The moderate La Niña event is expected to continue through the Northern Hemisphere winter 2020-2021 (a 95% chance during January-March), with a potential transition to ENSO-neutral during the Spring 2021 (55% chance during April-June).
Typical U.S. La Niña winter impacts include: above-normal temperatures and below-normal precipitation across the southern tier of States, below-normal temperatures and above-normal precipitation in the Northwest, below-normal temperatures in southeastern Alaska and the northern Rockies and Plains, and above-normal precipitation in the Ohio Valley and Hawaii.
However, other atmospheric phenomena (which can be difficult to predict) can influence impacts from La Niña. As of Jan. 26, 2021, drought (D1-D4) across the contiguous U.S. slightly declined during the past month, standing at 46% versus 49% on Dec. 29, 2020. The worst drought conditions (D3-D4) continued across the Southwest, with extreme to exceptional drought enveloping most of Nevada, Utah, Arizona, Colorado, and New Mexico.
During the past 30-days, subnormal precipitation and near to above-normal temperatures covered most of the Southwest, northern Plains, upper Midwest, Northeast, central Gulf Coast area, and Florida. In contrast, surplus precipitation fell on the Pacific Northwest, most of the southern and central Plains, western Corn Belt, Tennessee Valley, and from northwest Florida northeastward into southeastern Virginia.
Much of the drought improvement over the past month occurred in the southern Plains and Pacific Northwest. 30-day temperatures averaged slightly below normal in the central and southern Rockies and southern High Plains.
However, in late January, a change in the weather pattern brought stormy and colder weather to California and the Southwest, helping to boost the dismal SNOTEL Water Year to Date (WYTD) basin precipitation and snowfall totals, although much more was needed to get close to normal.
The updated monthly outlook favored a large area for above average precipitation probabilities, stretching from the northern and central Rockies eastward to the mid-Atlantic Coast, and southward into the Tennessee and lower Mississippi Valleys and southern Appalachians. Somewhat higher odds were located in the northern High Plains, Tennessee and Ohio Valleys, and mid-Atlantic.
Above-normal precipitation was also favored in western Alaska. In contrast, subnormal precipitation was likely along the Far West Coast and extreme southern sections of the Southwest, in the southern Plains, and along the central and eastern Gulf and southern Atlantic Coasts.
The updated Feb2021 temperature map was quite different from the initial mid-January release, with odds favoring a large area of subnormal readings in most of the West, northern half of the Plains, Midwest, and southeastern Alaska. Above-normal temperatures were limited to western and northern Alaska, southern Texas, and Florida.
Although above-normal precipitation is favored for the northern and central parts of the Rockies and Plains and upper Midwest, February climatology is quite dry and some of the drought was long-term. With recent dry and mild weather in many portions of the aforementioned areas, the expected near- to above-normal precipitation was most-likely not enough to make large-scale improvements during the cold season.
Some localized, small-scale improvements are possible in these areas, but overall, most of the drought will persist until warmer conditions in the spring thaw the ground and allow any snow to percolate into the soil. Exceptions to this included the small D1 areas in the central Corn Belt and Tennessee Valley where normal and forecast precipitation are higher, and recent precipitation has fallen there.
In California, although the updated Feb2021 precipitation outlook favored below-normal precipitation, very heavy rains inundated central coastal sections, with heavy snows blanketing the Sierra Nevada in late January.
Another potent storm is forecast to hit northern California in early February, thus some SHORT-TERM improvement was depicted for those two events in February; however, the rest of the month, climatologically one of the wettest for the state, is expected to be drier, as was the FMA2021 SDO, so these February improvements may only be fleeting when compared to the overall wet season (Nov-Apr).
Development is possible where D0 currently existed on the Jan. 26 USDM, the forecasts indicated good odds for subnormal precipitation and above-normal temperatures, and the past 30-days were drier than normal (e.g. southern Texas, central Gulf Coast, much of Florida, and coastal Georgia and South Carolina).
No drought is indicated or expected to develop across Alaska for February while conditions are frigid and the ground frozen. Drought persistence is likely for Puerto Rico as they enter the dry season. Across Hawaii, drought improvement is likely due to expected La Niña impacts (above-normal late winter and early spring rains) during the wet season (late winter ino early spring), along with dynamical model forecasts for surplus rains.
Forecast confidence is moderate to high for the Southeast.
- Much of the region is currently drought free, and with the 5-day QPF, 6-10 and 8-14 day ERFs, and updated February 2021 precipitation LLF all favoring mostly above-normal precipitation (except Week 3-4 with subnormal precipitation along the East Coast), no development is expected EXCEPT in far southern sections (e.g. along the central Gulf Coast, most of Florida, and southern Atlantic Coast).
- In this region, the 5-day QPF precipitation forecast had lower totals while the ERFs, Week 3-4, and updated February 2021 LLF have EC or subnormal precipitation odds, and the past 30-days were very dry in parts of the Southeast and much of Florida, with some areas of D0 already in place in southern Louisiana, southern Alabama, northern Florida, and eastern Georgia and South Carolina.
- The forecasted development area did not extend across the entire Gulf Coast as portions of the western and eastern Gulf received surplus rains the past 30 days, tempering any February development. Otherwise, development is favored in the drier southern areas as expected with typical La Niña impacts.
Forecast confidence is moderate for the South.
- Wet weather brought improvement to most of the region the past 30 days, particularly Texas, southern Oklahoma, Arkansas, and western Tennessee.
