Current Case Display

2017-02-15

Examples of Two February Squall lines Mesoscale features are often difficult to Predict On February 13 and 15 2017 two squall lines cross portions of the Mid-Atlantic region. The first squall line was observed during the evening hours of Sunday 12 February . Radar signatures implied 30 to 40dBZ echoes of 8-10,000 feet, low topped squalls which produced 73 reports of severe weather (Fig. 1). There were reports of lightning though lightning plots implied most of the lightning in Pennsylvania was located at turbine locations. The second squall line occurred in a slightly colder air mass and in central Pennsylvania the squalls contained snow. There were no known reports of severe weather. During the snow squalls visibilities dropped to 1/4SM in snow and blowing snow. Despite temperatures above freezing the rapid snow rates coated the ground . Similar to the 13 February event, there were isolated reports of thunder in the initial squall line though reports came from locations close to windfarms, the data showed most strikes along the line near or over wind turbines (Fig. 2) and larger windfarms. However, western portions of State College had several reports of thunder at least 12 miles from the nearest wind turbine. February snow squalls are not uncommon with strong cold fronts and arctic fronts. However, squall lines containing thunder are more unique and squalls producing severe weather are climatologically relatively rare in Pennsylvania and most of the Mid-Atlantic region. These two events offer a contrast in predictability. It will be shown that the squall line of 13 February was relatively well predicted and the squall line of 15 February was poorly predicted. Another interesting issue is the impact of the wind farms on lightning strikes.

2017-02-09

Northeastern United States Snowstorm of 9 February 2017: A strong shortwave produced a stripe of precipitation from the western Plains to East Coast on 8-9 February 2017 (Fig. 1). Most of the precipitation along and north of the track of the surface cyclone (Fig 2) fell as snow (Fig. 3). The highest snowfall totals were observed from northern New Jersey across Long Island and into New England. Though not shown, radar and lightning data showed an intense band of snow and a prolonged period of thunder-snow in the band from southern Maine into Massachusetts. Areas in this band received over 16 inches of snowfall.

2017-01-13

Minor Winter Flooding Event in northwestern Pennsylvania 12-13 January 2017 The combination of unseasonably warm air and an intrusion of humid air likely helped produce snow melt in northwestern Pennsylvania and southwestern New York. This likely reduced significant water from the snow pack. This combined with a widespread rainfall event of 1-2 inches (Fig 1) led to localized flooding over the region. Additionally, much of the region affected by the higher rainfall amounts received most of the rainfall in a 6-hour window. Many locations received 1 to 1.5 inches of QPE in a relatively short 6-hour window. The pattern in which the rainfall developed was relatively well forecast (not shown). The NCEP GEFS was able to predict the potential for 1 inch or more QPF but was limited in its ability to produce much more than 1.5 inches of QPF. Due to the frozen ground, snowmelt, and the 2 inches of rainfall, this relatively low end QPF/QPE event did produce minor flooding. The threshold for flooding is typically a bit higher, in the 3 inches and greater range in the warm season. However, in this case the antecedent and current conditions favored a better hydrologic response with relatively low rainfall amounts.

2017-01-12

Eastern United States Warm Episode of 11-12 January 2017: A large ridge of the southern United States (Fig.1) with +1 to +2500 hPa height anomalies brought period of warm weather to much of the eastern United States from 11 to 13 January 2017. During this period of time about 809 maximum temperature records were set or tied and 540 maximum low temperature records were set or tied (Table 1). The warmest day over much of the Mid-Atlantic region was 12 January 2017 when temperatures peaked in the 60s over much of Pennsylvania and Maryland. The warmest day over the Mid-Atlantic region was the 12th of January ahead of the cold front. There was a surge of high precipitable air ahead of the front (Fig. 3c) and the 850 hPa temperatures were well above 0C and were +1 to +2 above normal over much of the eastern United States with the highest standardized anomalies over the northeastern United States.

2017-01-08

The California Extreme Precipitation Event of 8-10 January 2017: A strong Pacific jet and a surge of high precipitable water (PW: Fig. 1) brought extremely heavy precipitation to portions of California on 8-9 January 2017 (Fig. 2). Rainfall amounts in excess of 200 mm were observed and there were reports of extreme snowfall to include. NBC news reported “epic snowfall” in the Sierras. The Los Angeles Times reported flooding, record snow, extremely high winds at higher elevations, extreme snowfall, and blizzard conditions in the Sierras on 8-9 January 2017. Squaw Valley reported winds of 99 MPH with gusts as high as 159 MPH . The strong Pacific system was associated with a near classic pattern for significant precipitation events in California which included a surge of high PW (Fig. 1) also known as an atmospheric river (AR: Neiman et al . 2008), a deep trough off the West Coast, a strong Ridge over the north Pacific and a ridge to the east (Fig. 3). This results in a strong 250 hPa jet (Fig. 4) moving into then over the affected region (Junker et al. 2008). It will be shown that the general pattern was well forecast and thus the NCEP GEFS was able to forecast the general regions of heavy rain in California. This probably was an easy forecast as the mountains act as fixed forcing for the impinging moisture and energy from the Pacific. This paper will provide an overview of the event and examine NCEP GEFS forecasts of the QPF. This was a multi-day event and the focus was on the heavy precipitation from 0000 UTC 8 to 0000 UTC 10 January. There was a significant event prior to this and another surge of precipitation after this time.

2017-01-07

East Coast Winter Storm of 7-8 January 2017:A winter storm produced snow and areas of heavy snow from North Carolina to southeastern New England on 7-8 January 2017 (Fig. 1). The snow overnight Friday into Saturday fell mainly from North Carolina to Virginia (Fig. 1: upper). Much of eastern North Carolina had ice pellets and freezing rain which limited the snow totals. Coastal Virginia and the Delmarva had heavy snow during the morning and early afternoon of 7 January. Later in the day the snow shifted to Long Island (Table 1) and southern New England (Table 2). Some of the higher snowfall amounts, in the 12 to 19 inch range were observed in southeastern Massachusetts and Rhode Island. The broad area of 12 to 16 inches of snow in southern Massachusetts was where the higher snow amounts were observed (Fig. 1: lower). The snow was the result of a relatively deep 500 hPa low (Fig. 2a), a surge of cold arctic air into the eastern United States (Fig. 2b), a surge of deep moisture along the East Coast (Fig. 2c), an elongated low off the east coast. The snow fell in the region of strong northwesterly flow between the cyclone and the strong anticyclone to the west (Fig. 2d). Note that during the peak of the event in the southeast the 850 hPa temperatures were in the -8 to -12C range as far south as northern Georgia (Fig. 2b). This paper will examine the East Coast Winter Storm (ECWS) of 7-8 January 2017. The focus is on the pattern and forecasts of the event. The storm occurred over a weekend which may have limited the overall impact of the storm. Additionally, with the exception of Boston, MA the major cities of the Mid-Atlantic and Northeast did not receive significant snowfall with this event.