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Northwest Flow Snow Events in the Southern Appalachian Mountains CSTAR Workshop 10705

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Title: Northwest Flow Snow Events in the Southern Appalachian Mountains CSTAR Workshop 10705


1
Northwest Flow Snow Events in the Southern
Appalachian MountainsCSTAR Workshop10/7/05
  • Blair Holloway

2
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

3
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

4
Example/Problem
  • 1.) Amount/local variability in upslope region
  • 2.) Spatial extent across mountains

6 March 2001
25 January 2001
5
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

6
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?)

7
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

8
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

9
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

10
Case 1 18-20 December 2003
11
Case 1 18-20 December 2003
19z 18th
00z 19th
06z 19th
19z 19th
12
Case 1 18-20 December 2003 18z 18th
13
Case 1 18-20 December 2003BNA and RNK, 12z 19th
14
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

15
Case 2 20-21 January 2001
16
Case 2 20-21 January 2001
21z 20th
18z 20th
00z 21st
03z 21st
17
Case 2 20-21 January 2001
18
Case 2 20-21 January 2001BNA and RNK, 00z 21st
19
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

20
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

21
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
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