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Clifford F. Mass
,
Robert Conrick
,
Nicholas Weber
, and
Joseph P. Zagrodnik

Abstract

On 27 January 2018, a highly localized, strong wind event occurred along the north shore of Lake Quinault, Washington. The resulting loss of large old-growth trees in a roughly 0.5-km2 region led to blocked roads and power outages. Nearby surface stations did not record anomalous winds, and no tree damage was reported in the surrounding region. Based on public accounts and a nearby seismometer, it appears that the strong winds lasted less than 10 min. Surface and aerial damage surveys showed that the trees fell from a different direction (northerly) than the synoptic or mesoscale f low (southwesterly to southeasterly). Based on high-resolution Weather Research and Forecasting (WRF) Model simulations, it appears that the damaging northerly winds were the result of a strong atmospheric rotor produced by a high-amplitude mountain wave. A simulation with 148-m grid spacing produced a rotor at the same time and location as the treefalls. Synoptic analysis and the high-resolution simulation showed that moderately strong southeasterly flow and a stable layer associated with the approaching occluded front interacted with a ∼750-m-high upstream mountain ridge to produce the mountain wave and associated rotor circulation. The combination of an inversion and strong shear at and above the upstream ridge were outliers in a climatology of soundings from the nearby Quillayute rawinsonde site, suggesting that such intense mountain-wave rotors are unusual in this valley.

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Vlado Malačič
and
Nedjeljka Žagar

Abstract

The marine icicles that form on coastal constructions (e.g., piers and railings) are very common in polar and subpolar areas. The occasional formation of icicles in the coastal zones of the Mediterranean Sea occurs in relation to cold-air outbreaks from the polar region, such as the one presented in this paper in February 2018. The air temperatures over the northern Adriatic Sea, the northernmost part of the Mediterranean, dropped below –2.1°C, a necessary condition for seawater to freeze, with salinity between 38.0 and 38.5 PSU. The formation of icicles on the coastal structures was further enabled by the bora wind and related high seas along the coast. Measurements presented in this paper confirm that the icicles in the Bay of Piran in the Gulf of Trieste (45.55°N) were formed from the seawater. The measured salinity level of the melted icicles, around 9 PSU, is a typical value reported for marine icicles in polar regions.

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Alan W. Black
,
John A. Knox
,
Jared A. Rackley
, and
Nicholas S. Grondin

Abstract

We examine the trajectories of four historical markers displaced during an enhanced Fujita scale 2 (EF2) tornado at the Fort Pulaski National Monument located on Cockspur Island, east of Savannah in southeast Georgia. The careful work of National Park Service employees in cataloguing the origin and landing points of the debris allows for an unusually accurate analysis of tornado debris trajectories for heavy objects. These markers, weighing around 68 kg (150 lb) each, traveled intact for distances of up to 220 m (750 ft). One of the historical markers was fractured into at least three pieces, the larger of which traveled 300 m (1,000 ft). Understanding the travel for these relatively heavy items is important, as they are similar in weight to household appliances that could commonly be part of a tornado debris field.

Open access
Daniel T. Lindsey
,
Dan Bikos
, and
Lewis Grasso

Abstract

Geostationary Operational Environmental Satellite-16 (GOES-16) was launched into geostationary orbit in late 2016 and began providing unprecedented spatial and temporal resolution imagery early in 2017. Its Advanced Baseline Imager has additional spectral bands including two in the “clear” window and “dirty window” portion of the infrared spectrum, and the difference of these two bands, sometimes called the split window difference, provides unique information about low-level water vapor. Under certain conditions, low-level convergence along a boundary can cause local water vapor pooling, and the signal of this pooling can sometimes be detected by GOES-16 prior to any cloud formation. This case study from 15 June 2017 illustrates how the technique might be used in an operational forecast setting. A boundary in western Kansas was detected using the split window difference more than 2 h before the first cloud formed.

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Yanluan Lin
,
Yuanlong Li
,
Qingshan Li
,
Minyan Chen
,
Fanghua Xu
,
Yuqing Wang
, and
Bin Huang
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Arunas P. Kuciauskas
,
Peng Xian
,
Edward J. Hyer
,
Mayra I. Oyola
, and
James R. Campbell

