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Eastern Mediterranean Cut-off low 6-10 January 2012-Draft
By
Richard H. Grumm
National Weather Service State College, PA
1. Overview
A deep 500 hPa low trekked across the eastern Mediterranean basin 6-10 January 2013 (Fig. 1).
The deep system mid-tropospheric low brought cold weather, rain, winds, and record snow to the
portions of the region. Jerusalem reported around 20 cm of snow (Jerusalem Post 2013) which
was the largest snowfall since 1992. The snow paralyzed the capitol and closed major roads
leading out of the Capitol, shut down bus service and caused problems for the regions light rail
system. The snow was observed in close proximity to the track of the deep mid-tropospheric low
and the pocket of -1 to -2 below normal 850 hPa temperatures (Fig. 2) that moved across much
of the eastern Mediterranean basin.
The BBC reported strong winds flooding, and coastal (BBC 2013) along the coast of Lebanon
and Israel. Snow was reported in Sawfar Village in eastern Lebanon and Damascus, Syria. The
Jerusalem Post (2013b) reported that rain, strong; winds and snow affected the normally
temperate nation for 5 consecutive days. Rainfall amounts in some areas ranged from 150 to 250
mm and 20 to 50 mm in the drier lower areas of the southern part of the country. The initial
surge of heavy rainfall was in close proximity to surge of deep moisture and above normal
precipitable water (PW) which moved into the Levant on 6-7 January 2013 (Fig. 3). PW values
were about 20-25mm which was 2 to 3 above normal. Surges of high PW air are often
associated with heavy rainfall amounts.
It will be shown that the event was relatively well predicted by both deterministic models and
ensemble forecast systems. This included the potential for snow, heavy rainfall, and wind. The
focus here is on the use of standardized anomalies to aid in identifying high impact weather
events.
This paper will document the pattern and standard anomalies associated with the eastern
Mediterranean winter storm of 6-109 January 2013. The focus is on the standardized anomalies,
short-range guidance and ensembles to aid in the prediction of this and similar winter storms.
2. Data and Methods
The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global
Forecast System as 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).
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The precipitation was estimated using the Stage-IV precipitation data in 6-hour increments to
produce estimates for various time periods. Snowfall data was collected from COOP and spotter
reports. The snowfall map in Figure 4 was produced from the NWS Eastern Region collective
dataset.
The NCEP global ensemble forecast system (GEFS) data were used to show the larger scale
forecasts. Overall, large scale models did not indicated a high probability of snow or mixed
precipitation in Pennsylvania.
The comparative cut-off low case of February 1992 is used as a comparison event. This event is
reconstructed using the Climate Forecast System version 1 data.
3. Pattern over the region
The large scale pattern over the region showed the deep 500 hPa trough over the eastern
Mediterranean basin with -1 to -2s below normal 500 hPa height anomalies in the early stages of
the event (Figs. 1a-c). As the system deepened the 500 hPa height anomalies fell into the -4
range (Figs. 1d-i). The 500 hPa heights bottomed out near 5340 m at 09/1200 UTC over southern
Turkey, about -3 below normal height anomalies.
The NCEP/NCAR re-analysis data (Fig 4) shows that 500 hPa heights over coastal Israel below
5450 m are rather rare events. During the cold and snow event of February 1992 (Fig. 5) the
5400 m contour moved into Israel along with anomalously low 850 hPa temperatures. These
deep cut-off 500 hPa lows and cold systems though rare can and do occur. A deep 500 hPa
trough with -4 500 hPa heights affect the region on 15 January 1973 (not shown) and the 850
hPa temperatures were -4 to -5 below normal during a cold outbreak on 19 January 1964.
During the early phase of the event, there was a surge of high PW air (Fig.3) into the region. The
6-hourly data showed above normal PW values from 05/0000 UTC through 07/0600 UTC.
Between 06/1200 and about 07/1800 UTC the high PW air was associated with a surge of strong
850 hPa winds (Fig. 6). The combination of above normal moisture and strong 850 hPa winds
resulted in high 850 hPa moisture flux value and at times +6 above normal MFLUX anomalies
over the eastern Mediterranean (Fig. 7). This event had strong winds, above normal PW, and
extremely high moisture flux anomalies during the early phase, an indication of the potential for
heavy rainfall. Though not shown, MFLUX anomalies remained high and peaked near +6s above
normal again over coastal regions of Israel at 08/1200 and 08/1800 UTC (not shown).
