northwest flow – watch out: experience with a dangerous canadian export david zaff science and...
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Northwest Flow – Watch Out:Experience with a Dangerous
Canadian Export
David ZaffScience and Operations Officer
Buffalo, NY Weather Forecast OfficeOct 25, 2006
A Dangerous Canadian Export
Northwest Flow and Severe Weather
• A classic severe weather outbreak over the Great Lakes region:– Warm frontal boundaries or within warm sector– High dewpoints
• BUT some of the worst events have to due with upper air patterns, particularly with northwest flow aloft
• Three-fourths of all Upper Ohio Valley events occur with northwest flow aloft Giordano et al. (1991)
• Derechoes and MCSs often form under these flow patterns
Northwest Flow and Severe Weather
• Definition of Northwest flow: (Johns 1982)
– Preceding event:• 500-mb long-wave ridge upwind of event• 500-mb long-wave trough downwind of event• 500-mb flow in event vicinity >280 degrees
– Following event• Little change in 500-mb long wave pattern• 500-mb flow in event vicinity >260 degrees
Jul Aug Sep
Apr May Jun
NCEP/NCAR Reanalysis 500mb Geopotential Height Composite Mean 1948-2006
Other Factors1. Northwest flow aloft
2. July
3. Short-waves in the cold upper level trough
4. Boundaries!!• In particular…lake breeze fronts
– Nearly 70% of tornadoes during VORTEX were associated with boundaries Markowski et al. (1998)
– Nearly 95% of storms with 60+ dBZ reflectivies were associated with radar-observed convergence lines in a High Plains (CO) study Wilson and Schreiber (1986)
Case studies
• Northwest flow events:– (and local interactions with a lake breeze)
– 30 July 1987 – F2 Cheektowaga Tornado
– 30 June 2006 – F1 Cheektowaga Tornado
– 23 July 2006 – Severe Weather
– 28 July 2006 – Severe Weather
30 July 1987
• ~2130Z: F2 Tornado in Cheektowaga – (about 2 mi from the BUF airport)
• No fatalities
• millions of dollars damage
30 July 1987
• 500 mb Heights (Contoured)
• 500 mb Temps (image)
No Sounding for KBUF12Z Upper Air Observations at 500 mb
30 July 1987
• At the surface– Niagara Falls:
• KIAG 301800Z 19016KT 12SM BKNXXX 29/21 A2995 KIAG 301900Z 17011KT 4SM TSRA OVC039 18/16 A2994 KIAG 302000Z 35006KT 4SM -SHRA OVC031 22/21 A2996 KIAG 302100Z 27005KT 5SM BR BKNXXX 27/22 A29.95KIAG 302200Z 32010KT 5SM BR SCTXXX 2722 A29.95
– Buffalo:• KBUF 301800Z 23015KT 20SM SCTXXX 28/19 A2995
KBUF 301900Z 23017KT 9SM TSRA BKN040 27/21 A2994 KBUF 302000Z 25009KT 10SM BKN040 A2995 KBUF 302100Z 23010KT 9SM TSRA OVC040 25/19 A2993KBUF 302200Z 34005KT 10SM BKNXXX 26/20 A2996
30 July 1987
Estimated location of lake breeze Boundary at 2130Z
Western NY
Lake Erie
Lake OntarioSouthern Ontario
Western NYLake Erie
Lake OntarioSouthern Ontario
30 July 1987
30 July 1987
1) Watch for the car drivingFrom Left to Right
2) Keep your eyes on the Chevy Chevette
Video of Tornado as it moved across parking lot
Tornado
30 June 2006
• ~1900Z: F1 Tornado in Cheektowaga – (about 3 mi from the BUF airport)
• No fatalities
• dollars damage??
