1. 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

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

5. NROW 2017 +3F to +4F temp anomaly
Combination of record warmth & drought conditions = low situational awareness for early season snows
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6. Synoptic Overview
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9. Secondary cyclogenesis across southern New England, trapping low level cold air
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10. Deep layer of cold/dry air below 800 mb available for evaporational cooling
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11. 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
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12. Deterministic Forecasts
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13. Not much snowfall expected initially
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14. Forecast trend is upward!
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15. Probabilistic Forecasts & Datasets
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16. Raw 70 Member Ensemble Output
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17. Automated SREF Derived Probability Winter Precipitation Rate ≥ 1”/hr
Valid 21z Thu Oct
Valid 21z Thu Oct –
21z RUN
03z RUN
Valid 00z Fri Oct –
Valid 00z Fri Oct –
21z RUN
03z RUN
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18. Model simulated reflectivity at 22Z up to 50 dbz over parts of CT/MA/NY!
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23. High Resolution 4km NAM nest Model Soundings & Reflectivity
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24. Topography
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25. 4 km NAM 06Z KBDL
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26. 4 km NAM 06Z KBAF
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27. 4 km NAM 06Z KORE
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28. NWA Annual Meeting
September 2017

29. Messaging Uncertainty
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30. Social Media Post

31. 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.

32. Remaining probabilistic during the event
Probabilistic info in a legacy text product
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33. Outcome & Impacts
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34. 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
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35. 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
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36. Observed Snowfall
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37. Damage Reports of down tree limbs & powerlines
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38. 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)
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