John Homenuk

Live Blog: Severe storms, heavy rain likely

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[alert type=”error”]Moderate Risk of severe thunderstorms for Southern New Jersey. Click for details.[/alert]

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An unusually strong storm system for June will move eastward from the Ohio Valley through the Mid Atlantic states on Thursday, providing the area with several hazardous weather threats. Severe thunderstorms, heavy rain and wind are all likely throughout the area at some point on Thursday as the system moves through. Throughout the day we will be updating this live blog post with the last information, discussion and links. We encourage you to keep reading along with us as we track the system.

5:00pm Update: The initial storm system helped to keep the warm front a tad further south than originally anticipated. Central NJ was originally on the border of the severe weather, but they are now safe from any severe weather threat, as are all areas north of Central NJ. An area of moderately strong (but NOT severe) thunderstorms are approaching northern and central NJ, so Sussex Counties through Flemington may have to watch out soon, and these storms should generally traverse I-80. Moderate to heavy rain, some lightning, and wind gusts up to 40mph can be expected.

However, strong and severe thunderstorms have developed a bit further south in the Mid-Atlantic, with numerous severe wind reports all throughout Virginia and Maryland. There was also a confirmed tornado by law enforecement near Colesville, Maryland, which is 15 miles north of Washington, DC. That tornado has lifted, but potent storms with strong winds still remain. They are heading east and could give southern NJ some wind gusts up to 50mph, and perhaps some exceeding 50mph. The strongest threat for these storms will be in Salem, southern Gloucester, Atlantic counties, and southward…perhaps clipping extreme southern Ocean county. We’ll keep you posted if anything changes and fine-tune things as well.

11:20am Update: Storms have cleared the NJ coast from the initial thunderstorm system. Rain continues over parts of New York, Long Island and Connecticut. Some clearing is expected over New Jersey during the next hour or two. A warm front is expected to push northward with time, with additional storms developing. A severe weather threat is expected over much of New Jersey this evening. Stay tuned.

9:20am Update: Severe Thunderstorm Warning for much of Southwest NJ and the city of Philadelphia. Strong winds, heavy rain and frequent lightning expected with the complex of warned storms. Click here to see the high resolution radar imagery from the time of the warning issuance.

9:00am Update: Heavy rain in association with the decaying thunderstorm complex will approach NYC, NJ, CT and LI over the next 30 minutes and continue through around 11am. A local threat for severe thunderstorms exists over Southern New Jersey through that time frame, with the potential for some hail and strong/damaging winds.

8:05am Update: Severe Thunderstorm Watch #304 remains in effect for Central and Southern NJ. A Moderate Risk for severe thunderstorms from SPC remains over Southern New Jersey through tonight. Our forecasters were tracking a weakening complex of thunderstorms moving through Southeast Pennsylvania. Periods of rain are expected to impact New Jersey, NYC, CT and LI within the hour from the system — with the severe storms impacting Southern and Central NJ. Stay tuned for any potential warnings. Additional severe thunderstorms are expected later today.

John Homenuk

Myriad of threats expected from midweek storm

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[alert type=”error”]Storm Prediction Center has placed parts of the area in a Moderate Risk for severe weather on Thursday. Details. [/alert]

A strong storm system traversing the Ohio Valley will strengthen and shift towards the Northeast US on Thursday, bringing the potential for several types of hazardous weather. Forecast models suggest the system may be anomalously strong for this time of year, with pressures dropping to near 990mb as it moves through the Mid Atlantic states. This is absolutely unprecedented for this time of year, as normally the jet stream is too weak this time of year to support such dynamic systems.

A warm front is forecast to lift northward from the Mid Atlantic states to a position near Central New Jersey on Thursday afternoon. The combination of building instability near and south of the warm front, as well as favorable wind fields and forcing for precipitation will lead to the potential for not only heavy rain and strong winds, but severe weather…including the potential for a few tornadoes.

If you have not already, I highly recommend you check out our previous technical discussion, as it will help to explain some concepts that I will talk about in detail for the rest of this post, and give you a pretext as to what is going on.

The exact position and timing of the surface low, cold front, and warm front will be crucial in determining where the tornado risk is, and where the northern boundary of the severe weather can be expected. Latitudes north of the surface low can expect heavy rain of 2-4″ and little in the way of severe weather, areas with the same latitude as the surface low can expect 1-2″ of rain with a chance of being hit by a squall line producing borderline severe winds, and latitudes south of the surface low can expect severe weather and a few tornadoes. The problem is, the exact position of the surface low is still a bit uncertain, but we’ll do our best to warn everyone of the possibilities.

The 00z 6/12 NAM valid for 2 p.m. on Thursday.

The 00z 6/12 NAM valid for 2 p.m. on Thursday.

