The synoptic-scale environments of predecessor rain events (PREs) occurring to the east of the Rocky Mountains in association with Atlantic basin tropical cyclones (TCs) are examined. PREs that occurred during 1988-2010 are subjectively classified based upon the synoptic-scale upper-level flow configuration within which the PRE develops, with a focus on the following: 1) the position of the jet streak relative to the TC, 2) the position of the jet streak relative to trough and ridge axes, and 3) the positions of trough and ridge axes relative to the PRE and to the TC. Three categories were identified from this classification procedure: ''jet in ridge,'' ''southwesterly jet,'' and ''downstream confluence.'' PRE-relative composite analysis for each category reveals that, consistent with previous studies, PREs typically occur near a low-level baroclinic zone, beneath the equatorward entrance region of an upper-level jet streak, and in the presence of a stream of water vapor from a TC. Despite these common characteristics, key differences exist among the three PRE categories related to the phasing of a TC with the synoptic-scale flow and to the interactions between a TC and its environment. Brief case studies of PREs associated with TC Rita (2005), TC Wilma (2005), and TC Ernesto (2006) are presented as specific examples of the three PRE categories.

Determining the inland extent (IE) of lake effect snow (LES) is an ongoing operational forecasting challenge at the Albany and Binghamton National Weather Service (NWS) forecast offices, and several other NWS forecast offices in the Great Lakes region. Assuming favorable conditions for development of LES, determining how far inland snow bands will extend is critical to forecasters making decisions supporting the NWS watch/ warning/advisory program and resulting impact-based decision support services. This research sought to identify which atmospheric parameters commonly have the greatest influence on how far inland LES bands travel, and to develop forecasting techniques to assist meteorologists. Single band LES events for the 2006-2009 winter seasons were examined downwind of Lake Ontario. The IE of LES bands was measured over the duration of each event and broken into quartiles. The quartiles were used to create categories for IE (short, moderate, and long). Several parameters were analyzed, using statistical correlations at data points within, and just outside of, LES bands. Box-and-whiskers plots were constructed for individual parameters relative to each IE category.The most strongly correlated parameters to IE included existence of a multi-lake/upstream moisture source connection (MLC), mixed-layer (ML) stability (represented by lake-air temperature differentials), 0-1-km bulk shear, and mean ML wind speed. LES bands featuring an MLC showed a greater tendency to progress farther inland, compared to those without. A predictive equation for forecasting IE of LES downwind of Lake Ontario was developed from a statistical model using a stepwise and backwards selection algorithm. A crossvalidation method was used to determine skill. ABSTRACT (Manuscript

Two banded, heavy snowstorms that occurred over the northern mid-Atlantic region are compared and contrasted. On 6-7 January 2002, a narrow, intense band of heavy snow was observed, along with several other weaker bands, embedded within a large area of moderate snow. On 19-20 January 2002, a single, broader band of heavy snow was observed, embedded within a broken area of light snow.The synoptic-scale settings associated with these two storms were strikingly dissimilar. In the first case, strong quasigeostrophic (QG) forcing for ascent was present just to the south of the heavy snowfall area. A highly amplified longwave trough was located over the Mississippi River valley, while a compact shortwave trough moved northward, up the east side of the longwave trough. The result was robust cyclogenesis off of the midAtlantic coast. In the second case, the relatively weaker QG forcing for ascent was located much farther southwest of the snowband. The flow aloft was much less amplified, with weaker cyclogenesis occurring off of the midAtlantic coast.Analysis of the frontal scale environments for both cases indicated that the snowbands were each associated with the collocation of midtropospheric frontogenesis and reduced stability. In the first case, evidence is shown that a layer of potential symmetric instability (PSI) was located just above a deep, sloping zone of frontogenesis, in the presence of deep near-saturated conditions. In the second case, evidence is shown that a layer of potential instability (PI), associated with rapidly decreasing relative humidity with height, was located just above a shallow, sloping zone of frontogenesis. In addition, it is shown that a particularly favorable thermal environment for snowflake growth and accumulation became collocated with the heavy snowband. It is hypothesized that the differences in the intensity and horizontal extent of the bands observed with these two events resulted from differing atmospheric responses associated with the areal extent of large-scale and frontogenetical forcing, moisture availability, degree of instability, and specific thermal profiles.

North American Mesoscale (NAM) model forecasts of the occurrence, magnitude, depth, and persistence of ingredients previously shown to be useful in the diagnosis of banded and/or heavy snowfall potential are examined for a broad range of 25 snow events, with event total snowfall ranging from 10 cm (4 in.) to over 75 cm (30 in.). The ingredients examined are frontogenetical forcing, weak moist symmetric stability, saturation, and microphysical characteristics favorable for the production of dendritic snow crystals. It is shown that these ingredients, previously identified as being critical indicators for heavy and/or banded snowfall in major storms, are often found in smaller snowfall events. It is also shown that the magnitude, depth, and persistence of these ingredients, or combinations of these ingredients, appear to be good predictors of event total snowfall potential. In addition, a relationship is demonstrated between temporal trends associated with one of the ingredients (saturated, geostrophic equivalent potential vorticity) and event total snowfall.Correlations between forecast values of these ingredients and observed snowfall are shown to decrease substantially as forecast lead time increases beyond 12 h. It is hypothesized that model forecast positioning and timing errors are primarily responsible for the lower correlations associated with longer-lead forecasts. This finding implies that the best forecasts beyond 12 h may be produced by examining the diagnostics of heavy snow ingredients from a single, high-resolution model to determine snowfall potential, then using ensemble forecasting approaches to determine the most probable location and timing of any heavy snow.

Supercell storms are the most prolific producers of violent tornadoes, though only a fraction of supercells produce tornadoes. Past research into the differences between tornadic and nontornadic supercells have provided some insights but are of little utility to a real-time warning decision process. Operational weather radars provide consistent observations in real time, but conventional radar techniques have not been able to effectively distinguish between tornadic and nontornadic supercells. After the national radar network upgrade to polarimetric capabilities in 2013, a polarimetric signature frequently observed in supercells is the separation of low-level enhanced differential reflectivity Z DR and specific differential phase K DP regions. We analyzed this signature in tornadic and nontornadic supercell cases and found that, although the separation distances are similar, the separation orientations are statistically significantly different. Tornadic supercells have orientations more orthogonal to storm motion and nontornadic supercells have more parallel orientations. Possible reasons for these differences are discussed. Plain Language Summary Supercell storms are responsible for a vast majority of violent tornadoes, but most supercells do not produce tornadoes at all. Finding differences between supercells that produce tornadoes and those that do not has been a goal of meteorologists for several decades and is important for issuing tornado warnings. Previous research has either used computer simulations or studied specific storms from specialized field campaigns. Neither of these are useful in a realistic tornado warning process. Weather radars provide constant monitoring of supercells, but techniques from the past few decades have been unsuccessful in finding differences between supercells that produce tornadoes and ones that do not. The national radar network was recently upgraded in 2013 and has provided new information. A new signature from the upgraded network is the separation of regions with large values of two different radar variables, which is analyzed in this study in a large number of tornadic and nontornadic supercells. We found that the separation distances are similar but the orientations are significantly different between tornadic and nontornadic supercells.