Flood Watch for Sunday – Remnants of Bill to Bring Heavy Rain
Widespread 1 to 2 inch rain totals are expected into today, with higher amounts and flooding also possible.
UPDATE as of 11:15 PM on Sat 6/20...
Moderate to heavy rain is arriving from west to east, and we're still looking at a prolonged period of steady, driving rain through the overnight hours. I'm not overly concerned about severe weather at this point - the heaviest bands of rain with the most energy have fizzled a bit, and the closest Severe T-Storm Warning (as of this writing) is way down in Virginia. Regardless, any outdoor activities are about to come to a rapid halt, and driving tonight could become difficult or even dangerous. A Flood Watch remains in effect until Sunday evening (unless cancelled early by the National Weather Service).
The good news is that the latest model runs have sped up the ultimate departure of the rain Sunday morning. I am in fact becoming increasingly optimistic that we'll see some breaks in the wet weather and even the clouds for Father's Day. We have said all week that Sunday would not be a total "washout"... and it really looks like at least part of New Jersey will see a period of decent (or at least dry) weather by the afternoon.
Best chance of dry skies will come after about 10 a.m. Keep an eye open Sunday evening though, as the backside of this system could bring some additional showers to close out the weekend.
UPDATE as of 9:00 PM on Sat 6/20...
The National Weather Service and Storm Prediction Center have issued a Severe Thunderstorm Watch for Mercer, Ocean, Burlington, Camden, Gloucester, Salem, Cumberland, Atlantic, and Cape May counties until 1:00 a.m. Sunday morning.
A line of strong thunderstorms is approaching from the west - as of this writing, the front of that line is 80 miles west of the Delaware River. These potentially dangerous storms may produce gusty winds, hail, frequent lightning, and torrential downpours.
A Watch is simply a "heads-up" that severe weather is possible - it's not a guarantee that thunderstorms will occur, nor an indication that severe weather is occurring. Be cautious and be smart as these storms come through, especially if you're out and about tonight. The main threat of heavy rain and flooding will continue through the overnight and early morning hours.
UPDATE as of 5:30 PM on Sat 6/20...
Saturday turned out all right for much of central and southern New Jersey, with breaks of sun and some highs in the 80s. Clouds are now thickening up, and we've been watching some widely scattered showers and thunderstorms pop up as well. So far, the worst of the weather has been well to the southwest of New Jersey.
After reviewing the evening forecast model package, we are holding steady on the very wet forecast for tonight. Widespread 1+ inch rainfall are expected, with 3+ inches amounts certainly possible.
Keep reading for more detail on what to expect in terms of rain, and the potential flooding and severe weather threat...
ORIGINAL POST from 10:00 AM on Sat 6/20...
I've been saying it here in my weather blog all week... Never underestimate the deep moisture of a storm of tropical origin. As the remnants of Tropical Depression Bill inch closer to New Jersey, it's becoming increasingly likely that New Jersey will experience very heavy rain for Father's Day morning. The National Weather Service has posted a Flood Watch for the entire state for Saturday night and Sunday.
Here is everything you need to know about this weekend's soggy, stormy forecast...
So far, Saturday morning has been fairly pleasant - mostly cloudy skies, comfortable temperatures, and just a few spot showers. As the day goes forward, clouds will progressively thicken and rain chances will increase, especially through the late afternoon and evening. Aside from a few localized torrential downpours perhaps, widespread heavy rain is NOT expected during the daytime hours on Saturday. Although there is also a slight chance of a stronger thunderstorm popping up today with gusty winds or small hail, you should be able to go through your Saturday without too much weather hassle. (Grab an umbrella, just in case.) Not the best beach or backyard day, unfortunately, but better than tomorrow for sure.
High temperatures should reach about 80° for most of New Jersey today. The Philadelphia metro area may push into the lower to maybe mid 80s. Temperatures will be stuck in the 70s at best today both along the coast and where any rain falls.
