forecasting heavy precipitation associated with cool-season 500-hpa cutoff cyclones in the northeast...
TRANSCRIPT
Forecasting Heavy Precipitation Associated with Cool-season
500-hPa Cutoff Cyclones in the Northeast
Melissa Payer, Lance F. Bosart, Daniel KeyserDepartment of Atmospheric and Environmental Sciences
University at Albany, SUNY
Neil A. Stuart and Thomas A. WasulaNOAA/NWS, Albany, NY
Eastern Region Flash Flood Conference2–4 June 2010
NOAA/CSTAR Grant NA07NWS4680001
Motivation
• Forecasting precipitation distributions associated with 500-hPa cool-season cutoff cyclones can be a challenge in the Northeast
• Forecast uncertainties often arise due to variation in cutoff speed/location and interaction with the complex topography in the Northeast
• Identifying signatures differentiating between precipitation distributions would help forecasters
Objectives
• Create composites of cutoff cyclones categorized by tilt, structure, and precipitation amount
• Investigate the 12–16 March 2010 cutoff cyclone that produced widespread flooding in the Northeast by examining conventional synoptic and dynamic fields
• Identify signatures differentiating between various precipitation distributions
Data
• 0.5° GFS analysis data
• 2.5° NCEP–NCAR reanalysis data– Climatologies created for 1979–2008
• Standardized anomalies fields were created from 1.0° GFS analyses with respect to climatology
• 6-h National Precipitation Verification Unit (NPVU) QPE
• NEXRAD base reflectivity
Methodology
• Analysis period:– Cool season (Oct–Apr)
– 2004/05–2008/09
• Cutoff cyclone domain: 35–52.5 °N, 90–60 °W
• Cutoff cyclone criteria:– 30-m height rise in all directions
– Duration >12 h
• Precipitation domain: New England, NY, PA, NJ
• Days were defined as the 24-h period from 1200 to 1200 UTC
Methodology
• Analysis period:– Cool season (Oct–Apr)
– 2004/05–2008/09
• Cutoff cyclone domain: 35–52.5 °N, 90–60 °W
• Cutoff cyclone criteria:– 30-m height rise in all directions
– Duration >12 h
• Precipitation domain: New England, NY, PA, NJ
• Days were defined as the 24-h period from 1200 to 1200 UTC
Composite Methodology
Each cutoff cyclone day was categorized by…
1) Precipitation amount observed: heavy precipitation (HP), light precipitation (LP), or no precipitation (NP)
– HP: > 1% of precipitation domain
received 25 mm (n=100)
– LP: < 1% of precipitation domain
received 25 mm (n=250)
– NP: no precipitation observed in
the domain (n=34)
Composite Methodology
Each cutoff cyclone day was categorized by…
1) Precipitation amount observed
2) Tilt: negative, neutral, or positive Scalora (2009)
Composite Methodology
Each cutoff cyclone day was categorized by…
1) Precipitation amount observed
2) Tilt
3) Structure: cutoff or trough
cutoff: presence of 250-hPa zonal wind standardized anomaly of −2.0 σ or below on the poleward side of the cyclone (i.e., purely separated from the background westerly flow)
trough: does not meet the cutoff criteria (essentially a closed low embedded within a large-scale trough)
Note: Since there were so few NP cutoff cyclone days they were not separated into cutoff/trough
Average Location of Cutoff Cyclones
m s–1
Composite: HP_neu_cutoff
250-hPa wind (m s–1, shaded), 500-hPa geo. height (dam, solid contours), and 850-hPa potential temperature (K, dashed contours)
n=14
Stand. anom. of precipitable water (σ, shaded), MSLP (hPa, solid contours), 850-hPa wind (>30 kt, barbs), and precipitable water (mm, dashed contours)
σ
Composite: HP_neu_cutoff
n=14
Cyclone-relative Composite Summary
12–16 March 2010 Cutoff Cyclone
• Long duration event (cutoff cyclone in domain for ~84 h)
• Widespread flooding occurred throughout southern New England
• High winds were also observed with this event — 64 kt wind gusts were observed at Kennedy International Airport around 0000 UTC 14 March
