Northwest Flow Snow Events in the Southern Appalachian Mountains CSTAR Workshop 10/7/05

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Northwest Flow Snow Events in the Southern
Appalachian MountainsCSTAR Workshop10/7/05

• Blair Holloway

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Outline

• Background
• Motivation
• Example/Problem
• Definition
• Questions Hypotheses
• Method
• Events
• Case 1 18-20 December 2003
• Case 2 20-21 January 2001
• Progress/Summary to Date
• Future Work

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Motivation

• Identified as a forecast problem by NWS offices
  in vicinity of Appalachians (e.g., through
  discussion with Larry Lee)
• Operational models struggle forecasting these
  events (resolution, moisture source, etc.)
• Main aspects in question include total
  accumulation, spatial extent

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Example/Problem

• 1.) Amount/local variability in upslope region
• 2.) Spatial extent across mountains

6 March 2001
25 January 2001
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Definition

• NW flow, in the lower levels, 850 mb
• Air parcels encounter mountains, orographic lift
  occurs, extracting low-level moisture and
  producing snowfall
• Study area is the southern Appalachians including
  areas from north Georgia to southeastern West
  Virginia

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Questions

• What parameters do forecasters currently examine,
  what other parameters could help?
• What determines degree of spatial precipitation
  variability / maximum accumulations in mountains?
  
• How far east of mountains will precipitation
  extend?
• Ability of high-resolution models (e.g. WRF) to
  predict these events (optimal configuration?)

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Hypotheses

• Great Lakes are a significant source of moisture,
  instability in some events (especially in heavier
  events) (Perry and Konrad study)
• Strength of synoptic forcing distinguishes
  localized from more widespread event
• Strength of low-level flow, moisture availability
  dictate snow accumulations, coverage
• High resolution WRF runs will provide more
  accurate and detailed forecasts

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Method

• Case study approach
• Identify key parameters that can be utilized in
  operational forecasting
• Use WRF model to test sensitivity to Great Lakes,
  mountains, model configuration and resolution

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Notable NW Flow Events

• 3 November 1999 Post Frontal
• 17-18 January 2000 NW Flow Aloft
• 20-21 January 2001 Cut-Off Low
• 25 January 2001 - Post Frontal
• 6 March 2001 Cut-Off Low
• 17 April 2001 Cut-Off Low
• 16-17 January 2003 Post Frontal
• 28-29 November 2003 Post Frontal
• 18-20 December 2003 Post Frontal
• 19 December 2004 Post Frontal
• 19 January 2005 NW Flow Aloft
• 20-21 January 2005 NW Flow Aloft

• Selected examples

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Case 1 18-20 December 2003
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Case 1 18-20 December 2003
19z 18th
00z 19th
06z 19th
19z 19th
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Case 1 18-20 December 2003 18z 18th
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Case 1 18-20 December 2003BNA and RNK, 12z 19th
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Case 1 18-20 December 2003

• Presence of shortwave trough at 500 mb (shear
  vorticity, cyclonic side of jet)
• After weak surface low passage, strong deep-layer
  W, NW flow
• Also, very moist around 850 mb in BNA and RNK
  soundings, instability?
• Additional lift from vorticity maxima aloft and
  enhanced orographic lift from strong low-level NW
  flow

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Case 2 20-21 January 2001
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Case 2 20-21 January 2001
21z 20th
18z 20th
00z 21st
03z 21st
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Case 2 20-21 January 2001
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Case 2 20-21 January 2001BNA and RNK, 00z 21st
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Case 2 20-21 January 2001

• Strong, 500 mb closed low tracks eastward across
  the mid-Atlantic
• East coast cyclogenesis taking place during peak
  NW flow time period
• BNA and RNK soundings moist with 20-25 kt NW flow
  at 850 mb

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Progress/Summary to Date

• Two cases identified for further study
• Identified other relevant cases
• Work with Perry and Konrad using detailed
  climatology studies on patterns, topography, and
  moisture source associated with NW flow snow

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Future Work

• Will identify cases for further hypothesis
  testing
• Great Lakes influence candidate Use model to
  quantify role of lakes
• Role of synoptic support in determining
  precipitation distribution remove mountains,
  isolate orographic signal
• Test WRF resolution, physics configurations to
  determine optimum configuration, required
  resolution for NW snow prediction