Characteristics of Upslope Snowfall Events in Northern New York State and

Northern Vermont

Diagnostics and Model Simulations of Several Northwest-Flow Cases

Daniel St. JeanNOAA, National Weather Service Forecast

Office, Burlington, Vermont

CSTAR (Collaborative Science Technology and Applied Research Program) --- improving prediction of cool season

heavy precipitation events over the northeastern U.S.

Paul Sisson, NWS Burlington VTDr. Lance Bosart, SUNY-Albany, Albany NYDr. Daniel Keyser, SUNY-Albany, Albany NYBrandon Smith, SUNY-Albany, Albany NY– 500 hPa cutoff cyclone climatology project

Tom Wasula, NWS Albany NYMike Cempa, NWS Albany NYGreg West, SUNY-Albany, Albany NY

Overview

Impetus for ResearchData SourcesSynoptic-scale and Mesoscale Structure5km vs. 40km Model Forecast FieldsResultsFuture Investigation

Why Study Northwest Flow and Cutoff Cyclone Cases?

Often produce significant sub-synoptic scale or mesoscale precipitation over the northeastern U.S.;Forecast models often handle cutoff cyclones and topographic precipitation inaccurately; Events are less frequent than those associated with rapid coastal cyclogenesis (i.e., heavy precip generally occurs only on upstream side of cyclone);Low-level flow pattern is generally orthogonal to local terrain enhanced orographic ascent;

Why Study Northwest Flow and Cutoff Cyclone Cases?

Often produce significant sub-synoptic scale and mesoscale precipitation over the northeastern U.S.;Forecast models often handle cutoff cyclones and topographic precipitation inaccurately;Events are less frequent than those associated with rapid coastal cyclogenesis (i.e., heavy precip generally occurs only on upstream side of cyclone);Low-level flow pattern is generally orthogonal to local terrain enhanced orographic ascent;

Why Study Northwest Flow and Cutoff Cyclone Cases?

Often produce significant sub-synoptic scale and mesoscale precipitation over the northeastern U.S.;Forecast models often handle cutoff cyclones and topographic precipitation inaccurately; Events are less frequent than those associated with rapid coastal cyclogenesis (i.e., heavy precip generally occurs only on upstream side of cyclone);Low-level flow pattern is generally orthogonal to local terrain enhanced orographic ascent;

Why Study Northwest Flow and Cutoff Cyclone Cases?

Often produce significant sub-synoptic scale and mesoscale precipitation over the northeastern U.S.;Forecast models often handle cutoff cyclones and topographic precipitation inaccurately; Events are less frequent than those associated with rapid coastal cyclogenesis (i.e., heavy precip generally occurs only on upstream side of cyclone);Low-level flow pattern is generally orthogonal to local terrain Enhanced orographic ascent;

Six events studied:

3 heavy snowfall-producing events:– 18-19 March 1999 (cutoff cyclone)– 15-16 November 1999 (cutoff cyclone)– 10-11 December 1999 (cutoff cyclone)

3 null events :– 02-03 March 2000 (cutoff, forecast bust)– 28-29 October 2000 (cutoff, forecast bust)– 12 November 2001 (short-lived open wave)

Data Sources

NCEP/NCAR Reanalysis grids (2.5 x 2.5 lat/lon grid)Operational ETA model (remapped to 40km grid)Workstation ETA model (5km grid)ETA model BUFR forecast sounding data (from 32km, 22km, and 12km operational native grids)Verification---NWS Burlington local storm data

Heavy Precip Case: 15-16 November 1999

Produced significant snowfall with clear upslope signature; Event length: 991115/18---17/00 UTC;500 hPa pattern: Persistent cyclonic flow;850 hPa pattern: Persistent, strong northwest flow;

Heavy Precip Case: 15-16 November 1999

Produced significant snowfall with clear upslope signature; Event length: 991115/18---17/00 UTC;500 hPa pattern: Persistent cyclonic flow;850 hPa pattern: Persistent, strong northwest flow;

Heavy Precip Case: 15-16 November 1999

Produced significant snowfall with clear upslope signature; Event length: 991115/18---17/00 UTC;500 hPa pattern: Persistent cyclonic flow;850 hPa pattern: Persistent, strong northwest flow;

Heavy Precip Case: 15-16 November 1999

Produced significant snowfall with clear upslope signature; Event length: 991115/18---17/00 UTC;500 hPa pattern: Persistent cyclonic flow;850 hPa pattern: Persistent, strong northwest flow;

