Transcript 17 August 2007 Severe Weather Episode: Fingerprints of a NorthwestFlow Event Matthew Steinbugl – NOAA/NWS/CTP 9th Northeast Regional Operational Workshop (NROW) November 7-8, 2007

17 August 2007 Severe Weather
Episode: Fingerprints of a NorthwestFlow Event
Matthew Steinbugl – NOAA/NWS/CTP
9th Northeast Regional Operational Workshop
(NROW)
November 7-8, 2007
Overview
Briefly Compare/Contrast Synoptic Patterns and
some Meteorological Parameters associated
with NWF events
Examine Large Scale Forcing/Jet Dynamics
Thermodynamic environment/Mesoscale
boundaries
Early Forecasts/Event Evolution
Display Model Data/Radar Imagery
Potential Near-Miss for 2007 LLWS…WHAT IFs
NROW November 7-8, 2007
NWF Synoptic Climatology Highlights
(Johns 1984)
SW flow (SWF) vs. NW flow (NWF) – seasonal
frequencies, jet streams, baroclinicity
NWF events usually characterized by strong
thermodynamic forcing in vicinity of surface boundaries
Mid/Upper jet typically plays an indirect role in
determining outbreak location
Influence of the 500mb short wave trough generally
weak
Differences from the basic (composite) pattern occur
frequently
Large directional and small speed contributions to the
wind shear vector
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17 August 2007 Event Highlights
Not a classic NWF case (digging short wave, anomalous
cyclonically curved upper jet, location vs. composite)
Primary foci for vigorous ascent and rapid convective
development included coupled upper level jet streaks
superimposed within region of maximum PVA
(divergence) aloft
Relatively “Early” event time (16-18Z) versus severe
weather climatology
Similar to NWF synoptic set-up in that outbreak occurred
– In the convergence zone near a sfc low center and
attendant warm front.
– South of the 500mb jet (anticyclonic-shear side).
– Other similarities noted as well.
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17 August 2007 Event Overview
38/216 reports from PA
Occurred between 12-3pm
Nickel to baseball size hail
Rotating supercells with TVSs
Wind damage but no
tornadoes
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500hPa event location comparison
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Anomalous 500 hPa trough and upper jet
500 hPa heights
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250 hPa u and v winds
Coupled jets and corresponding UVM
Primary
mechanism for
rapid
intensification
of convection
by enhancing
700-500mb
UVVELs
RUC 300mb isotachs (fill) ageostrophic wind and 700-500mb omega valid at
1700/1900 UTC. UVM increases from -6 to -12 microbars/sec.
NROW November 7-8, 2007
Jet Streak Analysis/Vertical Circulations
NROW November 7-8, 2007
- Vorticity Advection –
An approximation of divergence aloft
PVA (divergence)
superimposed
within coupled jet
circulation induced
significant large
scale ascent
This was
associated with the
explosive
convection over
Central PA
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X
Evolution of Surface/mesoscale boundaries and
the release of conditional instability
12Z
18Z
15Z
00Z
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Early Forecasts
Initial slight risk/general thunder areas where south and east of where
severe convection developed. Forecast was amended around 1245
pm.
Local and Regional WFO forecasts did not indicate the potential for
thunderstorms. A collaboration issue serving as the primary motivation
for this case study.
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SVR Tstorm Watch – 135pm
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Certainly not Cut and Dry…
MSLP 15Z
MSLP 18Z
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PWAT 18Z
1700 UTC IPT LAPS Sounding
TT = 50
FzgLvl= 11.8
Stm Motion =
294/25kts
0-3km SRH = 223
m2s2
LI = -6.1
CAPE = 1700 J/kg
CIN = 0 J/kg
70-80KT midlvl speed
max
Lolvl wnd backed
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High resolution NAM did very well
15Z
SB CAPE
LI
21Z
Conv Pcpn
Showalter
NROW November 7-8, 2007
SB CAPE
LI
Conv Pcpn
Showalter
SREF 6hr Prob of 0.05” and IPT CAPE Plume
Mean ~500 Jkg-1
A few members over 1000 Jkg-1
18 to 00z
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Water Vapor/300mb isotachs Loop
X
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1515-1630Z loop
Convection
develops over
Central PA in
coupled jet
“X” denotes S/W
crossing Lake Erie
providing
additional large
scale lift/PVA
Little to no
moisture in the mid
to upper levels
Radar Loop 1528-1726Z
Rapid initiation,
intensification
IPT storm
splits, right
mover
becomes storm
of interest
SVR issued
1610Z
TOR issued
1656Z
First Hail report
approx 1700Z
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IPT
Radar Loop continued 1730-1820Z
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Storm continues
ESE (rightmover)
Several large hail
reports
Wind damage
No reports of
tornadoes
Lancaster county
storm was also
severe
(hail/damaging
winds)
0.5° Reflectivity/SRM Close-up 1652Z
70+ dBZ
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IPT
TVS
0.5° Reflectivity/SRM Close-up 1704Z…
12 minutes later
70+ dBZ
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TVS
4-Panel View 1.3°, 2.4°, 3.1°, 4.0°
Four panel display
– 1.3 = 73dBZ, 12kft
– 2.4 = 72dBZ, 19kft
– 3.1 = 69dBZ, 24kft
– 4.0 = 66dBZ, 30kft
Height of:
0°C = 12kft, -20°C= 22kft
-60-70dBZ depth of
30kft
- Deep Well-defined,
persistent
mesocyclone (not
shown)
- Lost Archive Level II
data feed ( NO GR2)
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Storm Photos/Damage
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Williamson
Summary and Conclusions
NWF event awareness – watch out!
Sometimes difficult to forecast – large forcing, subtle features,
rapid evolution = need high situational awareness
Coupled jet circulation juxtaposed in max PVA/divergence
collocated in moist/unstable warm sector = vigorous UVM
Favorable shear profiles INVOF sfc warm front = supercell
environment
Unique NWF event - but many similarities or “fingerprints” to
synoptic climatology/parameters
Near-miss worst case scenario for the 2007 LLWS
NROW November 7-8, 2007
2007 LLWS WHAT Ifs!
Pre-Event Awareness Low (SPC Convective
Outlook/CTP HWO) – “Sudden”
Had a tornado struck the stadium (capacity of
around 9,000 with room for an additional 3035,000 on the terraced hills beyond the outfield
Storm hit just prior to the first pitch of the 2007
LLWS – time for evacuations? Where? Shelter
for approx. 40K +?
Attendees could have been subjected to large
hail/damaging winds – numerous
injuries/possible fatalities?
Lightning safety – considering the venue
NROW November 7-8, 2007
Questions
[email protected]
http://nws.met.psu.edu/severe/2007/17Aug2007
.pdf
NROW November 7-8, 2007