- The upcoming forecasts (QPF, ERFs, Week 3-4, updated Feb2021 LLF) depicted good odds for above-normal precipitation (and below-normal temperatures) across northern and eastern sections, while subnormal precipitation (and above-normal temperatures) were favored in extreme southwestern sections.
- February climatology is also rather dry in western sections. Therefore, improvements are likely in northeastern sections of the South (western Tennessee and northern Mississippi), with possible development in south and central Texas (with many areas already in D0), and persistence in western sections.
Forecast confidence is moderate for the Midwest.
- Varied precipitation amounts have fallen across the Midwest the past 30 days, with below-normal totals in northern and eastern sections, and above-normal amounts in western areas (e.g. Missouri, Iowa, southern Minnesota) and Kentucky. The 30-day temperatures have also been above normal.
- Based upon QPF, ERFs, Weeks 3-4, and updated 1-month LLFs, above-normal precipitation is expected across the Tennessee, middle Mississippi, and Ohio Valleys throughout the period, thus some improvement is likely in Illinois and Indiana where normals and surplus precipitation odds were higher than in western sections. Temperatures should also average below-normal during much of February.
- In western and northern Midwest sections, although the odds depicted near- to above-normal precipitation, the climatology is much drier than eastern sections, and with the drought in western Iowa being long-term, it would take an extended wet spell to alleviate the drought there.
- Therefore, although some small local improvements are possible in western parts of the Midwest, overall drought conditions should mainly persist to the end of February. This reasoning is also similar to much of the High Plains (below).
Forecast confidence is moderate for the High Plains.
- As just mentioned in the Midwest section, many portions of the High Plains are forecasting equal (EC) to above-normal precipitation chances during February.
- However, the past 1-2 months have been quite mild and dry in the northern Plains, with drought somewhat increasing during the past 30 days, February precipitation climatology quite dry, temperatures typically quite low and expected to be below-normal during February, keeping the ground frozen; therefore, overall persistence was depicted on the MDO map for this region, although some locally small improvements are possible where (and if) excessive precipitation occurred during the month.
- The above-normal precipitation chances also preclude any February drought development in the region. The upcoming spring months will be critical toward moisture infiltration into the soil.
Forecast confidence is moderate for the West.
- Until a few days ago, the wet season in the Far West was not faring well for California and the Southwest, with SNOTEL average basin Snow Water Equivalent (SWE) less than 50% of normal across much of California, Arizona, New Mexico, and southern Utah.
- Fortunately, a welcome change in the weather pattern brought an intense storm system that dropped heavy precipitation (up to 20 inches of coastal rain and more than 100 inches of mountain snow) to central California and the Sierra Nevada.
- Lighter but above-normal totals also fell on portions of the Southwest, increasing the SNOTEL basin average SWE (on Jan. 29) closer to 75% (~100% in central Arizona).
- February is normally one of the wettest months for California, so a continued wet pattern would be very beneficial to the state. The 5-day QPF forecast has another Pacific storm dropping heavy precipitation on northern and central California and the Sierras, but less so southward.
- Then the ERFs and Week 3-4 predicted subnormal precipitation as did the updated February 2021 LLF for the West Coast and extreme Southwest (and subnormal temperatures).
- Since the Jan. 26 USDM was used for the initial MDO drought conditions, precipitation during late January (Jan. 26-31) plus February forecasts can be used in the MDO outlook. As a result, the heavy precipitation in central California during Jan. 27-29, along with the expected early February storm, were shown as short-term improvements to northern and central California and southern Oregon.
- The expected subnormal February temperatures should minimize snow melt and evaporation, but the overall pattern for California was still for subnormal precipitation during their wet season (Nov-Apr).
- The SDO [FMA’21] has persistence for most of California except the extreme northwestern portion. Recent wetness in extreme southern California would most-likely hold off any development of D1 there until March if the long-range forecasts are correct about subnormal precipitation.
- Again, some small improvements are possible in the West where recent storms dropped locally heavy precipitation, especially on mountains, but overall, persistence of drought during February was most-likely on a general scale.
Forecast confidence is moderate for the Northeast.
- Although drought gradually receded from the Northeast in late 2020 due to the wet conditions and minimal impacts, the past 30-days have been drier and milder than usual, and recent USDM improvements have declined.
- With February precipitation climatology somewhat on the drier side as compared to other months; 5-day QPF keeping most of interior New England dry (immediate coast may see snow); 6-10 and 8-14 day ERFs hinted at slight odds for above-normal or equal chances (EC) precipitation; Weeks 3-4 expected subnormal precipitation; and the updated February precipitation has EC north and slight tilt toward above-normal precipitation south, the mixed signals did not favor either improvement or deterioration, thus persistence was forecasted for the two small D1 areas in northern New York and Vermont.
Forecast confidence is moderate to high for Hawaii, Purto Rico, and Alaska.
- In Hawaii, with a recent uptick in rainfall, the normal wet season ongoing, and La Niña usually bringing above-normal late winter and early spring precipitation, a general 1-category improvement was made for the islands.
- In Alaska, frozen ground and frigid air has the state in deep freeze mode, so no development is expected during February, although the southeastern Panhandle had favorable odds for subnormal February precipitation – something to watch in March.
- In Puerto Rico, the D1 area in northern portions of the island were most-likely to persist during the dry season. Without any definitive forecasts for above or below normal precipitation, no improvement or deteriorations (development) were made.