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Herr H.D.,National Weather Service - NWS | Krzysztofowicz R.,University of Virginia
Journal of Hydrology | Year: 2010

The problem is to provide a short-term, probabilistic forecast of a river stage time series {H1,...,HN} based on a probabilistic quantitative precipitation forecast. The Bayesian forecasting system (BFS) for this problem is implemented as a Monte-Carlo algorithm that generates an ensemble of realizations of the river stage time series. This article (i) shows how the analytic-numerical BFS can be used as a generator of the Bayesian ensemble forecast (BEF), (ii) demonstrates the properties of the BEF, and (iii) investigates the sample size requirements for ensemble forecasts (produced by the BFS or by any other system). The investigation of the ensemble size requirements exploits the unique advantage of the BFS, which outputs the exact, analytic, predictive distribution function of the stochastic process {H1,...,HN}, as well as can generate an ensemble of realizations of this process from which a sample estimate of the predictive distribution function can be constructed. By comparing the analytic distribution with its sample estimates from ensembles of different sizes, the smallest ensemble size M* required to ensure a specified expected accuracy can be inferred. Numerical experiments in four river basins demonstrate that M* depends upon the kind of probabilistic forecast that is constructed from the ensemble. Three kinds of forecasts are constructed: (i) a probabilistic river stage forecast (PRSF), which for each time n(n=1,...,N) specifies a predictive distribution function of Hn; (ii) a probabilistic stage transition forecast (PSTF), which for each time n specifies a family (for all hn-1) of predictive one-step transition distribution functions from Hn-1=hn-1 to Hn; and (iii) a probabilistic flood forecast (PFF), which for each time n specifies a predictive distribution function of max{H1,...,Hn}. Overall, the experimental results demonstrate that the smallest ensemble size M* required for accurate estimation (or numerical representation) of these predictive distribution functions is (i) insensitive to experimental factors and on the order of several hundreds for the PRSF and the PFF and (ii) sensitive to experimental factors and on the order of several thousands for the PSTF. The general conclusions for system developers are that the ensemble size is an important design variable, and that the optimal ensemble size M* depends upon the purpose of the forecast: for dynamic control problems (which require a PSTF), M* is likely to be larger by a factor of 3-20 than it is for static decision problems (which require a PRSF or a PFF). © 2010 Elsevier B.V.

Rappaport E.N.,National Weather Service - NWS | Blanchard B.W.,Federal Emergency Management Agency
Bulletin of the American Meteorological Society | Year: 2016

A review of storm-related reports and databases for 59 U.S. tropical cyclones from the past half-century tells us that the number of indirect deaths is almost as large as the number of direct deaths. Indirect deaths occurred most often in association with loss of electricity, cardiovascular failure, evacuation, and vehicle accidents. The prestorm phase presented significant threats to safety mainly from accidents during home preparations (e.g., fall from a ladder or building) and in association with evacuations, mainly from vehicle accidents and/or incidents involving the elderly. Most indirect fatalities, however, occurred after a triggering storm condition or after the storm had completely passed. The loss of electricity was noted in many of those cases. It was the antecedent to falls in the dark and/or down stairs, house fires, inoperative life-sustaining medical equipment, CO poisoning, hypothermia, electrocution during power restoration or from power lines that were unexpectedly live, and vehicle accidents associated with inoperative traffic lights. The importance of electricity is likely greater than indicated because the available data exclude the count of those who evacuated at least partly out of concern that power would be lost, and then died during the evacuation process (e.g., the Hurricane Rita bus tragedy) or at their evacuation destination. It also doesn't include some who died from the exacerbating impacts of heat stress resulting from a sometimes-extended period without air conditioning. Other incidents noted most often in the latter stages of the storm or after the storm's passage were recovery activities, vehicle accidents on slippery roads, and vehicle collisions with downed trees. We highlight further the involvement of trees in fatal incidents as, like loss of electricity, they appear in multiple types of fatalities. In addition to posing a deadly roadway obstacle, they were noted in storm clean-up accidents and heart attacks, some from physical overexertion. We recall that Rappaport (2000) noted toppling and fracturing trees were also causes of direct deaths. Cardiovascular failures occurred with especially great frequency, in about one-third of the indirect deaths. Evacuating and being evacuated also accounted for a significant number of indirect deaths. This observation supports the practice of care providers comparing the relative risk of conducting an evacuation - especially of the frail and elderly - to the risk of not evacuating. The data make clear that reducing the number of deaths associated with tropical cyclones in a significant way will require decreasing the occurrences of both indirect and direct deaths. Improvements in the hurricane forecast enterprise have led to a decrease in the number of direct deaths from what would have been expected otherwise (Willoughby et al. 2007). Among those life-saving advances over the study period has been a lengthening of the tropical cyclone forecast horizon from two days to five days. Paradoxically, the increase in lead time also provides opportunity for an increase in certain kinds of casualties, most notably indirect casualties during the lengthened preparation phase. It should then not be surprising that while the ratio of direct deaths to indirect deaths was more than two to one up through 1995, the ratio has reversed since approximately 2000 - though changes in reporting procedures (i.e., recognizing and providing more information explicitly about indirect deaths) are likely also partly responsible for the change. Minimizing indirect deaths will require focused outreach and education. We hope this study has identified the risks that can form the focus of those efforts to provide for greater public safety and the impetus to make significant progress quickly. ©2016 American Meteorological Society.