Heavy Rain Timing
In the graphic above, I've given the broad window of Midnight to Noon Sunday as the best chance for heavy rain. With a large-scale, tropical rainstorm like this, it's impossible to predict *exactly* where and when the narrow bands of excessively heavy rain will set up. So the safe forecast is to assume several hours of steady and, at times, heavy rain throughout Sunday morning.
Having said that, I've been pleasantly surprised with the consistency of the forecast models over the past few days, putting *the* heaviest rain over New Jersey in the window of 4 a.m. to 10 a.m. Sunday morning. That's about the time the center of the low (formerly known as Bill) will pass right over New Jersey. I don't want to pinpoint the geography too much, but it appears that central and southern New Jersey are in the bullseye for the heaviest rain totals. That includes the entire Jersey Shore.
We've been promising Father's Day won't be a "complete washout," with a clearing and drying trend expected by late Sunday afternoon. While that still holds true, this morning's models do paint some residual, wraparound showers and thunderstorms for Sunday afternoon and evening. So unfortunately, I can't guarantee a time on Sunday where the Garden State will be completely dry. While the afternoon and evening bands of rain won't be as heavy and steady as the early morning round, the chance for a thunderstorm or heavier downpour will remain.
Everyone in New Jersey will get *something* from this system. Expect rain gauges to collect 1 to 2 inches across almost all of New Jersey from Saturday night through Sunday night. Within the areas of heaviest rainfall, 3+ inches of rain is expected. Could we see a few 4 or 5 inch totals? Maybe... Again, never underestimate the potential rainfall from a tropical system.
So, with rainfall rates over an inch an hour expected during the peak of the storm, it's no surprise that flooding is a concern. Luckily, our "antecedent condition" - the current state of the ground and waterways - is fairly dry... While we've had some rain and heavy downpours lately, it has not been widespread or dramatic enough to warrant extra concern. The biggest problem will be travel - road closures due to flooding are likely.
There are two types of flooding we'll have to worry about from the remnants of Bill. The first and more dramatic type is flash flooding. This occurs when drainage systems and groundwater absorption can't keep up torrential rain, inundating streets and even buildings within minutes. Flooding is the number one weather-related killer in the United States. Just be smart and you'll be fine... Never attempt to drive, walk, or swim through flooded areas.
The second and more long-term type of flooding involves the swelling of streams, creeks, and rivers. Again, antecedent conditions are pretty good, so stream flooding should be minor to moderate at worst. It's still a concern and still something we'll be watching closely, even after the rain stops. The National Weather Service has posted a Flood Watch from 8 p.m. Saturday through 8 p.m. Sunday.
Coastal flooding will NOT be a concern this weekend. This storm is coming from the west, not from the ocean, so it won't be pushing a wall of water (storm surge) ahead of it. Regardless, as this storm heads out to sea, rough surf is expected for a time... A Gale Warning is in effect for the open ocean on Sunday and Sunday night.
While heavy rain and flooding will be the big concerns this weekend, severe weather will also be possible. The Storm Prediction Center has put the southern half of New Jersey in a Marginal Risk for severe weather through 8 a.m. Sunday. South of the Atlantic City Expressway is under a Slight Risk for severe weather.
Surface-based CAPE values top 1000 J/kg. That indicates a reasonable amount of instability or available energy (thunderstorm "food"). Given these dynamics, it's worth mentioning that gusty winds, hail, and even an isolated tornado may accompany the heavy rain and flooding threat. Be alert to changing weather conditions, and be aware of any warnings or advisories that are issued for your area.
As I've mentioned, the heaviest rain should taper off from west to east by around Noon on Sunday, although residual showers and thunderstorms are expected through Sunday night. Model consensus indicates that New Jersey will be rain-free by about 2 a.m. Monday morning.
Stay safe, stay dry, and stay tuned for regular weather updates both online and on-air.