Category Archives: Philadelphia Weather Outlook


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Tropical Storm Ophelia

Updated Fri 09/22 @ 4:39 PM — When I updated this site this morning, the storm that is forecast to affect our weekend was still referred to as Atlantic tropical depression 16. It’s been named Ophelia My regular “Weekend Weather Forecast” will cover this storm and its impact on our area.

Look for a new post shortly.

Update Fri 9/22 8:38 AM — A quick update. No significant changes from last night’s forecast. Rain moves in from the southeast between 2AM and 4AM. Very windy with wind gusts 38-46 mph. Heavy rain on Saturday, especially Saturday late morning. Rain continues through much of Sunday. Total rainfall by Sunday evening 2-3.5”

Updates later today.

Update Thu 9/21 9:30 PM — The latest NAEFS ( a combination of our GEFS and the Canadian ensemble models) captures the current consensus guidance for this storm—

Today’s NAEFS model forecast for 2 PM Saturday shows the deep surface low almost vertically stacked under the upper level low.

By Sunday, 24 hours later, the surface low has filled and weakened and the upper level low lingers. It’s still raining here—

The NAEFS model at 2 PM Sunday. (Click on image for larger view.)

The latest NBM model just available show rainfall of 2-3” by Sunday evening and wind gusts of 35-45 mph.

Update Thu 9/21 5:03 PM — This will a difficult forecast to get right. The model guidance is all over the map…literally. Most models now have the strong tropical low pressure moving inland. Most, but not all, have this surface low pressure system rapidly “filling” as it encounters blocking high pressure, leaving a closed upper low. Most models have about 1.5-3” of rain through Sunday, with the majority of the rain Saturday. Heaviest activity closer to the shore is a safe, but not necessarily accurate emphasis.

It’s going to be difficult to pin down specifics regarding locations of the heaviest rains and higher winds. Stay tuned

Update Thu 9/21 8:29 AM — Last night’s ECMWF has finally joined our GFS model with a more coastal track and with the low pressure system becoming quasi-stationary as it moves over southern Delaware. The system will encounter high pressure to our north and will stall and somewhat “fill” (weaken).

ECMWF forecast 3 AM Sunday morning.

High winds and heavy rains still likely with this system.

Update Wed 9/20 9:52 PM — This afternoon’s ECMWF and NAEFS models, just available, have become a bit more similar, with the storm either moving a bit eastward, or slower just to our south. This trend would reduce the impact of the storm in our immediate area. Will have to see the trends with tonight’s models to get a better handle on this system.

Previously Posted Wed 9:02 PM —

As mentioned over past days, a disturbance off of the Atlantic coast of Florida will develop into an intense coastal low with some tropical characteristics. It’s expected to begin to affect our area during the predawn hours of Saturday morning.

The majority of models have this system moving up the coastline, with the exception of the ECMWF, which continues to have the system bend away from the mid Atlantic coast. (Tonight’s GEFS does have the storm blocked from northward movement as it approaches the Delaware coast with a slight turn eastward. This somewhat supports the ECMWF track, but it’s too soon to hang our hats on this forecast.)

Heavy rain, high winds are likely on Saturday if it follows the track of the GFS, Canadian and German ICON models. Rainfall totals of 1.5 to 3 inches are likely, with wind gusts 35-50 mph. Impacts will be significantly less if it follows the ECMWF track.

Here’s the latest Canadian RGEM

18z Canadian RGEM forecast total rainfall for 2AM Sunday. (Click on image for larger view.)

So, this could be a major storm for our area or it might move eastward enough to significantly reduce its impact. Stay tuned.


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This Weekend’s Storm

Updated Wed 09/20 @ 8:37 AM — The models are in good agreement (with the exception of the ECMWF) that a major Nor’easter with some tropical characteristics will affect our area beginning Friday night.

The latest GFS shows this storm at 2PM Saturday—

06z GFS shows major low pressure system moving up the coast at 2 PM Saturday. (Click on image for a larger view.)

Anywhere from 1.5-4 inches of rain are possible with this system along with high winds and coastal effects.