Alcott T.I.,National Weather Service - NWS | Steenburgh W.J.,University of Utah
Monthly Weather Review | Year: 2013

Although several mountain ranges surround the Great Salt Lake (GSL) of northern Utah, the extent to which orography modifies GSL-effect precipitation remains largely unknown. Here the authors use observational and numerical modeling approaches to examine the influence of orography on the GSL-effect snowstorm of 27 October 2010, which generated 6-10mmof precipitation (snow-water equivalent) in the Salt Lake Valley and up to 30cm of snow in the Wasatch Mountains. The authors find that the primary orographic influences on the event are 1) foehnlike flow over the upstream orography that warms and dries the incipient low-level air mass and reduces precipitation coverage and intensity; 2) orographically forced convergence that extends downstream from the upstream orography, is enhanced by blocking windward of the Promontory Mountains, and affects the structure and evolution of the lake-effect precipitation band; and 3) blocking by the Wasatch and Oquirrh Mountains, which funnels the flow into the Salt Lake Valley, reinforces the thermally driven convergence generated by the GSL, and strongly enhances precipitation. The latter represents a synergistic interaction between lake and downstream orographic processes that is crucial for precipitation development, with a dramatic decrease in precipitation intensity and coverage evident in simulations in which either the lake or the orography are removed. These results help elucidate the spectrum of lake-orographic processes that contribute to lake-effect events and may be broadly applicable to other regions where lake effect precipitation occurs in proximity to complex terrain. © 2013 American Meteorological Society.

Rutz J.J.,National Weather Service - NWS | James Steenburgh W.,University of Utah | Martin Ralph F.,University of California at San Diego
Monthly Weather Review | Year: 2015

Although atmospheric rivers (ARs) typically weaken following landfall, those that penetrate inland can contribute to heavy precipitation and high-impact weather within the interior of western North America. In this paper, the authors examine the evolution of ARs over western North America using trajectories released at 950 and 700 hPa within cool-season ARs along the Pacific coast. These trajectories are classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over western North America. Interior-penetrating AR trajectories most frequently make landfall along the Oregon coast, but the greatest fraction of landfalling AR trajectories that eventually penetrate into the interior within an AR is found along the Baja Peninsula. In contrast, interior-penetrating AR trajectories rarely traverse the southern "high" Sierra. At landfall, interior-penetrating AR trajectories are associated with a more amplified flow pattern, more southwesterly (vs westerly) flow along the Pacific coast, and larger water vapor transport (qν). The larger initial qν of interior-penetrating AR trajectories is due primarily to larger initial water vapor q and wind speed ν for those initiated at 950 and 700 hPa, respectively. Inland- and interior-penetrating AR trajectories maintain large qν over the interior partially due to increases in ν that offset decreases in q, particularly in the vicinity of topographical barriers. Therefore, synoptic conditions and trajectory pathways favoring larger initial qν at the coast, limited water vapor depletion by orographic precipitation, and increases in ν over the interior are keys to differentiating interior-penetrating from coastal-decaying ARs. © 2015 American Meteorological Society.

Graham R.A.,National Weather Service - NWS | Grumm R.H.,National Weather Service - NWS
Weather and Forecasting | Year: 2010

Synoptic-scale weather events over the western United States are objectively ranked based on their associated tropospheric anomalies. Data from the NCEP 6-h reanalysis fields from 1948 to 2006 are compared to a 30-yr (1971-2000) reanalysis climatology. The relative rarity of an event is measured by the number of standard deviations that the 1000-200-hPa height, temperature, wind, and moisture fields depart from climatology. The top 20 synoptic-scale events were identified over the western United States, adjacent eastern Pacific Ocean, Mexico, and Canada. Events that composed the top 20 tended to be very anomalous in several, if not all four, of the atmospheric variables. The events included the northern Intermountain West region heavy rainfall and Yellowstone tornado of mid-July 1987 (ranked 5th), the Montana floods of September 1986 (ranked 4th), and the historic 1962 "Columbus Day" windstorm in the Pacific Northwest (ranked 10th). In addition, the top 10 most anomalous events were identified for each month and for each of the variables investigated revealing additional significant weather events. Finally, anomaly return periods were computed for each variable at a variety of levels. To place a given anomaly in perspective for a specific level or element, forecasters need information on the frequency with which that anomaly is observed. These return periods can be utilized by forecasters to compare forecast anomalies to the actual occurrence of similar anomalies for the element and level of interest to gauge the potential significance of the event. It is believed that this approach may allow forecasters to better understand the historical significance of an event and provide additional information to the user community.

