NOAA/National Weather Service Taunton, MA Communicating Uncertainty for an Anomalous Early Season Snow Event in New England Using High Resolution Ensembles & Probabilistic Datasets NOAA/National Weather Service Taunton, MA Frank Nocera NROW 2017

Outline Situational Awareness (SA) Antecedent conditions & potential impacts on SA Communicating/Messaging Uncertainty DSS emails, Social Media & Special Weather Statements Probabilistic, Ensembles & High Resolution guidance How did these datasets verify? NROW 2017

Antecedent Conditions NROW 2017

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NROW 2017 +3F to +4F temp anomaly Combination of record warmth & drought conditions = low situational awareness for early season snows NROW 2017

Synoptic Overview NROW 2017

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Secondary cyclogenesis across southern New England, trapping low level cold air NROW 2017

Deep layer of cold/dry air below 800 mb available for evaporational cooling NROW 2017

Forecast Challenge Checklist Warm ground given Oct & record warmth last 3 months - heavy QPF required for cooling via melting? Heavy QPF for wet bulb temperatures to materialize for evaporational cooling? Duration of cooling from vigorous upward vertical motion, evaporational cooling & from potential melting? How quickly would secondary low develop to cut off low level warming? Eventually robust WAA will win out given 850 mb low tracking northwest of CWA NROW 2017

Deterministic Forecasts NROW 2017

Not much snowfall expected initially NROW 2017

Forecast trend is upward! NROW 2017

Probabilistic Forecasts & Datasets NROW 2017

Raw 70 Member Ensemble Output NROW 2017

Automated SREF Derived Probability Winter Precipitation Rate ≥ 1”/hr Valid 21z Thu Oct - 27 - 2016 Valid 21z Thu Oct – 27 - 2016 21z 20161026 RUN 03z 20161027 RUN Valid 00z Fri Oct – 28 - 2016 Valid 00z Fri Oct – 28 - 2016 21z 20161026 RUN 03z 20161027 RUN NROW 2017

Model simulated reflectivity at 22Z up to 50 dbz over parts of CT/MA/NY! NROW 2017

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High Resolution 4km NAM nest Model Soundings & Reflectivity NROW 2017

Topography NROW 2017

4 km NAM 06Z 20161027 - KBDL NROW 2017

4 km NAM 06Z 20161027 - KBAF NROW 2017

4 km NAM 06Z 20161027 - KORE NROW 2017

NWA Annual Meeting September 2017

Messaging Uncertainty NROW 2017

Social Media Post

Decision Support Services Email Snowfall Potential... Snow will overspread western MA late this morning and early afternoon. By then temperatures will have climbed above freezing so any minor snow accumulations will be confined to non paved surfaces. However by mid afternoon there is a low risk that snow intensity may increase enough to drop temperatures back toward freezing. This would allow snow to accumulate on all surfaces with roads becoming snow covered for a time The greatest risk area for this to occur would be at elevations above 1000 ft across western MA. If this scenario were to materialize a slushy 1-3" of snowfall would be possible with highest amounts at elevations above 1000 ft. As a result a Special Weather Statement has been issued for this area. Toward sunset snow across western MA will likely change over to a period of sleet and then all rain as temperatures rise through the 30s and into the low 40s. Road conditions will improve during this time.

Remaining probabilistic during the event Probabilistic info in a legacy text product NROW 2017

Outcome & Impacts NROW 2017

Radar Loop From Albany, NY Reflectivity up to 50 dbz! Mid level dry air steepens lapse rates & precip becomes more convective Farther to the west mid level dry invades the dendritic growth zone & results in p-type changeover from snow to freezing rain/drizzle NROW 2017

NROW 2017 00z 28 Oct 2016 Observed Sounding, Albany, NY -15C 0C Robust warm advection being offset by cooling via strong ascent & melting snow aloft -15C 0C Quasi isothermal lapse rate = cooling via melting snow flakes Strong ascent likely reaching the dendritic growth zone given saturation Very steep lapse rates = brief heavy snow NROW 2017

Observed Snowfall NROW 2017

Damage Reports of down tree limbs & powerlines NROW 2017

Take – a - ways Challenging to communicate/message heavy snowfall potential given so much certainty with multiple parameters needed to come together for heavy snow to be realized Not having local probabilistic snow forecasts ready to go operationally (due to offseason changes) limited ability to communicate snowfall potential graphically Must utilize/leverage Hi Res guidance (NCAR ensembles & 4km NAM nest) for mesoscale/high terrain events 70 member low res. ensembles will not capture these mesoscale events. This is where forecasters can add value to snow probs (target of opportunity) NROW 2017