Category Archives: Philadelphia Area Weather Forecasts

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

Saturday Rain Totals

Posted Saturday 03/23/24 @ 5:25 PM — We had a lot of rain in just 14 hours or so. Here’s the MRMS rain totals as of 4 PM—

MRMS combined rain gauge & radar-based measurement 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.)

While I didn’t post these models, the GFS and the Canadian models correctly forecas the placement of the heaviest rain axis just east of the Delaware River into NJ, although the GFS rain totals were too low.

The NBM did well with a forecast 3.6″, as did the HREF forecast (PMM) version, but both had the rainfall axis too far west into Philadelphia and western suburbs. The ECMWF was slightly too far west as well. The experimental RRFS too far west and its rainfall forecast was too high!

Winds will be picking up over the evening hours tonight becoming quite gusty, but subsiding by the afternoon Sunday.

Rain Update

Posted Saturday 03/23/24 @ 12:58 PM — The heaviest rain has entered our area over the past hour or two, about two hours ahead of the model forecasts.

MRMS Instantaneous Precipitation Rate (PRATE) at 12:52 PM. (Click on image for a larger view.)

The latest hourly HRRR suggests that the rain will be tapering off between 2 and 3 PM and ending about 3-4 PM from the southwest.

At noon, many areas already had 2-2.75 inches of rain. according to the MRMS. I’ll post a ‘final’ rainfall total later this afternoon.

Saturday Rain Update

Posted Saturday 03/23/24 @ 8:09 AM — Some areas have already received 0.75 inches of rain and the heaviest rain is still to our southwest—

MRMS instantaneous precipitation rate at 8 AM Saturday. Heaviest rainfall is still southwest in Virginia and will arrive here between noon and 2 PM. (Click on image for a larger view.)

The expected time for the rain to taper off and end is between 4 PM and 7 PM. Clearing will occur from southwest to northeast later this evening.

Here’s the latest rainfall forecast (NBM) —

03-23-24 06z NBM accumulated rainfall by 1 AM Sunday. (Click on image for a larger view.)
Weekend Weather Update

Posted Friday 03/22/24 @ 8:07 PM — A few more afternoon models have come in since my last post. High rainfall accumulation is the trend and the HREF and [experimental] REFS show even higher rainfall for our area than the NBM model (shown in previous post.)

The HREF and REFS are “ensembles”, meaning that they combine and average different model runs, different models and/or different ‘perturbations’ of the same model. Both the HREF as well as the experimental RRFS-ensemble (REFS) also create means (averages) in several ways.

The HREF and the REFS include statistical averaging algorithms called PMM (“probability matched mean”) and LPMM (“local probability matched mean”), mathematical approaches which attempt to eliminate a weakness of ensemble mean forecasts that causes the magnitude of precipitation to be smoothed out in the mean— Important maxima are reduced in magnitude and low magnitude values are spread over too a large an area in simple arithmetic means.

Here’s the PMM and LPMM outputs for today’s HREF model which attempt to correct for problems with ensemble simple means —

HREF- Probability Matched Mean—

HREF Probability match mean (PMM) total precipitation forecast for Saturday. (Click on image for a larger view.)

HREF Local Probability Matched Mean precip forecast for Saturday—

HREF Local Probablity Matched Mean precip totals for Saturday (Click on image for a larger view.)

The LPMM can sometimes create artificially high values and the PMM version is often found to be more reliable. You can see that the PMM version is quite similar to the NBM model (shown just below in the previous post.)

Either way, it looks like a lot of rain tomorrow!

Posted Friday 03/22/24 @ 5:42 PM — This afternoon’s models continue to forecast impressive amounts of rain falling over a 16 hour period. The latest NBM, which tends to be conservative, has the following rainfall forecast through Saturday evening—

03-22-24 18z NBM accumulated rainfall through 11 PM Saturday night. As always, the exact placement of the heaviest rain bands can’t be taken literally, but it gives a good idea what to expect in our region tomorrow.

Saturday

Light rain begins after midnight tonight The heaviest rain about 2-3 PM Saturday, but like the Giant Food ad, where the shopper is being told about the melons, “they’re all good ones”, all areas in the Delaware Valley tomorrow will have heavy rain.

Winds will be elevated along the Jersey shore all day, but around Philadelphia, winds will pickup about 10 PM Saturday evening.

