wet week, water levels are reported to be 100%, and vegetation/crops are beginning to recover. The last 12 through 7 months (April-March through August-March) are still the driest on record, but the precipitation ranks for the last 4 months have improved. March (to date) ranks as the 8th driest March (compared to complete Marches) out of 31 years of record, which equates to the 26th percentile or D0; February-March ranks 5th driest out of 31 years (16th percentile, or D1), January-March ranks 3rd driest out of 27 years (11th percentile, or D1), and December-March ranks 3rd driest out of 25 years (12th percentile, D1). Considering the recent wetness and improving conditions, the status at Kapingamarangi was changed from D3-SL to D2-L.
With the previous 2 weeks wet, D-Nothing continued at Lukunor, and D-Nothing continued at the rest of the FSM stations where both this week and the month have been wet.
In the Marshall Islands (RMI), no data was received from Utirik this week and in recent months so an analysis could not be made there. The week was wet (above the 2-inch weekly minimum) at Mili (2.50 inches), Majuro (4.06) and Ailinglaplap (4.78), but dry at Wotje (0.36), Kwajalein (0.73), and Jaluit (1.22). Only Wotje (with 0.81 inch) and Kwajalein (6.21 inches) were dry (below the 8-inch monthly minimum) for the month, so D3-S continued at Wotje and D1-S continued for Kwajalein. The other stations were wet for March (through the 30th), with totals ranging from 15.00 inches at Majuro to 24.05 inches at Ailinglaplap, so D-Nothing continued.
Weekly rainfall totals were above the minimum needed to meet most water needs at Palau IAP (2.61 inches) and Koror COOP (2.15) in the Republic of Palau, and at Pago Pago (2.53) and the automated stations at Siufaga Ridge (1.75) and Toa Ridge (1.16) in American Samoa. Likewise, the monthly totals were above monthly minimums, so D-Nothing continued at Palau and Tutuila.
On March 23 and 24, heavy rainfall (more than 5 inches) resulted in a 2-category improvement to southeast Louisiana where soil moisture is currently above the 70th percentile. However, much less rain fell across northern Louisiana where an expansion of D0 (abnormal dryness) and D1 (moderate drought) was made. The recent heavy rainfall also led to elimination of abnormal dryness (D0) across the Mississippi Gulf Coast. Heavy rainfall triggered flooding across much of Tennessee during late March and the small areas of D0 were removed from the state. Additional improvements from the previous two weeks continued across the Texas Panhandle, while mostly dry weather and increasing precipitation deficits along with worsening soil moisture conditions resulted in expansion of D1 (short-term moderate drought) across parts of southeast Texas. Soil moisture rapidly declines from east to west across Texas.
A second severe weather outbreak during March occurred across Alabama and Georgia on the 25th, with numerous tornado reports and locally heavy rainfall. A 1-category improvement was made to areas that received more than 1.5 inches of rainfall including southern Alabama, the western Florida Panhandle, and parts of northern Georgia. D0 (abnormal dryness) was maintained for parts of eastern Alabama and northern Georgia where rainfall was lighter this past week, and based on 60-day SPIs. Increasing short-term precipitation deficits and low soil moisture conditions support the addition of D0 across the Shenandoah Valley in Virginia, extending northward to southern Pennsylvania. D1 (short-term moderate drought) was introduced to southern Palm Beach County in south Florida, based on 60-day SPI and soil moisture. The D1 areas currently designated for south Florida are consistent with the latest Keetch-Byram Drought Index. The D0 in central Florida was expanded slightly westward this week.
Widespread extreme (D3) to exceptional (D4) drought continues to be focused across the Southwest. According to the USDA (for the week ending March 28), nearly two-thirds of the winter wheat is rated as very poor in New Mexico. As of March 27, snow water content is running near 70 percent of average for the Sierra Nevada Mountains. Due to this snow water content along with WYTD (since Oct 1, 2020) precipitation deficits, long-term drought persists for nearly all of California. Since precipitation averaged below normal during March, the impact type was adjusted to include both long and short-term drought for California, along with parts of the Southwest. Based on SPIs at various time scales, D1 (moderate drought) was slightly expanded across north-central Montana while below average snow water content prompted an increase in abnormal dryness (D0) for northwest Montana. To the west across eastern Washington, below average precipitation during March and drying topsoil prompted an expansion of D0 and D1 (short-term moderate drought).
During the next 5 days (April 1 to 5), much drier weather is forecast for the eastern and central U.S. in the wake of a cold front. Mostly dry weather is also expected for the western U.S. with light precipitation limited to the Pacific Northwest and northern California. A brief period of below normal temperatures are forecast for the eastern U.S. and Gulf Coast States. A freeze may affect areas as far south as the Tennessee Valley and southern Appalachians from April 1 to 3. Meanwhile, a rapid warming trend is likely over the northern and central Great Plains with much above normal temperatures forecast during the first week of April.
The CPC 6-10 day extended range outlook (valid from April 6 to 10) favors above normal temperatures across the central and eastern U.S. with the largest probabilities centered over the central and southern Great Plains. Below normal temperatures are most likely along the West Coast and throughout Alaska. Probabilities of below normal precipitation are elevated from the Southeast westward to the southern Great Plains and southern Rockies. Near to above normal precipitation is favored for the Corn Belt, northern Great Plains, much of the western U.S., and Alaska.
Richard Heim, NOAA, NCEI
Brad Pugh, NOAA, NCEI