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Cold Episode of December 2013

Value of anomalies for situational awareness

By

Richard H. Grumm

National Weather Service State College, PA

Contributions by

Elyse Colbert

National Weather Service State College, PA

Trevor Alcott

National Weather Service, Salt Lake City UT

1. Overview

A period of cold weather affected much of North America during the first two weeks of

December 2013. During the peak of the event, over 100 daily low temperature records

were broken or tied (Fig. 1). The relative cold in North America and the western United

States was the result of a large ridge over the Gulf of Alaska (Fig. 2) with +2 to +3 500

hPa height anomalies and a downstream trough. During this time there was a persistent

ridge over the eastern Gulf of Mexico and western Atlantic.

The 850 hPa temperatures showed a surge of cold air into the western United States in

early December with -2 to -3 temperature anomalies over the region from 5 to 9

December 2013. The cold air slowly moved to the east, impinging on a region of

relatively warm air which initially extended from Texas to Quebec (Fig. 3a) but was

slowly displaced eastward as the cold air advanced ahead of strong surface anticyclone

(Fig. 4). A series of disturbances developed along the frontal boundary (Fig. 4) bringing

snow and ice to the southern plains, snow to the Midwest and a series of weak precipitation

events to the eastern United States on 6, 8, and 10 December 2013. The Philadelphia

Eagles bested the Detroit Lions in the “snowbowl” which produced nearly 8 inches of

snow during the game in Philadelphia on 9 December 2013 (AP 2013).

Standardized anomalies (Hart and Grumm 2001) can aid in identifying patterns associated

with extremes. When used with model data (Grumm and Hart 2001; Stuart and Grumm

2006; Junker et al. 2008) they can aid in predicting extreme events and identify when

forecast systems are predicting potentially extreme events, relative to a base climatology. It

is immaterial whether the climatology is re-analysis based (R-Climate) or model or

ensemble prediction system based (M-Climate).

This paper will document the cold episode of 4-12 December 2013. The focus is on how to

use standardized anomalies to characterize and identify; in this case; cold episodes. Some

limitations in using anomalies are presented and why return periods can aid in identifying

some extreme events better than standard deviations. The concept of using return periods

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and percentiles to compliment standardized anomalies is employed as a means to assess the

potential of a HIWE.

2. Data and Methods

The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global

Forecast System (GFS) as a first guess at the verifying pattern. The standardized anomalies were

computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter

1995). The standardized anomalies and the probability distribution functions are based on the re-

analysis climate (R-Climate). Though not shown here, they could be produce from internal

model system climatologies (M-Climate).

The traditional standardized anomalies were produced from the GFS 00-hour forecast using

Climate Forecast System based means and standard deviations. The climatology spans a 30 year

period. This 30-year period was used in the situational awareness (SA) tables to show the return

period of the anomalies.

The forecasts in the SA tables were derived from the 42 member North American Ensemble

Forecast System (NAEFS). These data are displayed using the NAEFS mean fields relative to the

climate. In addition to standardized anomalies, the return period and interval of the anomalies are

produced relative to the 21-day centered values. The return period is in years such that a 30-year

return period represents the extreme in the climatological data set. The percentile data represent

where along the climatological probability distribution the current NAEFS forecast lies.

Extremes in the tails would be in the 97th

to 100th

percentile (MAX) or in the 3rd

to 0th

percentile

(MIN). In addition to finding the MAX and MIN relative to the time, the MAX and MIN are

computed using the 4 climatological times of 00, 06, 12, and 18 UTC. This provides the ability

to estimate the MAX (MIN) at the specified time or for all 4 times in the same 21-day centered

climatological window.

Temperature records were obtained from the National Climatic Data Center (NCDC). Stories

related to the cold and winter precipitation events were obtained from both NWS and media

sources.

3. Observed weather

i. western United States

The number of record low and record low-high temperatures set during the event was displayed

in Figure 1. The corresponding tabular form of the data (Tables 1 & 2) showed the daily records

based on NCDC data. As indicated by the 850 hPa temperature anomalies (Fig. 3) and 500 hPa

height anomalies (Fig. 2) the coldest and most anomalous air affected western North America

and the West Coast. Thus during the early phases of the event the records were primarily set in

the western United States.