(Fast forward 19 years…)
30 June 2006
• 500 mb– Winds and Temp
30 June 2006• 15 min Water Vapor with lightning (red)
• 0945Z-1900Z (to time of F1)
“The Canadian Export”:
Notice the longevity of the storms as they move southeastward from Lake Huron into Southern Ontario
30 June 2006• 15 min Water Vapor with lightning (red)
• 0945Z-1900Z (time of F1)
•Northwest Flow
•(Almost) July
•Shortwave
•Look for lake breeze…
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
16Z 17Z 18Z
• 1.5 Refl shows lake breeze boundary better than 0.5 slice
Western NY
Lake Erie
Lake OntarioSouthern Ontario
• 1847Z1.5 Refl
• 1847Z1.5 SRM
• 1853Z1.5 Refl
• 1853Z1.5 SRM
30 June 2006
July 30 2006 and June 30 2006Tornado Track Comparison
NWS BUF
1987 and 2006 Tornadoes in Cheektowaga, NY
Reason for similarities• Northwest flow aloft• Time of year
– Climatology suggests July is the most active month for tornadoes over Western NY
• (Short-wave)• Lake breeze boundary
• These tornadoes probably take a local lake breeze circulation and tilt it into the vertical under a strengthening updraft
– Non-descending tornadoes• very difficult to pick up on radar• Little or no lead time!
23 July 2006
• Minor Severe event– The “Canadian Export”:
• A Strong outflow boundary moving into lake breeze front
• Microburst over KIAG (Niagara Falls)
• Funnel clouds
23 July 2006• 500 mb
– Winds and Temp
• *Note:– The flow aloft doesn’t
quite fit the Northwest flow definition
• The trough is still upstream (instead of downstream)
• Northwest flow definition fits for lower levels
– (ex 550mb)
• Trough Axis moving overhead
Lapse Rates from RUC at 21Z
23 July 2006• 15 min Water Vapor with lightning (red)
• 1212Z-2045Z (to time of Severe Weather)
“The Canadian Export”:
Southern Ontario convection starts by 16Z and slowly moves southeastward, then interacts with lake breeze boundary along Niagara Peninsula and develops
outflow boundary…
23 July 2006• 15 min Water Vapor with lightning (red)
• 1212Z-2045Z (time Severe Weather)
“The Canadian Export”:
Southern Ontario convection starts by 16Z and slowly moves southward, then interacts with lake breeze boundary along Niagara Peninsula and develops
outflow boundary…
0.5 Reflectivity 0.5 Velocity
Composite/lightning/Obs
Niagara Peninsula(Southern Ontario) Buffalo Airport
Lake Erie
Southern Ontario
Warning at 1933Z was issued With eye on outflow boundary
23 July 2006Funnel clouds, wind and ¾” hail
• Trough Axis moving overhead– Developing Northwest Flow
• July
• Short-wave
• Lake Breeze Boundary
28 July 2006
• One more case….
• Minor Severe event– Some wind damage.
28 July 2006
• 500 mb– Winds and Temp
• *Note:– The flow aloft
doesn’t quite fit the Northwest flow definition
• The trough is still upstream (instead of downstream)
28 July 2006• 15 min Water Vapor with lightning (red)
• 1315Z-2200Z (to time of Severe Weather)
“The Canadian Export”:
Low topped convection develops with a shortwave around 17Z and moves, then interacts with lake breeze boundaries along Niagara Peninsula and near Niagara Falls area
28 July 2006• 15 min Water Vapor with lightning (red)
• 1315Z-2200Z (to time of Severe Weather)
“The Canadian Export”:
Low topped convection develops with a shortwave around 17Z and moves, then interacts with lake breeze boundaries along Niagara Peninsula and near Niagara Falls area
28 July 2006Wind Damage
• Trough Axis moving through• July• Short-wave• Lake breeze front
• This was a localized event – All activity died before it got to KBUF
• Some enhacement possible due to extra low level moisture from morning precipitation.
30 July 1987500mb Temp over KBUF:-11C
Classic Northwest Flow Event
30 June 2006500mb Temp over KBUF:-17C
Classic Northwest Flow Event
23 July 2006500mb Temp over KBUF:-14C
Trough Axis Event
28 July 2006500mb Temp over KBUF:-7C
-Trough Axis Event
Summary
• A Mesoscale Severe weather effect:– Over Western NY, (primarily Niagara and
Northern Erie Counties) Northwest Flow aloft combined with short-wave energy and sustained upstream convection, can combine with lake breezes, developing rapid intensification with a severe weather potential. Lake breeze circulations can spin up under updrafts to produce funnel clouds and occasionally tornadoes.