regional variability and frequency of thundersnow over the u.s. kyle meier, lance bosart, and dan...
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Regional Variability and Frequency of Thundersnow over the U.S.
Kyle Meier, Lance Bosart, and Dan KeyserDepartment of Atmospheric and Environmental Sciences
University at Albany, State University of New York
CSTAR Focal Point: Michael JurewiczNational Weather Service WFO, Binghamton, NY
NROW XIV 10–11 December 2013
Importanceo Thundersnow events can produce mesoscale regions
of locally enhanced snowfall accumulations (6–12 in.) and intense snowfall rates (2–4 in. h−1)
o Relatively little is known about thundersnow• Rarity of such storms compared to both non-thundering
snowstorms and summertime thunderstorms• Lack of direct observations inside clouds that produce
lightningo The rarity of thundersnow events presents a
significant forecasting challenge when they do occur
o Convective storms require the collective contribution of:• Moisture• Instability• Lift
o Fourth ingredient needed specifically for thundersnow to occur: cold air • Below-freezing temperatures within clouds and near the surface
o Lightning production requires an interaction between different types of ice in clouds • Separation between ice crystals (+) and graupel (−) can result in an
electric field that becomes large enough to produce an electrical spark (i.e. the lightning stroke)
Background
Background Thundersnow can occur in a variety of mesoscale and
synoptic-scale settings: o Lake-effecto Orographic lift o Coastal storms and coastal frontso Thundersnow ahead of warm fronts (elevated
convection)o Thundersnow in the vicinity of cold fronts (anafronts) o Thundersnow associated with Alberta Clippers
Background Thundersnow can occur in a variety of mesoscale and
synoptic-scale settings: o Lake-effecto Orographic lift o Coastal storms and coastal frontso Thundersnow ahead of warm fronts (elevated
convection)o Thundersnow in the vicinity of cold fronts (anafronts) o Thundersnow associated with Alberta Clippers
o Objective: Construct a thundersnow climatology in order to establish the spatial and temporal distribution of TSSN reports across the contiguous U.S.
o Period of Study• 19 years: 1994–2012• Cool season: October–March
o Dataset: Total Surface Archives (Weather Graphics Technologies)• Comprehensive archive of hourly METAR surface observations • Off-hour (SPECI) surface observations also included • All valid AWOS and ASOS stations
TSSN Climatology (Methodology)
TSSN Climatology (Methodology)
o Scan observations for all instances of TSSN, VCTSSN, TSPL, and TSGS during the period of study
o Manually eliminate reports from Alaskan stations, Canadian stations, and eliminate “false positives”
o Tsurface < 4°C
o Multiple consecutive reports (i.e. separated by less than 12 h) at a single station constitute one count in the climatology
o Compile statistics o Plot the reports spatially on a map of the U.S.
TSSN Climatology (Overview)
o 2667 reports extracted o TSSN was reported at 680 stations in the contiguous
U.S.o 46 of 48 states reported thundersnow (exceptions:
Delaware and Florida)o Single, isolated TSSN reports were common
• Reinforces the notion that TSSN is a fairly localized phenomenon of limited duration
o Other instances where TSSN occurred at several adjacent stations and/or for several consecutive hours
1994–2012 Thundersnow Climatology
Legend1–2 reports3–5 reports6–10 reports10–20 reports20+ reports
Legend1–2 reports3–5 reports6–10 reports10–20 reports20+ reports
Intermountain West
1994–2012 Thundersnow Climatology
Legend1–2 reports3–5 reports6–10 reports10–20 reports20+ reports
Central U.S.
1994–2012 Thundersnow Climatology
Intermountain West
Legend1–2 reports3–5 reports6–10 reports10–20 reports20+ reports
Central U.S.
Northeast Coast
1994–2012 Thundersnow Climatology
Intermountain West
Legend1–2 reports3–5 reports6–10 reports10–20 reports20+ reports
Central U.S.