Heavy Precip Case (contd.): 15-16 November 1999

Relative humidity/V time-height diagram for BTV shows abundant moisture and significant cross-barrier flow from the surface to 850 hPa;BTV ETA BUFR forecast data suggest some measure of low static stability through the event;

BE

Heavy Precip Case (contd.): 15-16 November 1999

Relative humidity/V time-height diagram for BTV shows abundant moisture and significant cross-barrier flow from the surface to 850 hPa;BTV ETA BUFR forecast data suggest some measure of low static stability through the event;

Null Case: 02-03 March 2000

Produced no widespread significant snowfall;15 watches and 11 warnings issued---all unverified (no one lives atop Mt. Mansfield);Event length: 000303/00---03/12 UTC;500 hPa pattern: Cutoff stalls over eastern New England;850 hPa pattern: Persistent, fairly weak northwest flow;

Null Case:02-03 March 2000

Produced no widespread significant snowfall;15 watches and 11 warnings issued---all unverified (no one lives atop Mt. Mansfield);Event length: 000303/00---03/12 UTC;500 hPa pattern: Cutoff stalls over eastern New England;850 hPa pattern: Persistent, fairly weak northwest flow;

Null Case:02-03 March 2000

Produced no widespread significant snowfall;15 watches and 11 warnings issued---all unverified (no one lives atop Mt. Mansfield);Event length: 000303/00---03/12 UTC;500 hPa pattern: Cutoff stalls over eastern New England;850 hPa pattern: Persistent, fairly weak northwest flow;

Null Case:02-03 March 2000

Produced no widespread significant snowfall;15 watches and 11 warnings issued---all unverified (no one lives atop Mt. Mansfield);Event length: 000303/00---03/12 UTC;500 hPa pattern: Cutoff stalls over eastern New England;850 hPa pattern: Persistent, fairly weak northwest flow;

Null Case:02-03 March 2000

Produced no widespread significant snowfall;15 watches and 11 warnings issued---all unverified (no one lives atop Mt. Mansfield);Event length: 000303/00---03/12 UTC;500 hPa pattern: Cutoff stalls over eastern New England;850 hPa pattern: Persistent fairly weak northwest flow;

Null Case (continued): 02-03 March 2000

BTV RH/V time-height diagram shows drying occurring by 03/18, effectively ending event;BTV ETA BUFR forecast data show increasing low-level static stability through the event;

BE

Null Case (continued): 02-03 March 2000

BTV RH/V time-height diagram shows drying occurring by 03/18, effectively ending event;BTV ETA BUFR forecast data show increasing low-level static stability through the event;

Workstation ETA model

Run hydrostatically Kain-Fritsch Convective Parameterization5km horizontal grid spacing45 vertical levelsNested in 32km ETA for Lateral Boundary Conditions (NE U.S. Domain)

Results to Date:

Several factors may be significant in heavy precipitation production in the complex terrain of the BTV forecast area:

Factors:

Near saturated low-level conditions:– RH >75% from surface to ridgetop level;

Favorable conditions for dendritic growth:– -12c to -18C with 80% RH

Factors (cont.):

Strong low-level winds, with significant component orthogonal to terrain:– Wind direction of 270°--320°;– 925 hPa speed of at least 10 m/s;– 850 hPa speed of at least 15 m/s;

Low static stability below ridgetop level: e decreasing with height;– Steep low-level lapse rates;

Factors (cont.):

Long-lived upslope conditions:– >12 hour event duration—as you might expect with a

cutoff low;

990319 991116 991211 000303 001029 011112

RH >75%? Y Y Y Y N Y

925 hPa wind > 10 m/s?

Y Y Y N Y N

925 hPa wind 270°-320°?

Y Y Y Y N Y

850 hPa wind>15 m/s?

Y Y Y N Y N

850 hPa wind 270°-320°?

Y Y Y N N Y

e decrease with height?

Y Y Y N Y Y

900-850 hPaLapse rate >6°C?

Y Y Y N Y Y

Event Lifetime> 12 hrs?

Y Y Y N Y N

Possible Ingredients Checklist

Future Direction

Continued evaluation of operational 12km model and locally-run 5km ETA model:– Greater resolution better low-level depiction of features? – Which convective scheme works best? – How well does the model handle cutoff lows?

Applicability of the favorable ingredients to cutoff lows in other parts of the northeastern United States (incorporating CSTAR 500 hPa cutoff cyclone climatology);Incorporate the ingredients-based conceptual model into AWIPS procedures or Web graphics;

FIN