Category Archives: Philadelphia Weather Outlook

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Sat 10:58 AM —Forecast Review — We had much more precipitation and in some places, snow, than forecast. Looking back on last Thursday’s models, most forecast precipitation in the 0.13″ to 0.20″ range.

Looking at the final MRMS totals for yesterday’s storm, precipitation amounts (water or water equivalent) were in some cases 2-4 x as much as the models had forecast and banding occurred directly through Philadelphia—

MRMS actual precipitation amounts with select snowfall totals from storm spotters and submissions to the NWS. (BLUE – SNOW TOTALS IN SELECT AREAS) MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL PRECIPITATION RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

Snow totals can be seen at the NWS site The increase in precipitation rates caused dynamic cooling and temperatures dropped correspondingly in those areas.

Fri 9:18 PM —Forecast Review — My last update at 3:31 PM showed an area of banding setting up over Philadelphia. As is often the case, the models didn’t accurately forecast the location of precipitation banding until the event was well underway.

The MRMS shows where the most precipitation (water equivalent) fell —

MRMS rain-gauge measured precipitation (water) combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL PRECIPITATION RECEIVED as of 8 PM. Color shading is in inches (water, not snow) . Line contour numbers are in mm. (25.4 mm= 1 inch)    MRMS shows area of banding – precipitation (water) maximum. (Click on image for a larger view.)

With 0.30″ plus water having fallen in that band, I wouldn’t be surprised if some of those locations have received 2-3+ inches of snow on some surfaces.

Snow Update

Posted Friday 12/20/24 @ 3:31 PM — As predicted, the snow developed mid afternoon and there are signs that it will increase in intensity later this afternoon into this evening. The on-again, off-again forecast for light snow accumulation is on.

Here’s the current radar/water vapor image—

3 PM Current Water Vapor/Radar image with superimposed RAP model vertical velocity pressure (Omega) in yellow contours. Intense area of vertical motion over NJ shore area, but too warm for much accumulation. An elongated are of vertical motion lies over Philadelphia, where snowfall has increased in intensity. (Click on image for a larger view.)

The latest NBM has increased its snow accumulation forecast somewhat. Here’s the NBM (model blend) mean snowfall forecast—

Today’s 18z Model Blend snowfall accumulation totals through 1 PM Saturday. (Click on image for a larger view.)

There may be some snow showers early Saturday morning, but most of the accumulation will have ended by dawn.

Another Snow Update

Posted Thursday 12/19/24 @ 9:26 PM — What was a “low confidence forecast” this afternoon is even a lower confidence forecast this evening with the latest 00z NAM, NBM and HRRR models further downplaying accumulations, especially near the city. Here’s tonight’s NBM—

Tonight’s 00z Model Blend (NBM) shows a further decrease in forecast accumulation. This graphic is the mean snowfall prediction, a significant reduction from earlier today. (Click on image for a larger view.)

The above forecast snowfall may even be too high.

The reason for the change: less overall precipitation from the Atlantic storm being thrown our way. Add to that a slow-down in the temperatures dropping to 32º near the city, which may not occur until 6-8 PM. This forecast for accumulating snow here has been on-again, off-again since this past Sunday.

A Quick Snow Update

Posted Thursday 12/19/24 @ 5:22 PM — While I was preparing the last posting “A Coating of Snow Possible”, the ECMWF became available. It has significantly more snow than even the latest GFS. The ECMWF has as much as 2-3 inches in northwest suburbs and 1 maybe 2 inches just west of the city. I’m not going with the ECMWF forecast at this time and staying with the GFS and NBM.

A Coating of Snow Possible

Posted Thursday 12/19/24 @ 4:30 PM — Let me start by saying this is a low confidence forecast and weather scenario. Weak low pressure dropping down merges its energy with a storm in the western Atlantic. Those of you who are regular visitors here know that this was an on-again off-again forecast since Sunday

We’ll be receiving some precipitation from the initial impulse along witih some wrap-around moisture from the Atlantic storm, according to the models.

The models are “cranking out” anywhere from 0.06″ to 0.15″ of precipitation water equivalent. Much of this will be falling where near- ground temperatures are several degrees above freezing. Combine this with the actual surface ground temperature (“skin temperature”) above freezing and you have ingredients for a blown accumulation forecast.

Here’s the uncertainties— the actual precipitation (water equivalent) may be higher or lower than forecast with these merged-type systems. Temperatures at the ground and aloft may be colder…or warmer than forecast.

The error range of forecast snow accumulations with this system is magnified with snowfall amounts so low. And the degree of melting will be higher than usual.

The model blend (NBM) is the best choice here and below is is its mean (average) forecast snowfall accumulation which includes snow showers early Saturday morning—

NBM forecast mean snowfall at 1 PM Saturday. (Click on image for a larger view.)