Our first two images will illustrate the NAM’s boundaries and its movements throughout the day on Thursday. The NAM, at 2 p.m. on Thursday is forecasting a 990mb surface low in SW PA (top left panel). You can see two distinct temperature gradients — one to the south of the low, and one to the east of the low. I quickly highlighted the approximate warm front position in red and the cold front position in blue, at that time.

As I said in yesterday’s discussion, the cold front often triggers a linear convective mode. But look at how far east the warm front extends away from the front! Areas just south of the warm front and away from the cold front can expect a discrete supercellular convective mode. Thus, at 2 p.m., SE PA, SW NJ, DE, and MD could really be under the gun for a tornado threat, and perhaps severe hail as well.

A very key fact that one must understand for this threat: the slower the cold front advances, the longer the warm front will be the main boundary for thunderstorm initiation in the very favorable tornadic environment, thus discrete supercells. A faster moving cold front means that it would eventually be the dominating player in the favorable tornadic environment, potentially “wasting” the environment and turning the convective mode more linear, since it would more quickly intercept that environment. Linear squall lines can still provide embedded tornadoes, so a squall line does not mean there is no tornado threat, but it would be significantly reduced compared to that of more discrete supercells.

Also note the southeast surface winds that even exist to the south of the warm front in SE PA on the bottom left panel — when you combine that with the potent mid level winds from the WSW associated with this powerful storm, you get strong, deep-layer shear supportive for severe weather and even tornadoes. More will be discussed about the wind profiles later in this post.

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Doug Simonian

Severe Storms Likely for the Mid-Atlantic on Thursday. How Far North Will They Go?

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As well evidenced by the recent inclement weather, the northeast is under a very active weather pattern, and after a couple of days of general relaxation, the weather looks to turn unsettled again on Thursday.

A potent storm system with lots of vorticity will be the culprit for severe weather on Thursday.

A potent storm system with lots of vorticity will be the culprit for severe weather on Thursday.

A powerful storm system will quickly traverse the country, giving the Midwest and Ohio Valley a severe weather event on Wednesday. This will head eastward and approach our area for Thursday. At this point, all eyes will turn towards the northern Mid-Atlantic states: in particular, southeastern Pennsylvania, Maryland, Delaware, northeast Virginia, and perhaps central and southern New Jersey as well. This storm system will provide very strong winds — well in excess of 50 knots — at 500mb, coming out of the WSW; generally a very conducive environment for severe weather. In addition, the potent vorticity, among other factors that will be explained shortly, will also be conducive for lift, in order to generate thunderstorms.

What is often true about these powerful storm systems is that they possess strong boundaries, such as cold fronts and warm fronts. It is along these boundaries, and on the warm sides of them, where we pay particularly close attention, because the warm, moist air in the warm sector is lifted near the boundaries themselves, helping to initiate convection, and thus, thunderstorms. Usually, a cold front in itself is a boundary that is only conducive for squall lines, because they are expansive, they favor lots of lift over the entire front (as opposed to a dry-line, where the lift is more localized), and move quickly — so the best lift continually catches up with any storms that form, causing more and more storms to form in close proximity to them, thus quickly favoring congealing into a line. This is what is typical for northeast thunderstorm setups, and one reason why you tend to see large lines of storms, rather than the discrete supercells that you get in the Plains. However, in addition to this cold front, there will also be a warm front extending from west to east, well out ahead of the cold front.

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John Homenuk

Forecast: Unsettled week lies ahead

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Unsettled weather has gripped the areas weather pattern once again, after the remnants of Tropical Storm Andrea provided the area with 4-5 inches of rainfall. Multiple perturbations and individual disturbances will move through the Northeast US over the next several days, providing the chance for unsettled weather essentially throughout the week. After a prolonged period of clouds and showers on Monday, some clearing is expected on Tuesday with temperatures warming into the upper 70’s to near 80. By the afternoon, however, scattered showers and storms will return to the forecast. There could be a break in the action by midweek (Wednesday most likely), before chances of more steady rain return by Thursday and Friday.

Visible satellite imagery from the afternoon of June 10th, 2013 showing clouds and unsettled weather throughout the Northeast US.

Rest of Today (Monday): Cloudy with showers. Rain could be heavy at times. Highs in the lower 70’s. East winds around 10 miles per hour. Chance of precipitation 100%.

Monday Night: Cloudy with showers likely. Some thunder is also possible. Low temperatures falling into the mid 60’s. Southeast winds around 10 miles per hour. Chance of precipitation 100%.

Tuesday: Partly cloudy early and warmer with highs in the upper 70’s to lower 80’s. Showers and thunderstorms developing by afternoon. West winds around 10 miles per hour. Chance of precipitation 60%.

A look ahead: A break in the unsettled weather is possible around the middle of the week (Wednesday). But as another disturbance moves towards the Northeast/Mid Atlantic states by Thursday and Friday — the likelihood of showers and storms will increase once again. Behind this disturbance, early indications are that next weekend will feature dry but cooler than normal conditions.