This Week's Drought Summary (3/19)
The U.S. Drought Monitor week ending March 17 saw another round of winter storms, bringing above normal precipitation to parts of the northern High Plains, Southwest, southern plains, and Tennessee Valley. Many areas recorded totals that exceeded 200% of normal over the seven-day period, leading to improvements to areas of abnormal dryness and drought in areas where the excess moisture erased deficits and improved soil moisture and streamflow. Once again, precipitation over the Northwest and Gulf Coast states was below normal with most areas having received less than 50% of their normal amount over the last 30 days. The lack of precipitation, combined with warmer than normal temperatures, led to expansions in pockets of abnormal dryness and drought.
While much of the northeast received precipitation last week, totals in southern and coastal New England continued to be below normal, further contributing to deficits of 6-plus inches over the last 90 days. The lack of rain and snow combined with unseasonable warmth – with many stations reporting mean temperatures in the top five on record over the last month, low streamflow values, and low groundwater levels -- led to the introduction of a swath of D0 (abnormal dryness) from eastern Massachusetts southward to northern New Jersey. According to local media reports, the dry conditions combined with high winds contributed to brush fires in eastern Massachusetts, Rhode Island, and southeast New York.
The general pattern of rain in the northern tier of the region and dry weather in the south continued again last week. Much of the Tennessee Valley and Southern Appalachia received amounts in the range of 0.5 to 3 inches, while regions near the Gulf coast had little to no rainfall. Changes to this week’s map include small expansions of D0 (abnormal dryness) in southern Alabama, the extreme western Florida Panhandle, and the central and southern Florida Peninsula as rainfall shortages in these have reached about 3 to 4 inches over the last 30 days, drying soils and lowering streamflow. Should they miss out on next week’s forecast rainfall, areas to watch for deteriorating conditions include Virginia and northern North Carolina where moisture deficits are beginning to build.
Last week, a band of heavy rainfall fell across the northern half of the region, extending from West Texas to western Tennessee with amounts ranging from 1 to more than 4 inches (equivalent to more than 300% of normal in some locations). In southwest Oklahoma and northeast Texas, the excess moisture erased short-term precipitation deficits and recharged streamflow leading to reductions in D0 (abnormal dryness) and D1 (moderate drought). Additionally, the "S" was removed from the "SL" drought designation to indicate that drought and dry conditions are now only present at timescales longer than six months. With over an inch of rain falling after the close of the Drought Monitor week (Tuesday, 8:00 AM EDT) and more expected on the way, additional reductions may take place on next week’s map. Other areas seeing improvements include West Texas with reductions to D0 and D1. Unfortunately, the rain missed the parts of south Texas that need it most and conditions continued to deteriorate, resulting in expansions to ongoing areas of abnormal dryness and drought and the introduction of D4 (exceptional drought). Supporting data include rainfall deficits of 2 to 8 inches (25 to 50% of normal) over the last six months combined with mean temperatures consistently ranking in the top 10 warmest over the same time interval. The combination of dry weather and high temperatures has dried out soils and stressed vegetation with USDA reporting only 28% of topsoil as adequate for crops in the southeast and 3% in the southwest. Other areas seeing deterioration this week include southwest Louisiana and southeast Mississippi with expansions in D0.
The Midwest remains free of any drought or abnormal dryness. While Missouri and Kentucky saw precipitation totals in excess of 150% of normal, the rest of the region received amounts near or slightly below normal continuing a drying trend that began in February. In general, the relatively dry weather has been welcome, enabling producers to begin field work. Soil moisture and streamflow levels remain high from 2019's record-breaking precipitation and a wet January.
The map's drought depiction is unchanged this week in the High Plains. A winter storm during March 13-14 brought snow to the west and central parts of the region and rain to locations in the south and east. The Black Hills saw the highest totals, reporting from 6 to 12 inches of snow while portions of western and central South Dakota and Nebraska reported several inches of accumulation. Dry conditions continue to persist in the drought and abnormally dry areas in eastern Colorado, western Kansas, and southwest Nebraska where less than 0.50 inches of precipitation (about 50% of normal or less) has fallen so far this month. As we transition to normally wetter conditions in the spring, hopefully this area will begin to see relief from the deficits that have built over the last six to 12 months.