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The cold affected citrus growers and avocado growers in California (AP 2013) on 5-6 December

when temperatures fell into the 20s in crop growing regions. Temperatures in the lower 20s and

upper teens can cause critical damage to citrus and avocado trees. News stories indicated

growers spent over 24 million dollars combating the cold.

The push of cold air into the western United States began on 3 December (Fig. 5) and the cold air

moved southward with negative 850 hPa (Fig. 5) and 700 hPa temperature anomalies pushing

into the Baja of California by 5 December 2013. The cold air remained in place through 10

December but clearly began to loosen its grip over the region (Fig. 6) as the cold air moved east.

ii. central United States

The surge of cold air in the southern Plains (Fig. 7) was associated with 850 hPa temperatures in

the 0 to -26C range and temperature anomalies on the order of -2 to -4. A plume of warm moist

air was present ahead of the cold front. This warm moist air served as a focus for several wintry

precipitation events to include snow, ice and rapid temperature changes in the southern plains.

The cold air was accompanied by a strong surface anticyclone (Fig. 8), and by 7 December

pressure anomalies of 1 to 3 above normal dominated the region (Fig. 8f).

Surface ridging behind the cold air (Fig. 8a-c) implied strong northerly flow and the 850 hPa

winds showed strong northerly flow (Fig. 9) often associated with a “blue norther” (AMS 2011)

in the southern plains. The impact of the strong front impinging on the cold air resulted in

dramatic temperature changes. High temperatures in Texas and Oklahoma ranged from the 70 to

80s on 3-4 December and only reached the low 30s in southern Oklahoma on 6 December (Fig.

10). Dallas, TX saw temperature of 52F at 0200 UTC 5 December and was reporting rain and ice

pellets at 33F at 2000Z 5 December. As temperatures fell into the 20s, a prolonged period of

sleet, freezing rain and gusty northerly winds settled over the region from roughly 05/2100

through 07/0100 UTC (Observations in Appendix).

As the cold air settled southward, easterly flow developed over the Mississippi and Ohio Valleys

on 5-6 December. On the cold side of the boundary and with no distinct ‘storm’ or cyclone, areas

of snow developed over southern Illinois and Indiana late on the 5th

and early on the 6th

of

December. Radar imagery is shown of the snow bands near their peak (Fig. 11). Figures 7-9

show that the snow fell on the cold side of the baroclinic zone in the northeasterly flow

associated with the strong surface anticyclone.

iii. eastern United States

The cold air began to affect the eastern United States on 6 December with 3 precipitation events

along and over the shallow frontal boundary. Each of these events had distinct predictability

issues associated with QPF, timing of the onset of precipitation, and the precipitation types. All

of these issues lead to difficult predictions for three successive wintry precipitation events. The

focus here is on the snow in southeastern Pennsylvania and New Jersey during the “snowbowl”.

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The 850 hPa temperatures (Fig. 12) showed the cold air over the eastern United States and a

surge of warm air directed toward the frontal boundary. The 850 hPa winds (Fig.13) showed

easterly wind anomalies over the eastern United States. The surface pressure field showed a large

anticyclone to the northwest and enhanced cold air damming, with ridging down the coastal plain

(Fig. 14c-d). The snow fell during the period of intense frontogenesis and the low-level cold air

damming.

The strong frontogenesis snow bands developed over the area of cold air damming (Fig. 15).

This was the third significant winter precipitation event in 4 days with no significant surface

cyclone or “storm”.

The series of frontogenetic events left two distinct stripes of snow over the eastern United States

(Fig. 16). The satellite imagery was taken 12 December 2013.

4. NAEFS Forecasts

The forecast aspects of the event presented here focus on the larger scale pattern and the

potential for a cold episode. It is beyond the scope of this paper to cover the nuisances of the

forecasts associated with the winter weather events, each of which had serious issues related to

precipitation type and amounts. A follow-on study may address two of these events in further

detail.

The NAEFS anomalies and return periods of the anomalies for critical forecast fields over the

western United States are shown in Figure 17. These data show that the NAEFS predicted a

prolonged cold period over the western United States from 3 to 11 December 2013. During the

peak of the cold episode, the NAEFS indicated the return period for the anomalies in the

temperature and height fields was on the order of 30 years. The largest thermal anomalies of -

4.5 appeared about 1800 UTC 7 December.

