Monthly Archives: April 2020

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.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Updated  the Sunday forecast below with the newer model data.

The HIRESW model forecasts did pretty good today. We had sprinkles in the morning and again later in the afternoon and evening.  In between, we had clouds and sun.

Tonight’s 24 hour high resolution model data just became available.

Saturday— The models are showing the Atlantic low pressure system to still be affecting our weather on Saturday. Saturday will be similar to today in terms of cloud cover— periods of clouds and maybe some breaks of sun. The RAP model and one version of the HIRESW have it quite cloudy for most of the day. Another version of the HIRESW and the NAM -NEST have clearing breaks in the clouds later in the afternoon.  Less windy as well and winds become southerly late afternoon.  Highs 56-58º.

For Sunday— the low pressure system departs.  Southerly winds will bring milder temperatures. A mix of clouds and sun and perhaps some sprinkles in the late afternoon as a very weak front moves through. High 67-69º.