Updated Sat 11:16 PM — Some models are predicting as much as 0.6”-1.1” of rain Sunday night into Monday morning.
Updated Sat 9:53 PM —A warm front had been expected to move in late Sunday night. Things have speeded up a bit. The latest models show clouds moving in as early as 1-2 PM Sunday and becoming mostly cloudy by late afternoon. Some showers develop early evening.
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There’s a chance of some light accumulating rain early Monday morning. Updated Fri 10:52 PM — Tonight’s early models show the following minor forecast changes— Saturday’s high temperatures will be warmer, just under 90°. Originally Posted Fri 6:01 PM — A front moves through later tonight. As forecast in my morning update, very little rain is expected tonight in the immediate Philadelphia area; most of the light rain will move off to the northwest, affecting northwest suburbs, while another disturbance brings light rain to NJ. We may get some light showers this evening and again about 3 AM. Clearing by Saturday as high pressure builds in for the weekend. Not many chances of rain in the coming week. Very strong upper level high pressure builds in after Wednesday bringing very hot and dry weather for us. We may be seeing 100º temperatures again. Sunny and warm. High 86.6º sd 1.0º NBM model Blue Bell, PA Sunny and warmer. Some increasing cloudiness during the mid to late afternoon as a warm front will be approaching Sunday night. High 87.8º sd 2.8º NBM model Blue Bell, PA
Sunday has some significant mid level cloudiness developing during the afternoon with some light scattered showers in the evening south of the city.
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Sunday
Update Fri 07/29 @ 7:44 AM — Last night, our area received varying amounts of much-needed rain. Here’s the amounts we received Thursday evening according to the MRMS—
For this evening (Fri), the models are less enthusiastic about additional rain. The HRRR has much of the area with just spotty amounts, many areas with no rain. The HREF forecasts some rain, but scattered with light amounts. Not a drought-buster this evening.
The HREF has any showers/thunderstorms beginning this evening around 5-6 PM (a few isolated showers before then) and continuing through 11 PM or so.
HREF shows light, scattered showers—
My Weekend Weather Forecast will be posted this evening.
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Much more [needed] rain than had been forecast and some of the storms are blossoming in Philadelphia. The activity is still expected to dye off as it moves into NJ as vertical shear values drop off significantly. Update Thu @ 9:14 AM — We did have some showers shear off to the northwest last night, as some models had forecast— The models are advertising another round of showers and storms Thursday evening (tonight) about 6 PM- 9 PM. These showers are more likely to make it through more of Philadelphia before they diminish over NJ. Latest HREF forecast for 7 PM this evening— I am a bit more confident that some accumulating rain will make it into Philadelphia and immediate areas, although amounts may be small, variable and very non-uniform. Why am I more confident? First, several models have this forecast, including the model blend (NBM). A new model under development (FV3 LAMDAX) correctly predicted last night’s showers and it is predicting the showers making it into Philadelphia. Predicted vertical shear values are somewhat higher tonight in our immediate area as well. Finally, the 576 thickness line is flat and is no longer anticyclonically curved in our area as has been the case for several weeks now — Working against the chance of rain is the continued low soil moisture and the relatively weak vertical forcing of the trigger for tonight. Update Wed @ 8:13 PM — Not much happening. The dry spell continues and the models are over-forecasting rain in western PA as I write this. Several models have some showers developing in our area around and after midnight. Current water vapor imagery shows weak disturbances to our west— While some models have showers for us between midnight and 5 AM, the HRRR has them falling apart as they approach Philadelphia. I wish I could be confident in forecasting another 0.10 inches of rain tonight, but I’m not. The dry spell (perhaps “mini drought”) has played havoc with the model forecasts over past weeks. Rainfall has been consistently over-forecast by the models. There’s a front moving through Friday afternoon to clear out the weekend. The latest GFS has little to no rain for the Philadelphia area, while forecasting some for western areas and parts of NJ. Updated Tue 9:46 PM — Tonight’s models keep any light showers well to our south. The dryness continues. Revised Update Tue @ 8:29 PM — A weak wave in West Virginia along the stalled front will give us a chance of very light scattered showers towards daybreak Wednesday especially south of the city— Very light precip (0.10 inches or less) is expected with this disturbance. Again, most of the rain will be south of us. Another may affect us early Thursday morning. Update Tue @ 9:30 AM —There’s a stalled (stationary) front to our south. Low pressure waves will develop in the Midwest will move along it to our south. Precipitation from one wave will mostly pass to our south early Wednesday morning— Update Mon @ 8:37 PM —The front that passed through Monday afternoon will stall to our south. Low pressure is expected to develop along this front and bring a chance of rain late Wednesday evening into Thursday morning. Not a sure thing, since the forecast for this time period has changed several times. Update Mon @ 5:48 PM — Most areas did not get very much rain today. In this pattern, “dry begets dry”. The current MRMS shows the actual rainfall we received in the area— It will take a large storm (hurricane or tropical storm) to change the current pattern and break us out of this dry spell. Revised Update Mon @ 11:20 AM — Latest NAM-NEST has a line of storms moving through 3:30-5 PM. Cranks out 0.8+ inches of rain in some areas. Wouldn’t that be good! The HIRESW has the line moving through as early as 2 PM Update Mon @ 9:30 AM — Based on the latest ECMWF model just available, I’m concerned that we will be disappointed with the amount of rain we get today, despite the