The second surge of high MFLUX values was observed ahead of the cold front (Fig. 8) which
moved inland between 08/1200 and 08/1800 UTC. The 850 hPa temperatures were below 0C,
which can often support snowfall if the boundary layer is cold enough, and the 850 hPa
temperature anomalies were -1 to 2 below normal. Once the cold air had moved into the Levant
with 850 hPa temperatures below 0C the potential for snow at higher elevations and away from
the warm boundary layer air near the Mediterranean basin increased dramatically.
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A surface, a cyclone developed over the eastern Mediterranean. The cyclone had -2 pressure
anomalies over the eastern portion of the Mediterranean basin, east of Cyprus at 06/1200 UTC
(Fig. 9a), the cyclone deepened as it slowly moved eastward with pressure anomalies reaching 5s below normal as it tracked into northern Syria and into eastern Turkey (Figs. 9b-f). A second
low center developed over Syria on around 08/1200 UTC (Fig. 9i).
Figure 10 shows the regional geography and reference points used in the text.
4. Forecasts
The surge of high PW air into the region during the heavy rain, the QPF, and the cold are the key
forecast issue considered here with a focus on relatively long range forecasts. The surge of high
PW into the region (Fig. 11) for the period of rainfall shows that forecasts form 6 NCEP GEFS
predicted above normal PW air to imping on the coastal regions of the eastern Mediterranean on
5 January 2013. The resulting probabilities of 50 and 75 mm or more QPF during the 48 hour
period from 1200 UTC 5-7 January 2013 focused heavy rain along or just offshore (Figs 12&13).
The ensembles mean QPFs also indicated the potential for a significant 48 hour rainfall event
(Fig. 14).
The large scale 500 hPa trough (Fig. 15) and the cold intrusion, with sub-zero 850 hPa
temperatures (Fig. 16) were relatively well predicted by the NCEP GEFS. Clearly, shorter range
forecasts were more accurate and though not shown, there was considerable spread in longer
range forecasts precluding the larger anomalies. The overall 500 hPa pattern with the deep
trough, the surge of high PW air into the region, and the below normal 850 hPa temperature
surges were relatively well predicted at relatively long lead-times.
The key features with the heavy rainfall are shown as predicted from the NCEP GEFS on 1200
UTC 4 January 2013 (Fig. 17). The QPF is the 24 hour total for the period ending at 1200 UTC 7
January 2013 and the wind, PW, and 850 hPa moisture flux are as predicted at the mid-point of
the 24 hour period. Though not shown, the MFLUX was near +6 above normal at 06/1800 UTC
and 07/0000 UTC and above +4 for most of the 24 hour period of the heavy rainfall.
The key features with the cold rain and snowfall on 9-10 January 2013 are shown in Figure 18.
The deep 500 hPa low with -3s height anomalies is evident along with 850 hPa temperatures
below 0C with -2s anomalies over most of Lebanon and Israel. The black dot shows the
approximate location of Jerusalem where over 20 cm of snow fell. On the 9th there was strong
850 hPa moisture flux along the coast with 12.5 to 25 mm of predicted QPF during the period.
5. Summary
A slow moving and deep 500 hPa cyclone moved over the eastern Mediterranean basin from 510 January 2013. As the low deepened the strong frontal system pulled warm moist air into the
eastern Mediterranean leading to locally heavy rainfall on the 5th and 6th of January 2013. As the
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low deepened it both generated and advected cold air into the Middle East bringing with it subzero temperatures through a most of the atmospheric column. Once the cold air moved over the
region, this supported snow inland and at higher elevations. Jerusalem received in excess of 20
cm of snow. The resulting multi-day event produced heavy precipitation, flooding, cold,
damaging winds, and heavy snowfall.