February's dry spell over California finally broke as a late winter storm brought heavy showers to southern California and over 2 feet of snow to the Sierra Nevada. In southern California, the excess rainfall improved soil moisture and streamflow levels leading to reductions in areas designated as D0 (abnormally dry) or D1 (moderate drought). Despite the rain and snow, the maps depiction remained unchanged for the majority of the northern two-thirds of the state. Water year-to-date precipitation is more than 12 inches below normal (50% of normal or less) in the Sierras and the north coastal and north central regions. Soil moisture and streamflow values remain low and satellite based indicators of vegetation health continue to show stress across the Central Valley. Extreme northern California and southern Oregon missed out on the heavy precipitation further deteriorating drought conditions and leading to the expansion of D1 and introduction of D2 (severe drought). The Oregon state drought coordination team noted increasing water supply concerns in this region as many locations show record low streamflow values, declines in groundwater, and low reservoirs. Other changes in Oregon include minor improvements D1 areas in the west-central and eastern parts of the state where heavy precipitation fell. Having missed out on last week’s precipitation, Nevada and Utah both saw and expansion of D0 in the north. Further south, rainfall of 1 to 3 inches helped erase precipitation deficits, replenish soil moisture, and improve streamflow in southern Nevada, southwest Utah, and northwest Arizona resulting in reductions in D0, D1, and D2. Drought depictions in Washington, Idaho, Montana, Colorado, and New Mexico were left status quo.
Alaska, Hawaii, and Puerto Rico
After last week's improvements, Alaska remains free of abnormal dryness and drought. Above normal precipitation and snowpack has left the state in good shape heading into the drier summer months. In Hawaii, heavy rain events in progress at the close of the Drought Monitor week brought widespread flash flooding across the entire state and set a daily rainfall record in Honolulu, prompting the removal of the D0 (abnormal dryness) introduced on Oahu last week. With rain continuing the remainder of the week, additional improvements may be warranted on next week's map. Near to above normal precipitation continued this week across the majority of Puerto Rico. After last week’s removal of D0, the island remains free of any dryness on the map.
The weather conditions across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (3/11/20-3/17/20) consisted of a dry subtropical trade-wind regime which dominated western Micronesia and the northern Marshall Islands (RMI) contrasted against convection associated with the Inter-Tropical Convergence Zone (ITCZ) across southern portions of Micronesia. This is a typical synoptic pattern for this time of year when the North Pacific subtropical high dominates in the north and the ITCZ stays near the equator. In fact, March is the driest month of the year for the Marianas, Yap, and the Marshall Islands beneath the dominance of the North Pacific High. This week, the ITCZ was manifested by surface troughs, trade-wind convergence, and a couple disturbances and weak circulations. One of the circulations brought rain to the very dry regions of Chuuk State right at the end of the week. South of the equator, a ridge dominated the weather over American Samoa while a surface trough stayed to the south. On the last day of the week, remnants of a tropical system spread some limited moisture across the islands.
Satellite-based estimates of 7-day precipitation (QPE) showed areas of 2+ inches of rain across southern portions of the Federated States of Micronesia (FSM) and the RMI, with a band of 2+ inches stretching northward across Chuuk State. The QPE gradient dropped off sharply towards the north and west where very little to no rain was depicted. A similar pattern was evident in the Southern Hemisphere with 2+ inches of rain indicated south of the Samoan Islands and very little over American Samoa.
In the Republic of Palau, the last 2 weeks have been dry (less than the 2-inch weekly minimum needed to meet most water needs) at Koror, with 0.09 inch recorded this week and only 1.50 inches of rain so far this month. But with February wet and 3 weeks ago wet, D0-S continued for Palau.
Dry conditions continued in the Marianas, with rainfall totals well under the weekly minimum (1 inch). Dededo and Tinian received about a third to half of an inch of rain this week, but a tenth of an inch or less was observed at Guam (0.10), Saipan (0.06), and Rota (0.06). Conditions were variable across Guam and Saipan, with vegetation quite dry and cracks forming in the clay in the southern parts of Guam, but vegetation was still green in central areas; conditions were similar on Saipan. Many fires have developed on Guam in the southern mountains, and the Keetch Byram Drought Index (KBDI) reached 684 by March 17, which is in the high fire danger category. D2-S continued on Saipan and D1-S continued on Guam. With the last six consecutive weeks dry, D0-S was worsened to D1-S on Rota.
