Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Friday and the Weekend

Posted Thursday 04/04/24 @ 5:35 PM — Showers developed in Delaware and Chester counties early Thursday afternoon and moved northeastward. The clouds and showers are the result of instability from cold air aloft and a cyclonic flow around the deep low pressure system that affected our weather earlier this week.

NAEFS forecast for Friday at 11 AM. Deep low pressure over Maine will continue to rotate cold, unstable air over us. (Click on image for a larger view.)

For Friday, the cold cyclonic flow will continue with sunshine in the morning and some sun with instability cloudiness in the afternoon. Showers may wait until evening to move in from the northwest. High upper 40s to about 50º

For Saturday, the effects of the deep low will slowly diminish. A mix of sun and clouds but a light sprinkle can’t be ruled out Saturday in areas far northwest of the city. Gradually moderating temperatures.

For Sunday, more sun than clouds, but some instability cloudiness still possible.

Wednesday night through Thursday

Posted Wednesday 04/03/24 @ 5:30 PM — We’ve had a lot of rain and we’re not quite done with it. Rainfall over the past 72 hours has been in the 2-4 + inch range —

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.)

More showers tonight with the actual cold front moving through around midnight.

We may actually see some a bit of sun (with plenty of clouds) Thursday morning, but an upper air low will move directly over us resulting in thicker clouds and scattered showers developing early to mid afternoon—

HREF forecast 1 hour rainfall at 4 PM Thursday. (Click on image for a larger view.)

A similar scenario for Friday.

Wednesday Looking less Wild

Posted Wednesday 04/03/24 @ 9:02 AM — Last night’s models have backed off on the northern extent of the approaching warm front. That means that thermodynamic energy/instabliity (CAPE) will be significantly less than had been forecast, making thunderstorms and tornadoes less likely.

Vertical Shear and Helicity values remain highly elevated from 11 AM through the early afternoon. Since CAPE values are expected to be low, it’s unclear how that energy will come together into sensible weather. I would expect strong wind gusts during that period, along with heavy rainfall.

04-03-24 RAP model with current MRMS radar overlay at 9 AM. Very high upward vertical motion (yellow contours “Omega”) with blue contours (moisture convergence) will combine to produce heavy rainfall and strong wind gusts. (Click on image for a larger view.)
Wild Weather Wednesday

Update Tue 4/02 9:58 PM — Tonight’s models are increasingly forecasting the possibility of some severe weather Wednesday afternoon (3-4 PM) into the evening. This is especially true of areas from Philadelphia southward. Extremely high shear and helicity values coupled with moderate CAPE may allow some severe thunderstorms to form. High winds expected. Even tornadic activity is possible. Stay tuned.

Wednesday-Thursday Outlook

Posted Tuesday 04/02/24 @ 4:40 PM — I’ve been “doing the weather” for more years than I want to admit, but I don’t recall as many days of rain where 1-3″ accumulation has become the norm.

I’ve been able to access the MRMS rain accumulation data for the past two days, through 3 PM. (I’m not sure how accurate it is since the radar for Philadelphia, located in Fort Dix NJ, has been down for repairs.) —

MRMS combined rain gauge & radar-based measurement of actual rainfall received on Monday and today, Tuesday through 3 PM. Color shading is in inches. Line contour numbers are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)

Additional rainfall, probably the heaviest of this week, will be on Wednesday as a warm front moves across. Perhaps another 1.5- 2″!

We had some low level thunderstorm activity Tuesday afternoon and the possibility exists for additional thunderstorm activity on Wednesday and especially Wednesday evening when thunderstorms may be moderate to strong as the cold front moves through.

It would not surprise me if a severe thunderstorm watch is issued for the period 5 PM through 10 PM Wednesday.

The latest water vapor image shows the current weather well—

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. I’ve drawn in the warm front and cold front and indicated the influx of cold air behind the cold front (blue arrow) and the moist flow ahead (orange arrow) (Click on image for a larger view.)

Look for updates Wednesday morning if severe weather looks more likely.

Monday into Tuesday

Posted Monday 04/01/24 @ 7:50 PM — The forecast is unchanged. The latest GFS (which tends to slightly under-forecast rainfall) has 3 plus inches of rain by late Thursday.

Tuesday– An additional 0.8 -1″ of rain expected, especially north of the city.

Wednesday— Even heavier rainfall on Wednesday into Thursday.

By Friday—

04-01-24 18z GFS accumulated additional rain forecast from Monday night through 2 AM Friday. (Does not include Monday’s rainfall) (Click on image for a larger view.)

I’d like to post today’s rainfall, but the server I connect to is down.

Posted Monday 04/01/24 @ 9:45 AM — Last night’s models continued with a little-changed forecast. Several low pressure systems will move through our area from today (Monday) through Thursday, culminating in a closed low over Ohio and a developing Nor’easter for New England off of our coast by Thursday morning—

04-01-24 00z ECMWF total accumulated rain forecast from Monday through 5 AM Thursday morning. (3.2″ in our area) The mid-level low (upper low) in L light blue will still be in Ohio on Thursday. As it tracks over our area Friday, we’ll still run the chance of showers. (Click on image for a larger view.)

The trend has been for slightly higher rainfall totals in our area (now over 3″ expected by Thursday.

As is almost always the case, the upper low will trail the surface low. The upper low will track over our area on Friday, still with a chance of showers and considerable clouds at times.

Originally Posted Sun 7:49 PM —A series of low pressure systems will move across the country into our area over the next few days. The latest GFS shows these disturbances—

Today’s 18z GFS forecast for 11 AM Monday shows several low pressure systems to affect our area this week. The first system will move somewhat south of us on Monday, with the heaviest rain staying to our south. (Click on image for a larger view.)

The first of these systems will bring rain to our area Monday. The trend has been for the heaviest rain to remain to our south. Rain starts about daybreak and continues through about 2-3 PM. Most of the immediate PHL area will get only 0.25″ to 0.35″ of rain, but areas in southern Chester and Delaware counties will have heavier rainfall.

The second two systems will merge into a deeper low that moves to the Great Lakes, bringing heavier rain Tuesday.

This same low will spawn a coastal low bringing rain again on Wednesday. An upper closed low will slow the departure of the Great Lakes low.

GEFS forecast for 5 PM on Wednesday. Coastal low shown. Great Lakes low with upper closed low (lighter blue) will maintain windy conditions through Friday. (Click on image for a larger view.)

A cold front moves through early Thursday.