Evaluating Potential Impact of Significant East Coast Winter S torms by Analysis of Upper and Low-Level Wind Anomalies Neil A. Stuart NOAA/NWS Albany, NY Northeast Regional Operational Workshop VIII Albany, NY 1 November 2006

Considerations NWP guidance and forecasters can still miss important atmospheric features Recent storms illustrate NWP and forecaster busts: 25 January 2000 (Bosart 2003) 4-6 March 2001 (Grumm 2001; D’Aleo 2001) 6-7 January 2002 (Grumm 2002) February 2004 Small changes in forecasted and observed snowfall = big difference in how people prepare for the storm, and how they are impacted

Considerations (Cont.) Conventional NWP text and graphical guidance does not provide information on departures from normal Climatological anomalies are calculated as standard deviations from normal Based on a 30- year climatology from 1961 through 1990 (includes the snowy decade of the 1960s) Calculated on a 2.5  by 2.5  grid (Hart and Grumm 2001) U Wind anomalies were calculated at 850 hPa and 300 hPa, These anomalies can assist in identifying slow-moving storms with extended periods of enhanced precipitation Real-time model and ensemble forecasts of 250 Mb anomalies available - assumed nearly identical to 300 Mb

Why 850 Mb and 300/250 Mb? 850 Mb winds represent low level wind flow above the friction layer Easterly winds at 850 hPa – moisture advection off the Atlantic Ocean low-level convergence enhancement of low-level frontogenesis increased precipitation production 300 to 250 Mb winds represent upper-level wind flow affecting the movement of the upper-level vorticity center Below normal upper-level winds can signal a storm cut off from the steering flow, resulting in slower movement

Definitions Historical East Coast Winter Storm (based on analysis of snowfall amounts and wind anomalies in 85 winter storms from ) >12” of snow in the Carolinas and points south >18” of snow in Virginia and points north Anomaly – Departure from normal based on a 30 year climatology ( ), in units of Standard Deviations from Normal (SD) U and V wind anomalies – Departures from normal for Westerly and Southerly winds -U and –V wind anomalies = Departures below normal for easterly and northerly winds

4 historical storms provide overview December 1989 Southeast U.S. case – Affected Carolinas 12” to 18” of snow in areas that normally receive <6” of snow the entire season 5-7 March 1962 “Ash Wednesday Storm” mid-Atlantic U.S. case – Heavy Snowfall in Virginia, Maryland, and points north 12” to 36” of snow (greatest amounts near the Appalachian Mountains) 6-8 February 1978 “Blizzard of ‘78” Northeast U.S. case – Heavy Snowfall in southern New York and New England 2’ to 4’ of snow Over 30” of snow in Boston, Route 128 closed 3’ to 4’ of snow near Massachusetts/Rhode Island border January 2000 “Surprise Storm” Entire East Coast case – Heavy Snowfall from Carolinas through mid-Atlantic and northeast Widespread 12” to 24” of snow 20” of snow in Raleigh, NC

Peak 850 Mb U Wind Anomalies for Dec. 89, Mar. 62, Feb. 78 and Jan. 00 storms

Peak 300 Mb U wind Anomalies for Dec. 89, Mar. 62, Feb. 78 and Jan. 00 storms

What can we conclude? (Based on these 4 storms, and others not shown) 850 Mb U wind anomalies <-4 SD for potential extreme snowfall 250 Mb U wind anomalies <-2.5 SD for unusually slow-moving storm Heaviest snowfall generally within the nose of the 850 Mb –3 SD contour Rain/Snow conversion ratios not considered

Important note Past storm anomalies on a 2.5° X 2.5° grid, which is a resolution that is more coarse than NWP model output Forecasted anomalies from individual NWP models can be higher than reanalysis output, due to the finer spatial resolution in the NWP model output Snowstorms during the winter illustrated differences in reanalysis and NWP model anomalies Recent storms of will illustrate differences in anomalies due to resolution, and differences in model/ensemble-depicted evolution