The corresponding NAEFS-BC forecasts of 500 hPa heights, 850 and 925 hPa temperatures and

mean-sea level pressure (Fig. 18) showed the large anticyclone over the central United States and

the mass of cold air over the western half of the United States at both 925 and 850 hPa. The 500

hPa heights showed the sharp ridge over Alaska and the deep trough over the western United

States. The +3 height anomalies over Alaska in a 4-day forecast imply a convergence of the

forecasts. The sharp ridge provided northerly flow over northwestern North America while

providing cross polar flow and the flow of cold air into central and southern North America.

Earlier and latter forecasts continued to show this trend for the cold air to penetrate most of the

United States, sparing Florida under a persistent positive height anomaly at 500 hPa over the

western Atlantic.

5. Summary

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A relatively well predicted pattern set up over North America which funneled cold air into the

United States from 4-12 December 2013. The cold air brought snow and freezing conditions to

the West Coast and produced crop loss issues in California. As the cold air moved eastward, it

brought cold air to the Plains, and as the cold air moved into the southern Plains, near record

warm weather in Texas and Oklahoma was rapidly replaced by cold and snow as a blue norther

swept into the region. As the arctic air continued to move eastward, it produced snow and cold in

the Ohio Valley 5-6 December. The cold air moved into the eastern United States on 6-7

December producing several wintry precipitation events. The Mid-Atlantic heavy snow of 8

December will likely be well remembered for its impact on the Philadelphia-Detroit “snowbowl”

in Philadelphia, PA.

The data shown here indicated that standardized anomalies can be useful in characterizing these

events. When used with forecast data, this also shows how standardized anomalies can help

forecast these events. The NAEFS data and the NAEFS-BC data did a credible job with the

pattern and evolution of the cold air mass over the United States.

Though not the focus of this paper, the individual snow events and the lack of a well-defined

surface cyclone during these events may have contributed to some issues related to predictability

with the QPF and precipitation types. Most of the events; including the southern plains, Ohio

Valley, and Mid-Atlantic events; were overrunning events which lacked a distinct surface

cyclone. Some aspects of these events were difficult to predict. Further confusion occurred due

to identifying “storms” during this period of time despite the lack of a surface cyclone which is

within the superset used to define a “storm”.

.

6. Acknowledgements

The Pennsylvania State University for gridded data access and the Mid-Atlantic River forecasts

and Trevor Alcott for return period data and images. Elyse Colbert for NCDC data extraction and

summations.

7. References

Associated Press, 2013a: LeSean McCoy runs for 217yards, Eagles beat Lions 34-20 in snow

storm. And similar stories. 8 December 2013

Associated Press, 2013b: California citrus growers fight off cold spell. And similar stories 6-8

December 2013.

Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS.

Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H.

Bredekamp, NASA, Washington, D.C., 209-219.

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Graham, R.A, T. Alcott, N. Hosenfeld, and R. Grumm, 2013: Anticipating a rare event utilizing

forecast anomalies and a situational awareness display: The Western Region US Storms of 18-23

January 2010, BAMS, 2013.

Graham, R A. and R. H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale

Weather Events in the Western United States. Wea. Forecasting, 25, 428-445.

Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season

Weather Events: Preliminary Findings. Wea. and Fore., 16,736–754.

Grumm, Richard H., 2011a: The Central European and Russian Heat Event of July–August 2010.

Bull. Amer. Meteor. Soc., 92, 1285–1296.

Grumm, R.H. (2011b) New England Record Maker rain event of 29-30 March 2010. NWA. Electronic

Journal of Operational Meteorology, EJ4 12: 1-31.

Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic

scale events objectively. Mon. Wea. Rev., 129, 2426–2442.

Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential

for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the

Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society, 4

Article: pp. 445–453.

Junker, N. W., R. H. Grumm, R. Hart, L. F. Bosart, K. M. Bell, and F. J. Pereira, 2008: Use of

standardized anomaly fields to anticipate extreme rainfall in the mountains of northern California.

Wea. Forecasting,23, 336–356.

Stuart, N.A and R.H. Grumm 2006: Using Wind Anomalies to Forecast East Coast Winter Storms.

Wea. and Forecasting,21,952-968.

