The weather maps shown here are generated from the
NCEP Global Forecast System (GFS) model. GFS is the primary operational model framework underlying U.S. NOAA/NWS
weather forecasting. The model is run four times daily on a global T1534 gaussian grid (~13 km) to produce 16-day
forecasts. Here, we use 0.25°x0.25° (~30 km) output grids available from
NOMADS, and calculate daily averages from eight 3-hourly timeslices starting at 0000 UTC.
- Sea surface temperaure (SST) and SST anomaly maps are generated from
NOAA Optimum Interpolation SST version 2 (OISST V2).
OISST is a 0.25°x0.25° blendend dataset derived from satellite, ship, and buoy measurements. The SST anomaly
is based on a 1971-2000 NOAA climatology.
- Temperature refers to air temperature at 2 meters above the surface. The temperature anomaly is made in
reference to a 1979-2000 climatology derived from the
reanalysis of the NCEP Climate Forecast System (CFSR/CFSV2) model. This climate baseline is used instead of the
1981-2010 climate normal because
it spans a period prior to significant warming of the Arctic beyond historically-observed values. For context,
see this timeseries plot showing how various
climate baselines compare against the NASA GISS
1880-2014 global land-ocean temperature index.
- GFS Model Bias Correction — GFS has a model bias compared to CFSR/CFSV2 in which very warm and very cold
temperatures tend to be greater, leading to more pronounced temperature anomalies. In an effort to diminish the effect
of this bias, a simple correction factor (CF) is applied to each regional temperature anomaly. Correction factors were
derived by calculating the average percent difference between GFS-CFSR and CFSV2-CFSR using daily model output grids
from June, 2015. The resultant correction factors are 0.49 (World), 0.46 (Northern Hemisphere), 0.64 (Southern Hemisphere),
0.53 (Arctic), 0.63 (Antarctic), and 0.54 (Tropics). The terms are applied in the form:
Tanomaly region corrected = Tanomaly region * CF
Note that bias-corrected GFS-CFSR temperature anomalies will differ slightly from those calculated from CFSV2-CFSR
comparisons for June 30, 2015). CFSV2-CFSR is apples-to-apples; therefore, difference maps within the
Daily Reanalysis Maps image archive should be
considered reliable, and take precedent over the GFS-CFSR anomaly maps shown here as part of the current 7-day forecast.
See this NCEP/NWS discussion for additional
information on model bias.