Abstract

The landfall of Hurricane Juan (September 2003) in the Canadian Maritimes represents an ideal case in which to study the performance of operational forecasting of an intense, predominantly tropical feature entering the midlatitudes. A hybrid cyclone during its genesis phase, Juan underwent a tropical transition as it drifted slowly northward 1500 km from the east coast of the United States. Shortly after reaching its peak intensity as a category-2 hurricane, the storm accelerated rapidly northward and made landfall near Halifax, Nova Scotia, Canada, with maximum sustained winds of 44 m s⁻¹. Although the forecasts and warnings produced by the U.S. National Hurricane Center and the Canadian Hurricane Centre were of high quality throughout Hurricane Juan’s life cycle, guidance from numerical weather prediction models became unreliable as the storm accelerated toward the coast. The short-range, near-surface forecasts from eight operational models during the crucial prelandfall portion of Juan’s track are investigated in this study. Despite continued improvements to operational numerical forecasting systems, it is shown that those systems not employing advanced tropical vortex initialization techniques were unable to provide forecasters with credible near-surface guidance in this case. A pair of regional forecasts, one successful and one from the failed model set, are compared in detail. Spurious asymmetries in the initial vortex of the deficient model are shown to hamper structural predictions and to cause nonnegligible track perturbations from the trajectory implied by the well-described deep-layer mean flow. The Canadian Mesoscale Compressible Community model is rerun with an improved representation of the hurricane’s vortex in the initial state. The hindcast produced following the tropical cyclone initialization contains reduced track, structure, and intensity errors compared with those generated by the model in real time. The enhanced initial intensity produces a direct improvement in the forecast storm strength throughout the period, and the symmetrization of the vortex eliminates the interactions that plague the operational system. The southeastward relocation of the implanted vortex to Juan’s observed location eliminates a significant northwestward track bias under the influence of a broad area of southerly steering flow. The study concludes that the initialization of Hurricane Juan’s structure and position adds value to numerical guidance even as the storm accelerates poleward at a latitude where the implantation of a quasi-symmetric vortex may not be generally valid.

Abstract

The extreme precipitation index (EPI) is a coupled dynamic–thermodynamic metric that can diagnose extreme precipitation events associated with flow reversal in the mid- to upper troposphere (e.g., Rex and omega blocks, cutoff cyclones, Rossby wave breaks). Recent billion dollar (U.S. dollars) floods across the Northern Hemisphere midlatitudes were associated with flow reversal, as long-duration ascent (dynamics) occurred in the presence of anomalously warm and moist air (thermodynamics). The EPI can detect this potent combination of ingredients and offers advantages over model precipitation forecasts because it relies on mass fields instead of parameterizations. The EPI’s dynamics component incorporates modified versions of two accepted blocking criteria, designed to detect flow reversal during the relatively short duration of extreme precipitation events. The thermodynamic component utilizes standardized anomalies of equivalent potential temperature. Proof-of-concept is demonstrated using four high-impact floods: the 2013 Alberta Flood, Canada’s second costliest natural disaster on record; the 2016 western Europe Flood, which caused the worst flooding in France in a century; the 2000 southern Alpine event responsible for major flooding in Switzerland; and the catastrophic August 2016 Louisiana Flood. EPI frequency maxima are located across the North Atlantic and North Pacific mid- and high latitudes, including near the climatological subtropical jet stream, while secondary maxima are located near the Rockies and Alps. EPI accuracy is briefly assessed using pattern correlation and qualitative associations with an extreme precipitation event climatology. Results show that the EPI may provide potential benefits to flood forecasters, particularly in the 3–10-day range.

