Hail in the Gray ME County Warning Area

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Hail in the Gray ME County Warning Area

• Jim Hayes
• NOAA/NWS Gray ME

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Outline

• Provide an overview of hail days in the Gray
  (GYX) ME County Warning Area (CWA)
• Develop a synoptic climatology of hail days
• Evaluate traditional thermodynamic parameters
  used for hail forecasting

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Will Hail Occur?

• What is needed for hail to form, given Deep Moist
  Convection (DMC)?
• Adequate updraft to keep the hailstone aloft for
  an appropriate amount of time.
• Sufficient supercooled water near the hailstone
  to enable growth as it travels through an
  updraft.
• A piece of ice or snow for it to grow upon

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Hail Reports in the GYX CWA

• All hail reports for the GYX CWA (severe and
  non-severe) were collected
• For 1998 through 2006, inclusive
• 287 reports in all
• 134 severe hail reports
• Largest was tennis ball sized hail in Tamworth
  New Hampshire August 24th 1998

A hail day was any calendar day for a verified
hail report was received there were 75 hail
days for this study
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From 1998 through 2004, the average was about 6
hail days per year. The numbers increased
dramatically in 2005 and 2006, with MANY more
events, and a more aggressive approach for
acquiring severe weather reports
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Not unexpectedly, the peak months for hail are
June through August
June - 1.7 hail days July 2.6 hail days August
2.0 hail days
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Locations of hail reports in the GYX CWA
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Creating A Synoptic Climatology

• Computed for severe and non-severe hail events
  using the Daily Average NCEP NARR Composites Web
  page at the Climate Analysis Branch Composites
  computed using NARR Reanalysis

http//www.cdc.noaa.gov/cgi-bin/NARR/plotday.pl
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Mean Sea Level Pressure Composite
Cold front
Pre frontal trough
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925 mb mean meridional flow
Indicates a more southerly flow that climatology
925 mb meridional flow anomaly
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850 mb total wind composite (m/s).
Depicts a west southwest flow
850 mb total wind anomaly (m/s).
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500 mb height composite (m).
500 mb height anomaly. The maximum anomaly just
to the northwest is about 40 meters
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250 mb wind composite (m/s).
Note the implied jet coupling
250 mb wind anomaly (m/s).
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Precipitable Water composite
Precipitable Water anomaly. Note the maximum
anomaly is just to the east of the CWA
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Calculating Thermodynamic Parameters

• Thermodynamic parameters were calculated from
  proximity soundings for each hail event
• RUC native resolution BUFR data was used to
  construct a sounding closest to the hail report
  in time and space was used
• The surface conditions were interpolated from
  available surface observations

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Calculating Thermodynamic Parameters

• Proximity soundings were analyzed using the
  RAOB program
• To utilize parameters that forecasters would use
  in an operational setting
• Familiar software
• No thermodynamic parameters for hail reports
  before 2002
• Due to limited availability of RUC native
  resolution BUFR data

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Thermodynamic Parameters from RAOB

• Parameters collected from the RAOB soundings

Parameters were computed for all hail events
2002-2006 from proximity RUC soundings
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Which CAPE?

• For this study, Surface-Based CAPE (SBCAPE) was
  used
• Forecasters computed SBCAPE in the RAOB program
  while assessing convective potential
• This can be problematic in the case of elevated
  convection
• Resulted in a few ZERO CAPE values with severe
  hail

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CAPE
0.288
All half-dollar sized or greater hail coincided
with RUC CAPE values greater than 1200 J/kg
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Severe
Non-severe
4420
4243
1715
1464
1353
934
1009
279
0
0
There is MUCH less spread in the CAPE values for
severe hail reports than non-severe
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Shear
0-6 km shear value of 40 knots is generally
accepted as necessary for supercell development
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Severe
Non-severe
83
83
37
38
34
31
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8
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0-6 km shear value of 40 knots is generally
accepted as necessary for supercell development
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CAPE vs. Shear
-0.247
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All golf ball or large hail events occurred with
WBZ gt 9000 feet
Wet bulb zero values between 7000 and 9000 feet
generally considered prime for hail
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Severe
Non-severe
13275
13514
11036
10474
10369
9360
9043
5915
3999
2954
Wet bulb zero values between 7000 and 9000 feet
generally considered prime for hail
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Hail in the GYX CWA

• Typically, 6 hail days per year
• Increased to 22 in 2006
• Busier convective season, and better verification
  efforts
• Occurs mainly June through August
• Peak in July
• Occurs mainly in the mid to late afternoon

• On a pre frontal surface trough
• In a low level southwest flow
• Ahead of a mid level trough
• In an airmass with precipitable water values
  above climatology

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Assessing thermodynamic parameters

• Results appear to indicate instability (CAPE) is
  more important for discerning severe hail from
  non-severe hail than shear
• Most unexpected result in the higher Wet Bulb
  Zero values for severe hail events
• Limited dataset may have some effect here

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Questions?