Update Mon @ 7:51 PM — Weak thunderstorms that began in Chester county mid afternoon have moved up and diminished significantly in strength—
Current radar Monday at 7:49 PM. Line of weak storms moved through and are diminishing in intensity, as forecast by several models. Some additional activity may affect some areas before the front moves through tonight. (Areas moving up from Delaware into NJ have some advantage to staying together, passing over the Delaware Bay.) This sort of system will not make much of a dent in our dry weather. (Click on image for a larger view.)
Updated Mon 1:30 PM — A quick mobile update. There are 3 versions of the HIRESW model. The HIRESW-FV3 only shows one line of storms about 5-6 PM, closer to the NAM-NEST forecast. The earlier update based on the HIRESW-ARW is clearly not correct.
Updated Mon 11:14 AM — A quick mobile update. The HIREW model just available shows two lines of storms one moving through early afternoon and another about 7 PM
Update Mon @ 9:25 AM — This morning’s models aren’t available at the time of this update. There’s still a range of forecasts from the models. Some show storms developing in a more random fashion. Some show a distinct line moving through this evening.
The best forecast, based on current radar, water vapor and the NBM model suggests storms develop in mid afternoon western areas of Chester and Montgomery counties and gradually move eastward, arriving in the immediate PHL area between 6 and 9 PM.
With vertical wind shear values relatively low, I don’t see severe storms in our immediate area, despite very high CAPE values.
There is evidence that the storms may diminish in intensity just as they arrive in Philadelphia. Areas from Philadelphia and eastward may receive much lower amounts of needed rain.
I’ll be posting more about our dry current spell and some theories about it later this evening.
Previously Posted Sun 11:13 PM —
A complicated forecast for tonight into Monday as the models have a wide range of forecasts. Some show a chance of showers tonight into early morning Monday, while others have very little in the way of precipitation tonight.
Several high resolution models show scattered storms during the day, as early as 2 PM, with additional storms late afternoon and evening. The NAM has another line of storms around 11 PM.
So, the bottom line is an unsettled day with several chances of storms with locally heavy rain any time after 2 PM.
Adding to the uncertainty is some model suggestion that storms may fall apart as they move east into NJ.
Updated Sun 4:38 PM — We’ve had no showers this afternoon (as forecast by the NAM-NEST, not the ECMWF). There may still may be some scattered activity late tonight. The main area of showers will move off to our north tonight.
The cold front comes through early evening Monday. The NAM-NEST shows a line of storms, but they may diminish in intensity as they move through our area. Updates tomorrow.
Update Sun @ 10:14 AM —
Forecast Review— We had some scattered storms after midnight, not forecast by last night’s US models. Still not much rain in the immediate PHL area—
MRMS rainfall totals. The immediate PHL area received anywhere from 0.10″ to 0.44″, with a few localized exceptions. (Click on image for a larger view.)
Sunday will be partly sunny/partly cloudy very warm and humid. High 86º ± 1.1º Dew points near 70º will make it feel much warmer. (~90º)
Sunday satellite water vapor image shows current conditions: warm front in western PA that will move north. Some models have most of the showers with this warm front moving north of our area tonight. Some models have some storms 3-5 PM this afternoon ahead of the front.
A warm front will move through around midnight with showers and thunderstorms. Most models have the bulk of the rain moving to our north. The ECMWF shows some storms developing around the Delaware Valley abut 3-5 PM or so, ahead of the warm front passage this evening.
The ECMWF 3 previous hour rainfall forecast for 5 PM —
ECMWF forecast 3 hour rainfall at 5 PM today. Notice how the rainfall seems to bypass Philadelphia! (Click on image for a larger view.)
It should be noted that the NAM-NEST shows no showers here this afternoon. I’m including the ECMWF graphic because the ECMWF more correctly forecast the showers last night.
Monday afternoon shows the greatest promise of showers and thunderstorms here.
Updated Sat 11:11 PM —The rain most areas had today was minimal. For Sunday, there will be even less chance of rain.
Temperatures approaching 88° will feel hotter with dew points near 70°. There’s a chance of highly isolated showers around 6 PM. Any rain Sunday night is forecast to move north of us. We’ll have to wait for Monday late afternoon for any chance of significant rain. Even Monday may not offer relief from the dry conditions.
Update Sat @ 9:56 AM — Most models have no rain before 6 PM in the immediate PHL area. The European, ICON and HRDPS group of models maintain a dry area around PHL until about midnight. The latest RAP (12z) and HRRR (12z) show areas of showers and thunderstorms during the evening hours.
12z RAP shows no rain in the immediate PHL area before 6 PM (Click on image for a larger view.)12z HRRR shows showers/thunderstorms scattered around 11 PM.
Despite these forecasts, there is little indication that the dry spell we’re in will be ending today or Sunday.
Updated Fri 11:09 PM — Tonight’s NAM-NEST shows some impressive areas of thunderstorms around 5 PM.
NAM-NEST showers 5 PM Saturday.
Updated Fri 9:53 PM — Tonight’s early models are forecasting showers and thunderstorms Saturday evening. We’ll see if they’re correct; we could use the rain.
Previously Posted Fri 8:21 PM —
Low pressure in the Midwest will have mid-level and upper air disturbances move out ahead of the low.
Water Vapor image at 8 PM Friday. Upper and surface low over Michigan. (Click on image for a larger view.)
Ordinarily, summertime moisture will result in scattered thunderstorms. It doesn’t look like much for us.
I think it’s time to use the “d” word— it’s looking like there’s a mini drought situation developing in this area. It seems as though we’ve run into a situation where soil moisture levels have been reduced to low levels. A negative feedback loop can develop with thunderstorms where ‘dry begets dry’.
The last time I saw this in our region was back in 2000 and 2001 when very little rain had fallen. Radar images were similar…rain approaching from the west would appear to hit a wall and fall apart.
It often takes something like a strong tropical storm or hurricane to shake up the weather pattern. Nothing like this on the horizon yet.
But I digress….let’s talk about the weekend.
Saturday
Some upper air troughing will result in periods of cloudiness and sun, similar to what we had on Friday. Most of the day dry, although some very isolated popup storms may develop late afternoon. While some models have some rain moving in from the west during the evening, it appears that this will fall apart as it approaches the Delaware Valley.
High temperature 86.0º ± 2.2º NBM model Blue Bell, PA
Sunday
Previously Posted Fri 8:21 PM —
Some low clouds, maybe fog, in the morning. Perhaps a few sprinkles early. Most of the day will be a mix of sun and clouds. Little or no rain until evening.
GFS forecast for Sunday at 2 PM. Not much rain. (Click on image for a larger view.)
In fact, the latest NAM-NEST shows heavy storms in far western suburbs about 11 PM, falling apart as they try to move east towards us.
High temperature 86.5º ± 2.6º NBM model Blue Bell, PA
