Category Archives: Winter Weather


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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 8 AM
So, yesterday’s models were significantly off about the amount of rain we had early this morning. (We had double the amount.) Can we depend on them for a 5 day forecast?

The current trend for Sunday night: the NAM maintains about a 2-3 inch brief snowfall before daybreak Monday. The GFS has very little. I’m going with the NAM at this time.

For Monday night into Tuesday, it appears it will be briefly snow changing to freezing rain and then all rain for most of the storm. The secondary low coastal formation doesn’t appear to be very strong. Stay tuned.

from yesterday…

As mentioned in my previous post, high pressure will move in for the weekend behind a cold front that moves through Friday morning. The frontal boundary will stall to our south.

For Saturday and most of Sunday, we will have mostly sunny skies but with temperatures below average, in the 30s.

The stalled frontal boundary will become associated with a developing low pressure system in the Midwest and will redevelop as a warm front, moving slowly north as it tries to displace the cold air at the surface.

Warm air will start to overrun the cold air with an initial weak impulse that may bring some light snow late Sunday night or before daybreak Monday.  (This looks to be very insignificant.)

Thursday evening 2/7- Latest NAM has 1-3 inches of snow Sunday night.

As the warm front slowly approaches Tuesday, snow may develop Monday night into Tuesday, but it appears that it will mix with sleet and change to all rain sometime Tuesday. This is the most likely scenario.

But, there’s significant disagreement among the models with this system. While the change to all rain (after some freezing rain) is the most likely scenario, the GFS and FV3-GFS suggest a secondary low develops along the Delmarva coast, possibly allowing more snow. The Canadian model is similar but maintains much more cold air, with more snow.  The European model has a much later development into Wednesday, with the secondary low developing north in New England, giving us mostly rain.  

The Tuesday through Wednesday timeframe looks interesting.  Stay tuned.

Winter Weather

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2 PM Radar shows snow falling heavily much further north than had been predicted by the models.
Weather Update Fri 12:30 PM
The latest HRRR (High Resolution Rapid Refresh) and NAM high resolution show about 1 to 1.5 inches snow in Philadelphia and about 2 inches of snow in southern Delaware and Chester counties. Less in northern suburbs.

As mentioned last night, this clipper was insignificant until last night’s model runs showed there might be some actual QPF associated with it.

from this morning–

Weather Update Fri 11:30 AM
The NAM probably did the best with the QPF last night. This morning’s models (GFS & NAM) continue to show about a coating to 1 inch of snow, ending about 2-3 PM. (The National Blend of Models did the worst, where it was still showing no snow here even with this morning’s runs.)

from last night–

An upper air disturbance (an area of vorticity) will move across our area, staying mostly to our south.  The models have not been very impressive with this disturbance until tonight’s model run. The NAM has a QPF of 0.10 inches of water while the GFS has 0.03.

The National Blend of Models has nothing for Philadephia. Not surprising—  these clipper systems have a way of being a bust, but occasionally they surprise.

So, flurries or a coating to an inch is possible in and south of Philadelphia for late morning into afternoon on Friday. Even less north.


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Weather Update Monday 10:50 PM
Tonight’s GFS just became available. QPF value about 0.38 inches water. The GFS is colder than the NAM and shows most of the precipitation falling as wet snow from PHL, west and northwest. The GFS supports my feeling that a changeover to snow might occur earlier. Snow accumulations would be 2-3 inches, especially west and northwest of the city if the GFS is correct.
Weather Update Monday 10 PM
Tonight’s NAM just became available. QPF value about 0.28 inches water. The models are consistent that the upper atmosphere is cold enough for snow, BUT temperatures in the lowest 5000 feet are expected to be above freezing during much of the afternoon, so the changeover in and around PHL will be about 6 PM. According to the NAM, about 1 inch of snow when it ends about 9 PM Tuesday night. I still think it’s possible the changeover to snow might start earlier with higher accumulations. The latest NBM has a mix starting between 5 and 6 PM.
Weather Update Monday 5:10 PM
As soon as I posted the forecast below, the 1PM run of the GFS model has raised the snow totals to 3.5 inches. Read the rest of post immediately below to see the context.

So, I’ve been trying to get a handle on the rain –> snow precipitation that is forecast to accompany an arctic front passage late Tuesday afternoon.

Let’s first go with the models and their algorithmic snow predictions. The NAM has a total QPF of 0.28 inches water falling as rain changing to snow before ending in the evening.    The GFS maintains the highest QPF of 0.53 inches water falling as rain changing to snow before ending.  The European ECMWF has a QPF of about 0.40 inches water.  The National Blend of Models (NBM) which has been fairly accurate recently has a QPF of 0.32 inches water.

So how much snow?

The uncertainty with this weather event is determining when the changeover will occur.    Most of the models have the rain -snow changeover occurring late in the period, about 6-7 PM, after most of the precipitation has fallen as rain. 

The models are showing a total snow accumulation of about 1 inch, possibly 2, in the Philadelphia area, somewhat more west and northwest of the city.  I think that’s a good guess, especially since the initial surfaces will be wet, melting some of the first snow that falls.

I hate to go against the models,  but based on the thermal profiles of the NAM and GFS which are cold enough for snow except near the surface, I think it’s a possibility that the changeover to snow may occur earlier than the preconfigured snow algorithms are currently forecasting, raising the amount of accumulation.  (I’ve also noticed that the GFS predicted surface temperatures were too high today. Add dynamic cooling and we would get more snow.)

So you can expect the 1, possibly 2 inches, but don’t be surprised if another inch or so falls, if the the snow changeover occurs earlier than the models forecast.

I guess we’ll see … that’s what makes this so interesting.  Stay tuned!

One has to wonder why there’s no “flash-freeze” hysteria with this snowfall. If anything, this event will have a lot more icing than the last one.