La Niña conditions continue over the equatorial Pacific, with a 95-percent chance of persisting through the end of March 2021. The MJO is also active, with an Indian Ocean event underway.
While the MJO may influence the midlatitude pattern, especially early in the outlook period, climate anomalies associated with the La Niña response will likely play the dominant role throughout the Boreal winter months.
Due to this, the CPC monthly outlook for December 2020 and the December 2020 – February 2021 seasonal outlook both closely resemble composites of previous La Niña events.
During the past month, drought conditions expanded rapidly across much of Texas as the central and southern parts of the state received less than 25 percent of normal rainfall. Drought and abnormal dryness also slowly expanded across parts of the Southwest, Plains, and Midwest, but a swath of heavy precipitation associated with the remnants of Hurricane Delta and a potent midlatitude system brought drought relief to Oklahoma, southern Missouri, parts of the Ohio River Valley, and the Northeast.
Overall, nearly half (45.9 percent) of the Contiguous U.S. was experiencing drought conditions as of 10 November 2020, with the greatest coverage and most intense drought conditions located over the southwestern quadrant of the CONUS.
La Niña winters frequently result in a dipole of above (below) average precipitation across the Northwest and north-central US (southern tier and the central Plains), and these conditions are anticipated to play out during the upcoming winter months.
Therefore, drought reduction is favored for the Northwest and northern Rockies, while persistence or expansion of drought is favored for California, the Four Corners region, the central and southern Plains, and parts of the Southeast that are currently experiencing growing precipitation deficits.
Additionally, the La Niña response tends to shift the favored winter storm track towards the Ohio Valley through interior New England, favoring continued drought improvements for Illinois, Indiana, and the Northeast.
While the climate signal favors above-average snowfall for the northern Plains and upper Midwest, little change to drought conditions is forecast, as frozen soils and streams will inhibit absorption of moisture until the spring thaw commences.
Elsewhere, abnormal dryness and warmth have promoted drought expansion across much of Hawaii, but given the oncoming wet season and enhanced tradewinds in association with La Niña, drought improvements are forecast, especially across the leeward sides of the islands. No drought conditions are currently present or expected to develop during the winter months across Alaska and Puerto Rico.
Forecast confidence is high for the Western region.
- Mostly dry weather and above-normal temperatures did little to alleviate drought conditions across California, the Great Basin, and the Four Corners region, but recent storminess over the Northwest and northern Rockies brought some relief to ongoing drought conditions during the past few weeks.
- The short term pattern looks similar, with storm systems bringing additional coastal rain and mountain snows to the Cascades, northern Rockies, and the northern Sierra Nevadas, with accumulations tapering off towards the south.
- Both the CPC Dec and DJF outlooks reflect climate anomalies associated with La Niña, favoring above-median precipitation for the Northwest and northern Rockies, below-median precipitation for central and southern California through the southern Four Corners region, and equal chances for near-, below-, and above-median precipitation straddling the California Oregon border eastward to southern Idaho and northern Utah.
- DJF is a very wet time of year for the western U.S., and winter moisture is critical to build mountain snowpacks that feed reservoirs during the dry season months. Therefore, the wet signal favored for the Northwest favors further drought reductions across the Northwest and northern Rockies, with drought elimination possible for Washington state.
- In contrast, drought persistence is favored for the southern portions of the Western Region, with drought expansion possible across southern California. Above-normal temperatures favored for DJF may exacerbate the drought conditions by keeping mountain snow levels abnormally high.
Forecast confidence is moderate for the High Plains region.
- Drought changes across the High Plains were chaotic during the past four weeks, with degradations observed across North Dakota, and improvements or degradations fairly localized in other states.
- The La Niña favored anomalous precipitation dipole is reflected in the seasonal outlook, where above-median precipitation is favored for the Dakotas and northern Wyoming, while below-median precipitation is favored for southern Colorado and most of Kansas and Nebraska.
- DJF is a dry, cold time of year for the Plains, and any anomalous snowfall during the winter months across eastern Montana and the Dakotas will likely remain above the frozen soils until the spring thaw beyond the DJF period.
- Therefore, little change in the incipient drought conditions is likely during the outlook period despite the tilt towards above-median precipitation.
- Further west, some drought improvement is possible across the higher elevations of northwestern Wyoming due to a potential for above-average snowpack building.
- Further south, despite the dry time of year, a potential for above-normal temperatures and continued dryness favors eastward expansion of drought conditions across Kansas.
Forecast confidence is high for the Southern and Southeastern regions.
- The most substantial drought development over the past month occurred in Texas due to abnormal dryness and near- to above-normal temperatures.
- Further east, a patchwork of increasing dryness along the Gulf Coast States and Southeast was interrupted by swaths of heavy rainfall from tropical cyclones, including Hurricane Delta, which impacted the central Gulf Coast, and Tropical Storm Eta, which brought flooding rains to parts of the Florida peninsula.
- La Niña winters tend to be warm and dry across the south-central and southeastern CONUS, and this is reflected in the probabilistic monthly and seasonal outlooks. Therefore, drought development is likely for the rest of Texas (except extreme eastern Texas), and along a swath extending from southeastern Louisiana eastward across southern Alabama, Georgia, South Carolina, and northern Florida, as these areas have growing precipitation deficits out to 60 days.
- In contrast, areas that received heavy rainfall from tropical cyclones are favored to begin drying out during DJF, but widespread drought development is unlikely.
Forecast confidence is high for the Midwestern region and moderate for the Northeastern region.
- Small pockets of drought currently extend across parts of northern Missouri, central Illinois, and northern Indiana, as these areas missed out on heavy precipitation that overspread parts of the Ohio River Valley in association with a strong midlatitude cyclone and the remnants of Hurricane Delta.
- Additionally, unusually long-lasting drought conditions also persist across central Pennsylvania, upstate New York, and much of New England, though recent storminess brought some relief to these regions.
- During La Niña winters, the climatological winter storm track shifts to the west, favoring the Ohio Valley through interior New England. This is reflected by enhanced chances for above-median precipitation in the DJF seasonal outlook.
- Therefore, drought removal is favored for the Corn Belt, and additional drought reductions are the favored outcome across the Northeast.
- While enhanced chances for above-median precipitation do not extend to coastal New England, climatology favors frequent coastal storms during the winter, and a few such storms should be sufficient to erode the ongoing drought.
Forecast confidence is high for Hawaii.
- No drought is currently in place or anticipated to develop across Alaska or Puerto Rico.
- Persistent dryness and above-average temperatures promoted widespread drought development in Hawaii, with nearly three-quarters of the state currently experiencing drought conditions.
- The start of the wet season in January, as well as enhanced trade winds associated with the La Niña response, favor above-average rainfall, particularly across the leeward sides of the islands. Therefore, drought improvement or removal is favored during DJF.