Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Weekend Storm Update -Late Thursday Edition

Posted Thursday 01/04 @ 10:08 PM — A quick update. Some of the large changes in this afternoon’s 18z model runs have reverted back to previous forecasts. In fact, tonight’s 00z HRRR backed off on some of the snow with more rain. The NAM just available is similar again to other models and has the precip ending Sunday morning.

Here’s the very latest model blend (NBM) snow forecast—

Tonight’s 01z NBM snowfall forecast remain similar to previous runs. (Click on image for a larger view.)

Updates tomorrow.

Posted Thursday 01/04 @ 5:13 PM — There are some unexpected changes in the forecast, based on this afternoon’s models.

It now appears the low snow totals that had been consistent across models may need to be revised upwards. Both this afternoon’s GFS and NAM show the storm undergoing intensification Sunday afternoon with the possibility of prolonged snowfall from wrap around precipitation.

Specifically, the water equivalent precipitation that had been forecast (about 0.6-0.9″ water) is now up to 1.2″ water equivalent.

Instead of having the precipitation end around daybreak Sunday, the latest NAM and GFS has heavy precipitation continuing into early afternoon—

01-04-24 18z NAM forecast for Sunday 1 PM. Superimposed simulated radar and Precipitation type shown as shading. Upper atmosphere is much colder, supporting snow, even though temperatures are forecast to be about 33º.

Here’s the latest NAM snow totals.

01-04-24 18z NAM snow totals by Sunday afternoon. (Click on image for a larger view.)

So, what looked to be a straightforward warm-storm snowfall is now possibly shaping up to be more of a forecast challenge with greater snowfall possible. A mix of marginal surface temperatures, high precipitation rates (dynamic cooling) is going to make accurate snowfall forecasts incredibly challenging

This may be just a glitch in this 18z run of the models, but it’s going to be more interesting than I thought. Stay tuned.

Weekend Storm Update – Thursday Edition

Posted Thursday 01/04 @ 10:18 AM — The “model blend” (NBM model) does a major update of its precipitation forecast four times daily. The most recent was the 13z model run.

Here’s the latest snow totals by Sunday—

01-04-24 13z NBM calculated snow totals through Sunday. (Click on image for a larger view.)

Posted Thursday 01/04 @ 6:37 AM — Last night’s models (00z and 06z) continue with the following trends—

• Light snow moves in between noon and 3 PM, earlier in southwest Chester county.

• Snow changes to sleet in some areas by 6-7 PM, according to the NBM and NAM —

• In NJ, the city, and northwest immediate suburbs, little to no accumulation by Sunday morning.

• Areas northwest and far northwest will have some accumulation (see below)

• Wrap around flurries, snow and rain showers Sunday.

01-04-24 06z NBM Precipitation type probability at 7PM Saturday evening. (Green =rain, Blue = snow) (Click on image for a larger view.)

NAM at 7 PM Saturday—

01-04-24 06z NAM Precipitation type at 7 PM Saturday (Click on image for a larger view.)

By 7 AM Sunday morning, the storm will be moving away. Wrap around mixed precipitation continues—

01-04-24 06z NAM Precip type at 7AM Sunday morning. (Click on image for a larger view.)

NAM snow totals by 10AM Sunday—

NAM snow accumulation totals by 10 AM Sunday. (Click on image for a larger view.)

Canadian RGEM snow totals are somewhat higher with a sharper gradient —

01-04-24 06z Canadian RGEM total snowfall by 10 AM Sunday (Click on image for a larger view.)

The next big fine tune in the storm forecast will occur in the 48 hour time frame, where numerous other high resolution model forecasts come into range (HREF, HRRR, HRDPS, NAM-NEST). So Friday’s 06z and 12z forecasts available Friday morning, should seal the deal.

Next update later this afternoon, about 5:30 PM

Weekend Storm Update

Posted Wednesday 01/03 @ 10:06 PM — Tonight’s NAM forecast extends into Sunday morning. Let me just preface that this model is not the state of the art, but over the years, it’s done very well with snowstorms.

At 3 PM, it’s mostly snow, except rain in NJ —

NBM at 4 PM Precipitation type (rain or snow or a mix)

By Saturday 10 PM, it’s changing over to rain—

10 PM Saturday (Click on image for a larger view.)

By near daybreak Sunday, here’s its accumulation totals—

NAM snowfall totals by 4 AM Sunday (Click on image for a larger view.)

These numbers will likely change by tomorrow, but the general trend of snow –> mix –> rain is shared by numerous other models.

Updates tomorrow.

Posted Wednesday 01/03 @ 6:11 PM — The storm is just entering into the forecast range of some of the higher resolution models— NAM and the RGEM (84 hours).

Regular readers of this blog know my mantra, “Never ignore the NAM“, when it comes to snowfall here. Since the storm has just entered the forecast range of the NAM, here’s what I’ve been waiting to see—

01-03-24 18z NAM forecast precip type and simulated radar at 1 AM Sunday morning (84 hours) All rain in the immediate area. (Click on image for a larger view.)

The next NAM becomes available about 9:40 PM tonight. I’ll update here about 10 PM.

Posted Wednesday 01/03 @ 5:38 PM — Update with the latest GFS model, just available—

01-03-24 18z GFS Total accumulated snowfall by 9 AM Sunday. This is likely too high…there will be melting from rainfall. (Click on image for a larger view.)

Posted Wednesday 01/03 @ 5:17 PM — The trends that began yesterday continue with the Nor’easter expected to move in Saturday into early Sunday.