The heavier rainfall in the event occurred as a surge of high PW air and strong winds moved into
the eastern Mediterranean. This produced high moisture flux and moisture flux anomalies on the
order of 6s above normal. The result was heavy rainfall which was relatively well predicted by
the NCEP GEFS, likely due to the strong forcing within the model atmosphere.
The snow and cold phase of the event occurred when the deep 500 hPa cyclone moved into
Israel, Lebanon, and Syria. Beneath this deep cold 500 hPa cyclone the 850 hPa temperatures
were in the -2 to -5C range. This was sufficiently cold to allow the precipitation to fall as snow in
the higher terrain and away from the warm boundary layer air from the Mediterranean to the
west. The deep cyclone and cold air was relatively well predicted, with long lead-time by the
NCEP GEFS.
Synoptically, a deep slow moving cyclone, with a close mid-level cyclone brought a surge of
strong winds and above normal moisture into the eastern Mediterranean basin. The result was a
multi-day precipitation event, with locally heavy rainfall, inland and elevation dependent
snowfall, and strong winds. The precipitation resulted in regional flooding and some areas of
heavy snow fall. The heavy snow portion of the event involved a deep 500 hPa closed cyclone
and a core of cold air at 850 hPa. This produced the largest snowfall in Jerusalem since February
1992 when a similar deep cut-off low and deep cyclone moved over the region.
6. Acknowledgements
NCEP for the data and IMS for information related to the storm
7. References
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.
Miller, J.E. 1946: Cyclogenesis in the Atlantic coastal region of the United States. J. Meteor.,3,31-44.
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Figure 1. Return to text.
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Figure 2. Return to text.
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Figure 3. Return to text.
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January 500 hPa height bins -Tel Aviv
2500
2000
Count
1500
Series1
1000
500
0
5875 5870 5750 5700 5650 5600 5550 5500 5450 5400 5350 5300
height bin (m)
Figure 4. NCEP/NCAR reanalysis data of 500 hPa heights over a point near Tel Aviv for the month of January from 1948-2010
Return to text..
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Figure 5. Reanalyis data of the deep trough and cold air with the snow event of February 1992. Return to text.
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Figure 6. Return to text.
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Figure 7. Return to text.
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Figure 8. Return to text.
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Figure 9. Return to text.
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Figure 10. Reference map of the affected countries and region of the eastern Mediterranean to include terrain.
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Figure 11. Return to text.
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Figure 12. As in Figure 11 except for QPF from 6 NCEP GEFS Cycles showing the probability of 50 mm or more QPF in the 48 hours ending at 1200
UTC 7 January 2013. Return to text.
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Figure 13. As in Figure 12 except for 75 mm QPF threshold. Return to text.
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Figure 14. As in Figure 12 except for the ensemble mean QPF (mm) as in the shading bar and the each member 50 mm contour. Return text.
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Figure 15. NCEP GEFS forecasts of 500 hPa heights and 500 hPa height anomalies valid at 0000 UTC 09 January 2013 from the NCEP GEFS initialized
at a) 0000 UTC 02 January, b) 0000 UTC 03 January, c) 0000 UTC 03 January, d) 0000 UTC 04 January, e) 1200 UTC 04 January, f) 0000 UTC 05
January, and f) 0000 UTC 06 January 2013. Return to text.
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Figure 16. As in Figure 15 except for 850 hPa temperatures and anomalies valid at 0000 UTC 10 January 2013. Return to text.
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Figure 17. NCEP GEFS initialized at 1200 UTC 4 January 2013 showing a) 850 hPa winds and u-wind anomalies, b) precipitable water and
precipitable water anomalies, c) 850 hPa moisture flux and moisture flux anomalies, and d) the mean precipitation for the 24 hour
period ending at 1200 UTC 7 January 2013. The black dot is the location of Jerusalem. Return to text.
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Figure 18. NCEP GEFS forecasts from the 1200 UTC 8 January GEFS valid at 1200 UTC 19 January 2013 showing a) 500 hPa heights and 500 hPa
height anomalies, b) 850 hPa temperatures and temperature anomalies, c) 850 hPa moisture flux, and d) the 30h QPF ending at 1200 UTC 10
January 2013. The black dot is the location of Jerusalem. Return to text.