forecast for heavy thunderstorms. The ECMWF has only 0.2 inches of rain falling (Blue Bell) It’s rare that I hope I’m wrong about a forecast. This is one of those times. Updated Mon 8:45 AM —Last night’s models from 2 AM (06z) explain last night’s uncertainties and differences in the timing of the showers and storms today. The showers/storms (if any) are forecast to come in two batches. The first batch (as forecast by last night’s RAP and ECMWF models) will come through scattered at 1-2 PM. The front is expected to slow down and eventually become stationary after it slowly moves through. The second batch will result in weak low pressure developing along the slowing front, with more showers and storms between 7 PM and 11 PM. The 06z NAM-NEST shows rainfall amounts as little as 0.4 inches of rain and as much as 1.5 inches of rain (in some lucky areas). This type of two batch forecast occurred with a frontal passage in June, but the second batch was a dud, leaving us wanting for more rain. Updated Sun 11:02 PM —Tonight’s models are having a rough time with tomorrow’s forecast. The RAP and ECMWF have showers as early as 1 PM. The NAM-NEST has showers and thunderstorms as late as 7-8 PM. The three versions of the HIRESW are all different; the HIRESW-FV3 has no rain for immediate Philadelphia area, with all showers passing to our north and south. I’m leaning towards showers/thunderstorms between 2 and 6 PM with rainfall amounts on the low side. A low confidence forecast leaning towards the HRRR and HIRESW-FV3. We need the rain. I hope I’m wrong. Originally Posted Sun 8:24 PM — For several weeks now, I’ve noticed a trend towards very dry conditions here. I first discussed it here and then put together a more complete discussion here. I’m hoping I’m wrong about this trend. Tomorrow, Monday, may put it to the test. A cold front will move through between 2 and 7 PM Monday. Based on the usual parameters of thunderstorm development, tomorrow’s setup would ordinarily provide some heavy thunderstorms and heavy rain. We have an approaching cold front, high CAPE values (~2800-3000 j/kg), high PWAT values (2.1″) and decent areas of vertical lift. What’s missing is medium to high shear and reasonable soil moisture values (which are thought to provide positive feedback for thunderstorms.) The models are predicting unimpressive accumulated rain for this frontal passage. The forecast average quantity of precipitation falling is only about 0.3-0.6 inches of rain. No torrents of rain here tomorrow. Even more concerning…we may have even less rain that that. This afternoon’s HRRR and Canadian models have very little rain falling in the immediate PHL area— Some models have the ridge eroding enough for showers to pick up in NJ. Some have the entire area of rain diminishing as it moves into Philadelphia and eastward. I guess we’ll see tomorrow
Thu @ 8:50 PM —Forecast Review— The storms tonight came in fairly close to the time forecast of 6-9 PM. (This morning’s forecast had been based on the HREF and the 06z HRRR.)
Update Sun @ 9:54 AM — New 12z NBM model out continues with the somewhat lower temperatures for Sunday—
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Updated Sun 8:37 AM — Last night’s model blend (NBM) shows high temperatures will not reach the extreme highs forecast yesterday. It appears that highs will be 96-98º with some areas just reaching 99º. Nonetheless, dew points will be higher and heat indexes still forecast to be over 100. Update Sat @ 6:18 PM — Today’s high temperatures were inline with this morning’s NBM temperature forecasts. Sunday appears to be on track for extremely hot weather. The latest NBM high temperature— And heat index (apparent temperature)— Of interest— some models are showing some widely scattered showers and thunderstorms Sunday evening. Update Sat @ 10:17 AM — The latest NBM model forecast high temperatures have trended downward slightly for today, Saturday, from previous model runs— Even heat indexes have trended downward for Saturday, as dew points will also be somewhat lower. I don’t see any 100 heat indexes here today . No change in Sunday’s extreme heat. In fact, the NBM shows higher heat indexes >100 more widespread for Sunday. Update Fri @ 10:08 PM —Updated the Rain Outlook for Monday A continued hot and increasingly moist flow of air will be with us for the weekend. Little chance of rain in our area until later on Monday (more about that later.) Current model jet stream position— Current water vapor imagery (Friday late afternoon) — A southwesterly flow will bring heat and increasing humidity. Sunny and hot. High temperature 97.2º ± 1.2º NBM model Blue Bell, PA Dew points will be still be moderate, despite the temperatures, although higher than Friday— Sunday will continue to be extremely hot and increasingly humid. I expect to see actual temperatures reach 100º+ in several areas. I’m usually not ‘into’ the heat index (what meteorologists call the “apparent temperature”) since it sensationalizes things and I think is confusing. (If a 100 degree temperature always feels different than 100º, how do you know what a 100 degree temperature really feels like? ….And I hate when TV weather people start talking about temperature and then they talk about heat index and then talk about temperature. Too sensational and too confusing. But don’t get me started. ) Anyhow, the heat index for Sunday is forecast to be impressive. All areas will feel like it’s over 100 (whatever that means. I think it means it will feel unusually hot.) Heat Index for Sunday— Just a teaser…we need the rain and the first chance is Monday late afternoon and evening. Will we see this fall apart as it has for several past forecast rains? More about that over the weekend. Rain Outlook for Monday— Latest models suggest a repeat of a low rain amount event for Monday evening. As has been the case for weeks now, the front will run into a wedge of very dry, hot stable air. Total rainfall may be less than 0.3- 0.5 inches, much less than we need. Updated Fri 10:52 PM — Tonight’s NAM shows heavy rain for late Monday ( over 2 inches). I’m not sure I believe it.
Forecast Review— Actual high temp recorded at Philadelphia Airport on Saturday was 98º and heat index was 100º 
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Sunday
Rain outlook for Monday
