Category Archives: Basic Weather Facts

Basic Weather Facts

HIRESW MODELS (Geeky)

My regular forecast followers may have noticed that I’m spending more time using  some new, high resolution models for these forecasts— these models are referred to as HIRESW  (High RESolution Window) models.  The models are run by NOAA/NCEP. I’ve been really impressed with these models;  I have only recently gotten access to them.

These models were developed through open-source university development of the WRF (Weather Research Forecast) Model around 2002 and later enhanced and further developed.  (In fact, the old ETA model starting using the WRF physics packages about 2005; subsequently, the ETA then became known as the NAM.  

–> There are two major forks and development paths of the HIRESW models, an ARW (Advanced Research Weather version) and an NMM version (a Non-hydrostatic Mesoscale Model version). 

The ARW and NMM versions use different “physics packages” and different “initializations”.     (There are different sets of equations  or “packages” used in each that make different assumptions and approximations about the atmosphere.  These equation packages are used to predict things such as rain, clouds etc..  The “packages” are referred to by the name of the researcher who developed these advanced, applied equations.)

It’s complicated, but in a loose way,  the HIRESEW-ARW  version is closer to the physics of the GFS model and the HIRESW-NMM version is closer to the physics of the NAM model. 

In recent years, there have been further improvements in both versions (ARW, NMM) of these HIRESW models.   These improvement included increases in resolution, (now 3 kilometers).

More interestingly, new statistical versions have been developed for each model — groups (“ensembles”) of models with intentionally introduced known errors called “perturbations. These allow forecasters to see how known errors statistically affect the computed forecast outcomes.

Out of these ensembles, it emerged that two perturbed versions (called “members”) of each seemed to offer improved forecasts:

  • HIRESW-ARW-MEM2   (MEM 2 refers to member two)
  • HIRESW-NMMB2      (B2  refers to grid B, member two)

Both of these versions are run twice a day at 8 AM and 8 PM EDT., for the continental US by the National Center for Environmental Prediction (NCEP-NOAA).  These high resolution models forecast up to 48 hours out in time.

The forecast output of these models becomes available between 10:25 AM/PM  EDT for the first 24 hour forecast and about 10:58 AM/PM EDT for the next 24 hour forecast.) 

That’s why I’ve been holding back posting my forecasts until after 11 PM in recent days.  (The model data available at 11 PM  Friday forecasts out to 8 PM Sunday. )

Sincere there remain differences in the forecasts of both version’s ensembles, you’d think there would be a model that combines them.  There is.  It’s called the HIREF.  I haven’t found it very useful to date.

The latest development for the HIRESW models still in experimental stages, is use of a new grid arrangement, the FV3 geometry, that is being used for the latest GFS model released in May 2019.

Basic Weather Facts, Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK

The snow-free winter continues…

A cold front moves through on Thursday and cold high pressure moves over us for Friday and Saturday. The National Blend of Models (NBM) is showing highs of 33° for both Friday and Saturday although some statistical models (ensemble models) show highs to be a bit colder.  Skies will be sunny both Saturday and Sunday.

The pattern is fast moving and progressive; temperatures rebound into the mid 40s on Sunday as winds shift to southerly with the departure of the high.

The Extended Range GFS model attempts forecasts 384 hours (16 days) into the future. No potential snow storms are currently predicted, except a possibility the last day in February.  Don’t change your plans.

This winter has been characterized by a lack of large, dense, cold air masses sinking straight south in the middle of the country.  

There is a  common misunderstanding, perpetuated by TV weather entertainers, that the jet stream determines where the air masses go.  Actually, it’s really the other way around— the size, shape and temperature-density of the cold air masses in winter determine the position, shape and configuration of the jet stream. The ‘valley’ between the polar air mass and the tropical air mass is where the jet stream is positioned and where major low pressure systems develop and track.

The current pattern has been with us for almost three months. Like bull stock markets, weather patterns don’t last indefinitely. Weather patterns tend to change as the seasons change.