Tag Archives: Snow


As often happens with the afternoon “off-hour” runs of the models, the forecast can be pushed off course.  Such was the case with this afternoon’s NAM. One of these days, I’ll stop looking at them.

Tonight’s latest NAM and GFS models have become available. The trend is towards colder temperatures and higher QPF values.

The NAM has a QPF of 0.89 inches water and the GFS has 0.73.

If this were a straight forward forecast, this would be a 7-10 inch snowfall, even for Philadelphia.  However, there’s a few glitches.

The temperatures at critical levels of the upper atmosphere are a bit warm for snow. We’ve had similar data where we have had snow but other times, there was significant sleet and rain in the mix, making the snowstorm forecast a bust.

Here’s my take. Having seen the new NBM correctly predict precipitation type, I’m leaning on it heavily for this forecast, along with NAM thermal profiles.

A mix of rain sleet and snow will begin about 3-5 PM The mix will continue, leaning towards a shift to more sleet and rain between 7 and and 11pm in the area south of the white line in graphic below.

Transition line snow – sleet rain

Areas northwest of this white line will be all snow. Total snow accumulation In those areas about 5-8 inches by daybreak Monday.

Areas south of this white line will have a considerable amount of sleet and rain in the mix for at least 1/3 of the total storm QPF, during the heaviest precipitation time. This will significantly reduce total snow accumulations. The sleet line moves back south after midnight, as shown in my earlier afternoon post.

It’s an educated guess but the area from the Delaware river north to that white line may have 2-4 heavy inches. I think the warmer temperatures aloft will play a big role in keeping those numbers lower than they would be otherwise as a result of the sleet rain mix.  Very cold temperatures follow later Monday.

I will update again with tomorrow morning’s data.


The latest NAM data has become available.  According to the NAM, the snow starts between 9 and 11 AM.  Total QPF falling as snow  varies by location, but in and around the city, about 0.30 inches water or about 3 inches of snow before mixing with sleet and freezing rain.

North and west of the city, QPF values falling as snow increase to about 0. 40 inches water, with an additional delay in the changeover.   About 4-5 inches possible in areas north and west of the city.

Because it’s already the third week of February,  the sun angle is high; solar insolation through clouds may reduce accumulation on dark pavement and roadways.

The changeover to sleet is expected between 4:30 and 6 PM.  An hour or two of sleet and freezing rain before changing to rain.

Most of the heavy precipitation will fall as rain, leaving a soggy mess by morning.  Temperatures are expected to stay above freezing towards morning, so additional freezing of fallen precip is not expected with this storm.

These sort of warm air over-running cold air scenarios  with cold air damming are tough to predict accurately.  Let’s see how it unfolds.


I’ve been looking at the latest GFS and NAM models from this morning. Sunday night into Tuesday promises some interesting (at least for this season) winter weather.

Two systems will affect us, one Sunday evening and the other starts Monday evening. The first approaches Sunday with light snow developing sometime during the evening and ends early morning Monday.

There are differences in the models with the QPF.  The NAM is showing 0.23 inches water, the GFS less. With the prior two winter weather events, the NAM out-performed the GFS.   So it appears that 2, possibly 3 inches of snow will have accumulated when it ends Monday morning.

Monday will be mostly cloudy as another more intense storm moves to our west.  Cold air in place at the surface with warm, moist air over-running the the cold air will result in snow initially.

The GFS has the snow starting early on Monday evening.  Critical temperatures in the upper atmosphere warm by 3-5 AM Tuesday morning, with the snow changing to sleet and freezing rain at that time.  It’s possible that 3-4 inches of snow will have accumulated before the changeover.

Surface temperatures may remain at or below freezing until 7 AM Tuesday, so things will be icy and messy Tuesday morning.

North and west of the city, the freezing rain and sleet may be prolonged into late morning.

By noon on Tuesday everything should have transitioned to all rain.   This looks messy and the temperature profiles and QPF predictions will likely change in the next few days.  Stay tuned.


A front moved through today, associated with a dip in the jetstream that will allow cold air to move in for the next few days.

The frontal boundary stalled to our south and multiple models are showing the development of low pressure that will move along the front bringing significant precipitation.  Both the NAM and GFS show QPF values, as high as 0.98 inches of water.

There are currently differences between the timing and thermal profiles  of the models and those differences result in significant differences in the precipitation type (rain /sleet/snow).

The NAM is warmer and shows little snow until the afternoon.  The GFS is colder and faster, and shows as much as 4-6 inches of snow!

We also need to include consideration of sun angle in April and resultant solar insolation through clouds along with daytime vs nighttime Spring snows.   Too early to make a call, but interesting weather for April.  Stay tuned.

Wednesday 10:45 pm -Tonight’s NAM continues with a thermal profile that is too warm for snow during much of the storm around Philadelphia.

Upper Bucks and Montgomery County as well as western Chester county may have over 4 inches, as measured on grassy surfaces, but the immediate PHL area is mostly rain and some sleet, changing to snow late in the game.  A light coating possible.   Expect changes


The latest NAM data is coming in as I type this. Looking at the trends-

QPF values have stabilized at about 1.00 inches water.

Temperatures at critical levels levels of the atmosphere decrease enough to support a changeover to snow about 9-11am in Philadelphia, earlier north and west of the city.

Prior to the changeover to snow, expect continued sleet and freezing rain.  So snow changeover may be a bit later than previously thought .

[su_note note_color=”#ebf2d9″]Based on the actual QPF expected to fall as snow and factoring in March sun angle through clouds, I’m thinking that about 6-9 inches fall in Philadelphia and immediate suburbs, 10-12 in far northwest suburbs.

Winds will be high at 30 -35 mph with higher gusts.   [/su_note]

So, the snow transition starts a bit later here and QPF values have decreased somewhat and winds remain a big problem.

important update 11:30pm. Correct winds to 20-25 mph with higher gusts. Reduce snowfall to 4-7 inches in Philadelphia  based on GFS NAM blend