Northeast Coast
Great Lakes
1994–2012 Thundersnow Climatology
Intermountain West
NLDN Data to Supplement Reports
12 February 2006
Legend
METAR report
NLDN lightning flash
Hypotheses
o The intermountain west maximum in TSSN reports is likely due in part to orographic forcing (and lake-enhanced effects near the Great Salt Lake)
o The maximum in the central U.S. is likely associated with the relatively high frequency of extratropical cyclone activity
o Reports near the Great Lakes stations suggest a lake influence (if not actual lake-effect events)
o Some East Coast events may have benefited from mesoscale forcing provided by coastal fronts associated with coastal cyclones and their ability to tap warm, moist oceanic air
Thundersnow Annual Distribution
**Cursory analysis suggests there is not a strong diurnal preference for TSSN to occur
October November December January February March0
100
200
300
400
500
600
700
800
Month
19 Y
ear C
ount
s
Thundersnow Capital of the U.S.?
Copper Mountain, CO 135 reports (5.07%)
Thundersnow Capital of the U.S.?
Wolf Creek Pass, CO 114 reports (4.28%)
Rounding out the top 10…
3. Beckley, WV
4. Salida Mountain, CO
5. Pagosa Springs, CO
6. Telluride, CO
7. Ely, NV
8. Sunlight Mountain, CO
9. Ogden Hill, UT
10. Bedford, MA
50 reports
48 reports
29 reports
28 reports
27 reports
24 reports
19 reports
19 reports
Thundersnow Reports by State
**The top 10 states comprise ~68% of the total reports
1. Colorado
2. Minnesota
3. Illinois
4. Oklahoma
5. Nebraska
6. Texas
7. Wisconsin
8. Michigan
9. West Virginia
10. New York
516 reports
379 reports
219 reports
197 reports
100 reports
99 reports
98 reports
84 reports
73 reports
71 reports
Two Case Studies from the 2012–2013 Winter Season
o 8–9 February 2013• A historic blizzard associated with a deep cyclone produced
widespread snowfall totals of 20–40 inches across parts of New England. TSSN was reported in five states: NY, CT, MA, RI, and NH.
o 16–17 February 2013• A strong cold front moved across the Carolinas on the morning of
16 February. Later in the afternoon, the main upper-level trough moved across the Carolinas, resulting in a second round of precipitation. TSSN was reported across several locations in NC and SC.
Methodology
o Objectives: (1) Compare the synoptic-scale and mesoscale features associated with the two events and (2) Identify the relevant dynamical and thermodynamic reasons for the observed thundersnow
o Datasets• Plan view charts (0.5° GFS data) • Cross-sections and proximity soundings (13-km RUC) • 0.5° WSR-88D radar reflectivity mosaics and NLDN data
1000–500-hPa thickness (dashed, every 6 dam), mean sea level pressure (black, every 4 hPa), and 250-hPa wind speed (filled, every 10 m s−1 starting at 40 m s−1)
0000 UTC 9 February 2013 0000 UTC 17 February 2013
Plan View Analysis
1000–500-hPa thickness (dashed, every 6 dam), mean sea level pressure (black, every 4 hPa), and 250-hPa wind speed (filled, every 10 m s−1 starting at 40 m s−1)
* Coupled jet system with a strong jet entrance region to the north
* Strong jet core (90 m s−1) well to the east of observed TSSN
Plan View Analysis0000 UTC 9 February 2013 0000 UTC 17 February 2013
1000–500-hPa thickness (dashed, every 6 dam), mean sea level pressure (black, every 4 hPa), and 250-hPa wind speed (filled, every 10 m s−1 starting at 40 m s−1)
Plan View Analysis
* 984 hPa surface cyclone SE of Long Island, NY, and a 1036 hPa anticyclone in Quebec
* Absence of a strong cyclone. 1024 hPa anticyclone over WI advecting cold air into the Southeast
0000 UTC 9 February 2013 0000 UTC 17 February 2013 * Coupled jet system with a strong jet entrance region to the north
* Strong jet core (90 m s−1) well to the east of observed TSSN
500 hPa geopotential height (black, every 6 dam), geostrophic absolute vorticity (filled, every 4 x 10 -5 s−1), and wind barbs (kts)
0000 UTC 9 February 2013 0000 UTC 17 February 2013
Plan View Analysis
500 hPa geopotential height (black, every 6 dam), geostrophic absolute vorticity (filled, every 4 x 10 -5 s−1), and wind barbs (kts)