Christmas 2002 Snowstorm Interior Pennsylvania through upstate New York and northern New England Widespread 18” to 36” of snow 24” to 36+” of snow in a large area of upstate New York

Reanalysis - Peak 850 Mb U Wind Anomaly –3.70 SD Eta Model - Peak 850 Mb U Wind Anomaly <-5.5 SD

Reanalysis - Peak 300 Mb U Wind Anomaly –3.23 SD Eta Model - Peak 250 Mb U Wind Anomaly <-3 SD

Another recent example – 5-7 December ” to 36” of snow northern mid-Atlantic through New England in northern New England) ETA, GFS and SREF depict different anomaly values – initial and forecasted values Due to spatial resolution differences Due to differences in model-depicted evolution ETA, GFS and SREF anomaly values all validate the anomaly thresholds for historical snowstorms

Initial and forecasted 850 hPa wind anomalies

Initial and forecasted 250 hPa wind anomalies

SREF wind anomalies from 09Z 12/6 10 member ensemble: 5 RSM, 5 ETA

15-16 December of snow Lake Champlain area to northern Maine Isolated amounts to near 30” some orographically enhanced 850 Mb anomalies exceeded the threshold for >18” of snow Eta (-6.35 SD), GFS (-4.83), SREF SD) 250 Mb anomalies did not reach the threshold for a long duration storm until it reached Maine Eta (-2.49 SD), GFS (-1.90 SD), SREF SD)

Carolinas Example February ” of snow in central and western North and South Carolina Very little impact in eastern North Carolina No SREFs available, but Eta and GFS both exceeded the lower and about equaled the upper anomaly thresholds One of the best lead times in the study

Forecasted wind anomalies

Storms and peak anomalies EventMaximum Observed Snow (cm) Region AffectedPeak 850 Mb AnomalyPeak upper level anomaly Ash Wed. Storm 5-7 Mar Interior mid-Atlantic Blizzard of ' Feb New England Dec Carolinas Blizzard of ' Jan Mid-Atlantic and New England Surprise Storm Jan Carolinas to New England Dec Interior Northeast (-5.9)-3.23 (-3.3) 3-4 Jan Interior Northeast (-5.8)-1.99 (-3.3) President's Day Feb Mid-Atlantic and Northeast (-5.5)-2.03 (-2.5) 6-7 Dec New England (-5.5)(-2.7) Dec Northern New England (-6.4)(-2.5) Great Canada Maritime Blizzard Feb Southeast Canada (-5.5)(-3.1) Feb Carolinas (-5.3)(-2.7) Blizzard of ‘ Jan New England and Southeast Canada -6.3 (-5.3)-2.5 (-2.5) Feb Northern New England -6.3 (-4.5)-3.5 (-2.5)

Two important exceptions April Fool’s Day 1997 Storm in New England February 2006 Storm in southern NY and NJ Each storm was 0.5 to 1.0 SD short of the thresholds for historical storm Exceptional frontogenesis and varying snow ratios may have contributed to extreme snowfall

850 Mb anomalies from 12 February 2006

Upper-level anomalies and frontogenesis for 12 February 2006

Final thoughts Upper-level and 850 Mb wind anomalies <-2.5 SD and <4 SD No false alarms, but POD may be impacted due to small scale processes - frontogenesis and liquid/snow ratios MREF and SREF anomalies based on different blends of operational models How will WRF resolve anomalies? This is just one tool for evaluating the potential for a significant storm Model-derived forecast anomalies for 850 Mb and 250 Mb are available in real-time at: Thanks to Rich Grumm of NWSFO State College, PA, for his assistance in this study, and for maintaining the eyewall website with Dr. Bob Hart of Penn State University. Thanks also to Wayne Albright, John Billet, and everyone at NWSFO Wakefield, VA for their support of this study.