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Southern Appalachian and eastern Maine Early Season Snow:
Forecasting an extreme weather event (EWE)
By
Richard H. Grumm
National Weather Service State College, PA
contributions from
Trevor Alcott, Randy Graham, and Lance Bosart
1. Overview
A deep trough (Fig. 1) brought unseasonably cold air into the Deep South. As the trough moved
over the Carolinas the 500 hPa height anomalies were -5 to -6below normal (Fig.2) and the
850 hPa temperature anomalies were on the order of -5below normal (Fig. 3). The deep trough
and cold temperatures produced record snows in the southern Appalachian Mountains on 1
November 2014. Many sites in eastern Tennessee, western South and North Carolina had the
earliest snowfall and the earliest measureable snow on record (AP 2014). The significant
accumulating snows were confined to higher terrain of the southern Appalachian Mountains on
1-2 November (Fig. 4). The 1300 UTC surface plot over South Carolina showed temperatures
1C above freezing with snow falling in Columbia, South Carolina (Fig. 5).
The deep trough and cold air set snowfall records in the mountains of the Carolina’s and would
move up the coast to produce snow on 2-3 November in the Boston area and record snowfall in
northern Maine (Fig. 6). Along the coast of Maine and in New England the low-level cold air
(Fig. 7) was accompanied by a strong northeasterly low-level jet (Fig. 8) which was -2 to 4below normal at 850 hPa. A strong easterly jet north of surface and 850 hPa cyclones has
been associated with many significant East Coast Winter Storms (Stuart and Grumm 2006).
There are many interesting aspects of this event worthy of study. Some aspects include the
tropical activity over the Pacific which may have amplified the pattern over North America.
Another interesting aspect of this event was the strong tropopause fold which moved into the
eastern United States. This produced +5 to +6 250 hPa temperature anomalies over the central
and southeastern United States (Fig. 9).
This paper will focus on the NCEP GEFS and NAEFS forecasts of this record event using
ensemble forecasts and climate based anomaly data (R-Climate). It will be shown that this highly
anomalous extreme weather event (EWE) was well predicted several days in advance.
2. Methods and Data
The 0.5 degree CFSR was used to construct the larger scale pattern and show the traditional
standardized anomalies.
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The NAEFS situational awareness table and anomalies were retrieved from the NWS Western
Region site.
Snowfall data was retrieved from the National Snow site.
3. Traditional Standardized Anomalies and return periods
The NAEFS and GEFS began to show a period of below normal 850 hPa temperatures and 500
hPa heights in the eastern United States about 7 days prior to the onset of the cold episode. The
WR SATABLE main viewer correctly showed positive temperature anomalies as the 200 hPa
anomalies had the largest magnitude.
GEFS 0000 UTC forecasts from 24 through 29 October 2014 of the 500 hPa heights (Fig. 10),
850 hPa temperatures (Fig. 11) and 250 hPa temperatures (Fig. 12) show the evolution of the
cold episode and early snow. These forecasts imply that the first coherent signal appeared in the
GEFS from 25/0000 UTC (Fig. 10b). Successive 500 hPa height forecasts showed a deeper
trough farther south and west of previous forecasts. As the forecasts members of the GEFS
converged, the standardized anomalies in the 500 hPa heights (Fig. 10f), 850 hPa temperatures
(Fig. 11f) and 250 hPa temperatures (Fig. 12) indicated larger values of the absolute anomalies.
The shorter range GEFS forecasts from 0000 UTC 28 October through 0000 UTC 31 October
showed a sharper and deeper trough and more significant 500 hPa height (Fig. 13), 850 hPa
temperature (Fig. 14) and 250 hPa temperature (Fig. 15) anomalies. The 250 hPa temperature
forecasts of the deep trough and warm stratospheric air to 250 hPa initially indicated that the
system would slide off the New England coast (Fig. 12a). But slowly the GEFS trended toward
deeper trough (Figs. 10 & 13) and warmer 250 hPa anomalies. The forecast anomalies peaked at
+4s above normal in the 1200 UTC 31 October 2014 (Fig. 15f) with a closed -36C contour.
The GEFS 500 hPa heights and height anomaly forecasts followed the same trend as the 250 hPa
warm pocket. As the forecast length decreased, the forecast error and convergence decreased
which likely produced a deeper trough and more negative height anomalies. Examining
individual members and spread (not shown) the GEFS also produced a deeper trough in shorter
ranges.