• After a start with light snow and flurries about 1-2 PM Saturday, snow will become somewhat heavier, but will begin mixing with rain during the late afternoon or early evening. There’s simply not enough cold air with this system.

• A changeover to mostly rain will occur before midnight Saturday. Most of the area will be mostly rain. There may be about an inch or so by midnight before the changeover.

The latest ECMWF shows temperatures above freezing by 10 PM Saturday at critical levels of the atmosphere—

01-03-24 12z ECMWF forecast for 10 PM Saturday night. Heavy precipitation, but temperatures will be above freezing at critical levels of the atmosphere; i.e., the freezing levels will be to our north. Snow—> Mix —>Rain. (Click on image for a larger view.)

By 3 AM and likely sooner, it’s mostly rain —

01-03-24 18z NBM (Model Blend) Precipitation type. Green- rain Blue- snow. While line is 32º line at surface. (Click on image for a larger view.)

Saturday into Sunday Storm

Update Wed 1/03 1:42 PM — A quick mobile update. I got a peek at the new GFS and the trend towards more rain is appearing definite. Snow totals an inch or less in the city. I’ll be updating about 5:30 PM with additional new models and graphics.

Posted Wednesday 01/03 @ 8:32 AM —

I’ve movied the coverage of this storm from my Winter Outlook page to this main page starting with this posting this morning. See earlier coverage on that page.

The expected storm Saturday into Sunday should be entering the forecast range of the shorter range models (84 hours) later today. I’ve reviewed the latest models from last night. Here are the trends, somewhat similar to last night’s update.

• The snow starts earlier, sometime early afternoon, but with some snow flurries possible in the morning. Starting as snow, it will change and mix with rain in most areas south and east as shown below, possibly changing back to snow before ending. The NBM show’s little to no accumulation—

Latest NBM 12z, “just off the wires”. Forecast for 4 AM Sunday morning. Too warm for much snow here. (Click on image for a larger view.)

Incredibly, the latest NBM shows NO SNOW accumulation in most areas in around Philadelphia due to a changeover to rain.

The final word isn’t in yet… the ECMWF was somewhat colder, as was the GEFS.

I’ll update later today, when the storm comes in range of the NAM and Canadian RGEM higher resolution models.

Wednesday Forecast Update

Posted Tuesday 01/02 @ 5:55 PM — It appears that Wednesday will start sunny and cold, but clouds will move in during the early afternoon ahead of another system, expected to pass to our south. A few snow flurries (non-accumulating) looks possible for early Thursday morning.

Current Water Vapor image

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. System 1 will move to our south on Thursday. System 2 will affect us on Saturday into Sunday. (Click on image for a larger view.)
I’ve just updated my Winter Outlook Page today, about the weekend storm late Saturday into Sunday Jan 7th.
Forecast Update

Posted Tuesday 01/02 @ 9:37 AM — No change in Tuesday’s or Wednesday’s forecast.

I’ve just updated my Winter Outlook Page today, about a potential future storm late Saturday into Sunday Jan 7th.
Tuesday Outlook

Posted Monday 01/01 @ 7:34 PM — The sun makes a much needed appearance on Tuesday. After a cold morning, temperatures are expected to warm up to the lower 40s by 3 PM.

It looks like a storm will likely miss us and move east of us late Wednesday into Thursday. The weekend looks interesting.

I’ve just updated my Winter Outlook Page today, about a potential future storm late Saturday into Sunday Jan 7th.
Mon 12:03 PM —Forecast Review — The coastal low seems to have developed a bit further south and east than forecast yesterday. The current position, based on 3 hour pressure changes is off the Virginia coast—
Current (noon) HRRR/RAP pressure changes with superimposed MRMS Radar and Philadelphia area composite Radar. Isobars are in yellow. Pressure change is color shaded with green contours.. Low pressure development is indicated at center of maximum negative pressure change. (Click on image for a larger view.)

It should remain cloudy much of the remainder of the day, perhaps with a brief break in the cloud cover. As forecast, much of the rain will miss the immediate PHL area.

I’ve updated my Winter Outlook Page today, about a potential future storm coming Sunday Jan 7th.

Previously Posted Sun 4:50 PM —

New Year’s Day Forecast

Happy New Year! The persistent upper level low will drift over us and a very weak secondary low pressure system will form off of the Delmarva coast. (Yes, this forecast has changed from this past Friday.)

Today’s GFS forecast for New Years Day at 10 AM. Upper low (blue L) and weak secondary coastal low (black L) Cloudy with light showers expected. (Click on image for a larger view.)

For Monday, considerable cloudiness with light, spotty showers expected, more to our west and to our east in NJ. Philadelphia and its immediate suburbs may have the least rain, about 0.01-0.02″. High low 40s, which is near to slightly above season average. Winds light, increasing towards evening.

Today’s 18z NBM total precip forecast through 7 PM Monday (Click on image for a larger view.)

Skies clear on Tuesday, but it will be colder than average.

Looking Ahead

Several areas of coastal low pressure are expected to develop in the coming days— one late Thursday and another on Sunday afternoon.

With temperatures near to slightly above normal , we’re on the edge of a few possible light snow/dustings in far northern and western suburbs. As mentioned over past weeks, the pattern may be changing the end of this week. (see my Winter Outlook Posts, updated today, for more speculation.)

Things have trended a bit colder and the storm next Sunday needs to be watched, although right now, it looks like it will either be too warm or to far to the east for snow here.