The latest German ICON model shows very high rainfall possible—

Latest ICON model shows large areas of 3-4 inches of rain by Sunday afternoon. (This model is at the upper end of the rainfall forecast range.)

Wednesday Through Sunday Outlook

Update Tue 9/19 9:28 PM — Continued fair weather through early Friday. Increasing cloudiness expected during the day on Friday.

The models have come together forecasting a slow moving coastal storm, held back by a westward closed upper low. This is expected to move in Friday night.

NAEFS forecast for 8 AM Saturday. ( Click on image for larger view.)

The ECMWF, which had been holding back on the development of this system, has joined the other models forecasting this storm. It is a bit slower than the GFS and NAEFS, with the rain moving in during the day Saturday. Currently, it looks like it will be raining most of Saturday with somewhat light showers on Sunday.

Sometimes these storms move out faster than the model forecasts, so there’s some hope for Sunday.

Tuesday – Friday Outlook Update

Updated Mon 09/18 @ 6:23 PM —The low pressure system off of the coast will move northeast and high pressure builds in for Tuesday through Thursday.

Fair skies and seasonably mild temperatures expected.

ECMWF Temperatures forecast through Friday (Blue Bell, PA) (Click on image for a larger view.)

Clouds move in sometime on Friday.

There’s uncertainty with the low pressure development off the Florida coast. The NAEFS (combined GEFS and Canadian Global Ensemble) continues to forecast an “inverted coastal trough” moving in Friday into Saturday with clouds and an increasing chance of showers. The ECMWF is NOT currently on-board with this system moving northward. I’m leaning towards the NAEFS—

NAEFS forecast for Friday. Inverted trough along the Atlantic coastline of Florida is forecast to move northward toward us. (Click on image for a larger view.)

Update Sun 9/17 11:09 PM — Tonight’s models are forecasting a slower exit of the showers Monday. We may still see some showers in Philadelphia and eastward as late as 1-2 PM.

Previously Posted Sun 8:03 PM —

A weak coastal low will move northeast Monday. Showers may linger until 10 -11 AM and clouds will linger into the early to mid afternoon.

High pressure builds in for Tuesday through much of Friday—

NAEFS forecast for Wednesday at 11 AM. High pressure over the Mid Atlantic and Northeast will keep sunny skies and and seasonable temperatures. (Click on image for a larger view.)

Average seasonal high temperatures this week are 75º-76º. Temperatures this week will be close to slightly above seasonal averages.

So an uneventful weather week is in store for us.

Of interest is a potential tropical or semi-tropical system expected to develop off the Atlantic coast of Florida sometime on Friday. This is forecast to move north and bring clouds and rain late Friday into Saturday. Things will likely change by the time Friday rolls around.


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Updated Sat 9/09 9:23 PM — Regular updates here to This Week’s Weather and Weekend Weather Forecast should resume next weekend…. Certainly in enough time for coverage of Hurricane Lee, which the ECMWF model is forecasting to possibly affect our region.

Thunderstorms Thursday Night

Updated Thu 09/07 @ 11:55 AM — A quick update. This morning’s models are suggesting that most of the showers and thunderstorms stay to our west tonight. We need the rain, but it looks like very little will make it into the immediate Philadelphia area tonight.

Updated Thu 09/07 @ 9:27 AM — A slowly advancing cold front will trigger thunderstorms tonight. Most of the stronger storms will affect far northwestern areas (Pottstown, Reading, Allentown) this later this evening.

Most (but not all models) have the storms moving through the immediate Philadelphia area and adjacent suburbs between midnight and 2 AM. Since these storms will be occurring well after the sun has set, surface level CAPE values will not be that high. Severe storms are NOT expected.

Rainfall amounts in our immediate area are not expected to be that high—

Today’s 06z HREF total rainfall for Thursday night. (Click on image for a larger view.)

Thursday Forecast

Updated Wed 09/06 @ 5:40 PM — A preliminary check of the RTMA shows that we ‘only’ hit 94º in Philadelphia, but areas just west of Atlantic City hit 96º. Dew points were in the mid to upper 60s during the high temperature period.

