Update Thu @ 7:49 AM — A front will move through between 2 PM and 5 PM today. The front is clearly visible on current satellite water vapor imagery—
Water Vapor at 7:41 AM shows distinct front moving through central PA this morning. This front will move through here later this afternoon. The front represents more of a dew point change/wind shift than a temperature change. (Click on image for a larger view.)
Some models have a few very widelyscattered showers and thunderstorms between 2-3 PM around Philadelphia and about 5 PM into NJ. Little in the way of accumulating rain is expected.
Today’s HREF forecast for rain accumulation 2-3 PM Thursday. This may be over-predicting the amount of showers. (0.02-0.20 inches of rain) (Click on image for a larger view.)
The frontal passage will bring lower dew points (less humidity) later this afternoon and evening. Higher humidity gradually returns by Sunday. The heat continues.
NBM meteogram for Blue Bell, PA temperature dew point and “heat index” (apparent temperature) through Sunday. Note that the dew points are lower Friday and Saturday. (Click on image for a larger view.)
Update Wed @ 8:09 AM — Temperature- dew point meteogram says it all—
NBM meteogram (temperature dew point, max temp and heat index (apparent temp) with standard deviations for Blue Bell PA (Click on image for a larger view.)07z NBM forecast Apparent Temperature (“heat index”) for Thursday at 2:55 PM. (Click on image for a larger view.)
The extended 90º+ stretch we’re entering into has been well-advertised by the media.
Current satellite water vapor image shows plenty of hot air streaming up from the southwest—
Hot air (orange-red) “conveyor belt” with heat and humidity streaming up from the southwest. Stalled frontal boundary (1) just to our south (Keep in mind that water vapor imagery is NOT showing reflected light from clouds. Water Vapor shows the temperature (and therefore the height) of the very top layer of substantial water vapor. In effect, it shows a three dimensional view of the atmosphere. Cold temperatures, shown in blue and green, indicate rising air — vertical motion. (Click on image for a larger view.)
This week’s jet stream is typical for a summer pattern, albeit the temperatures are well above seasonal averages. Our area has been in a consistent, slight upper trough since May and this trough and the current pattern is forecast to remain unchanged for the next 10 days—
GEFS 250 mb jet stream winds on Thursday. Slight upper trough over the Northeast and the hot dry H over the southwest has been with us for a few months/
The big item for us will be the heat—
NBM meteogram temperature/dew point/heat index for Blue Bell PA. Philadelphia is generally 2s-3º hotter. “Apparent Temperature” is the blue line (heat index) Dew points (magenta lines) are in the upper 60s to near 70. (Click on image for a larger view.)
There’s a chance of thunderstorms Thursday evening as a weak front move through.
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.
