Tools used in the U.S. Seasonal Drought Outlook (SDO) included the official Climate Prediction Center (CPC) temperature and precipitation outlooks for January 2018 through March 2018 (JFM 2018), various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) totals from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC extended-range forecasts (ERFs), Weeks 3-4, dynamical models at the seasonal time scale, 384-hour total precipitation forecasts from several runs of the GFS, 240-hour total precipitation forecasts from the ECMWF, climatology for the JFM season including median soil moisture changes, initial conditions (the U.S. Drought Monitor valid on December 19, 2017), and canonical climate anomalies associated with La Nina conditions.

Over the past month, drought coverage - D1 and above as depicted on the U.S. Drought Monitor - nearly doubled across the Western Climate Region, increasing from 11.4 percent to 21.2 percent, with most of the expansion occurring over Arizona, Utah, and western New Mexico. The increase in drought coverage is due to persistent ridging over the West, which has promoted both dryness and much above normal temperatures. The wet season has gotten off to a slow start across southern California, but more beneficial rainfall and mountain snows have fallen across the Northwest and northern California. Overall, reservoir levels remain in good shape, and soil moisture is adequate across California and Nevada, so drought expansion in these states has been more limited than across the Four Corners region. The canonical atmospheric response to La Nina conditions favors a northward shift in the favored location of the midlatitude jet over the West, bringing above-normal precipitation to the Northwest, and below-normal precipitation across the Southwest. This pattern is reflected in the CPC seasonal outlook, and is broadly consistent with dynamical model guidance at the seasonal time scale. There is increased uncertainty at the subseasonal time scale, however, due to an active MJO pattern, which may briefly destructively interfere with the La Nina pattern and promote an atmospheric river event during January. The ECMWF guidance during the Week 3-4 period bears this possiblity out. Therefore, drought development is favored across a broad area of the Southwest, but development is not anticipated north of the San Francisco Bay area, where incipient water supply is in better shape and there is a greater potential for precipitation during the period. The potential for subseasonal pattern changes yields a lower confidence in this outlook across California and Nevada, despite the ongoing La Nina conditions. The potential for enhanced precipitation along the northern tier favors some amelioration of short term drought impacts across western Montana, with persistence favored across eastern Montana due to a drier winter climatology.

Forecast confidence for the Western Region is low to moderate.

Little change to the broad drought area across the northern Plains occurred over the past four weeks, but drought has expanded and intensified across Kansas due to dry conditions and much above normal temperatures. With frozen ground and a dry climatology during the winter, little additional change to the drought conditions is anticipated for the northern Plains. Therefore, persistence is favored for the Dakotas and western Nebraska. The CPC seasonal outlook favors below-normal precipitation across the central High Plains during the outlook period, thus further drought development or intensification is possible across southern Kansas.

Forecast confidence for the High Plains Region is moderate to high.

Substantial drought expansion and intensification has occurred during the past month across eastern Texas, eastern Oklahoma, and the lower Mississippi River Valley, with some stations across Oklahoma and Texas recording zero precipitation or just trace amounts. Climate anomalies associated with La Nina vary meridionally in this region, with suppressed precipitation favored across southern areas, while the lower and middle Mississippi Valleys sit on the southwestern edge of the favored Ohio River storm track. Recent heavy precipitation across eastern Texas and Oklahoma, Arkansas, Louisiana, and northern Mississippi likely provided some relief to short term drought conditions, and this area is favored for additional locally heavy rainfall during the next two weeks. Despite the dry signal at the seasonal time scale, JFM is a climatologically wet time of year for the lower Mississippi Valley region, and even below-normal precipitation may be sufficient to keep impacts at bay. Therefore, based on forecasts for wet conditions through the end of December and the wet climatology, drought improvement is favored for far northeastern Texas, northern Louisiana, central and northern Mississippi, and Arkansas. In contrast, drought development is possible along the Gulf Coast, where the La Nina signal is strongest.

Forecast confidence for the Southern Region is low.

Dry, unseasonably warm conditions plagued the Midwest Climate Region during the past few weeks, promoting the expansion of abnormal dryness and moderate drought across Missouri, Illinois, and Iowa. Drought impacts often lag the meteorological dryness, so further drought expansion due to incipient conditions in this region is possible, despite any short term precipitation. An enhanced precipitation climate signal associated with the La Nina favored storm track across the Ohio Valley region may yield improvements to southeastern Missouri, but peristence is favored elsewhere due to actively expanding impacts and the lack of opportunity for any precipitation that falls to substantially reduce impacts once the ground is frozen during the winter months.

Forecast confidence for the Midwestern Region is moderate.

Outside of a stripe of heavy rainfall over the Nature Coast of Florida, below-normal rainfall promoted drought expansion across the Southeast during the past month. The WPC 7-day QPF forecast shows widespread rainfall in association with a slow moving frontal system extending across much of Alabama and northern Georgia, with lesser amounts forecast for the Carolinas and Virginia. This rainfall, coupled with additional rains anticipated towards the end of December, may bring relief to moderate drought areas of western Alabama. In contrast, mostly dry conditions are favored to persist across Florida in the short term. La Nina composites and the CPC seasonal outlook both favor suppressed rainfall across the Southeast during the JFM period, with the highest probabilities over extreme southern South Carolina, southern Georgia, Florida, and the immediate Gulf Coast. Due to the fairly strong climate signal, drought persistence and development are anticipated across the South Atlantic coastal Plain, all of Florida, and the Gulf Coast.

Forecast confidence for the Southeast Region is moderate.

Abnormal dryness (D0) expanded across much of the mid-Atlantic region and parts of the Northeast during the past few weeks, with pockets of moderate drought developing around the Washington DC/Baltimore corridor and the Susquehanna River Valley of central Pennsylvania. During the next week, widespread light to moderate (0.75in to 2in) precipitation is forecast over the entire region, followed by a cold air outbreak. This precipitation may be sufficient to prevent additional short term degradations. The CPC 6-10 and 8-14 day outlooks both favor above-normal precipitation across the mid-Atlantic, and the seasonal outlook maintains equal chances for below-, near-, and above-normal precipitation. Typical rainfall across the Northeast is plentiful and evenly distributed across the year, so it is difficult to maintain long term drought conditions given climatological rainfall. Therefore, in the absence of a clear dry signal, drought removal is anticipated across Maryland and Pennsylvania, with no additional development in the Northeast.

Forecast confidence for the Northeast Region is low to moderate.

Small areas of drought persist across the southeastern Hawaiian Islands. JFM is a climatologically wet time of year for Hawaii, and La Nina climate anomalies favor an enhancement of the rainy season. Therefore, drought improvement or removal is anticipated. There is no current or forecasted drought across Alaska and Puerto Rico.

Forecast confidence for Hawaii is moderate to high.

Forecaster: Adam Allgood

Next Seasonal Outlook issued: January 18, 2018 at 8:30 AM EST