1. 1 Ensemble Reforecasts Presented By: Yuejian Zhu (NWS/NCEP) Contributors:

2. 2 Operational System Attribute(s) System NameAcronymAreal Coverage Horz Res Cycle Freq Fcst Length (hr) Global Ensemble Forecast System Reforecast (retrospective) GEFSGlobal34km/ 52km 4 days 384hrs SystemAttributes GEFSDA:NCEP hybrid GSI/EnKF; INI: ETR 6hr cycling + TS relocation; Model: GFS System Data Assimilation or Initialization Technique GEFSv11 reforecast configuration: 20 years (start from 1995), every 4 days, 00UTC only, 5 ensemble members Cost: 30% of GEFS forecast resource in real-time

3. 3 Why System(s) are Operational?  Primary stakeholders and requirement drivers NCEP/CPC; NCEP/WPC; OST/MDL; NWC; Public users References: ESRL run GEFSv10 reforecast (off-line) NCEP will run GEFSv11 reforecast (off-line)  What products are the models contributing to? Forecast calibration NDGD - National blender project Week-2 anomaly forecast Hydrological application – precipitation extreme  What product aspects are you trying to improve with your development plans? Precipitation, temperature, and etc… Extreme weather events  Top 3 System Performance Strengths Enhance forecast skills (and/or reduce systematic error) Improve forecast reliability Improve prediction of extreme weather events  Top 3 System Performance Challenges Limited reforecasts due to resources Not enough ensemble members to represent forecast uncertainty Reanalysis – inconsistent initial analyses

4. 4 System Evolution Over the Next 5 Years  Major forcing factors Requirement from broader users (and applications) Severe weather guidance Week-2 hazard weather Hydrological application of extreme events  Science and development priorities To have a new reforecast for each major upgrade To have a new reanalysis for each major upgrade To have optimum configuration (considering trade-off)  What are your top challenges to evolving the system(s) to meet stakeholder requirements? Resources – labor, computation and storage To satisfy user’s minimum requirement: MDL – 2y retrospective – daily (00UTC and 12UTC) and full ensemble CPC – 20+y – every 5 day, 5 members NWC – 30+y – daily, 10 members – 4 factors to real time GEFS cost  Potential opportunities for simplification going forward If NWS has dedicated HPC for reanalysis and reforecast, … If we could use “cloud machine” and storage,...

5. 5 Top 3 Things You Need From the UMAC 1.Currently, model bias is still one of the main error sources of forecast. Therefore, “bias correction” is very important for user application. When do we expect that the “bias correction” is less important? 2.In order to balance our computational resources (HPC), which one is relatively important – increase model resolution or increase reforecast samples? In particularly for hydrological application? 3.If reforecast is in our production suite, which mode (“real time” or “off-line”) is the right way to go? Real time: running past “m” years reforecasts each day, and moving forward – seamlessly for implementation/change Off-line: running past “m” years reforecasts at once, pass to users several months ahead before model is implemented How do we balance requirements