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2016-05-21

redictability Horizons and the cut-off low 21-22 May 2016:A late season cut-off 500 hPa low developed over the Mid-Atlantic region on 21-22 May 2016 (Fig. 1). Near the track of the cut-off clouds and rain kept temperatures unseasonably cold. Areas north of the cut-off were relatively warm. On 21 May temperatures in the Mid-Atlantic region and Pennsylvania were in the 50s while central and northern New England, near the larger scale ridge were in the 70s and 80s. Forecasts of the cut-off low from 6 GEFS forecast cycles indicated considerable uncertainty with the evolution of the 500 hPa wave and cyclone (Fig. 2). As will be shown, this had a significant impact on the weather near the surface, specifically the rainfall and temperature forecast. Many of the forecast issues associated with this event were quite similar to those often associated with a winter storm. The large uncertainty with the evolution of the cut-off cyclone affected the track of the precipitation shield. Thus the concept here apply to many events including winter storms. This paper will summarize the pattern and the forecast issues in the NCEP GEFS associated with cut-off cyclone and rainfall event of 21-22 May 2016. Focus is on the predictability horizon (PH) of this event and how to deal with uncertainty in the forecast process.

2016-05-01

Recent Trends in Northern and Southern Hemispheric Cold and Warm Pockets:The climate forecast system was used to examine trends in warm and cold pools and trends in the precipitable water in both the northern and southern hemispheres from 1979-2010. The data were examined by season focusing on potential extremes in warm and cold season in both hemispheres. The data revealed a stronger signal in 850 hPa temperatures and 500 hPa heights in the northern hemisphere. Generally, areas covered by extreme cold air; defined as the areas where the 850 hPa temperatures were equal to or less than -30C (243K); in the northern hemisphere winter are decreasing. During the warm season, the areas covered by precipitable water values equal to or greater than 60mm are increasing. Similar but weaker 850 hPa temperature and 500 hPa height trends were found in the southern hemisphere. However, the trends in southern hemispheric precipitable water were similar to those in the northern hemisphere.

2016-04-18

Southeastern Texas historic rain of 18 April 2016:Extremely heavy rainfall across southeastern Texas (Fig.1) produced major flooding in the Houston Metropolitan area during the morning hours of 18 April 2016. The flooding disrupted most human activities closing roads and causing the Houston areas schools to close for two days due to water on roads, and led to at least eight confirmed deaths (Houston Chronicle, AP 2016). Observed rainfall was likely over 17.60 inches (Table 1 based on NWPNS) based on gage data. The Stage-IV data (Fig. 1) indicated 250 to 300 mm (14 inches) of rain for the 24 hour period ending at 1800 UTC 18 April 2016. The Stage-IV gridded rainfall 24 hour total ending at 1800 UTC 18 April (Fig. 2) indicated areas in southeastern Texas had 75 to 125% of the 100 year recurrence interval in the 24 hour period. The Average Recurrence Interval (ARI) is based on the blended NOAA14 and NOAA40 data . The 6-hour period of heaviest rain was observed in the period of 0600 to 1200 UTC (Fig. 2 lower panel). The impacts of the event included extreme flooding, closed schools, and 8 fatalities. A critical issue in the Houston area is underpasses. The majority of the fatalities involved individuals driving into deeply flooded under passes. One individual died after she drove around a barrier before the steep drop-off into the under pass. The other 7 people drove into flood under passes. The City is investigating better means to prevent this type of tragedy. Many of the deaths occurred in under passes where similar deaths have occurred before (Houston Chronicle 2016). This paper will document the pattern in which the heavy rainfall occurred and present methods which may aide anticipating and characterizing this and similar events. The focus is on using climate data to identify the potential for heavy rainfall including the pattern in which the event occurred and the value of Average recurrence intervals (ARI: NOAA14) to characterize the observed and forecast precipitation. The estimate precipitation (QPE) and forecast precipitation (QPE) are shown along with ratios relative to the ARI values. Forecasts from several NCEP forecasts systems are also shown relative to the ARI values as ratios.