Abstract

The priority of an operational forecast center is to issue watches, warnings, and advisories to notify the public about the inherent risks and dangers of a particular event. Occasionally, events occur that do not meet advisory or warning criteria, but still have a substantial impact on human life and property. Short-lived snow bursts are a prime example of such a phenomenon. While these events are typically characterized by small snow accumulations, they often cause very low visibilities and rapidly deteriorating road conditions, both of which are a major hazard to motorists. On the afternoon of 28 January 2010, two such snow bursts moved through the Ottawa River valley and lower St. Lawrence River valley, and created havoc on area roads, resulting in collisions and injuries. Using the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR), these snow bursts were found to be associated with an approaching strong upper-tropospheric trough and the passage of an arctic front. While convection or squall lines are not common in January in Canada, snow bursts are shown to be associated with strong quasigeostrophic forcing for ascent and low-level frontogenesis, in the presence of both convective and conditional symmetric instability. Finally, this paper highlights the need for the development of a standard subadvisory criterion warning of short-lived but high-impact winter weather events, which operational forecasters can issue and quickly disseminate to the general public.

Abstract

Freezing rain is a major environmental hazard that is especially common along the St. Lawrence River valley (SLRV) in southern Quebec, Canada. For large cities such as Montreal, severe events can have a devastating effect on people, property, and commerce. In this study, a composite analysis of 46 long-duration events for the period 1979–2008 is presented to identify key synoptic-scale structures and precursors of Montreal freezing rain events. Based on the observed structures of the 500-hPa heights, these events are manually partitioned into three types—west, central, and east—depending on the location and tilt of the 500-hPa trough axis. West events are characterized by a strong surface anticyclone downstream of Montreal, an inverted trough extending northward to the Great Lakes, and a quasi-stationary area of geostrophic frontogenesis located over Quebec. Central events are characterized by a cyclone–anticyclone couplet pattern, with a deeper surface trough extending into southern Ontario, and a strong stationary anticyclone over Quebec. East events are characterized by the passage of a transient well-defined cyclone, and a weaker downstream anticyclone. In all cases, cold northeasterly winds are channeled down the SLRV primarily by pressure-driven channeling. Northeasterly surface winds are associated with strong low-level temperature inversions within the SLRV. Additionally, west events tend to have a longer duration of weaker precipitation, while east events tend to have a shorter duration of more intense precipitation. The results of this study may aid forecasters in identifying and understanding the synoptic-scale structures and precursors to Montreal freezing rain events.

Abstract

Two high-impact convective snowband events (“snow bursts”) that affected Calgary, Alberta, Canada, are examined to better understand the dynamics and thermodynamics of heavy snowbands not associated with lake effects or the cold conveyor belt of synoptic-scale cyclones. Such events are typically characterized by brief, but heavy, periods of snow; low visibilities; and substantial hazards to automobile and aviation interests. Previous literature on these events has been limited to a few case studies across North America, including near the leeside foothills of the U.S. Rockies. The large-scale dynamics and thermodynamics are investigated using the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR). Previously, high-resolution convection-explicit Weather Research and Forecasting Model (WRF) simulations have shown some ability to successfully reproduce the dynamics, thermodynamics, and appearance of convective snowbands, with small errors in location and timing. Therefore, WRF simulations are performed for both events, and are evaluated along with the NCEP North American Mesoscale (NAM) model forecasts. Both the NARR and WRF simulations show that while the two snow bursts are similar in appearance, they form as a result of different dynamic and thermodynamic mechanisms. The first event occurs downstream of an upper-tropospheric jet streak in a region of little synoptic-scale ascent, where ageostrophic frontogenesis helps to release conditional, dry symmetric, and inertial instability in an unsaturated environment. The inertial instability is determined to be related to fast flow over upstream high terrain. The second event occurs in a saturated environment in a region of Q-vector convergence (primarily geostrophic frontogenesis), which acts to release conditional, convective, and conditional symmetric instability (CSI).