Those who remember their basic statistics courses know that when the mean and the median forecast totals differ significantly, it suggests wide standard deviation and high uncertainty. Such is the case here.

Some light wet snow or snow with mixed with rain starts about 12-2 PM Friday, especially northwest areas. More coverage and and change to snow with melting on many surfaces through midnight Friday. Very widely scattered snow showers possible Saturday morning into the afternoon.

Not much accumulation expected on roadways for most of the event. The snow showers Saturday morning may leave a dusting.

Stay tuned for updates.

Forecast Update

Posted Wednesday 12/18/24 @ 5:32 PM — Some afternoon models are again showing precipitation moving in from a system out in the Atlantic, although the ECMWF AI model have backed away from that. This forecast has been on-again, off-again.

Here’s the tricky thing about the ‘snow’ forecast for Friday afternoon/evening— critical temperatures several thousand feet above ground are all cold enough for snow. However, temperatures of the ground and near the surface are several degrees above freezing—

The latest NAM (18z) precipitation, wind and temperatures (contours) forecast for 4 PM Friday. Temperatures near the surface are too warm for accumulation, except in northern Bucks county. (Violet 32º contour is several thousand feet above ground. ) (Click on image for a larger view.)

Temperatures aloft support snow, but ground temperatures are forecast to be too warm for accumulation, except in northern Bucks county where a coating is possible.

So we may see some wet snow or snow showers Friday afternoon and evening, but it likely won’t accumulate in the immediate Philadelphia area and surrounding suburbs. (Unless precipitation rates are greater than forecast and dynamic cooling occurs.)

Update Wed 12/18 11:50 AM — No sooner than when I posted the “on-again off-again” very light snow forecast for Friday night that today’s morning models became available. They’ve substantially backed off on any accumulations and snow showers for Friday night.

This may change again, but for now, we’ll have to wait later in the season for any snow here.

Posted Wednesday 12/18/24 @ 9:22 AM — First, light rain is expected to move into western suburbs about 4 PM today (Wednesday) and the rest of the city about an hour later according to the model blend (NBM) . The heaviest rain is forecast to move to our north, according to the latest HRRR.

Today’s 12z NBM simulated radar forecast for 4 PM Wednesday. The heaviest rain slides up to our north. (Click on image for a larger view.)

For Friday, according to the NBM, most of the area will see snow or rain mixing with and changing to snow Friday night into Saturday morning. How much accumulates will be tricky, since ground temperatures have been warm. Here’s the Canadian RGEM snow accumulation forecast—

Today’s 06z Canadian RGEM Snow accumulation by 7AM Saturday morning. This is in line with the NAM, ICON and NBM (Click on image for a larger view.)

The NBM shows additional light snow flurries early Saturday morning. Very gusty winds Saturday.

Stay tuned for updates later today.

Models again showing light snow late Friday.

Posted Tuesday 12/17/24 @ 6:01 PM — The GFS and ECMWF are again showing light snow or snow showers Friday evening/night through daybreak Saturday. This snow shower forecast has been on-again off-again since Sunday.

Today’s latest GFS shows snow showers starting as early as Friday evening, continuing through the night. This shows light snow showers at 4 AM. (Click on image for a larger view.)

Areas in northwestern Chester/Montco/Bucks counties may see a coating up to 3/4″ fading to zero near the city, according to the latest GFS.

The ECMWF shows similar very low accumulation totals, with total water equivalent in the 0.1″ range. Very cold weather through Christmas eve with a slight warmup Christmas day. The above average temperatures I spoke of yesterday may not get here until later that week.

How Much Rain Did We Get?

We’ve had two periods of rain since Sunday early evening. The total rainfall we received is shown by the MRMS—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

Another 0.20″-0.40″ of rain is expected starting later Wednesday afternoon into Thursday morning.

Forecast Update

Update Mon 12/16 9:44 PM — Tonight’s models have moved the start of the rain on. Wednesday earlier,, to early afternoon.

Posted Monday 12/16/24 @ 7:39 PM — Following the warm front that moves through tonight with showers, high temperatures on Tuesday will be in the upper 50s to near 60º with mostly sunny skies by late morning. It will be somewhat windy.

NBM mean high temperatures on Tuesday. There’s a large standard deviation (3º) indicating a wide spread in the model forecasts. It will likely be warmer than the mean. (Click on image for a larger view.)

A cold front moves through Wednesday night with more rain.

Things have already changed for Friday. No snow expected, as the dip in the jet will be further east than previously forecast and will not be as amplified.

The extended forecast for Christmas and Christmas day is for temperatures to be above average. No chance of snow currently seen.

Rain

Posted Monday 12/16/24 @ 8:29 AM —As discussed yesterday, we’ll have several rounds of rain this week. The GFS and ECMWF are showing about 1″ to 1-1/2″ of rain total, including this morning, tonight and Wednesday night.