2016-04-16

Western Plains heavy rain and Mountain snows of 15-17 April 201: A significant precipitation event affected the western Plains on 15-17 April 2016 (Fig. 1). The event included heavy snows along the front range of the Rocky Mountains and heavy rains in the plains of eastern Colorado and western Kansas and Nebraska. The multi-day event in the plains was the result of an amplifying ridge in the eastern North America (Fig. 2) as a deep trough moved into the southern Rocky Mountains. Deep southerly flow developed between the large ridge to the east and the trough to the west. High precipitable water (PW) air, with PW anomalies over 2above normal moved into the Plains from the south (Fig. 3) with the maximum PW anomalies peak at over above 3normal. This paper will document the pattern in which the heavy precipitation occurred and present methods which may aide anticipating and characterizing this and similar events. The focus is on using climate data to identify the potential for heavy rainfall including the pattern in which the event occurred and the value of Average recurrence intervals (ARI: NOAA14) to characterize the observed and forecast precipitation. The estimate precipitation (QPE) and forecast precipitation (QPE) are shown along with ratios relative to the ARI values. Forecasts from several NCEP forecasts systems are also shown relative to the ARI values as ratios.

2016-03-31

A Review of warmth of March 2016 Globally averaged, the month of March 2016 was the warmest March on record dating back to the 1880, the first year of the NOAA global temperature dataset (NOAA, 2016). Based on the NOAA data (Fig.1) most of the record warmth was focused over the Indian and Pacific oceans. It was much above normal over a large extent of North America. The pattern over North America (Fig. 2) for the month of March showed a trough (Fig. 2a) over eastern Canada and a ridge over the southwestern Atlantic and Caribbean. Positive height anomalies were located in the ridge and these anomalies extended into the eastern United States. This note summarized the pattern and conditions of March 2016.

2016-03-09

Southern Heavy rain and floods of 8-10 March 2016:Heavy rains (Fig. 1) produced record flooding in northeastern Texas and northwestern Louisiana 9-11 March 2016. The period of heaviest rain was the 24 hour period from about 1800 UTC 8-9 March 2016. Some areas of northwestern Louisiana had over 300 mm (11 inches). Point maximum data indicated during the longer period of time rainfall may have exceed 20 inches in some locations. The heavy rains produced record flooding along the Sabine River which runs along the Texas and Louisiana border. Record crests along the Sabine were measured at several locations including Burkeville, Deweyville, and Bon Wier (TWC, USA Today, AP 2016). Several of the sites broke records which were had stood for over 100 years. This paper will document the pattern in which the heavy rainfall occurred and present methods which may aide anticipating this and similar events. The focus is on using climate data to identify the potential for heavy rainfall including the pattern in which the event occurred and the value of Average recurrence intervals (ARI: NOAA14) to characterize the observed and forecast precipitation. The estimate precipitation (QPE) and forecast precipitation (QPE) are shown along with ratios relative to the ARI values. Forecasts from several NCEP forecasts systems are also shown relative to the ARI values as ratios.

2016-02-24

Eastern United States Severe and Tornado Event of 24 February 2016:A rare late winter severe weather event produced widespread severe weather from the Carolinas to New England on 24 February 2016. The strong frontal system produced over 20 tornadoes and basically tripled the recorded February tornado climatology for Pennsylvania. The first February EF2 tornado and the first tornados since February 1990 were observed on 24 February 2016. The tornadoes occurred in an environment which rarely develops in February. The surge of deep moisture and anomalous PW along with a strong low-level jet contributed to the instability and shear necessary for severe weather. The PW, 850 hPa winds, and temperatures were all well above normal for February. Surface temperatures were in the 50s and 60s over much of the mid-Atlantic region where the convection was observed and farther south temperatures were in the 70s. The CAPE was relatively low but the strong shear allowed for significant convection to develop. The satellite and lightning data showed a line of storms which produced lightning from Pennsylvania to southern Virginia. The WV image implied a dry slot moving over the low-level moist air may have contributed to the event. The radar data indicated an extremely strong mesocyclone with winds varying by over 80kts across the mesocyclone which crossed Lancaster County, PA. The strong shear in the mesocyclone was detected by the 3 WSR-88D which cover Lancaster County. Base V in the area was observed to be over 100 to 110kts at times. Strong winds, unseasonably warm and moist air came together to produce a rare February tornado event across Pennsylvania and the Mid-Atlantic region.