Abstract

During the spring and summer months, the greater Caribbean region typically experiences pulses of moderate to heavy episodes of airborne African dust concentrations that originate over the Sahara Desert and propagate westward across the tropical North Atlantic basin. These dust episodes are often contained within the Saharan air layer (SAL), an elevated air mass (between 850–500 hPa) marked by very dry and warm conditions within the lowest levels. During its westward transport, the SAL’s distinct environmental characteristics can persist well into the Gulf of Mexico and southern United States. As a result, the Caribbean population is susceptible to airborne dust levels that often exceed healthy respiratory limits. One of the major responsibilities within the National Weather Service in San Juan, Puerto Rico (NWS-PR), is preparing the public within their area of responsibility (AOR) for such events. The Naval Research Laboratory Marine Meteorology Division (NRL-MMD) is sponsored by the National Oceanic and Atmospheric Administration (NOAA) to support the NWS-PR by providing them with an invaluable “one stop shop” web-based resource (hereafter SAL-WEB) that is designed to monitor these African dust events. SAL-WEB consists of near-real-time output generated from ground-based instruments, satellite-derived imagery, and dust model forecasts, covering the extent of dust from North Africa, westward across the Atlantic basin, and extending into Mexico. The products within SAL-WEB would serve to augment the Advanced Weather Interactive Processing System (AWIPS-II) infrastructure currently in operation at the NWS-PR. The goal of this article is to introduce readers to SAL-WEB, along with current and future research underway to provide improvements in African dust prediction capabilities.

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James R. Campbell
,
David A. Peterson
,
Jared W. Marquis
,
Gilberto J. Fochesatto
,
Mark A. Vaughan
,
Sebastian A. Stewart
,
Jason L. Tackett
,
Simone Lolli
,
Jasper R. Lewis
,
Mayra I. Oyola
, and
Ellsworth J. Welton
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Peter Stucki
,
Stefan Brönnimann
,
Olivia Martius
,
Christoph Welker
,
Ralph Rickli
,
Silke Dierer
,
David N. Bresch
,
Gilbert P. Compo
, and
Prashant D. Sardeshmukh
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Joseph C. Picca
,
David M. Schultz
,
Brian A. Colle
,
Sara Ganetis
,
David R. Novak
, and
Matthew J. Sienkiewicz

The northeast U.S. extratropical cyclone of 8–9 February 2013 produced blizzard conditions and more than 0.6–0.9 m (2–3 ft) of snow from Long Island through eastern New England. A surprising aspect of this blizzard was the development and rapid weakening of a snowband to the northwest of the cyclone center with radar ref lectivity factor exceeding 55 dBZ. Because the radar reflectivity within snowbands in winter storms rarely exceeds 40 dBZ, this event warranted further investigation. The high radar reflectivity was due to mixed-phase microphysics in the snowband, characterized by high differential reflectivity (Z DR > 2 dB) and low correlation coefficient (CC < 0.9), as measured by the operational dual-polarization radar in Upton, New York (KOKX). Consistent with these radar observations, heavy snow and ice pellets (both sleet and graupel) were observed. Later, as the reflectivity decreased to less than 40 dBZ, surface observations indicated a transition to primarily high-intensity dry snow, consistent with lower-tropospheric cold advection. Therefore, the rapid decrease of the 50+ dBZ reflectivity resulted from the transition from higher-density, mixed-phase precipitation to lower-density, dry-snow crystals and aggregates. This case study indicates the value that dual-polarization radar can have in an operational forecast environment for determining the variability of frozen precipitation (e.g., ice pellets, dry snow aggregates) on relatively small spatial scales.

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Randy Graham
,
Trevor Alcott
,
Nanette Hosenfeld
, and
Richard Grumm

From 18 January to 23 January 2010, a series of winter storms impacted the western United States. During this period, a record-setting system produced severe convection, high winds, and heavy rain and snow on 21–22 January. The severe weather included tornadoes in California and gusts in excess of 40 m s−1 associated with an intense squall line affecting southeast California and Arizona. One of the primary impacts of the storms was a heavy precipitation event across Arizona. Rainfall amounts of 125–250 mm were recorded along the Mogollon Rim in central Arizona, while higher elevations in northern Arizona received 100–150 cm of snow, with one site setting the state's 24-h snowfall record. The heavy snow and high winds resulted in widespread power outages and paralyzed travel across portions of northern Arizona. All-time minimum pressure records were set across a large portion of the western United States from Oregon to Arizona.

This was an extraordinary event that was well predicted. Standardized anomalies derived from the GFS Ensemble Forecast System (GEFS) indicated a potentially historic storm one week in advance. The forecast synoptic-scale anomalies were well correlated with high-impact weather across the western United States. This case demonstrates the utility of using standardized anomalies to increase situational awareness, which enables operational forecasters to provide decision makers with information regarding the potential significance of pending weather events. The event will also be utilized to demonstrate an anomaly-based situational awareness display for streamlining the identification, and analysis, of significant forecast anomalies.

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