Overview:

Overall spring is favored to be warm in Alaska, the Pacific Northwest and Eastern US with seasonal average temperatures in the Upper Midwest and California. Meanwhile, a slightly cooler spring is likely for the Southwest US and the flood-weary central and southern Plains, where ongoing floods, high soil moisture, and a wet weather pattern will keep flood chances ongoing through most of the season. California will likely start wetter than usual through April but dry with the onset of summer and the diminishing impact of El Niño. Flow from the warm Gulf of Mexico, Caribbean Sea, and southwest Atlantic Ocean will keep most of the Eastern US wet through spring, especially when the southern stream is still active during the first half of the season.

The Details:

The 2018-2019 Winter Season will best be remembered by its historic cold and snowfall for much of the western two-thirds of the country. For these regions, Spring’s arrival may appear as sweet relief from a relentless series of winter storms that have battered the Western US and portions of the Upper Midwest since December. Unfortunately, there is no rest for the weary. Winter’s corpse will prove hard to bury throughout the 2019 Spring season as snow melt, warm temperatures, and above-normal precipitation across most of the US contribute towards more widespread flooding, especially across the Central US, where flooding of the Missouri and Mississippi River basins have already resulted in historic and catastrophic destruction.

The March flooding in the Midwest was instigated by a powerful winter storm that dumped heavy snow and soaking rains alongside hurricane-force wind gusts over a brief period of time across the Mississippi and Missouri River Basins. Contributing to the flooding in the storm’s aftermath, temperature swings and additional rainfall encouraged rapid snow-melt over an already saturated watershed. Historic devastation has ensued. The still-ongoing flooding has already resulted in over $3 billion of damage and is responsible for taking at least three lives. Nebraska alone incurred over $1.3 billion in losses. The Missouri and Mississippi Rivers are finally receding, but as alluded in our outlook below, the flooding disaster is far from over.

The three month period from April through June is poised to feature above-normal temperatures across much of the nation, especially throughout the Pacific Northwest, Gulf Coast, and the East Coast. In the Pacific Northwest it is expected to be dry throughout the season, whereas spring is expected to start wet in California but end dry. The Gulf Coast and East Coast have high chances of above normal precipitation persisting through spring. The central US is more likely to undergo periods of below normal temperatures and above normal precipitation, especially during April. However, temperature swings will be fairly common. This outlook is based on support from a weak El Niño, sea surface temperature (SST) anomalies, soil moisture, and guidance from dynamic climate models.

Spring weather patterns are largely influenced by winter’s legacy, especially with respect to water. Oceans cover 71% of the Earth’s surface and maintain a pivotal role in driving the Earth’s weather and climate system. It therefore makes sense to use the oceans as a foundation for any seasonal outlook. Of most relevance to this spring’s outlook for North America are the Pacific and Arctic Oceans, with a specific focus on the Bering Sea, Gulf of Alaska, and Gulf of Mexico.

The most renowned oceanic phenomenon this year is not necessarily the most important factor in this spring’s outlook. The weak El Niño hyped since last fall was finally declared in February (NOAA announces when there are El Nino/La Nina conditions because there are different standards throughout the globe), when equatorial Pacific SST anomalies exceeded 0.5° Celsius and were forecast to persist through at least early Fall. El Niño’s impacts on global weather peak during late winter and wane as spring progresses into summer. Earlier in spring though, its impacts tend to be more reliable, especially in the West and the southern tier of the US. Links between weak El Niño events and temperature trends elsewhere, including the Northeast, are dubious at best during winter (let alone spring), so it is not considered as a ubiquitous factor across the US.

Tropical Pacific Sea Surface Temperatures Anomalies Animation
Equatorial Pacific SST anomalies for the past twelve weeks. SST analysis is the optimum interpolation (OI) analysis, while anomalies are departures from the adjusted OI climatology (Reynolds and Smith 1995, J. Climate, 8, 1571-1583). Data provided by NOAA/CPC
from their Web site https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml
Time series of weekly sea surface temperatures anomalies for the 4 Niño regions
Time series of weekly SST anomalies for the 4 Niño regions. NOAA uses Niño region 3.4 in declaring El Niño or La Niña. Data provided by NOAA/CPC from their Web site https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

Early this spring, the weak El Niño is expected to contribute to warmer than normal conditions in the Pacific Northwest, with anomalies strongest toward the coast. Springs are typically warm in the Upper Midwest during El Nino events, but abundant soil moisture from recent flooding and heavy snowfall is expected to limit the extent and intensity of warmth. Consequentially, April’s mean temperatures are expected to be around normal for this region, with above-normal warmth likely by June.

