Communicating Uncertainty via Probabilistic Forecasts for the January 2016 Blizzard in Southern New England Frank M Nocera, Stephanie L. Dunten & Kevin J. Cadima NOAA/National Weather Service Boston, MA.

2014-2015 Probabilistic Snow Method WPC 90th Percentile 2014-2015 Probabilistic Snow Method

What QPF is driving WPC Percentile Forecasts?

Probabilistic Winter Precipitation Forecast Ensemble Composition (2016-2017 Season) 70 members total (up from 63 last season) 26 NCEP SREF members 25 ECMWF ensemble members, randomly selected 1 NAM operational run 1 NCEP GFS operational run 1 ECMWF latest operational deterministic run 1 CMC latest operational run 1 ECMWF latest ensemble mean 1 GEFS latest ensemble mean (6-h SLRs) 1 WPC deterministic snow/ice forecast 10 GEFS members, randomly selected (5 last season) 1 WRF ARW run (Day 1 only) 1 WRF NMMB run (Day 1 only)

General Overview of Methodology WPC creates an ensemble (63 members) derived PDF For every grid pt. on the 2.5 km CONUS grid domain WPC deterministic forecast becomes the mode Percentile accumulations are derived from this PDF 2 CDFs created (WPC & WFO), reshape CDF to account for differences Step 1 Snowfall accumulation  Probability 90th Percentile Value 10th Percentile Value Mode Step 3 The forecast process begins with ensemble data generated at WPC. WPC creates an ensemble-derived probability distribution function, or PDF, twice daily from a 63-member ensemble which is influenced by their human-created winter weather desk forecast. This occurs at every grid point on the 2.5km CONUS grid domain. Percentile accumulations are derived from this PDF, at the 5, 10, 25, 50, 75, 90 and 95th levels, and sent over the SBN to AWIPS. A cumulative distribution function (CDF) is then created from these WPC percentile amounts in GFE. Next, the official WFO Storm total snowfall amount or STS, is considered in addition to a GFE-created “WPC Storm Total Snowfall Amount”. Recall that WPC only produces set time accumulations (eg., 6 hr, 24 hrs), so a WPC Storm Total Amount is automatically created in GFE to align the WFO and WPC storm total snowfall accumulation periods. A difference between the two storm total snowfall grids are computed and the original WPC ensemble-derived CDF is shifted to create a new, adjusted CDF at each grid point, re-centered on the official WFO storm total snow amount. This is shown in Step #3. It’s at this step that the WFO adds their local expertise and value to the forecast process. Exceedance probabilities, percentile amounts and a suite of graphics depicting the probable minimum, maximum and most likely scenario are then output to the web. Step 2

January 23-24th 2016 blizzard

WPC 10th Percentile 48 hour Forecast 24 hour Forecast 12 hour Forecast Cut!

WPC 90th Percentile 48 hour Forecast 24 hour Forecast 12 hour Forecast Cut!

Multi-Faceted Storm First snow storm of the 2015-2016 season Heavy snow and P-type Issues Sharp snowfall gradient on the Northern edge High Winds Coastal Flooding

Headlines

10 Percentile – 48-60 hour Forecast (Expect at least this much snow) WPC 10th Percentile

90 Percentile – 48-60 hour Forecast (Potential for this much snow) WPC 90th Percentile

Storm Total Snowfall – 48-60 hour Forecast (Most Likely)

10 Percentile – 12 hour Forecast (Expect at least this much snow) WPC 10th Percentile

WPC 10th Percentile – 12 hr Fcst Valid 12z /23 - 12z/24 Radar Imagery 07z to 09z, 23 Jan 2016

90 Percentile – 12 hr Forecast (Potential for this much snow) WFO WPC The original unedited 90th percentile had 20-24” to MA/NH border

Storm Total Snowfall – 12 hr Forecast

Forecast Snowfall Amounts (DSS slide) Highest Uncertainty

Observed Storm Totals: Jan 23-24th, 2016 Boston: 6.1” 2-5” 4-7” 4-7” 8-10” 2-5” 4-7” The forecasters edited the 90th percentile to bring the goal posts down. The 12 hour 90th percentile was very close to what actually happened. This is a big improvement to the original and how forecasters can add value to allow Ems to better prepare. 10-15” 10-14”

Challenges/Lessons Learned Goal posts of the ensemble guidance were very wide Local office edits provided more reasonable best/worse case scenarios WPC 10th & 90th Ensemble: Boston: 0-18 inches Providence: 0-21 inches NWS 10th & 90th Forecasts: Boston: 0-6 inches (6.1”) Providence: 1-9 inches (8.0”) Narrow the goal posts Lessons Learned Forecasters need to validate WPC 10th & 90th percentile forecasts upstream Load & go not acceptable Finish!

Challenges Cont. High SREF data incorporated in the snow probability graphics 40 percent (26/63 ) of the ensemble composition SREF was not removed as it is more reliable for mesoscale events especially across the Great Lakes If no editing occurred on the high-end probabilities then the City of Boston would have spent 2 million dollars prepping for this particular storm Load and go was not acceptable

Satellite Image on January 24th after the Blizzard moved through

Communicating Uncertainty via Probabilistic Forecasts for the January 2016 Blizzard in Southern New England Frank M. Nocera, Stephanie L. Dunten & Kevin J. Cadima NOAA/National Weather Service Boston, MA.