Category Archives: Winter Storm Outlook


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.


This afternoon’s NAM model just became available. This is the “off hour” model run that does not include new weather balloon upper air (radiosonde)  data.

Here are the changes—Temperatures are warmer and precipitation ends earlier, about 4 AM Monday. Precipitation starts about 4 PM Sunday, probably as snow or a mix.Temperatures at the surface are above freezing, so the first amounts won’t accumulate very much.

Temperatures at critical levels of the atmosphere become too warm for snow about 8:00 PM in Philadelphia and move north.

8:00 PM Critical Temperatures

The warm air aloft moves north to a position shown below by midnight. This is the northern extent of the sleet/rain line.

Areas  north and west of the white line below are all-snow for the storm: 

Midnight Sunday critical temp- Maximum northward movement- white line is transition line

By 2 AM, cold air line moves back through Philadelphia:

Sunday 2 AM critical temp
So this means that between 8:30 PM and 2 AM, areas below the white line in the middle graphic are a mix of sleet and rain instead of snow.

This will seriously limit the snow accumulations in the immediate Philadelphia area. Combine this with marginally freezing surface temperatures in the 32-33 degree range means limited snow.

This is an off-hour run.  We really will need to wait until Sunday morning’s model runs for the final call.




I’ve been reviewing the latest NAM and GFS data.  Here’s the latest on the expected storm for Sunday afternoon into Sunday night.  Total QPF values 0.50 GFS and 0.74 NAM

Basic scenario: snow to sleet and rain in the city, then back to snow. Details below:

Precipitation starts as wet snow in the afternoon, about 2-4 PM.  Initially, surface temperatures are above freezing in areas near the city, preventing accumulation.  Temperatures  then drop due to dynamic cooling and there’s some snow accumulation in the city.  However…

During the early evening, the snow in the city will mix with and change to sleet and rain.  By 9 PM, it will be mostly rain and some sleet in the city.  The northern extent of this changeover is depicted in the following map at 10 PM:

Northernmost extent of changeover to sleet at 10 PM Sunday night is depicted as white line.

North of this white line, snow continues throughout the storm.

South of this white line (10PM), the sleet and rain will gradually change back to snow about 12-1 AM  and will continue as snow until morning.

North, the QPF values are less, but the temperatures are colder; calculations for the snow totals become a bit complicated.

North of this white line, final accumulations will be 4-8 inches.

South of this white line, the final accumulations will be 2-5 from the city northward.  These are preliminary estimates.

This is a complex storm. We won’t have a final handle on the thermal profiles  and final QPF until tomorrow morning’s model runs.


Winter Weather Update: Tuesday 8 PM
A quick review of the today’s models. We’re back to 4-5 inches of snow, changing to sleet about 5 pm. Tonight’s NAM data starts becoming available after 9 pm and I’ll update sometime after 9:40.
Winter Weather Update: Tues 9:30 AM
A change in the forecast, based on the latest NAM. Precipitation starts about 10 AM, earlier to the west and southwest. Snow until about 4-5 PM, then a changeover to sleet and then some freezing rain.

The revision in the forecast- most of the QPF will be arriving after the changeover to sleet.. about 3 inches of snow expected, more north and especially west, before a changeover to an extended period of heavy sleet and sleet mixed with rain, then rain.

I’ll be updating this evening. Likely update time is around 9:30 PM.

I’m waiting for the latest NAM data.  Last night’s NAM and GFS moved the precipitation start time later to 10-11 AM (with the exception of the FV3-GFS which still maintains an earlier start.)

NAM QPF values for the morning and afternoon increased to 0.80 inches water, but some of that will be falling as sleet.   The NAM maintains freezing temperatures into the evening.  At the current time, I’m still with 4-6 inches, leaning towards the higher amount, before a changeover to an extended period of sleet.   Snowfall will be heavy in the early afternoon.

I’ll  be updating with the NAM data this morning, before 10 AM.

A very busy schedule at work today, so I probably won’t get a chance to further update until this evening.  (It will be this evening’s data that will be most important.)