Federated States of Micronesia:
In the FSM, Fananu has been missing for several months, so this station could not be analyzed. Some eastern and southern stations had a wet week (more than the 2-inch weekly minimum), with 4.06 inches of rain reported at Kosrae, 5.56 inches at Kapingamarangi, 3.50 inches at Chuuk, 2.58 inches at Pohnpei, 2.63 inches at Lukonor, and 2.00 inches at Pingelap. It was a dry week at the rest of the stations, with rainfall reports ranging from 0.73 inch at Woleai to 0.01 inch at Ulithi and no measurable rainfall at Yap and Nukuoro as of March 17. With a wet week and wet February, D-Nothing continued at Kosrae and Pohnpei. Reports of low water levels in the tanks prompted the introduction of D0-S at Pingelap. With measureable rain falling on 6 of the 7 days this week, the 5.56 inches of rain received at Kapingamarangi this week raised the monthly total to 6.46 inches and changed the D0-S status to D-Nothing. Woleai has had the seventh driest March (based on data received so far in the month) and fifth driest January-March; these translate to the 15th to 19th percentiles, which rank in the D1 category, so D1-S continued for Woleai. With the last 3 consecutive weeks very dry, and only 0.42 inch of rain received so far this month, the status was worsened to D1-S at Ulithi. At Yap, the last 11 consecutive weeks have been dry, and both January-February and January-March ranked as the fourth driest, so the status was worsened to D2-S.
In Chuuk State, reports have been received from municipalities in the Northwest and in the Mortlocks of rain water catchments becoming exhausted and deep wells running low or drying up or having brackish water. Crops and vegetation in the Mortlocks (PiisEmwar and Namoluk) are suffering – some coconuts do not have juice, some food crops and vegetation have leaves turning yellow, and taro patch leaves are turning yellow. A weak circulation and associated surface trough moved over Chuuk State right at the end of this USDM week, dropping rain which helped to refill water catchments, but the agricultural impacts continued. Over 3 inches of rain was reported for the week at Chuuk (3.50 inches) and over 2 inches was measured at Lukonor (2.63). As of March 17, the date of this week’s USDM, no rain was observed at Nukuoro for the week, but heavy rains were on the horizon and expected in the next 24 hours. Since the USDM depicts drought conditions as of the date of the report, those upcoming rains could not be considered for the depiction at Nukuoro for this week. The previous 5 weeks were dry at Chuuk, and Nukuoro had the third driest February-March and second driest January-March (through March 17). Even though Lukonor had 2.63 inches of rain this week, March (3.77 inches) is still below monthly minimums (for this point in the month), February-March 2020 ranked as the third driest February-March, and January-March ranked second driest on record. The status at Nukuoro was worsened to D1-S, Lukonor was kept at D1-S, and Chuuk was kept at D0-S – to reflect the lingering agricultural impacts.
Southern stations in the Marshall Islands had a wet week (above weekly minimums), while it continued dry in the north. With 1 or 2 days missing at some stations, more than 2 inches of rain was reported at Majuro (3.49), Mili (2.56), Ailinglapalap (2.05), and Jaluit (2.06). The rain raised Majuro’s reservoir level, but (at last report) it was still below the 80% threshold for concern. And, in spite of this week’s rain, March was still mostly below monthly minimums (for halfway through), so D0-S continued for Jaluit, Majuro, Ailinglapalap, and Mili. Less than an inch of rain fell at Kwajalein (0.67). With month-to-date totals continuing dry, and the previous 3 months dry, D1-S returned to Kwajalein. Utirik had two-thirds of an inch of rain this week, while Wotje had none. March-to-date totaled only 1.31 inches at Utirik, so D2-S continued. According to reports from Wotje, water tanks were almost empty and vegetation was starting to turn brown. Based on these reports, and no measureable rain at all for March so far, the USDM status at Wotje was worsened to D3-S.
In American Samoa, Pago Pago reported over an inch of rain (1.24), while one to two tenths of an inch was measured at the automated stations at Siufaga Ridge (0.12) and Toa Ridge (0.17). With the previous week wet and February very wet, D-Nothing continued at Tutuila.
Tropical storms and hurricanes are a main source of rain for the U.S. Virgin Islands (USVI). With the North Atlantic hurricane season running from June through November, the winter half of the year is the dry season. In fact, March is the driest month of the year. The weather pattern across the USVI during this USDM week (3/11/20-3/17/20) consisted of mid- to upper-level troughs which created instability that gave rise to showers. Moisture was provided by an old frontal boundary at the beginning of the week, with northeast trades bringing moisture into the region later in the week. These troughs were interrupted by a drier air mass at mid-week. The showers were mostly light.
Radar-based estimates of rainfall for the 7 days ending at 12z on Tuesday depicted areas of half an inch of rain over the northern islands with less than a half inch over the southern islands.
The Hamilton/Henry Rohlsen AP had half an inch (0.50) of rain this week (through March 17), while the Christiansted 1.8 ESE CoCoRaH