Search

Though prolonged freezing rain events are rare, they can result in substantial damage when they occur. While freezing rain occurs less frequently in the south-central United States than in some regions of North America, a large number of extremely long-duration events lasting at least 18 h have been observed there. We explore the key synoptic--dynamic conditions that lead to these extreme events through a comparison with less severe short-duration events. We produce synoptic--dynamic composites and 7-day backward trajectories for parcels ending in the warm and cold layers for each event category. The extremely long-duration events are preferentially associated with a deeper and more stationary 500-hPa longwave trough centered over the southwestern United States at event onset. This trough supports sustained flow of warm, moist air from within the planetary boundary layer over the Gulf of Mexico northward into the warm layer. The short-duration cases are instead characterized by a more transient upper-level trough axis centered over the south-central U.S. region at onset. Following event onset, rapid passage of the trough leads to quasigeostrophic forcing for descent and the advection of cold, dry air that erodes the warm layer and ends precipitation. While trajectories ending in the cold layer are very similar between the two categories, those ending in the warm layer have a longer history over the Gulf of Mexico in the extreme cases compared with the short-duration ones, resulting in warmer and moister onset warm layers. [ABSTRACT FROM AUTHOR]

Freezing rain is an especially hazardous winter weather phenomenon that remains particularly challenging to forecast. Here, we identify the salient thermodynamic characteristics distinguishing long-duration (six or more hours) freezing rain events from short-duration (2–4 h) events in three regions of the United States and Canada from 1979 to 2016. In the northeastern United States and southeastern Canada, strong surface cold-air advection is not common during freezing rain events. Colder onset temperatures at the surface and in the near-surface cold layer support longer-duration events there, allowing heating mechanisms (e.g., the release of latent heat of fusion when rain freezes at the surface) to act for longer periods before the surface reaches 0°C and precipitation transitions to rain. In the south-central United States, cold air at the surface is replenished via continuous cold-air advection, reducing the necessity of cold onset surface temperatures for event persistence. Instead, longer-duration events are associated with warmer and deeper >0°C warm layers aloft and stronger advection of warm and moist air into this layer, delaying its erosion via cooling mechanisms such as melting. Finally, in the southeastern United States, colder and especially drier onset conditions in the cold layer are associated with longer-duration events, with evaporative cooling crucial to maintaining the subfreezing surface temperatures necessary for freezing rain. Through an improved understanding of the regional conditions supporting freezing rain event persistence, we hope to provide useful information to forecasters in their attempt to predict these potentially damaging events. [ABSTRACT FROM AUTHOR]

Freezing rain can cause severe impacts, particularly when it persists for many hours. In this paper, we present the climatology of long-duration (6 or more hours) freezing rain events in the United States and Canada from 1979 to 2016. We identify three focus regions from this climatology and examine the archetypal thermodynamic evolution of events in each region using surface and radiosonde observations. Long-duration events occur most frequently in the northeastern United States and southeastern Canada, where freezing rain typically begins as lower-tropospheric warm-air advection develops the warm layer aloft. This warm-air advection and the latent heat of fusion released when rain freezes at the surface erode the cold layer, and freezing rain transitions to rain once the surface temperature reaches 0°C. In the southeastern United States, a larger percentage of events are of long duration than elsewhere in North America. Weak surface cold-air advection and evaporative cooling in the particularly dry onset cold layers there prevent surface temperatures from rising substantially during events. Finally, the south-central United States has a regional maximum in the occurrence of the top 1% of events by duration (18 or more hours), despite the relative rarity of freezing rain there. These events are associated with particularly warm/deep onset warm layers, with persistent low-level cold-air advection maintaining the cold layer. The thermodynamic evolutions we have identified highlight characteristics that are key to supporting persistent freezing rain in each region and may warrant particular attention from forecasters tasked with predicting these events. [ABSTRACT FROM AUTHOR]

Several classes of significant cold-season precipitation events occurring in Burlington, Vermont (KBTV), during the 33-yr period from 1963 to 1995, are studied with the objective of identifying large-scale circulation precursors to the more extreme events. Several physically interesting and unique features that correspond to 24-h totals of 25 to 50 mm of precipitation are found. Preferential southerly and more maritime surface geostrophic flow occur in the heavier cases, in association with a strong cyclone (anticyclone) to the west (east) of KBTV. The 1000–500-hPa positive thickness anomaly corresponds to a depth-mean virtual temperature anomaly of +10.5°C in the heavy events. Additionally, statistically significant negative thickness anomalies, responsible for triggering these significant precipitation events, can be traced westward to a position in the Pacific Ocean at least 6 days prior to the event. Significantly heavier precipitable water amounts and preferentially strong water vapor transports from maritime regions are also associated with the heavier cold-season precipitation events. [ABSTRACT FROM AUTHOR]

Warm, moist southwesterly airflow into the northwestern United States during the cold season can result in rapid snowmelt and flooding. The objectives of this research are to document characteristic synoptic flow patterns accompanying cold-season (November-March) flooding events, and isolate flow anomalies associated with the moisture transport during a representative event. The first objective is accomplished through a 46-case composite spanning the years 1962-88; the second objective is addressed through diagnosis of a flooding event that occurred on 17-18 January 1986. The 46-case composite is constructed for a 6-day period centered at 1200 UTC on the day of heavy precipitation onset (denoted τ[sub 0]). Composite 500-hPa geopotential height anomaly fields reveal anomalous ridging over the Bering Sea preceding the precipitation event, a negative anomaly over the Gulf of Alaska throughout the composite evolution, and a positive anomaly over the southwestern Unites States and adjacent eastern Pacific Ocean during and after the event. The gulf trough and southwestern ridge lead to enhanced southwesterly geostrophic flow into the northwestern United States at τ[sub 0]. A positive temperature anomaly at the 850-hPa level advances northeastward into the northwestern United States by τ[sub 0], and expands over much of the United States by τ[sub +48. Piecewise geostrophic moisture transport computations for 17-18 January 1986, based on quasigeostrophic potential vorticity inversion, demonstrate that the transport of moisture into the northwestern United States is largely associated with a duo of mobile cyclones that track from the subtropical Pacific Ocean toward British Columbia. There is also a smaller contribution from a stationary anticyclone over the southwestern United States. These results indicate that the role of the planetary-scale flow, as depicted in the composite analyses, is to provide a persistent storm track, while the moisture flow... [ABSTRACT FROM AUTHOR]