We have a very wet and unstable weather pattern setting up for much of this week. Thunderstorms, some severe, are possible most days of the upcoming week.
Right now (Monday early evening), a severe line of thunderstorms is affecting Washington DC. (Possibly adding more water to the “swamp” there.)
In all seriousness, the storms approaching DC are extremely fast moving (excess of 60 mph), appear as a ‘bow echo’ but almost take the form of something called a derecho. I don’t usually focus on weather outside the PHL area, but this radar image is impressive, with likely tornados. We may have similar weather late tomorrow afternoon.
So far, the forecast is going well. The showers and thunderstorms have moved east and there should be a significant brightening of skies mid to late morning. Still not clear how much sunshine we’ll get, but the cloud cover should thin considerably.
Expect dry conditions for much of today. High temperature 77 with increasingly humid conditions.
A wave of low pressure develops just south of PHL late this afternoon and this evening. Considerable instability will result in showers and thunderstorms, some heavy. Expect the showers and thunderstorms to move in from the west between 5 and 7 PM and continue through the evening.
This weekend’s Philadelphia weather forecast will be lower confidence than usual. As mentioned, a frontal boundary moves south and stalls as waves of low pressure move along the front. The waves of low pressure may make the boundary quasi-stationary causing the boundary to undulate slightly north and south with each wave.
The timing of the waves and the exact placement of the front with each wave will determine our sensible weather. There remain differences in timing with the GFS and NAM models. I’m going to blend the two to attempt a forecast with an emphasis on the NAM.
(Last week’s GFS over-predicted precipitation and it resulted in a poor forecast for both days; last week’s weather was forecast better by the NAM.)
Saturday starts damp and cloudy with possible showers ending early. Expect skies to brighten mid-day and early afternoon, with even some sunshine possible. Light winds and increasingly humid. Showers and thunderstorms develop later in the afternoon and continue into the evening hours. Some of the showers may be heavy. High 82
NAM forecast 8 PM Saturday
The front sinks to our south Saturday night.
On Sunday as an easterly flow develops. Cloudy and cool conditions (High 67) but not too wet, according to the GFS, but the National Blend of Models (NBM) and NAM maintains a good chance of showers during the day Sunday and especially Sunday late afternoon into evening.
