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Late Afternoon Update
The NAM is still showing 0.17 inches water for snow tonight. (1-2 inches snow.) The GFS is maintaining much less QPF – 0.08 inches and the statistical models and NBM seem to side on the lower amounts of (0.5 to 1 inch of snow.)

Of interest is that the NAM and to some extent, the GFS has some light snow into Monday morning and again late afternoon Monday.

Later Monday-  Temperatures will fall below freezing early evening Monday and snow will develop and become moderate for some hours during the night.  It appears that temperatures at critical levels will rise above freezing about 6AM Tuesday, BUT temperatures at the surface may remain freezing for well into the morning. After a few inches of snow, significant sleet and freezing rain is a possibility for Tuesday morning until about noon. I’ll update this evening.

from earlier this morning:

Morning Update with GFS Data
This morning’s GFS maintains a low QPF of about 0.10 inches for the general Philadelphia area, more further south into Delaware. It also has the snow starting later than the NAM, after midnight tonight. So we’re down to 1 or 1.5 inches of snow.

The blocking effect of greater colder air with the GFS suggests that the icing potential from cold air damming might be considerable on Tuesday.   I’ll keep an eye.   And I’ll update with this evening’s NAM  about 9:15 PM, when the early data becomes available.

from earlier Sunday morning...

With such large QPF differences last night between the NAM and GFS, something had to give.  This morning’s NAM model run just becoming available shows a decrease in the NAM QPF to 0.17 inches water.  This is still double the GFS, but more of a reasonable discrepancy.  So we’re back to about 1.5 to 2 inches of snow at daybreak Monday morning.

For today–  While it’s sunny right now, clouds should increase about noontime and thicken during the afternoon.  Light snow starts about 9 PM in the western suburbs and ends about daybreak Monday.

The National Blend of Models (NBM), based on some warming in the upper atmosphere is suggesting that the final precip Monday morning may include some sleet or freezing rain.  There’s also the chance of some spits of precipitation during the day on Monday.

For late Monday afternoon and early evening, snow is expected to redevelop with several inches fallling before a transition to sleet and freezing rain before daybreak Tuesday. An extended period of freezing rain, especially north and west of Philadelphia appears likely Tuesday morning.

Let’s see how well the models do with tonight’s light snow before tackling the Monday into Tuesday mix.


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An Interesting and active weather period is unfolding for the next several days.  Model differences promise to make the forecast more challenging.

Tonight’s NAM has snow developing early Sunday evening and extending into early Monday morning.  QPF values have increased over the day and are now at 0.40 inches water. If the NAM is correct, 4-5 inches of snow will accumulate by early Monday morning in and around the immediate Philadelphia area.

The same event is depicted differently by the GFS, which has high pressure blocking the moisture’s northward movement.  QPF values tonight are about 0.10 inches water, even less just north of Philadephia.

I’m leaning towards the NAM, which has done the best for the last two wintery weather events. So the current predicted amount is 3, possibly 4 inches of snow Sunday night, ending early Monday morning.

Another larger system will affect us late Monday afternoon through Tuesday.  Let’s get through the first one before focusing on details, but the trend is for snow, several inches, changing to sleet and freezing rain Tuesday morning. An extended period of freezing rain is possible even in Philadelphia.  An eventual change to all rain later Tuesday morning.

Further complications include the possibility of a secondary low formation.

Stay tuned.





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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.


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Weather Update Fri 10:30PM
Tonight’s NAM has a QPF of 0.13 water falling as light snow Sunday evening ending before daybreak Monday. This translates into 1-2 inches.

High pressure builds in for Saturday and most of Sunday.  As mentioned previously this week, the frontal boundary remains not far to our south.  A disturbance will move along this front bringing light snow to our area Sunday evening and night.

Saturday will be sunny and cold.  It will be windy with high temperatures 33-35.  Clear and cold Saturday night.

Sunday will start sunny, but the aforementioned disturbance approaches Sunday and cloudiness will become significant by about noon.  High temperatures near 38.

Light snow is expected Sunday evening and night. Currently, the GFS gives us a coating, the NAM and ECMWF about 1 inch, slightly more south of Philadelphia.

On the horizon is a more significant storm expected to move to our west on Tuesday and spawn a weaker coastal low off the New England coast.

Warm, moist air overspreading cold air at the surface will allow snow to develop Monday night but the snow will change to sleet and freezing rain Tuesday morning, likely early. Then all rain.

An extended period of freezing rain and sleet is possible in the northwest suburbs into Tuesday morning as a “cold air damming” effect occurs. A changeover to all rain will occur in all areas, but later further north.

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