National Blend of Models (NBM) Upgraded to Version 4.3

May 27, 2025

NBMv4.3: Note that the intrusion of stronger inland wind speeds in southeast Georgia, southern South Carolina, southern Alabama and Mississippi, and the western portions of the Florida Panhandle has been muted and conforms more to reality.

National Blend of Models (NBM) Upgraded to Version 4.3

On May 27, 2025, NOAA’s National Weather Service (NWS) upgraded its operational product, the National Blend of Models (NBM) — a nationally-consistent and calibrated suite of forecast products generated by combining numerical weather prediction and post-processed data. This upgrade (version 4.3) primarily focused on improvements to the NBM Tropical Cyclone Feature-Matched wind speed product and incorporated updates provided by the Storm Prediction Center (SPC) to the probabilistic severe weather products. While the Tropical Feature Matching product was originally planned to be part of the planned NBMv5.0 upgrade in April 2026, the National Hurricane Center expressed its desire to have these NBM tropical wind improvements placed into the operational job stream for the 2025 tropical season. Given the urgency of this request, it was decided to implement an intermediate version (NBMv4.3) prior to implementing NBMv5.0. Two noteworthy enhancements were made to NBMv4.3

Tropical Feature Matching 10m wind enhancements:  

  •  Land/Water adjustments of Direct Model Output (DMO) wind speed and u-v wind components when displaced to match the National Weather Service’s Tropical Cyclone Wind Model’s (WTCM) center. These adjustments are determined using the surface roughness values from the Hurricane Analysis and Forecast System (HAFS).  The goal is to remove unrealistic fast speeds over land (and resulting discontinuities) when a wind field around a storm is adjusted.  The two figures below demonstrate the improvement to the 10m wind speed field before (Figure 1a) and after (Figure 1b) the change.    
  • Swapping of the existing NBM tropical wind direction with the derived 10m wind direction from tropical feature-matched 10m u- and v- components. 
Screenshot of 0700 UTC NBMv4.2
Figure 1a: 0700 UTC NBMv4.2 Tropical Feature-Matched 10m wind speed and barbs at the 41h forecast hour for Hurricane Helene as it makes landfall over Florida. Note the intrusion of stronger inland wind speeds in southeast Georgia, southern South Carolina, southern Alabama and Mississippi, and the western portions of the Florida Panhandle.

 

Screenshot of 0700 UTC NBMv4.3
Figure 1b: Same as in Fig. 1a, except for NBMv4.3. Note that the intrusion of stronger inland wind speeds in southeast Georgia, southern South Carolina, southern Alabama and Mississippi, and the western portions of the Florida Panhandle has been muted and conforms more to reality.

SPC probabilistic severe weather products enhancements:

Probabilistic severe hazards for tornado, hail, and wind are expanded out to Day 2 and are now calibrated from a Global Ensemble Forecast System (GEFS) and High Resolution Ensemble Forecast (HREF) blend - usage of the Short-Range Ensemble Forecast (SREF) system to compute these probabilities has been terminated. New cycles for these severe hazards are 0000 UTC and 1200 UTC to complement the existing products available from the 0300, 0600, 1500, and 1800 UTC cycles. 

New SPC guidance for combined hazard severe weather probabilities issued for each day over the Days 3-8 period, available for the 06Z and 18Z cycles. A case example is shown in Figure 2 and SPC’s storm reports map for the relevant period can be found in Figure 3. 

NBM v4.3 SPC-Post 24-hr Prob of Total Std Severe Init: Wed 2025-03-26 06z
NBM v4.3 SPC-Post 24-hr Prob of Total Std Severe Init: Thur 2025-03-27 06z
NBM v4.3 SPC-Post 24-hr Prob of Total Std Severe Init: Fri 2025-03-28 06z
NBM v4.3 SPC-Post 24-hr Prob of Total Std Severe Init: Sat 2025-03-29 06z
NBM v4.3 SPC-Post 24-hr Prob of Total Std Severe Init: Sun 2025-03-30 06z
NBM v4.3 SPC-Post 24-hr Prob of Total Std Severe Init: Mon 2025-03-31 06z
Figure 2: SPC’s Total Severe probabilities for all severe hazards combined covering lead times from Day 8 to Day 3, valid at 1200 UTC 3 April 2025, as one moves left to right and top to bottom, respectively. Note that the probabilities increase with shorter lead times as the uncertainty in the forecast diminishes.

 

Screenshot of SPC’s Total Severe storm report
Figure 3: SPC’s Total Severe storm report for the period of 20250402 1200 UTC - 20250403 1159 UTC.


It is anticipated that these improvements and enhancements will benefit the NWS in its mission to support Impact-based Decision Support Services (IDSS) in both the deterministic and probabilistic space.  For more information about NBMv4.3, please see the corresponding Service Change Notice [PDF].

The next major upgrade of the NBM, Version 5.0, will include new probabilistic products supporting fire weather and ocean wave forecasting, additional improvements to the suite of winter fields, precipitation products, an extension of hourly guidance from 36 to 48 hours, and much more.