Data suggest that the arrival of winter's permanent snowpack impacts daily high and low surface temperatures in Fairbanks, Alaska. Given temperatures at 850 hPa ranging from 0 °C to - 5 °C in October, high temperatures on days with snow on the ground are 4.9 °C colder than high temperatures on days with no snow on the ground. The difference for low temperatures is 7.3 °C. While the exact date the snowpack is established varies from year to year, standardizing this date in time as "S-Day" reveals that the drop in daily high and low temperatures through the period from 5 days before S-Day to 5 days after S-Day is from 5 to 7 °C greater than the gradual cooling associated with the change of seasons from fall to winter.

Rappaport E.N.,National Weather Service - NWS
Bulletin of the American Meteorological Society | Year: 2014

The article describes the findings obtained by extending the record of hurricanes that occurred in the US between 1963 and 2012. During that period, about 650-700 Atlantic tropical cyclones occurred, comprising 578 hurricanes and tropical and subtropical storms and around 100 tropical depressions. These additional cases add confidence to findings and conclusions. For context, while operational breakthroughs occur intermittently, several critical components of today's operational tropical cyclone forecast and warning program started or had their origins in innovations that took place near the beginning of the extended study period, making that period to some degree representative of the current era. Katrina took around 1,100 lives. It stands apart not just for the enormity of the losses, but for the ways in which most of the deaths occurred. Research published by Brunkard and colleagues in 2008 found that 84% of the deaths in Louisiana occurred in Orleans and neighboring St. Bernard Parishes.

Wolfe J.P.,National Weather Service - NWS | Snider J.R.,University of Wyoming
Journal of Applied Meteorology and Climatology | Year: 2012

An important application of radar reflectivity measurements is their interpretation as precipitation intensity. Empirical relationships exist for converting microwave backscatter retrieved from precipitation particles (represented by an equivalent reflectivity factor Z e) to precipitation intensity. The reflectivity-snow-rate relationship has the form Z e 5 αS β, where S is a liquid-equivalent snow rate and α and β are fitted coefficients. Substantial uncertainty exists in radar-derived values of snow rate because the reflectivity and intensity associated with snow tend to be smaller than those for rain and because of snow-particle drift between radar and surface detection. Uncertainty in radar-derived snow rate is especially evident at the few available high-altitude sites for which a relationship between reflectivity and snow rate has been developed. Using a new type of precipitation sensor and a NationalWeather Service radar, this work investigates the Ze-S relationship at a high-altitude site (Cheyenne, Wyoming). The S measurements were made 25 km northwest of the radar on the eastern flank of the Rocky Mountains; vertical separation between the radar range gate and the ground was less than 700 m. A meteorological feature of the snowstorms was northeasterly upslope flow of humid air at low levels. TheZe-S data pairs were fitted with β=2. The finding of this study for Cheyenne, α=110 mm 4 h 2 m -3, is bounded by previous determinations made at other high-altitude NationalWeather Service sites. Also investigatedwas the temperature dependence of α.Apositive α-T relationship is evident and is hypothesized to result from ice crystals produced by heterogeneous ice nucleation, at cloud top, followed by diffusional crystal growth during sedimentation. © 2012 American Meteorological Society.

Schultz C.J.,National Weather Service - NWS | Askelson M.A.,University of North Dakota
Weather and Forecasting | Year: 2012

Despite great strides in understanding the tornadic near-storm environment (NSE), at times it remains difficult to determine why some storms produce significant tornadoes, while others produce none, given similar pretornadic radar reflectivity and velocity signatures. Previous studies have shown that this is likely related to the potential buoyancy (θep) of the rear-flank downdraft (RFD) air. Unfortunately, to date there are few ways to operationally anticipate possible RFD thermodynamic character. Based upon previous research indicating that capping inversionsmay restrict much of the low-level RFD air to come from within the boundary layer, this study considers the relation of δθep (vertical change in θep within the boundary layer below the cap) to tornadogenesis potential. This is because when a cap exists above a boundary layer and the descent of lower-θep air from aloft to the surface is potentially limited, then minimal δθep may indicate more RFD air that has greater potential buoyancy. The Rapid Update Cycle (RUC) soundings used in this study and several observed soundings taken in the vicinity of violent tornadoes suggest that boundary layer δθep shows promise as an additional means of discriminating between tornadic and nontornadic NSEs. © 2012 American Meteorological Society.

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