• Models did well forecasting precipitation would occur but the forecast amounts were lower than observed and they didn’t capture the terrain influences
12–16 March 2010500-hPa mean geo. height (dam)
and track of cutoff cyclone every 6 h
12–16 March 20104-day NPVU QPE (mm)
12–16 March 2010 Cutoff Cyclone
mm
13 March 2010
24-h NPVU QPE (mm) ending 1200 UTC 14 March 2010
mm
13 March 2010: 250 hPa
250-hPa geo. height (dam, solid contours), wind (m s–1, shaded), and divergence (10–5 s–1, dashed contours)
1800 UTC 13 March
m s–1
0000 UTC 14 March
Standardized anomalies of 250-hPa zonal wind (σ, shaded) and 250-hPa geo. height (dam, solid contours)
13 March 2010: 250 hPa
σ
1800 UTC 13 March 0000 UTC 14 March
700-hPa geo. height (dam, solid contours), temperature (°C, dashed contours), Q vectors (>5 x 10−7 Pa m−1 s−1, arrows), and Q-vector
convergence/divergence (10−12 Pa m−2 s−1, shaded)
13 March 2010: 700 hPa
10−12 Pa m−2 s–1
1800 UTC 13 March 0000 UTC 14 March
13 March 2010: 850 hPa
850-hPa geo. height (dam, solid contours) and wind (m s–1, shaded)
m s–1
1800 UTC 13 March 0000 UTC 14 March
Standardized anomalies of precipitable water (σ, shaded), 850-hPa geo. height (dam, solid contours), and precipitable water (mm, dashed contours)
13 March 2010: 850 hPa
σ
1800 UTC 13 March 0000 UTC 14 March
13 March 2010: Radar
Base reflectivity (dBZ) and surface observations
1800 UTC 13 March
dBZ
13 March 2010 Summary
• Heaviest precipitation (>100 mm) observed in northern New Jersey and southern New England
– Favorable forcing for ascent downstream of upper-level cutoff in the region of a shortwave trough
– Weak Q-vector convergence
– Strong (>70 kt) low-level southeasterly jet advected Atlantic moisture
– Precipitable water of +1 to + 3 σ above normal along the coast
• Low-level flow interacting with topography suppressed precipitation in the Hudson Valley
24-h NPVU QPE (mm) ending 1200 UTC 15 March 2010
14 March 2010
mm
250-hPa geo. height (dam, solid contours), wind (m s−1, shaded), and divergence (10−5 s−1, dashed contours)
14 March 2010: 250 hPa
m s–1
1200 UTC 14 March 1800 UTC 14 March
500-hPa geo. height (dam, solid contours), absolute vorticity (10−5 s−1, shaded), and wind (kt, barbs)
14 March 2010: 500 hPa
10−5 s–1
1200 UTC 14 March 1800 UTC 14 March
14 March 2010: 850 hPa
850-hPa geo. height (dam, solid contours) and wind (m s–1, shaded)
m s–1
1200 UTC 14 March 1800 UTC 14 March
14 March 2010: 850 hPa
850-hPa θe (K, solid contours), θe advection [K (3 h)−1, shaded], and wind (m s–1, barbs)
K (3 h)−1
1200 UTC 14 March 1800 UTC 14 March
14 March 2010: 925 hPa
925-hPa frontogenesis [K (100 km)−1 (3 h)−1, shaded], potential temperature(K, solid contours), and wind (kt, barbs)
K (100 km)−1 (3 h)–1
1200 UTC 14 March 1800 UTC 14 March
14 March 2010 Summary
• Heaviest precipitation (>80 mm) observed in northern Massachusetts and coastal New Hampshire
– Favorable forcing for ascent within the entrance and exit regions of an easterly upper-level jet poleward of the cutoff cyclone
– Strong low-level southeasterly jet continued to advect Atlantic moisture
– Quasi-stationary region of frontogenesis developed along coastal New England
• Lobe of cyclonic absolute vorticity moving westward contributed to heavy precipitation (>25 mm) in New Jersey
Conclusions
• Long-duration cutoff cyclone associated with widespread heavy precipitation and flooding primarily caused by…
– Advection of anomalous moisture (+1 to +3 σ) by a strong (>60 kt) southeasterly low-level jet
– Favorable forcing for ascent in the entrance and exit regions of an easterly jet poleward of the cutoff cyclone
– Presence of a quasi-stationary region of low-level frontogenesis that developed along coastal New England
• Signatures contributing to heavy precipitation for this event agree well with the composite for the heavy precipitation, neutral tilt, cutoff category