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Date Broken Records Ties Sum Cumulative Sum

Total Possible

Locations

12/4/2013 49 10 59 59 5456 West

12/5/2013 142 31 173 232 5429 West

12/6/2013 244 55 299 531 5255 West and Central

12/7/2013 273 57 330 861 4898 West and Central

12/8/2013 250 37 287 1148 4913 West, South, and Central

12/9/2013 47 18 65 1213 4641 West and South

Table 1. List of dates and records of record low high temperatures set for the periods of 4 December and 9 December 2013. Data from NCDC. Return to text.

Date Broken Records Ties Sum Cumulative Sum

Total Possible

Locations

12/4/2013 47 10 57 57 5439 West

12/5/2013 161 23 184 241 5414 West

12/6/2013 136 27 163 404 5252 West

12/7/2013 152 30 182 586 4900 West and Central

12/8/2013 67 19 86 672 4878 West and Central

12/9/2013 17 5 22 694 4629 West

Table. 2. As in Table 1 except low temperatures. Return to text.

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Figure 1. Return to text.

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Figure 2. GFS 00-hour forecasts of 500 hPa heights and standardized anomalies in 24 hour increments from a) 1200 UTC 04 December 2013 through f) 1200 UTC 09 December 2013. Heights every 60 m and anomalies in standard deviations as in the color bar. Return to text.

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Figure 3. As in Figure 2 except for 850 hPa temperatures zoomed over the United States. Data show isotherms and the standardized anomalies. Return to text.

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Figure 4. As in Figure 2 except for surface pressure (hPa) and pressure anomalies. Return to text.

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Figure 5. As in Figure 3 except for 850 hPa temperatures in the western United States from a) 1200 UTC 1 to f) 1200 UTC 6 December 2013. Return to text..

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Figure 6. As in Figure 5 except for a) 1200 UTC 7 December through f) 1200 UTC 12 December 2013. Return to text.

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Figure 7. As in Figure 3 except 850 hPa temperatures focused over the central United States from a) 1200 UTC 4 through f) 0000 UTC 7 December 2013. Return to text.

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Figure 8. As in Figure 7 except for mean sea-level pressure. Return to text.

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Figure 9. As in Figure 7 except for 850 hPa winds and wind anomalies in 6-hour increments from a)1200 UTC 4 December through f) 1800 UTC 5 December 2013Return to text.

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Figure 10. High temperatures ( F) on 4 and 6 December 2013. From the PSU ewall website. Return to text.

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Figure 11. Composite radar in 3-hour increments from top) 2338 UTC 5 December through bottom) 0538 UTC 6 December 2013. Return to text.

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Figure 12. As in Figure 3 except 850 hPa temperatures over the eastern United States from a) 0000 UTC 8 December through f) 0600 UTC 9 December 2013. Return to text.

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Figure 13. As in Figure 11 except 850 hPa winds over the eastern United States from a) 0000 UTC 8 December through f) 0600 UTC 9 December 2013. Return to text.

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Figure 14. As in Figure 3 except mean sea-level pressure over the eastern United States from a) 0000 UTC 8 December through f) 0600 UTC 9 December 2013. Return to text.

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Figure 15. As in Figure 11 except focused on the eastern United States at top) 1436, 1636 and bottom) 1836 UTC 8 December 2013. Return to text.

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Figure 16. AWIPS display of visual satellite imagery showing the residual snow in the Ohio Valley and the snow in the Mid-Atlantic region from the series of storms from 5-10 December 2013. The image is from satellite data valid at 18Z 12 December 2013. Return to text.

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Figure 17. NAEFS based situational awareness tables of anomalies by variable. The anomalies by standard deviations from normal are shown in the table on the left side and the right side shows the return period for the largest, in a magnitude sense, anomaly for each variable shown. The height, temperature, u,v, wind speed, and Q fields have several levels of data from 1000, 850, 700, 500 and 200 hPa. Only the largest anomaly at any level is depicted. Return to text.

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Figure 18. NAEFS bias corrected forecasts initialized at 0000 UTC 3 December 2013 showing the forecasts valid at 1800 UTC 7 December 2013 of 500 hPa heights, mean sea-level pressure, 850 hPa temperatures and 925 hPa temperatures. All fields are shown with the standardized anomalies. Return to text.

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Appendix Dallas Observations screen capture image