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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—

02-11-24 00z NAM showing snow accumulation. Blue contours are in 1/2″ increments Above freezing near surface air temperatures, warm ground temperatures and rapid precipitation rate with dynamic cooling will make forecasting snow accumulation a challenge. (Click on image for a larger view.)

Sat 5:40 PM —Forecast Review — It looks like the temperatures at the two local airports didn’t get above 57º (KLOM -Blue Bell) and 54º (KPHL- Philadelphia). The NBM, my go-to-model for temperatures, really didn’t impress today, while the GFS and HREF were right on the mark. Not surprised with the lack of any really strong sunshine. For the record, the RRFS model (experimental, being developed) was too warm.

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—

02-10-24 12z NBM high temps for Saturday. All highs are ± 1.6º (Click on image for a larger view.)

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

02-10-24 06z GFS accumulated snow totals Tuesday morning. (Click on image for a larger view.)

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—

Current satellite water vapor image (Friday afternoon) with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Most of the activity on Saturday stays to our north and to our south. (Click on image for a larger view.)

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.


Cloudy. Showers mostly north west and south of our area as described above. The latest NBM has temperatures close to 60º, despite the cloudiness. The Canadian HRDPS, which tends to run warm, has us up to 60º while the GFS has us only in the mid 50s. Windy during the evening hours.

NBM high temperatures: Blue Bell, PA 59º Philadelphia, PA 60º
slightly above average uncertainty (based on standard deviation): ± 2.0º


Cloudy, cooler, but still temps above average.

NBM high temperatures: Blue Bell, PA 49º Philadelphia, PA 51º
slightly above average uncertainty (based on standard deviation): ± 2.1º

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.

GFS shows secondary low formation. Precipitation may ‘skip over our area’ (Click on image for a larger view.)