Abstract

A precipitation climatology is compiled for warm-season events at Montreal, Québec, Canada, using 6-h precipitation data. A total of 1663 events are recorded and partitioned into three intensity categories (heavy, moderate, and light), based on percentile ranges. Heavy (top 10%) precipitation events (n = 166) are partitioned into four types, using a unique manual synoptic typing based on the divergence of Q-vector components. Type A is related to cyclones and strong synoptic-scale quasigeostrophic (QG) forcing for ascent, with high-θ _e air being advected into the Montreal region from the south. Types B and C are dominated by frontogenesis (mesoscale QG forcing for ascent). Specifically, type B events are warm frontal and feature a near-surface temperature inversion, while type C events are cold frontal and associated with the largest-amplitude synoptic-scale precursors of any type. Finally, type D events are associated with little synoptic or mesoscale QG forcing for ascent and, thus, are deemed to be convective events triggered by weak shortwave vorticity maxima moving through a long-wave ridge environment, in the presence of an anomalously warm, humid, and unstable air mass that is conducive to convection. In general, types A and B feature the strongest dynamical forcing for ascent, while types C and D feature the lowest atmospheric stability. Systematic higher precipitation amounts are not preferential to any event type, although a handful of the largest warm-season precipitation events appear to be slow-moving type C (stationary front) cases.

Abstract

The development of Hurricane Catarina over the western South Atlantic Ocean in March 2004 marks the first time that the existence of a hurricane has been confirmed by analysis and satellite imagery in the South Atlantic basin. The storm undergoes a complex life cycle, beginning as an extratropical precursor that moves east-southeastward off the Brazilian coast and toward the midlatitudes. Its eastward progress is halted and the system is steered back westward toward the Brazilian coast as it encounters a strengthening dipole-blocking structure east of the South American continent. Entering the large region of weak vertical shear that characterizes this blocking pattern, Catarina begins a tropical transition process over anomalously cool 25°C ocean waters above which an elevated potential intensity is supported by the cold upper-level air associated with the trough component of the block. As the convective outflow from the developing tropical system reinforces the ridge component of the dipole block, the storm is accelerated westward toward the Santa Catarina province of Brazil and makes landfall there as a nominal category-1 hurricane, causing extensive damage with its heavy rains and strong winds.

The complex evolution of the system is analyzed using a suite of diagnostic tools, and a conceptual model of the tropical transition and steering processes in the presence of a dipole block is developed. Once the essential properties of the upper-level flow are established, an analog study is undertaken to investigate lower-atmospheric responses to similar blocking regimes. Persistent dipole-blocking structures are found to be rare east of South America; however, the evolution of systems occurring during these periods is shown to be complex and to exhibit various subtropical development modes.

Abstract

A persistent feature of the Southern Hemisphere upper-level time-mean flow is the presence of a split jet across the South Pacific east of Australia during the austral winter. The split jet is composed of the subtropical jet (STJ) on its equatorward branch and the polar front jet (PFJ) on its poleward branch. The NCEP–NCAR reanalysis is used to investigate the structure and evolution of the split jet. Results show that the presence/absence of the PFJ determines the degree of split flow, given that the STJ is a quasi-steady feature. A split-flow index (SFI) is developed to quantify the variability of the split jet, in which negative values represent strong split flow and positive values nonsplit flow. Correlations with teleconnection indices are investigated, with the SFI positively correlated to the Southern Oscillation index and negatively correlated to the Antarctic oscillation.

The SFI is used to construct composites of heights, temperature, and wind for split-flow and non-split-flow days. The composites reveal that relatively cold conditions occur in the South Pacific in association with non-split-flow regimes, and split-flow regimes occur when relatively warm conditions prevail. In the latter situation cold air bottled up over Antarctica helps to augment the background tropospheric thickness gradient between Antarctica and the lower latitudes with a resulting increase in the thermal wind and the PFJ. It is surmised that frequent cold surges out of Antarctica moving into the South Pacific are associated with non-split-flow regimes. In this context, the variability of the split jet responds to large-scale baroclinic processes and is further modulated by synoptic-scale disturbances.