The hot air ‘dome’ is being squeezed by a very slowly moving push from the west and a stagnant low pressure system in the Atlantic (the non-tropical remains of what was Hurricane Idaila!)—

WV image Wednesday afternoon shows the ‘squeezed dome’ of hot air directly over us, delineated by the 576 thickness lines (yellow). (Click on image for a larger view.)

Thursday will likely be the hottest day, with temperatures truly hitting 96º+ and a much wider area around Philadelphia hitting heat indices of over 100º.

As the system in the Ohio tries to move eastward, it will be blocked somewhat but some slow progress is expected.

This means an eventual decrease in temperatures and an increasing likelihood of thunderstorms and rain from Thursday evening through at least Sunday.

Areas to our west and northwest will likely see thunderstorms Thursday night. They may not make it into Philadelphia or much further eastward.

From Friday through Sunday, more of our area will see showers and thunderstorms. Sunday looks to have the most widespread activity as the advancing front slows over our area.

Wednesday Forecast

Updated Tue 09/05 @ 5:19 PM — High temperatures today (usually reached between 3:45 PM and 4:00 PM) were a degree or two lower than Monday. The increased dew points (69º -71º) more than made up for the marginally lower temperatures.

For Wednesday, temperatures are expected to move back to those reached on Monday, and the higher readings will be more widespread—

Today’s 18z NBM forecast high temperatures for Wednesday. (Click on image for a larger view.)

The dew points on Wednesday will be a bit higher as well. As a result, the heat indices will be quite hot in many areas—

18z NBM Apparent Temperature forecast (Heat Index) for Wednesday. (Click on image for a larger view.)

An approaching front will bring a good chance of thunderstorms Thursday evening and Friday evening.

Tuesday Forecast

Update Mon 9/04 10:10 PM — Tonight’s 00z NBM’s high temperatures for Tuesday are coming in a 1-2 degrees lower than Monday, but dew points look to be be higher. So heat indices may be the same or hotter. Another hot day!

Tonight’s 00z NBM “apparent temperature“ forecast (aka heat index) for Tuesday (Click on image for a larger view.)

Updated Mon 09/04 @ 5:22 PM — Actual high temperatures on Monday—

1945z RTMA Temperature analysis at 3:45 PM Monday. Contours are 1º increments (Click on image for a larger view.)

It is very similar to last night’s NBM model forecast (but not this morning’s NBM) —

Last night’s 00z NBM forecast highs. While it is often the desire to ‘get the latest model’, due to model spin up issues, the most recent model forecast is often not the most accurate. (Click on image for a larger view.)

We can see that the HRRR (below) over forecast the high temperatures. When it comes to temperature forecasts, the NBM is often the best model.

Previously Posted Mon 10:10 AM —

The large heat dome continues to nudge north and eastward towards us. The “576 thickness line” provides a clue into the three dimensional outline of this hot air mass—

This morning’s water vapor image (capturing the middle and upper atmosphere) and the RAP model 500-1000mb thickness line (mid level contour showing the outline of the very hot air at 576 decimeters) The hottest air is still to our west and south. (Click on image for a larger view.)

This morning’s NBM model’s high temperature forecast has moved down a degree or so from last night’s forecast highs temperatures.

Last night’s forecast—

This is last night’s NBM high temperature forecast. Today’s is similar, but without that patch of violet 95.6º High temperatures will be closer to 93º-95º today in our region.

Today’s 12z NBM high temperature forecast—

Today’s 12z NBM High temperature forecast (Click on image for a larger view.)

As good as the NBM has been regarding temperature forecasts, it’s not forecasting the showers affecting northern parts of the Jersey Shore at this time—

Current radar with Satellite Channel 13. (Click on image for a larger view.)

The latest HRRR does show these showers and it has a much hotter forecast for us today than the NBM—

Here’s the latest HRRR forecast high temps for today—

Today’s 12z HRRR is forecasting much hotter high temperatures than the NBM (Click on image for a larger view.)

The NBM is my preferred model for high temperatures, but the NBM tends to be a lagging forecast by design.

We’ll see if the HRRR is forecasting too hot for today or whether its forecast represents a more up to date forecast.