Tag Archives: Secondary Low


The latest model data has just come in.  NAM QPF values are 1.89 inches water and GFS 1.20 inches water (1.82 in Chester county!).

Current NAM and GFS thermal profiles show temperatures at critical levels of the atmosphere compatible with snow, with some possibility of mixed precip in PHL at some points during start of the storm.   As was the case with the last storm, they keep the surface temperatures above freezing, in the 33-37 degree range.

I think the warm surface temperature prediction is incorrect. As was the case with the last storm, dynamic cooling should bring it down to 33 or 32 once the precipitation cranks up, allowing for accumulation.

Accumulation potential  for the immediate PHL area (which doesn’t take into account melting due to March sun factors) is 12-16 inches, potentially more in immediate western suburbs.

NAM Wednesday 1PM forecast

Timing: Some wet snow falls during the evening  and night time Tuesday and before daybreak on Wednesday.  Depending upon temperatures, there will be some accumulation. Snowfall becomes heavier during Wednesday morning and peaks during the afternoon on Wednesday, ending early Wednesday night.

I need to let you know that the model preconfigured snow accumulation algorithms currently show rain more likely than snow for our area.  I think this wrong, as the preconfigured algorithms often are.    I’m basing my forecast on predicted QPF and thermal profiles from the NAM FOUS data which has served me well in the past.   So changes are possible with the forecast.  I, for one, will be getting the snow blower ready tomorrow. 



Last night’s models showed significant agreement that a classic coastal snow storm (“Miller type B”) will develop Tuesday night into Wednesday and intensify as it moves up the coast.  Current QPF values are very high- as much as 1.80 to 2.00 inches water.  This has the potential of translating into 14-20 inches of snow assuming a ratio of 1:10.

Current track is a ‘sweet spot’ for snow development in PHL and westward.  Highest amounts on either side of the I-95 corridor.

NAM model Forecast 4 PM Wednesday

Right now, current thermal profiles support snow, not a mix.

I’ll update later this morning with the latest models.




The models have been advertising another coastal snow storm development for Tuesday night into Wednesday.  Not all of the major models are totally on-board with this scenario, but it still appears more than likely.   The wind intensity of this storm will NOT be anything like this past Friday, although snowfall rates may look impressive during the day.

NAM Forecast Wednesday 1 PM

Incredibly, the current range is anywhere from 6-10 inches of snow for Philadelphia , based on the GFS (6″) and the NAM (10″).

Current timing- light snow develops during the evening Tuesday and increases in intensity after daybreak Wednesday, as the secondary coastal low develops and intensifies.

There’s still some thought that warm air might mix in, but right now the NAM and GFS critical temperatures all support snow.

I’ve given much thought about this past storm  I’m not going to get into the March sun angle, solar insolation and ground temperature effects, since it made giving an accurate accumulation forecast impossible.   I guess in March, it’s really about snowfall rate, not accumulation since accumulations mean much less in March— the snow can start melting as soon as it falls.

I’ll keep an eye on this one and will update tomorrow!

[su_note note_color=”#ebf2d9″]Sunday 10pm: The latest NAM data shows a significant snowstorm for Philadelphia on Wednesday.  QPF values are almost two inches water.  Critical temperatures show it falling as heavy wet snow.  Still too early to be sure, but this could be a major snowfall.

11pm:  Tonight’s GFS confirms a major snowstorm likely for PHL.  We’ll be waking up to heavy snow falling on Wednesday, snowfall increases for the rest of the day.  Current models suggest as much as 15 inches!  Too soon to hang our hat on this.