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Sunday Forecast Update

Posted Sunday 02/18/24 @ 9:42 AM — Sunny and windy today. The cloudiness that I thought would impact northern areas today will remain even further to our north. Highs 41-43º. Wind gusts near or over 30 mph.

This Week’s Peek

The coming week looks tranquil. A coastal storm has been consistently forecast for this Friday. It looks like rain, not snow here—

Philadelphia Inquirer Article on the Snow

Sunday’s Philadelphia Inquirer had an article on the snow forecast that didn’t quite make the grade. I would have added a comment about the ground temperatures that were well above freezing and melted much of what fell initially and allowed accelerated melting in the morning.

Sat 9:05 AM —Forecast Review — A beautiful sight out the window this morning and a not-so-good forecast. Let me cut to the chase: what went wrong with the forecast—

• Temperatures remained well-above freezing before the storm (35º Blue Bell Wings Field) until it started snowing at 12:15 AM.
• Temperatures remained somewhat warmer than predicted throughout the storm.
• Ground temperatures (‘skin temperatures and soil temperatures”) were above freezing for much of the storm near the city.
• Precipitation amounts (in water) were very different from what was predicted.
• Banding of precipitation and snowfall occurred far north of our area.

The MRMS summary gives a handle on how much precipitation (water or water equivalent) actually fell and where—

From the above, we can see that the storm’s precipitation occurred in the city and immediate suburbs in the range forecast (0.30″) , but south of the city in Delaware county and Chester counties, precipitation was much lower than forecast. The banding occurred far north with heavy snow in that region.

It’s funny. I pointed out that the media weather people were under-forecasting the snowfall. Eventually they raised their forecast to a “Winter Storm Warning” last night, only to see that their original forecast was probably better.

Meanwhile, I saw signs of this forecast change, in last night’s HRRR, but I was reluctant to call it and attributed the reduction in snow to model spin up errors. Go figure.

Snow Update

Posted Friday 02/16/24 @ 9:00 PM — Some trends are becoming apparent in reviewing additional models. The high snow accumulation values predicted this morning and this afternoon have been eroding downward.

The latest HRRR (00z) just available more than captures this significant downward tend—

When I saw the trends with tonight’s HRRR and other afternoon models, I wasn’t sure if I would post it. I can think of many good forecasts that were derailed by my posting the ‘latest model’ and going with it.

There is a known phenomenon called “model spin up time”. Basically, it takes several hours before model forecasts become accurate They can take as long as 6 to 10 hours to be valid in an area, especially when a storm is on one’s doorstep. Put another way, the ‘latest model’ is often inaccurate in the short term. That could be the case here.

The latest satellite and radar shows the storm may be splitting into two centers, explaining the HRRR’s corridor of low snowfall above.

I guess we’ll find out when we wake up tomorrow morning.

Originally Posted Fri @ 4:57 PM — —A fast moving storm will bring snow to our area tonight (Friday) late evening and night. The storm is visible on WV imagery—

The light snow will move in from the west as early as 11 PM, but most models have it moving in about 1 AM.

Some banding in snowfall has been expected to develop and the heavier snow is now forecast to be north of the city, instead of south of the city as forecast earlier. A generalized 1.5-5 inches of snow is expected.

As is always the case, the models differ in their placement of the heaviest snowfall. Here’s the latest HRRR (18z) —

Many long time followers of this blog know my preference for the NAM for snowfall forecasts here. Here’s the latest NAM (18z)—

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Monday Night into Tuesday Outlook

Posted Saturday 02/10/24 @ 10:16 PM — The storm for late Monday night into Tuesday will be a forecasting challenge. A secondary low will form off of the Delmarva coast, perhaps bringing in more cold air aloft. 1.1″ of water will fall, initially in the form of rain, mixing with and changing to wet snow, then tapering off during Tuesday afternoon. The changeover to snow or a wet mix near the city will occur between 5 and 8 AM Tuesday morning.

Here’s tonight’s NAM snow depth forecast by Tuesday afternoon—

Posted Saturday 02/10/24 @ 10:33 AM — Many models are showing highs today in the 60-62º range, but the GFS and HREF continue to keep us in the mid to upper 50s.

Here’s the latest NBM—

As for the ‘snow’ for Tuesday morning, little accumulating snow is expected. The latest GFS captures it well—

Updated Fri 9:12 PM — Tonight’s early models are trending a bit more towards some widely scattered light sprinkles during the day Saturday and a few heavier showers early evening with the cold front. The NBM shows low 60s in NJ.

Previously Posted Fri 6:05 PM —

The blocked pattern giving us this past week’s fair weather and now increasingly warm weather will become a thing of the past by late Monday. But first things first.

A slow moving cold front will approach Saturday, preceded by clouds and the chance of light widely scattered sprinkles in mostly far western and northern suburbs. (An area of very light showers may move in during the early evening Saturday just ahead of the cold front. )

Most of the showers are expected to stay to our north and to our south on Saturday. Current water vapor shows the setup—

On Sunday, the front stalls to our south and moisture moving to our south will spill up towards us in the form of cloudiness. No rain expeacted.

The moisture plume to our south will be the launch pad of a developing storm for late Monday into Tuesday morning.

Monday into Tuesday Outlook

Another rainstorm, begins towards midnight Monday and changes to wet snow from northwest to southeast into the city. A heavy 1.4″ of water falling (rain) with about 0.5″- 1.5″ (preliminary estimate) wet snow accumulation upper Montgomery, western Chester and northern Bucks counties.

Forecasts have trended to show increased intensification of this low pressure system. Current lack of cold air will be a major factor in snow totals. The latest GFS shows secondary coastal low formation. Stay tuned for updates.