A cold front will slowly approach from the west later tonight. The front will begin to bring shower chances along with a few thunderstorms to the Lehigh Valley, Poconos, and Lancaster area towards dawn.
On Monday, the cold front will slowly move closer to the region. At some point, it is expected to become stationary and stall out. Where it does so will be critical in determining the axis of heavier rain for Monday Night and most of Tuesday as waves of low pressure will move along the front causing an enhancement in the shower and thunderstorm activity along the boundary.
At this point, Monday could be fairly gray and showery in the western suburbs and it could start off with mostly cloudy skies with some peaks of sun closer to the coast. However, deteriorating conditions are expected throughout the day. Monday Night and Tuesday look fairly wet for everyone, especially along the immediate boundary. A widespread one to three inches of rain is anticipated. Thunderstorms, especially those that train over the same areas (which is increasingly likely at this point for the upcoming event) may produce over three inches of rain in localized areas. Any areas that receive three inches or more of rain will be prone to flash flooding, especially around smaller creeks and streams...poor drainage...fields....and low spots. Depending on where this occurs, an isolated larger creek may go above flood stage.
Severe weather will be somewhat limited due to the extensive cloudiness keeping the instability in check. With shear values increasing during the event and a very moist atmosphere...the potential does exist for one or two wet microbursts of damaging wind. However, the primary concern appears to be the potential for heavy rainfall.
On Wednesday, a secondary cold front will arrive in the region. This will bring the chance for scattered showers and thunderstorms. With a little more in the way of sun to heat the atmosphere, Wednesday may bring the best chance for any strong to severe thunderstorms.
For the weekend, an onshore flow will bring a lot of clouds our way with the potential for low clouds and drizzle...especially closer to the coast. With this still far out, the temperatures currently reflect a more optimistic outlook....but they could end up being much cooler.