NBM high temperatures: Blue Bell, PA 54º Philadelphia, PA 56º
uncertainty slightly above average (based on standard deviation): ± 2.3º

Sunday

Lingering showers at the shore early morning. Clouds around Philadelphia break for sunshine during the mid to late morning. Temperatures mild early, peaking around noon, then falling during the afternoon.

NBM high temperatures early : Blue Bell, PA 55º Philadelphia, PA 56º

Early Edition- Friday and Saturday

Posted Fri 8:34 AM — Two low pressures systems near the Gulph of Mexico will interact and move towards us late Friday into Saturday.

Current satellite water vapor image 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. Two low pressure systems will interact over our area .

But first some coverage of today, Friday— It will be sunny with relatively light winds. The area of cloudiness mentioned last evening to move in mid afternoon today is looking less impressive with last night’s models. So sunny for much of the day, with high clouds late.

What will be deceptive today will be the high temperatures. While the the TV/radio forecasts will give you the high temperatures, they don’t often tell you when in the day they will occur. Today, the high temperature will be reached late in the afternoon and it will be fairly chilly through at least noon. Basically, the winds will shift to the south about 2 PM, bringing the warmer temperatures by about 4-5 PM.

Getting back to Saturday, the two systems developing will interact and a supply of Gulph moisture will result in heavy rain on Saturday.

03-22-24 06 z NAEFS model statistical “mode” version overlayed with GEFS model “bias-corrected” precipitation model forecast for 2 PM Saturday. The heaviest rain moves in early afternoon. (Click on image for a larger view.)

Rain moves in 2-3 AM Saturday morning and will be heavy during the day with 2″-3″ inches of rain expected.

The latest NBM confirms yesterday’s trend for heavier rain, with an axis of heaviest rain though the immediate PHL area—

03-22-24 12z NBM accumulated rainfall by 8 PM Saturday. contours are 0.1 inch increments. (Click on image for a larger view.)

It becomes very WINDY after 8-9 PM Saturday. More details later this afternoon or early evening.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

Rex Blocking Pattern for through Wednesday

Posted Saturday 03/16/24 @ 9:44 AM — An upper air stable pattern, called a Rex Block (named after the meteorologist Daniel Rex, who identified this pattern) will keep a colder flow over our area through Wednesday.

ECMWF upper air/jet stream forecast winds at 300 mb for early Sunday. Rex block consisting of a stable couplet- upper high pressure directly north of upper low pressure, both cut off from the main jet flow (white arrows)

Here’s the current satellite water vapor image showing the developing Rex Block —

Current satellite water vapor image 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. (Click on image for a larger view.)

Originally Posted Fri 7:04 PM — A cold front with its dynamics and showers to our south will sink further to our south Friday night.

Another cold front is waiting in the wings for Sunday morning Here’s the current water vapor satellite image —

Current satellite water vapor image 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. Cold front to our south and another in the Great Lakes area. (Click on image for a larger view.)

High pressure will build in for Saturday. With the NBM recently running too cold, I’ll give the HRDPS a chance in the temperature forecast.

Saturday

Mostly Sunny.

HRDPS high temperatures: Blue Bell, PA 63ºº Philadelphia, PA 65º

The second cold front moves through Sunday morning.

Sunday

Several periods of cloudiness in the morning with the second front, becoming partly sunny in the afternoon. Increasingly WINDY and gusty. Temperatures decreasing in the mid afternoon.

HRDPS high temperatures: Blue Bell, PA 60º Philadelphia, PA 62º

The current trend is for colder temperatures next week through Wednesday. A generally dry period with no storms expected.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

Sunday Forecast Update

Posted Sunday 03/10/24 @ 5:54 PM — As forecast, rain and snow squalls are moving through now. Upper low is just north of us —

RADAR with RAP model overlay – mid level low just north of us. Snow squalls and rain squalls passing through (Click on image for a larger view.)

Posted Sunday 03/10/24 @ 4:40 PM — Here’s the MRMS tally of Saturday’s heavy rainfall—

MRMS combined rain gauge & radar-based measurement of actual rainfall received. Color shading is in inches. Line contour numbers are in mm (25.4 mm= 1 inch) Most areas west of the Delaware received about 1- 1.4″ (Click on image for a larger view.)

Posted Sunday 03/10/24 @ 10:26 AM — I had hoped to have a MRMS estimate of the rainfall we received last night. Unfortunately, the server I access for that info is down today; I’ll do that tomorrow.