The spread in the GEFS 500 hPa forecasts (Fig. 16) implied considerable difference in the
forecasts between members. A similar signal was present in the 850 hPa and 250 hPa
temperatures (not shown). As the spread decreased, the magnitude of the anomalies increased
(Fig. 13) showing a direct relationship between the standardized anomalies and the ensemble
spread. In regions of large anomalies, the spread can be relatively large due to depth,
displacement and timing issues. But the generally significant feature can produce a coherent
signal but of the wrong magnitude due to averaging.
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The traditional standardized anomalies produced a strong signal with 5-7 days of lead-time of
this extreme trough, the potential for unseasonably cold air and the general conditions favorable
for a record early snow1.
4. Return periods and percentiles
The summarized SA Table from the NAEFS from the 0000 UTC forecasts cycles on 26-28
October 2014 are shown in Figure 17. Focusing on the height (Z) and temperature (T) columns
the potential for a deep trough in the southeastern United States increased with each successive
NAEFS cycle. By 0000 UTC 28 October the deep blues showed -3 height anomalies.
The temperature anomalies were a bit difficult to decipher in this case due to the highly
anomalous 200 hPa temperatures, which at longer ranges were larger in magnitude than the
anomalies at other levels. Thus, the 200 hPa temperature anomalies dominated the table with
high values and the warmer colors. Examining the temperature2 data from the 0000 UTC 28
October 2014 NEAF (Fig. 18) revealed that there would be a prolonged period of above normal
200 hPa temperatures with anomalies in the +3 to +3.4 range accompanied by negative thermal
anomalies in the lower troposphere.
These NAEFS data (Fig. 18) show that the 200 hPa temperature anomaly of +3.4s was the
largest value in the 30 year CFSR climate for that date and time and it had a return period of
30 years. Additionally, for several days these extreme 200 hPa thermal anomalies would be over
the southeastern United States with anomalies near the positive tail of the probability distribution
function (PDF) at the 99.5 interval. During this period of time the return period ranged from 30
to 10 years. At lower levels the -3temperature anomalies showed that some of the values were
in the negative tail of the PDF with shorter return periods.
The 1200 UTC 30 October NAEFS SA TABLE for all variables (Fig. 19) show the traditional
standardized anomalies, the percentiles, and the return periods. These shorter range forecasts
show that as the forecast length decreased, the magnitude of the anomalies increased. This
reinforces the concept that the anomalies for EWE often contain uncertainty information. After
0000 UTC 1 November, the forecasts showed that the lower tropospheric anomalies had a large
magnitude than the 200 hPa anomalies. This shorter range forecasts also showed that in many
fields, the NAEFS was forecasting a record event in the southeastern United States. Many daily
and all-time for the day temperature, height, and wind records were forecast by the NAEFS.
Additionally, the NAEFS indicated some extremely dry air over the region.
The temperature fields showed that (Fig. 20) showed that from 1800 UTC 30 November the 200
hPa temperatures were record or near record warm and remained so until about 0000 UTC 2
1
Precipitation and precipitation type forecasts showed some signals but the snow was likely the least well
predicted feature.
2
Clicking on a variable in the live tables provides anomalies for the variable select with levels if appropriate.
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November 2014. Later in the event 700 hPa, 850 hPa, and finally 1000 hPa temperatures were in
the extreme tails of the PDF, but on the cold side.
The 200 hPa anomaly at its peak in the 1200 UTC 30 October 2014 NAEFS is shown in Figure
21. The maximum warm anomaly was over the western Carolinas at this forecast time. The data
in Figure 20 show that the anomaly was a maximum and the return period spanned the entire 30
PDF. The 5 NAEFS temperature anomalies from these forecasts and the full SA Table are shown
in Figure 22. These data show that record or near record anomalies were forecast at 200, 700,
and 850 hPa. This display does not allow the user to discern the sign of the anomaly. But clearly
shows a record event in many fields, including the temperature fields. The 3 display concepts
need to be effectively employed to relate the anomalies back to the sensible weather
implications.
5. Summary
A deep trough brought cold air and record early snow fall to the southern Appalachian mountains
and northern Maine on 1-3 November 2014. The larger scale pattern showed a deep trough and
unseasonably cold air which moved over the affected region. In addition to the deep trough and
low-level cold air, extremely warm air at 200 and 250 hPa was present over the low-level cold
air. A record warm lower stratospheric event occurred over the southeastern United States with
this deep early season trough.
The NCEP GEFS and the NCEP-CMC NAEFS provided relatively good guidance as the
potential for an early season cold episode. The broader region to be affected was relatively well
predicted with 4-7 days lead-time. The more specific region and the magnitude of the event had a
2-4 day lead-time. The anomalies peaked in magnitude as the forecast length decreased and the
forecasts converged. Predictability horizons for the general region to be impacted were longer
than those related to the more refined region to be impacted.
