Tag Archives: Secondary Low


Here’s the latest on the snowstorm expected Wednesday. Tonight’s NAM data along with this afternoon’s model runs show high continuity suggesting that this is likely to be a high confidence forecast.

Latest NAM has a QPF of 1.34 inches water for PHL, higher amounts north and west of Philadelphia. (Expect those numbers to change with each model run.  It’s the nature of weather prediction.)

NAM Forecast Wednesday 1pm. A little less intense than this afternoon’s NAM

Still a significant snowstorm, with snow totals 12-18 inches as a current best guess.

Snow starts as a wet mix between 7-10 pm Tuesday night but changes to snow overnight. Snow falls moderate to heavy after daybreak Wednesday , tapering off about 6pm.

I’ll try to update tomorrow morning about 9:15 am.

[su_note note_color=”#ebf2d9″]Tonight’s GFS has an almost identical QPF of 1.33 inches water. GFS has heavier snow earlier on Wednesday [/su_note]


This afternoon’s model runs continue to show a major snowstorm for Philadelphia and its suburbs, especially north and west of I-95.

There isn’t much to add to my previous posts from earlier today.

The thermal profiles, especially the NAM, make this storm to be mostly snow.  Latest QPF values from the NAM is over 2.0 inches water! (24 inches of snow potential!). The GFS, as it almost always does, shows a lower QPF of about 1.0 inches ( 12 inches snow).

Usually in these situations, the NAM is overstating things, so for now, the average is 1.5 inches water.  At a 10-1 ratio (which may be too low), we’re still looking at a potential 18 inches.  With the thermal profiles getting colder Wednesday afternoon, the ratio will increase, so 18 inches might be conservative.  This looks like it could be a major snowstorm if these numbers hold up.

Timing: Snow or a mix of rain and snow starts about 9 PM Tuesday evening in PHL.  It will be light and may not accumulate much by morning, however, the storm will be forming before daybreak along the coast and snowfall will be increasing as temperatures in the upper atmosphere drops.  Snow ends early evening Wednesday.

So why haven’t the regular TV people been on the bandwagon sooner?  Not sure…I know they were heavily criticized for a poor forecast that impacted the Flower Show a few years ago.  So my guess is they’re treading lightly.


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.