Two cold fronts move through Thursday and again later Friday. A secondary low may form off the coast, now likely Friday instead of Saturday (as discussed yesterday) and the GFS is showing light snow Friday afternoon and evening. This low will move far off the coast, so this won’t be any sort of snow storm here. Temperatures will be borderline here near Philadelphia, so there may be little or no accumulation around the city.

As far a precipitation type—

GFS Precipitation Type. (not radar) Friday afternoon. (Click on image for a larger view.)

Stay tuned for updates.

Originally Posted Sun 9:28 PM —A warm front will slowly move across our area tonight through Monday with light rain in our area. Some areas of fog and drizzle appear likely to occur Monday morning.

Heavier rain is expected with the actual warm front, followed by a cold front Monday night into Tuesday morning. Clearing Tuesday afternoon.

Another disturbance ahead of stronger cold front is expected late Wednesday into Thursday with additional rain. Colder weather expected Friday through next weekend.

What about snow?

Like last year, there hasn’t been much of a signal or pattern for snow here, although there are elements in recent jet stream dips that show some potential.

The first is later on Thursday, following the cold front. Some snow showers are possible, although not currently predicted.

A few disturbances may pass through either Friday or Saturday.

ECMWF forecast for Saturday morning. Clipper disturbance may bring light snow, but low pressure off the coast may rob the disturbance of energy and moisture. (Click on image for a larger view.)

The ECMWF is showing the possibility of light snow from a clipper disturbance moving through on Saturday. A developing secondary low off the coast may rob our area of the energy and moisture and the forecast jet configuration is too flat for any substantial storm. That said, the current ECMWF is showing the possibility of a period very light snow on Saturday. I’ll keep an eye on it.

Philadelphia Area Weather Forecasts, Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Cold and Dry

Posted Thursday 12/12/24 @ 7:39 PM — We’ll have cold and dry weather Friday through most of Sunday. Sunny skies Friday and Saturday, cloudy on Sunday. An area of rain will move towards us later Sunday, likely not moving in until late evening or after midnight Sunday.

GFS forecast for Sunday midnight. Clouds with rain just to our west, which may move mainly north of our area. (Click on image for a larger view.)

More active weather expected next week.

Rain early Monday and again Monday night into Tuesday. Another possible rainstorm next Thursday ahead of another cold front. No clear signal for any snow seen in the long range

Wed 10:05 PM —Forecast Review — The rainfall was considerably less than forecast and the wind gusts, while high, fell short of the extremes forecast yesterday. The two bands of heavier rain were captured by several models.

Here’s the MRMS though 9 PM tonight—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

Wednesday Forecast Update

Posted Wednesday 12/11/24 @ 11:14 AM — The models have backed down on the severe 50-60 mph wind gusts forecast yesterday but are still showing gusts near 40 mph. They’ve also backed off on some the extreme precipitation amounts.

Based on 3 hour pressure changes, several areas of low pressure are expected to develop and move northeastward—

HRRR 3 hour pressure change shows areas of expected low pressure development and intensification at 1 PM
++ indicates the front moved through and high pressure is building in.

The cold front moves though Philadelphia about 4 PM, earlier to our west according to the latest hourly HRRR model. The heaviest activity is forecast for east of us, in NJ about 3 PM, less severe near Philadelphia and western suburbs. Rain continues until about 7-9 PM. There may be a mix of rain and wet snow showers in the northwest suburb about 10 PM to mid night.

Latest hourly HRRR model (14z) shows a line of heavy showers/thunderstorms in NJ at 3 PM
Wednesday Wild Weather

Posted Tuesday 12/10/24 @ 4:58 PM — Some light showers have moved into western suburbs ahead of the main disturbance that will move in later this evening with heavy rain and high winds on Wednesday—

Current (4:45 PM)satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow contours), mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Cold air plunging towards Texas with a developing jet stream trough. Rain moving towards our area. (Click on image for a larger view.)

The latest ECMWF and the latest GFS has raised their rainfall forecasts for our area. Here’s the ECMWF (the GFS is very similar)—

12-10-24 12z ECMWF total rainfall through early Thursday morning. Several models are showing this banding of heavier rain in this general location. (Click on image for a larger view.)

The GFS is showing high wind gusts in the morning in PA —

Lastest GFS (18z) wind gust forecast 10 AM Wednesday (Click on image for a larger view.)

Both the ECMWF and the Canadian RGEM are showing very high wind gusts, especially in New Jersey during the mid to late afternoon—

Today’s 18z Canadian RGEM showing very high wind gusts about 3 PM. (Click on image for a larger view.)

The latest GFS shows high wind gusts near Philadelphia during the morning hours—

Rain tapers off and ends 9 PM Wednesday evening Some wet snow flakes may mix in with the rain as it ends in the western suburbs.