0000 UTC 9 February 2013 0000 UTC 17 February 2013
Plan View Analysis
* Northern and southern stream short-wave troughs merge ~ 0200 UTC, coincident with a maximum in observed lightning
* Deep long-wave trough associated with lowest heights of ~528 dam
0.5° radar reflectivity mosaics and observed CG lightning flashes (black plus signs)
0230 UTC 9 February 2013 2145 UTC 16 February 2013
A
A’
B
B’
Radar and Observed Lightning
++++++ +
+ ++++
Cross sections of θe (red, every 4 K), absolute geostrophic momentum (black, every 10 m s−1), and relative humidity (filled, every 10% starting at 80%)
0300 UTC 9 February 2013 2100 UTC 16 February 2013
Cross-section Analysis
Cross sections of θe (red, every 4 K), absolute geostrophic momentum (black, every 10 m s−1), and relative humidity (filled, every 10% starting at 80%)
0300 UTC 9 February 2013 2100 UTC 16 February 2013
WMSSWMSS
Cross-section Analysis
Cross sections of θes (black, every 4 K), negative ω (dashed, every 3 μbar s−1 starting at −12 μbar s−1), frontogenesis [filled, every 2 K (100 km)−1 (3 h)−1], and the −10°C and −20°C isotherms
0300 UTC 9 February 2013 2100 UTC 16 February 2013
Cross-section Analysis
Cross sections of θes (black, every 4 K), negative ω (dashed, every 3 μbar s−1 starting at −12 μbar s−1), frontogenesis [filled, every 2 K (100 km)−1 (3 h)−1], and the −10°C and −20°C isotherms
0300 UTC 9 February 2013 2100 UTC 16 February 2013
Mixed-phase region
Mixed-phase region
Cross-section Analysis
Cross sections of θes (black, every 4 K), negative ω (dashed, every 3 μbar s−1 starting at −12 μbar s−1), frontogenesis [filled, every 2 K (100 km)−1 (3 h)−1], and the −10°C and −20°C isotherms
0300 UTC 9 February 2013 2100 UTC 16 February 2013
Cross-section Analysis
ω = −21 μbar s−1
ω = −33 μbar s−1
Mixed-phase region
Mixed-phase region
Vertical temperature and dewpoint profiles at New Haven, CT (HVN)
Vertical temperature and dewpoint profiles at Rock Hill, SC (UZA)
0300 UTC 9 February 2013 2100 UTC 16 February 2013
Sounding Analysis
Vertical temperature and dewpoint profiles at New Haven, CT (HVN)
Vertical temperature and dewpoint profiles at Rock Hill, SC (UZA)
0300 UTC 9 February 2013 2100 UTC 16 February 2013
* CAPE: 0 J kg−1
* 700−500 hPa lapse rate: 6.3°C km−1
* LCL: 669 hPa* Winds veer with height in lower troposphere
* CAPE: 210 J kg−1
* 700−500 hPa lapse rate: 7.4°C km−1
* LFC: 940 hPa* EL = 535 hPa* Winds back with height in the lower troposphere
Sounding Analysis
Vertical temperature and dewpoint profiles at New Haven, CT (HVN)
Vertical temperature and dewpoint profiles at Rock Hill, SC (UZA)
0300 UTC 9 February 2013 2100 UTC 16 February 2013
* CAPE: 0 J kg−1
* 700−500 hPa lapse rate: 6.3°C km−1
* LCL: 669 hPa* Winds veer with height in lower troposphere
* CAPE: 210 J kg−1
* 700−500 hPa lapse rate: 7.4°C km−1
* LFC: 940 hPa* EL = 535 hPa* Winds back with height in the lower troposphere
* Entire tropospheric column below freezing
0°C Isotherm
Sounding Analysis
Conclusionso Two thundersnow events from February 2013 occurred in
very dissimilar synoptic-scale environments• Case #1: NW quadrant of a strong coastal cyclone and was
associated with the merger of two shortwave troughs • Case #2: Post-cold-frontal environment and associated with a deep
500 hPa trough
o Similarities: Near-saturated conditions, weak MSS, and strong updrafts in the lower-to-middle troposphere over the range of temperatures corresponding to the mixed-phase region of a thundercloud
Future Worko Surface observations alone will not reveal all TSSN events
• NLDN observations can fill in these gaps (provided the lightning strokes are CG)
o Generate constant-pressure and vertical-profile composites of the environment preceding and during the occurrence of thundersnow• The composites may help determine the dynamical and
thermodynamic processes that contribute to regional TSSN frequency and variability
o Conduct representative case studies of the various TSSN pathways
o Determine discriminators between TSSN events and non-thundering snow events