Increasing winds and an increasing chance of light showers/squalls as the day progresses. Max probability is between 3 and 8 PM this evening as the upper low moves over us.

NAM-NEST forecast showing mid level low (L) over our area at 6 PM. Tightly packed isobars (black contours) show rapid gradient in pressure change responsible for the winds. (Click on image for a larger view.)

Winds/Gusts

NAM-NEST winds and wind gust meteogram for location Blue Bell, PA (Click on image for a larger view.)
Rain Update

Posted Saturday 03/09/24 @ 8:06 PM — The heaviest rain has moved in this evening and it is expected to taper off from southwest to northeast between 10:30 PM and midnight.

The surface low will be directly over us around this time and wind should diminish for about an hour or two. (Almost like being in the eye of a hurricane.) The winds will pick up and become gusty again. The windy conditions should continue through Sunday.

GFS surface forecast at 8 PM EDT Sunday evening. Upper (mid level) low is over NYC. Squalls with showers move through Pennsylvania. (Click on image for a larger view.)

Sunday is looking more and more unsettled, with plenty of clouds and wind. Wind gusts 40-45 mph. Some breaks of sun also. Intermittent light showers are possible, especially Sunday afternoon as the upper level low moves over us.

High temperatures will plateau around noon or 1 PM in the upper 40s and then drop during the afternoon. Continued fast moving light showers and even some squalls possible late afternoon. We may even see some snow flurries mixed in with the rain showers about 6-7 PM!

Saturday Rain Update

Posted Saturday 03/09/24 @ 9:35 AM — Last night’s models show little change in the storm forecast. Rain is showing already on radar, but much is not reaching the ground. A more significant batch of rain is quickly moving in from the southwest —

03-09-24 RADAR 9:22 AM Light precip not reaching the ground in many areas. Heavier rain just passing the Maryland – Pennsylvania border is moving fast and will be here shortly. (Click on image for a larger view.)

Satellite water vapor imagery shows the main event—

Current 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. I’ve drawn in the cold front, expected to reach us 2 AM Sunday, accompanied by strong winds. (Click on image for a larger view.)

Forecast rainfall totals have shown good continuity in recent model runs. Here’s the latest HREF ensemble forecast —

03-09-24 06z HREF total rainfall forecast. (Click on image for a larger view.)

Update Fri 3/08 10:07 PM — Tonight’s models continue with a trend towards an earlier start to the rain on Saturday. Scattered light sprinkles or drizzle may begin as early as 7- 8 AM, according to tonight’s NAM-NEST.


For Sunday there’s a chance of light widely scattered sprinkles with considerable cloudiness. High temperatures will be reached early in the day and temperatures will fall in the afternoon.

Previously Posted Fri 6:17 PM —

A strong low pressure system with plenty of moisture will move directly over Philadelphia Saturday night. The latest models are on-board with heavy rain especially in the evening.

As posted this morning, the energies of two jet streams will be coming together over our area—

GFS jet stream wind forecast for 9 PM Saturday. Highly cyclonic northern jet with strong southern jet. Faster areas in the jet stream are called “jet streaks”. The red box is an area called the ‘left exit region’ of the jet streak , an area of maximal upward motion. Heavy rain is most likely in this area, which will be just northwest of the Philadelphia region. Small changes in the position of the jet streak can result in major changes in forecast rainfall. (Click on image for a larger view.)

This energy will result in low pressure intensifying as it moves directly over Philadelphia Saturday night —

This afternoon’s GFS (03-08-24 18z) shows low pressure and heavy precip in the left exit region of the jet streak at midnight Saturday. (Click on image for a larger view.)

The NBM is cranking out high rainfall amounts with a fairly high chance of thunderstorms about 10 PM —

A cold front moves through about midnight. Sunday will be somewhat colder and very WINDY.

Saturday

Cloudy in the early morning. The latest models continue a trend of an earlier start of the rain. It now appears that it may start as early as 10-11 AM, earlier in western suburbs. Heaviest rain as we approach midnight. Increasing winds and a chance of thunderstorms about 10 PM.

NBM high temperatures: Blue Bell, PA 52º Philadelphia, PA 54º
Moderately high uncertainty (based on standard deviation): ± 3º

Sunday

Considerably cloudy and increasingly windy.

NBM high temperatures: Blue Bell, PA 49º Philadelphia, PA 51º
average uncertainty (based on standard deviation): ± 1.8º