Scattered snow showers possible Thursday morning.

Forecast Update

Posted Tuesday 12/10/24 @ 8:37 AM — A quick update. Here are the forecast trends—

  • Fog lifts and visibility improves later this morning (Tuesday). Low clouds hang tough.
  • Light sprinkles move in from southwest between 5 PM and 8 PM
  • Heavy rain with increasingly strong wind gusts (35 mph) on Wednesday
  • Trend is for greater rainfalln now to be near 2″ but the ICON and Canadian RGEM still maintain a band of 3″+ through Philadelphia.
  • Rain tapers off earlier with winds,between 6 PM and 8 PM Wednesday.
  • Some wet snow showers possible near the end of the precip.
  • Falling temperatures Wednesday evening and night.
Updated Forecast Wednesday-Thursday

Posted Monday 12/09/24 @ 7:54 PM — Fog and low clouds may hang on during the early morning hours Tuesday. The GFS has some bright skies Tuesday morning after the fog lifts; the NAM-NEST keeps us in low clouds. Either way, it becomes cloudy in the afternoon and some light sprinkles develop from the southwest as early as 5-6 PM around Philadelphia ahead of the main system.

The system for Wednesday is already visible on radar and water vapor imagery—

Current satellite water vapor image (Monday evening) 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. Cold air descending will re-contour the jet flow into a trough as low pressure (1) moves up towards us late Tuesday night through Wednesday. (Click on image for a larger view.)

Wednesday will be cloudy and rainy and increasingly windy as the day progresses. Heavy rainfall expected throughout the area and continues into the evening.

The NBM has backed off from the 2.2″ rainfall and is now predicting 1.8″ over much of the area. The GFS and ECMWF are in that ballpark, about 1.6″. The latest ICON model and Canadian show well over 3 inches for our area. There will be the usual banding of the heavy rainfall, difficult to predict in advance.

The cold front moves through about about 6 PM and the rain tapers and ends towards midnight Wednesday.

Posted Monday 12/09/24 @ 8:47 AM — For today, Monday, light rain and has just moved into western Chester County moves into the rest of our area between 9 AM and 11AM . Heaviest rainfall today in about 2 PM and tapers off and ends between 5 PM and 6 PM.

The system to affect us on Wednesday now appears to move in as early as early as Tuesday evening with light scattered rain. Rain on Wednesday throughout the day, but especially heavy late afternoon into early evening, ending about midnight. The range of forecast rainfall accumulation is large, with the ECMWF 1.2″ GFS- 1.8″ and Canadian RGEM 3.3″. The latest model blend (NBM) is essentially unchanged from what was forecast last night.

The model blend is also forecasting a possible change to wet snow showers as the precipitation ends around midnight Wednesday with possible coating on grassy surfaces Thursday morning. Not all models are on board with this, but the latest ECMWF shows instability snow showers Thursday morning.

Heavy Rain on Wednesday

Posted Sunday 12/08/24 @ 5:17 PM —The latest NBM forecasts heavy rainfall for us on Wednesday—

12-08-24 18z NBM total rainfall forecast for Wednesday. Black contours are 0.1″ increments.

Some models (Canadian) are forecasting much greater amounts, in the 3.5″ range, while the latest GFS and ECMWF are forecasting total rainfall in the 1.6″ range.

This is in addition to Monday‘s light drizzle/rain forecast totaling 0.13″- 0.28″.

Active Pattern Change

Originally Posted Sun 11:21 AM —This week’s weather will be active, as cold high pressure dives south, pushing the jet flow into a sharp highly amplified trough.

The current satellite water vapor image captures the main elements and the graphic’s caption below goes over the details—

Current satellite water vapor image (Sunday morning) with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours).
A disturbance (1) will move eastward as another disturbance (2) moves up from the southwest. This will bring drizzle and rain Monday as part of a warm front. At the same time, cold high pressure will be pushing the jet flow southward (blue arrow) creating an amplified jet trough. Disturbance (3) moves up from the Gulf ahead of the developing trough to bring heavy rain Wednesday. (Click on image for a larger view.)

By Monday, the NAEFS captures the beginning of the rapid pattern change. Drizzle and rain with a warm front and developing low pressure system. About 0.20 inches of rain expected —

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Monday afternoon. Low pressure develops over our area along a warm front. (Click on image for a larger view.)

By early Wednesday afternoon, a deep trough and strong cold front moves through—

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Wednesday at 7 AM Low pressure brings heavy rain (Click on image for a larger view.)

Heavy rain expected late Tuesday night into Wednesday. Perhaps as much as 1.5-2 inches of rain with some localized higher amounts! High winds behind the front later Wednesday.

Stay tuned this week for updates.