The deep trough at 500 hPa was associated with a lowered height of the stratosphere. This
produced extremely warm 250 hPa and 200 hPa temperatures in both the GEFS and NAEFS. The
warmest air at 250 hPa with -36C temperature and +5 thermal anomalies moved over the
western Appalachians. The GEFS initially had the 250 hPa thermal ridge too far north and east
of the verifying position. These data imply that following forecasts of 250 hPa warm pockets
could be of value in predicting extreme trough and EWE associated with deep troughs. This
could be an avenue of further study. Confidence in longer range forecasts can be gleaned from
the standardized anomalies in terms of when a significant event might occur. But the true
magnitude of this and many similar events had a relatively short lead-time.
This system was relatively well predicted, especially with 2-4 days lead-time, assuming a
forecast of a -4 trough is a good forecast. The general hints of an intrusion of cold air into the
eastern United States had a lead-time on the order of 4-7 days. Clearly, these longer lead-time
forecasts had considerable errors with the location, magnitude, and timing of the cold episode.
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It takes time and experience to use the SA Tables effectively. This includes getting into the
details as each event has its own subtle nuances. In this event, the record warm 200 hPa
temperatures obfuscated the cold 850 hPa temperatures. This requires the forecaster to examine
where the anomalies occur and be mindful that in cold outbreaks the lower stratosphere can
become extremely warm as deep cold air moved in at lower levels.
The standardized anomalies are relatively easy to interpret. They also provide a good first guess
of an EWE. However, using the entire PDF, expressed as percentages or as return periods
provides an extra measure of the potential extremity of an event. Thus all of these data are
complimentary and with experience may be used to effectively anticipate a wide range of EWE
and HIWEs.
The anomalies may also provide some sense of uncertainty which can be leveraged by the
forecaster. As shown in Figure 19, large anomalies and anomalies in the tail of the PDF require a
convergence of forecasts. This demonstrates the concept that the anomalies for EWE often
contain uncertainty information. They do not provide information in terms of spatial and
temporal errors, but the large magnitude of the anomalies requires relatively low uncertainty.
An historic snow event of 4 November 1927 produced 15 inches of snow and killed 200 people
in Vermont. This event, viewed form the 20th Century re-analysis data may be a good
comparative event to the early season snow of 1-3 November 2014. Other events which may bear
consideration include the 14-15 November 1995 Appalachian snow event.
6. Acknowledgements
Albany MAP for large scale perspectives and predecessor concepts as to the reasons for the cold
outbreak. WR of the NWS for support for and access to the WR-SATABLES. Trevor Alcott,
Randy Graham, and Lance Bosart provided guidance and collaboration on EWE and HIWE
concepts, forecast techniques and displays.
7. References
Associated Press, 2014: November snow surprises Southeast. And similar stories 1-2 November
2014.
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. http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-1100181.1
Graham, R A. and R. H. Grumm, 2010: Utilizing Normalized Anomalies to Assess SynopticScale Weather Events in the Western United States. Wea. Forecasting, 25, 428-445
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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., 2011: The Central European and Russian Heat Event of July–August
2010. Bull. Amer. Meteor. Soc., 92, 1285–1296. doi:
http://dx.doi.org/10.1175/2011BAMS3174.1
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
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Figure 1. Climate Forecast System 500 hPa heights and 500 hPa height anomalies in 24 hour increments from a) 0000 UTC 30 October 2014 through f)
0000 UTC 04 November 2014. Contours every 60 m. Return to text.
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Figure 2. As in Figure 1 except of 6-hour increments from a) 0000 UTC 1 November 2014 through f) 0600 UTC 02 November 2014. Return to text.
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Figure 3. As in Figure 2 except for 850 hPa temperatures and temperature anomalies. Return to text.
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Figure 4. Observed snowfall 1 and 2 November over the southern Appalachian Mountains. Images courtesy of the
National Snow Analysis Site. Return to text.
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Figure 5. Surface plot with weather valid at 1300 UTC 1 November 2014. Return to text.
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Figure 6. As in Figure 4 except for snowfall over New England on 2 and 3 November 2014. Return to text.
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Figure 7. As in Figure 2 except for the 850 hPa temperatures and anomalies over the northeastern United States from a) 0000 UTC 2 November
2014 through f) 0600 UTC 03 November 2014. Return to text.
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Figure 8. As in Figure 7 except for 850 hPa winds and u-wind anomalies. Return to text.
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Figure 9. As in Figure 7 except for 250 hPa temperatures and temperature anomalies in 12 hour increments from 0000 UTC 31 October through
f) 1200 UTC 2 November 2014. Return to text.