El Niño will keep the southerly Pacific jet stream active through at least the first half of spring. California eastward across the Southwest can therefore expect continued bouts of unsettled weather, which will make for extended periods of cooler than normal temperatures from time-to-time. This storm track will also keep the weather pattern active across the Southeast US, but a building ridge of high pressure growing from the Southwestern Atlantic Ocean will offset the overall impact of what El Niño alone suggests would be a cool start to spring. This ridge will impede on the eastward transit of Pacific-based storm systems over Texas northward toward the Central Plains. The slow progression of these frequent storm systems will result in strong chances that cooler and wetter than normal weather dominates the first half of spring here.

Unfortunately, this kind of weather pattern is highly conducive for flooding in the Central US as storms that traverse the region have access to a deep moisture tap from the Gulf of Mexico. With the Missouri and Mississippi Rivers still above flood stage, their drainage basins have little capacity for additional runoff. Any heavy rain event therefore has the potential to trigger the return of widespread floods. Making matters worse, the high soil moisture across the Central US will help fuel thunderstorms when steamy temperatures intermittently bask the region, perpetuating flood chances through the season.

The limited scope and diminishing influence of El Niño suggest one look elsewhere across the oceans for insight on spring. The immediate waters surrounding North America have been abnormally warm this winter. Until last week (March 23th to be exact), The Bering Sea, Gulf of Alaska, Gulf of Mexico, and southwestern Atlantic Ocean all contained vast regions in which SSTs were observed to be at least 1°C above 1981-2010 averages as depicted by the anomaly chart below from NOAA Earth Systems Research Laboratory.

Monthly SST Anomaly chart for the month ending March 23, 2019. Data/image provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.esrl.noaa.gov/psd/

Consider the Bering Sea and Gulf of Alaska, where SSTs are as much as 2°C above normal after a very warm winter. Winters are quickly becoming more benign in Alaska due to the rapidly warming arctic. This winter was no exception. High temperatures on March 19 recently soared into the 60s and 70s in eastern and southeastern parts of the state, annihilating records by up to 15°F. Klawock’s high temperature of 70°F was the earliest observation of a temperature of at least 70° anywhere in Alaska by 12 days. The old state record was only set in 2016, also in Klawock. It should therefore be no surprise that sea-ice is much below normal, particularly in the Bering Sea, where the concentration of sea ice was less than 50% below normal in February and where sea-ice extent was much smaller compared to its 1981-2010 median.


Sea Ice Concentration in the Arctic for February 2019. Data/image provided by Fetterer, F., K. Knowles, W. N. Meier, M. Savoie, and A. K. Windnagel. 2017, updated daily. Sea Ice Index, Version 3. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: https://doi.org/10.7265/N5K072F8.
March 29, 2019 ].
Sea Ice Concentration Anomalies in the Arctic for February 2019. Data/image provided by Fetterer, F., K. Knowles, W. N. Meier, M. Savoie, and A. K. Windnagel. 2017, updated daily. Sea Ice Index, Version 3. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: https://doi.org/10.7265/N5K072F8. [March 29, 2019].

The below average coverage of sea-ice in the waters surrounding Alaska is part of a climate-change induced trend of decreasing sea-ice cover during transition seasons. With less sea-ice, the albedo of the arctic is drastically reduced, so more solar radiation is absorbed by the arctic and subarctic waters. This is why the arctic is warming so much faster than the rest of the globe, where the proportion of incoming solar radiation absorbed by Earth’s surface has changed little.

A warm spring can naturally be expected in Alaska. Ridging of high pressure from this warmth will intermittently expand and progress across western North America. This ridging, alongside El Niño, supports a warm weather pattern across the Pacific Northwest. Additionally, the persistent presence of high pressure will increase odds for spring to be drier than normal across the Northwest, especially east of mountains.




Although much of the West is expected to experience a warm spring, the most favorable area for above-normal warmth is actually in the East. The warm Caribbean Sea, Southwest Atlantic Ocean, and Gulf of Mexico per the observations presented earlier will help spur the development of ridging east of the Mississippi River. Height and temperature anomaly charts from the CFS and the North American Multi-Model Ensemble of climate models–of which the CFS is a constituent–reflect this ridging in near unanimous agreement. The unanimous agreement among climate models regarding this ridge placement is why the East has a particularly high chance for a warm spring overall. Abnormally warm conditions will become especially common place by the end of April across most of the Eastern US as the ridge intensifies with the increasing sun angle and subtropical heat.

