The GDPNow model forecast jumped to 5.4% on February 1, only to come crashing down the next day.

Pat Higgins, creator of the GDPNow model addresses the question GDPNow’s Forecast: Why Did It Spike Recently? on today’s Atlanta Fed Macroblog.

If you felt whipsawed by GDPNow recently, it’s understandable. On February 1, the Atlanta Fed’s GDPNow model estimate of first-quarter real gross domestic product (GDP) growth surged from 4.2 percent to 5.4 percent (annualized rates) after a manufacturing report from the Institute for Supply Management. GDPNow’s estimate then fell to 4.0 percent on February 2 after the employment report from the U.S. Bureau of Labor Statistics. GDPNow displayed a similar undulating pattern early in the forecast cycle for fourth-quarter GDP growth.

What accounted for these sawtooth patterns? The answer lies in the treatment of the ISM manufacturing release.

In the Atlanta Fed’s GDPNow factor model, the last month of ISM manufacturing data have large weights when calculating the terminal factor value right after the ISM report. These ISM weights decrease significantly after the employment report, when about 50 of the indicators have reported values for the last month of data. [Mish note: the next day]

A possible shortcoming of the GDPNow factor model is that it does not account for the previous month’s forecast errors when forecasting the 127 indicators. For example, the predicted composite ISM PMI reading of 54.4 in December 2017 was nearly 5 points lower than the actual value.

If we decide to incorporate adjustments to GDPNow’s factor model, we will do so at an initial forecast of quarterly GDP growth and note the change here. [Mish Note: “Here” is download of a PDF of GDPNow Model Modifications]

Would the adjustment have made a big difference in the initial first-quarter GDP forecast? The February 1 GDP growth forecast of GDPNow with the adjusted factor model was “only” 4.7 percent. Its current (February 9) forecast of first-quarter GDP growth was the same as the standard version of GDPNow: 4.0 percent. These estimates are still much higher than both the recent trend in GDP growth and the median forecast of 3.0 percent from the Philadelphia Fed’s Survey of Professional Forecasters (SPF).

Most of the difference between the GDPNow and SPF forecasts of GDP growth is the result of inventories. GDPNow anticipates inventories will contribute 1.2 percentage points to first-quarter growth, and the median SPF projection implies an inventory contribution of only 0.4 percentage points. It’s not unusual to see some disagreement between these inventory forecasts and it wouldn’t be surprising if one—or both—of them turn out to be off the mark.