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Figure 10. NCEP GEFS mean of 500 hPa heights and 500 hPa height anomalies valid at 1200 UTC 01 November 2014 from GEFS forecasts
initialized at 0000 UTC from a) 24 October through f) 29 October 2014. Return to text.
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Figure 11. As in Figure 10 except for 850 hPa temperatures ( C) and 850 hPa temperature anomalies. Contours every 2C. Return to text.
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Figure 12. As in Figure 11 except for 250 hPa temperatures and anomalies. Contours every 4C. Return to text.
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Figure 13. As in Figure 10 except for GEFS initialized at a) 0000 UTC 28 October, b) 0000 UTC 29 October, c) 1200 UTC 29 October, d) 0000 UTC 30
October, e) 1200 UTC 30 October and f) 0000 UTC 31 October 2014. Return to text.
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Figure 14. As in Figure 13 except for 850 hPa temperatures and anomalies. Return to text.
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Figure 15. As in Figure 13 except for 250 hPa temperatures and anomalies Return to text.
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Figure 16. As in Figure 13 except 500 hPa heights and spread about the ensemble mean. Return to text.
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Figure 17. SA Table showing the more significant standardized anomalies in the NAEFS from Forecasts initialized at (left to right) 0000 UTC 26 October, 0000
UTC 27 October and 0000 UTC 28 October 2014. The values represent the, by magnitude the largest standardized anomaly for the variable for single level
parameters (SLP,PW, IVT) and at the level where the largest anomaly exist for multi-level variables (Z,T,U,V,WSP,Q) . In real-time the anomalies are clickable
providing a plan view display. The variables too are clickable to examine the data for various levels . Return to text.
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Figure 18. As in Table 17 except showing the temperature anomalies at each level were NAEFS temperatures are available. The standardized anomalies, the
percentiles, and return periods from the 1200 UTC 28 October 2014 NAEFS was used to produce the table. Return to text.
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Figure 19. As in Figure 17 except from the 1200 UTC 30 October 2014 NAEFS showing (left to right) the standardized anomalies, the percentiles, and the return
period for the anomalies. Return to text.
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Figure 20. As in Figure 19 except from the 1200 UTC 30 October 2014 NAEFS. Return to text.
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Figure 21. The SA Table from the 1200 UTC 30 October 2014 NAEFS showing the 200 hPa warm pocket forecast valid at 1200 UTC 01
November 2014 when the anomaly was +4.1s above normal and had are return period of 30 years as shown in Figure 20. Return to text.
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Figure 22. The full SA TABLE for return intervals showing the temperature and return interval of the temperature anomalies at the 5 NAEFS forecast
levels. The scale is time and the deep red is outside the CFSR climate range for the 21-day centered day. Return to text.
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Appendix
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CARIBOU ME
743 AM EST MON NOV 3 2014
...RECORD SNOWFALL ACROSS NORTHERN AND EASTERN MAINE...
A TOTAL OF 10.1 INCHES OF SNOW WAS OBSERVED AT CARIBOU, MAINE YESTERDAY.
THIS SMASHED THE DAILY SNOWFALL RECORD FOR NOVEMBER 2ND OF 1.3
INCHES IN 2002. THIS WAS ALSO THE LARGEST CALENDAR DAY SNOWFALL
EVER OBSERVED SO EARLY IN THE SEASON BREAKING THE RECORD OF 9.4
INCHES ON OCTOBER 29, 1963. IT WAS BY FAR THE EARLIEST DOUBLE DIGIT
SNOWFALL ON RECORD AT CARIBOU. THE PREVIOUS RECORD OF 12.8 INCHES
WAS OBSERVED ON NOVEMBER 20, 1945.
AT BANGOR...A TOTAL OF 12.0 INCHES OF SNOW WAS OBSERVED YESTERDAY.
THIS CRUSHED THE PREVIOUS DAILY RECORD FOR NOVEMBER 2ND OF 0.5"
SET IN 1951. THIS WAS BY FAR THE LARGEST CALENDAR DAY SNOWFALL
EVER OBSERVED SO EARLY IN THE SEASON BREAKING THE RECORD OF 7.8
INCHES ON OCTOBER 26, 1962. IT WAS BY FAR THE EARLIEST DOUBLE
DIGIT SNOWFALL ON RECORD AT BANGOR. THE PREVIOUS RECORD OF 18.9
INCHES WAS OBSERVED ON NOVEMBER 15, 1962.
It is snowing in South Carolina (CAE - Columbia). This is the earliest snow ever recorded at the CAE
airport (per WFO CAE forecast discussion). I've attached surface plots and put my cursor over CAE to
show the METAR. The regional radar is also included.
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