|WPC's 7 day rainfall outlook made in the evening of Feb 17.|
|The NWS significant river flood outlook made on the evening of Feb 17.|
All of this will be made possible by the advent of a nearly stationary deep upper trough in the western US and a ridge on the eastern Seaboard. The passage of several upper-level shortwave troughs moving through the mean trough will allow several extratropical cyclones to pass through the Midwestern states but none of them will be able to sweep a cold front through the Southeast US, and at least a few of them will produce substantial atmospheric rivers. The biggest one will occur in mid- to late week, potentially lasting up to a full day over northern AL to KY. Water vapor transport will possibly exceed 1000 kg per meter per second. This pattern fits closely to what Moore and co-authors found in 2015 to be associated with extreme precipitation events in the southeast US during the cool season. It's likely this atmospheric river event will come to pass and quite soon after the last one.
|The forecast integrated water vapor transport forecast from the largest of three atmospheric river events forecast over the period from Feb 18 to Feb 23, courtesy of the North American Ensemble Forecast System.|
Atmospheric rivers have been in the news lately after the flooding rains and huge mountain snows that fell on California last week. And that's with good reason. The sounding site at the National Weather Service, San Diego, CA just registered its highest precipitable water for the cool season. The integrated water transport was at least 750 kg per meter per second. Perhaps the more remarkable aspects of this river was its length and depth. Sheldon Kusselson, retired research scientist at NOAA/NESDIS and an expert in satellite analysis of water vapor, remarked that he's seen few rivers visible all the way up to 300 mb in the layer precipitable water product (see his analysis below). The accompanying rainfall broke the daily record at Palm Springs, CA with nearly 4". Nearby Mount Polamar recorded over a foot of rain. Meanwhile the Sierra added prodigious new snow accumulations to their expanding snow base. Mammoth Mountain added almost six feet. The floods that followed were substantial, especially around Palm Springs. Multiple news sources mentioned atmospheric rivers, including LiveScience, Wired, the Washington Post.
|The 13 Feb 2019 Atmospheric River is analyzed by Sheldon Kusselson, NOAA/NESDIS retired.|
|A radiosonde climatology of San Diego precipitable water where the observed value on 13 February far exceeds any previous precipitable water maximum (thin red trace) during the cool season.|
However, I haven't heard any mention of the upcoming atmospheric river in the news media, or for any past flooding events in the Southeastern US. In my recollection, the media only mentioned the term 'atmospheric river' when one struck California, subjecting the state to all the impacts we've heard about last week. And it's probably no surprise either. The precipitation events occurring in the Western US, especially California, are almost completely dominated by atmospheric rivers. As Mahoney and co-authors in 2015 pointed out, heavy precipitation events in the southeastern US, on the other hand, can come from a multitude of synoptic and mesoscale patterns during the warm and cool seasons. They found that 41% of heavy precipitation events were matched to an atmospheric river. I suspect that the term hasn't really caught on without the dominance of atmospheric rivers controlling southeast US heavy rain events.
But that's not to say that atmospheric rivers shouldn't be recognized as major flood producers in the southeast. Consider the Nashville, TN flood of early May of 2010 where 12-15" of rain flooded the downtown, resulting in huge losses. The event was big enough to trigger the National Weather Service to deploy the only service assessment team that year. The huge rain event was fed by a persistent strong low-level jet, rapidly feeding tropical moisture into mesoscale convective complexes training over the same area for nearly a day. That feed was identified by Moore in 2012 to be an atmospheric river drawing moisture northward from Central America. Moore determined that this atmospheric river differed from ones existing solely over the ocean because the low-level jet was partially governed by lee cyclogenesis, and with a stationary midlevel trough to the west, the river wound up stalled.
This time may be a little different upon considering that multiple shortwave troughs will be traversing through the mean western trough. There will be similarities too. Each transient extratropical cyclone will have a low-level jet forming an atmospheric river with a tap deep into the tropics, particularly the central Carribean. The moisture is already rich along the Gulf Coast with the onset of the first river. There will be no fronts to displace the moisture and thus, little time needed for each cyclone to re-establish an atmospheric river.
