Winter Weather Update

The afternoon NAM model continues to show colder surface temperatures around PHL.  Chances of freezing rain or drizzle appear more likely for later tonight and Monday closer to PHL.

I don’t think the models are capturing this low pressure system well.

A small  cyclonic surface wind circulation is developing off the NJ coast.  Could an unexpected secondary low be developing?   Stay tuned.

Weather Update

A difficult forecast period for late Sunday night through Monday, as the recent models have significantly changed the track of a developing low pressure system and associated frontal boundary.   There is low confidence with the weather forecast for Monday.   (Indeed, there has been more sunshine both Saturday and today, so even in the short range, the models have not been capturing this setup very well.)

Previously, the models had the low tracking across PA and we would be in the warm sector on Monday.  However, the new track of the developing low will be to our south, allowing significant cold air to mix in at some levels of the atmosphere.  While snow is not expected for much of the day in the immediate PHL area, the precipitation type is looking to be more in the sleet and possible freezing rain variety.  This mixed precip will likely change to snow before ending.  QPF values remain light, so only 1 or 2 inches of snow is expected by late Monday night.  However, the real hazard may be the sleet and freezing rain.

I should point out that there is uncertainty about the development of this storm as it reaches the coast late Monday night.  Right now, it’s expected not to affect us, but there is low confidence with that scenario.

Basic Weather Model Facts

Here is some basic weather information for understanding my blog:

GFS model is a global weather model and is the “Global Forecast System” model.  (It is what used to be called the AVN/MRF model (AVN=aviation, MRF= medium range forecast model

The GFS is a global spectral model (its physics is based on waveform physics)  and it models the entire globe.  It’s a relatively coarse resolution model, not as fine a resolution as the NAM described below, however it is very advanced. The GFS is run four times a day. While not as ‘fine’ a resolution as the NAM model, the GFS model has wonderful features and is the one I usually bet on.  The GFS is constantly improved and its most recent update was in January 2015.

Another subset of the model, the GFSX (X for extended) also predicts over longer ranges, as much as 144 hours to 384 hours into the future.

NAM stands for the North American Mesoscale model. Previously known as the Eta model, it was renamed on 1/25/05.

On 6/20/06, the NAM was converted from using older Eta physics to WRF (Weather Research Forecast) model physics.  There are also variations of the NAM model which use different physics for their initial conditions.

As mentioned NAM has different physics than the GFS and is a higher resolution model. It attempts to predict in areas as small as 4 km in area. In meteorology,  that scale is referred to as a “mesoscale” area. (Thunderstorms are mesoscale weather events.)

The NAM is not a global model and only predicts for North America. It’s run  four times daily and the usual form predicts 84 hours into the future. Experimental extended versions are being developed.

NAVGEM is the model developed and used by the US NAVY. A global model, somewhat similar in coverage as the GFS, but different physics. Run four times a day. I find it very useful for hurricane predictions

GFS MOS and the NAM (Eta) MOS: MOS stands for Model Output Statistics. MOS data are statistically based forecasts for up to 84 hours (NAM and GFS) or 144 hours (GFSX or GFS eXtended) and

The MOS predicts temperature, humidity and precipitation probabilities every 3, 6 and 12 hours, depending upon the specific MOS output . MOS forecasts use historical data and reinterpret the raw data for specific locations. However, MOS forecasts do not correct for model biases.

RAP: Rapid Update Cycle model.  Based on the NAM model, this model is a short term model that is updated hourly.  There is also a High Resolution Rapid Refresh (HRRR) model.

LAMP Forecast:  An hourly forecast, rerun every hour and based on the GFS

Other models include the DIGEX, Canadian GEMS, MM5, UKMET, ECMWF.

All models have their specific biases, inaccuracies, etc. New forms of each model are constantly being developed and tested.

The Acronyms and other meteorological concepts

QPF is “quantity of precipitation falling”. Most of the models predict the amount of precip that will fall in a given period of time, based on the total amount of available moisture that can precipitate (PWAT = precipitable water) and the physical conditions (lift, convection, etc.) that will cause it to precipitate. The amount of snow (snow:water ratio) is usually calculated by multiplying the QPF in inches by a factor of 12-20, depending upon the temperature.

Atmospheric Thickness Levels: Heights in the atmosphere are often measured based on where the pressure is a constant value. The ‘thickness’ of the atmosphere is the three dimensional depth of the atmosphere between two pressure values. A useful thickness value is the thickness (or depth) of the atmosphere between the pressure of 500 millibars (mb) (about 18,000 feet) and 1000 mb (millibars) (about the earth surface near sea level). The thickness values become higher when the upper atmosphere is warmer and become lower when the upper atmosphere is colder. Thickness values are useful in predicting rain/snow or sleet. They correlate with temperatures at specific heights that are correlated with snow or sleet or rain.

Useful Temperature Levels: 800 millibars and 900 millibars- Both of these levels must be at or below 0 degrees C (freezing) for snow to form.

Possibly Colder on Monday –

Latest NAM model data has a shift southward in the position of the low pressure system and front Sunday night through Monday.  A greater possibility of sleet and freezing rain would result.  It’s a significant shift.  This trend needs to be watched.  The QPF values are low so we’re not looking at heavy precipitation, but even light amounts could make travel difficult Monday and Monday night.

Storm Forecasts & Other Things "up in the air".