2016-02-14

Eastern United States Big Chill of 13-15 February 2016: A surge of arctic air with a strong and deep 500 hPa trough brought a short-lived shot of extremely cold air to the eastern United States on 13-14 February 2016. The pocket of coldest air had 850 hPa temperatures values between -30 and 32C. In recent years, there has been a dearth of large pockets of sub -30C air at 850 hPa. The crème de la crème of the cold air at 850 hPa used to denote arctic is in North America has been fleeting in recent years. This contraction of the cold air has been document by Martin (2014). This short-lived but intense arctic air mass focused over New York and New England. This led to temperatures below 0F at New York City, Providence, and Boston. New York had not seen a sub-zero reading in decades. Temperatures were well below zero over much of the eastern United States from northern Pennsylvania to Maine with readings in the -20 to -30F range over northern New York and New England. The NCEP GEFS and the joint NCEP-CMC NAEFS should considerable skill in predicting this intense arctic outbreak to include the short duration of the event. The larger scale features to include the large ridge to the west were relatively well forecast. The cold pool over the Northern hemisphere was examined using Python to find the areas -20 and -30C. Total area covered by the -20C contour peaked over North America at 0000 UTC 14 February with a deep cold pocket over eastern North America and Greenland. The total area was about 6.46% of the area of the Northern Hemisphere. The area covered by the -30C air was smaller, about 0.74 percent of the entire Northern Hemisphere. The two notable cold pockets are over Siberia and eastern North America with a third smaller area over northern Greenland. The area of Siberia persisted for several days while the pool over North America quickly dissipated.

2016-01-23

Historic Eastern United States Winter Storm of 22-24 January 2016: Record snow along the megalopolitan corridor:The storm developed as a strong 500 hPa shortwave moved into the southern Plains (Fig. 3) and across the Gulf States. This short-wave originated over the North Pacific Ocean (not shown). As this wave moved eastward, strong convergence developed over the northeastern United States (Fig. 3a-d). A strong 250 hPa jet developed in this region (Fig. 6). At 1200 UTC 22 January 2016 (Fig. 6b) a well-developed “banana jet” was present over the eastern United states with an implied jet entrance over the Ohio Valley. At 850 hPa on 22 January a strong 850 hPa LLJ was present with -4850 hPa u-wind anomalies (Fig. 5a-c). The heavy snow (Fig. 2) in south-central Kentucky fell in close proximity to this strong 850 hPa easterly jet. As the pattern shifted eastward, the 250 hPa winds peaked along the East Coast around 0000 UTC 23 January 2016 (Fig. 7c). Between 0000 and 0600 UTC the 850 hPa jet had strengthened along the Mid-Atlantic coast (Fig. 8a-c) with -6850 hPa u-wind anomalies. This strong LLJ was associated with the QPE and snow maximum across the Mid-Atlantic region from West Virginia across northern Maryland and southern Pennsylvania. During this period the coastal cyclone deepened as it moved up the coast (Fig. 9). The surface cyclone pressure in the CFS was about -4 below normal. The strong 250 hPa jet and the strong easterly flow at 850 hPa are well known features associated with many major and historic East Coast Winter storms. This storm with -6 850 hPa jet was one of the strongest ECWS in recent memory and the 850 hPa u-wind anomalies were on the same scale as those observed during Hurricane Sandy in October 2012. The strong 850 hPa winds were a known signal for a potential record setting event. The storm was generally well forecast with the notable exception of the heavy snow which fell well north of many forecast system predictions.

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