The one question may be, why bother call the upcoming moisture-laden low-level jets atmospheric rivers? Does naming them add value to the awareness of the upcoming rain event, or improve the accuracy of the forecasts? I can't say that the forecasts will be improved. But I do say it's important that we recognize that these processes occur globally. Multiple papers have been written about the role of atmospheric rivers in transporting moisture from the tropics toward the poles (see Gimeno and co-authors in 2014, and Waliser and co-authors in 2012). They've documented that there are several going on around the Earth at any one time. Thus if so much effort has been made to name these moisture feeds into California, then perhaps we should spend the same time doing the same wherever they occur. As Ralph and co-authors just recently documented in 2018, the AMS Glossary of Meteorology now has an official definition of an atmospheric river. The definition defines an Atmospheric River to be:
Atmospheric river–A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources. Atmospheric rivers frequently lead to heavy precipitation where they are forced upward—for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapor transport in the midlatitudes occurs primarily in atmospheric rivers and is focused in the lower troposphere. Atmospheric rivers are the largest “rivers” of fresh water on Earth, transporting on average more than double the flow of the Amazon River.
There is no mention that atmospheric rivers are confined to some arbitrary geographical location. And so neither should anyone else. Call them for what they are anywhere.
Marshall Shepherd pointed out to me a comprehensive study on the climatology of southeast atmospheric resource events by Debbage and co-authors published in 2017 (including Marshall). Their study provided more information on the frequency of these events impacting the coastline from Brownsville to Cape Hatteras. They report an atmospheric river affects somewhere between these end points about 45% of all days where an average of 26 events per year affect each of the approximately 100 mi sections along the midAtlantic coastline to about 18 events per year along similar sections of the Texas coastline. Their synoptic climatology of 500 mb ridge/trough positions and 850 mb flow for Gulf Coast atmospheric rivers, also shows similarities to some of the forecast events coming up the next week.
|Debbage, N. , Miller, P. , Poore, S. , Morano, K. , Mote, T. and Marshall Shepherd, J. (2017), A climatology of atmospheric river interactions with the southeastern United States coastline. Int. J. Climatol, 37: 4077-4091. doi:10.1002/joc.5000|
Dettinger, M., F. M. Ralph, and D. Lavers, 2015: Setting the stage for a global science of atmospheric rivers. Eos, Trans. Amer. Geophys. Union, 96, https://doi.org/10.1029/2015EO038675.
Gimeno, L., R. Nieto, M. Vázquez, and D. A. Lavers, 2014: Atmospheric rivers: A mini-review. Front. Earth Sci., 2, doi:https://doi.org/10.3389/feart.2014.00002.
Lavers, D. A., and G. Villarini, 2013: Atmospheric rivers and flooding over the central United States. J. Climate, 26, 7829–7836, https://doi.org/10.1175/JCLI-D-13-00212.1
Mahoney, K. M., and Coauthors, 2016: Understanding the role of atmospheric rivers in heavy precipitation in the southeast United States. Mon. Wea. Rev., 144, 1617–1632, https://doi.org/10.1175/MWR-D-15-0279.1.
, 2018: Investigation of Atmospheric Rivers Impacting the Pigeon River Basin of the Southern Appalachian Mountains. Wea. Forecasting, 33, 283–299, https://doi.org/10.1175/WAF-D-17-0060.1
Moore, B. J., P. J. Neiman, F. M. Ralph, and F. E. Barthold, 2012: Physical processes associated with heavy flooding rainfall in Nashville, Tennessee, and vicinity during 1–2 May 2010: The role of an atmospheric river and mesoscale convective systems. Mon. Wea. Rev., 140, 358–378, https://doi.org/10.1175/MWR-D-11-00126.1.
Moore, B. J., K. M. Mahoney, E. M. Sukovich, R. Cifelli, and T. M. Hamill, 2015: Climatology and environmental characteristics of extreme precipitation events in the southeastern United States. Mon. Wea. Rev., 143, 718–741, doi:https://doi.org/10.1175/MWR-D-14-00065.1
, 2018: Defining “Atmospheric River”: How the
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