Probability of a location observing an above or below normal three month average 2m temperature anomaly between April and June. Probabilities are calculated using the ensemble mean of the eight dynamic climate models that comprise the NMME. NMME project and data dissemination is supported by NOAA, NSF, NASA and DOE. A description of the NMME, NMME data, and development is given by
Ben P. Kirtman, Dughong Min, and Johnna M. Infanti. 2014. The North American Multimodel Ensemble: Phase-1 Seasonal-to-Interannual Prediction; Phase-2 toward Developing Intraseasonal Prediction . Bulletin of the American Meteorological Society, 95, 585-601.
https://doi.org/10.1175/BAMS-D-12-00050.1

Given the ridge’s varying center of circulation between the northern Caribbean and Southeastern US, deep plumes of tropical moisture will frequently be pumped northward, keeping most of the East Coast and Central US wet throughout the spring season, especially through May. This is much to the detriment of flood-weary communities of the Missouri, Mississippi, and Tennessee River Valleys. The ongoing flooding in the former two river basins has drawn attention away from the Tennessee Valley, which already experienced widespread flooding from historic rainfall in February.

Flooding woes in the central US will likely peak by early May, long after the snow melts in the Upper Midwest and after snow-melt from the Rocky Mountains into the overwhelmed river basins begins to subside. Additionally, warming temperatures and increasing sun angle will dry out soggy soils at an increasingly fast pace. Note that the NMME probability for precipitation anomalies is not considered verbatim due to low skill.




The warmth won’t be as persistent everywhere in the East. From time-to-time the eastern ridge will succumb to reinvigorated Pacific storms from the west, in which case temperatures will swing across the Mid-Atlantic and Northeast as they typically do during spring alongside heavy precipitation. But areas of the coastal Northeast may be more susceptible to sudden temperature swings from back-door cold fronts than usual, especially across coastal New England and Long Island. When high pressure builds across the New England or Canadian Maritime coast, east and northeast winds from the cold North Atlantic will expel summer-like warmth in an instant. Tight pressure gradients arising from the differential heating between the cold Atlantic and northern extent of the subtropical warmth make back-door cold fronts more likely. On multiple occasions this spring, it will be possible for inland communities, including mountainous areas, to bask in 80-90 degree warmth while coastal communities remain stuck in the 50s. Adding to the dreariness, fog and clouds will be common-place wherever these fronts stall.

Further encouraging pesky back-door cold fronts, the active southern branch of the jet stream must ultimately merge with the northern branch somewhere. With a mean storm track expected to cut across the Mississippi and Ohio River Valleys toward Ontario, confluence of the jet stream will commonly occur over southeast Canada. Where there is confluence, there will be sinking motion and high pressure at the surface. By the same token, high pressure develops on the warm side of the “jet exit” region, where winds in the jet stream decelerate. The mean position of the jet stream will commonly cut through or just north of New England under the regime of a springtime subtropical ridge, increasing the chances for high pressure to develop off the coast of New England. Fortunately, the back-door cold fronts will lose their punch as the ocean warms such that they become less frequent and less extreme by mid-May.




Overall spring is favored to be warm in Alaska, the Pacific Northwest and Eastern US with seasonal average temperatures in the Upper Midwest and California. Meanwhile, a slightly cooler spring is likely for the Southwest US and the flood-weary central and southern Plains, where ongoing floods, high soil moisture, and a wet weather pattern will keep flood chances ongoing through most of the season. California will likely start wetter than usual through April but dry with the onset of summer and the diminishing impact of El Niño. Flow from the warm Gulf of Mexico, Caribbean Sea, and southwest Atlantic Ocean will keep most of the Eastern US wet through spring, especially when the southern stream is still active during the first half of the season.




Author

As Head Meteorologist, Josh bridges together weather forecasting with product quality and innovation. He vigilantly monitors weather threats across the country and directly engages with clients to outline hazards posed by expected inclement weather. He also offers insights into meteorology and numerical weather prediction to aid the development team in improving and expanding the diverse set of products. Feldman graduated from Stony Brook University in 2018 with Bachelor of Science degrees in Atmospheric and Oceanic Sciences and Physics.

Comments are closed.