Doug Simonian

Heavy rain and embedded thunderstorms tonight

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We are currently watching a complex of showers and thunderstorms in the Midwest and Ohio Valley as it swings eastward, along a stationary frontal boundary. Additionally, a mid-level trough will be working its way into the region from the north, which will allow the storm system to strengthen, as they interact with each other. The strengthening storm system will provide plenty of lifting in the atmosphere, as well as greatly increase the strength of a southerly low-level jet, which will help to pump plenty of moisture into the region. The combination of plenty of moisture in our atmosphere and the lift will lead to the threat for heavy rain and embedded thunderstorms. The primary threat is flash flooding, as some areas may pick up 1-2″ of rain in a short period of time.

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Today’s NAM model valid for 5:00am tomorrow morning shows very high precipitable water values, which is an indicator of a very moist atmosphere profile. This supports the threat for heavy rain. (weather.cod.edu)

However, the threat for severe thunderstorms with severe winds should only be confined to the southern third of New Jersey, and that threat will be isolated. This is because the frontal boundary that the storm will be tracking on should remain in Central New Jersey. North and northeast of the warm front is where more broad lifting is expected, thus the threat for heavy rain over the entire New York Metro region.

But south of the warm front is where the lifting for precipitation is not as widespread, but the atmosphere overall is more unstable — thus the threat for isolated severe weather. This means that southern NJ may not see widespread rain, but could have a brief localized heavy downpour, with flash flooding, and a strong wind gust. The instability overall, however, is only marginally strong for severe weather — thus the severe threat being marginal and isolated. Plus there is also the fact that the instability may not be strong enough for the complex of storms to entirely maintain itself as it crosses the Appalachian Mountains.

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Doug Simonian

Showers and thunderstorms expected this evening

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Although a lot of the area has been able to salvage sunny skies for most of the day, clouds are on the increase and thunderstorms are forming in W NJ. The very warm and muggy airmass in association with a trough moving into the Great Lakes is the main trigger for these storms. The trough being quite far away also serves to keep wind shear quite weak. What this does is it prevents thunderstorms from becoming severe — but it also means that the storms move slowly. This, combined with the incredibly moist atmosphere leads to the potential for heavy downpours for a decently long period of time — thus the concerns for flash flooding. The strongest storms could have 40mph wind gusts and small hail — though that risk is more isolated.

As far as timing, the storms should move into Central New Jersey between 4:30 and 5:00pm, the NYC area between 5:00pm and 5:30pm, SW CT and Nassau County between 5:30pm and 6:00pm, and Suffolk County sometime after 6:00pm. Not everyone will get heavy downpours, but most of the area should see rain, with the potential for 1″ — with locally higher amounts. Definitely keep an eye on the latest radar and have the umbrella handy this evening.

Although the heaviest rain will probably fall within an hour of the storm’s arrival, showers and an isolated thunderstorm still cannot be ruled out for the rest of the overnight, and patchy fog may develop as well. Temperatures will remain in the 60s tonight with muggy conditions.

Wednesday: The showers and storms will clear the area by Wednesday morning, and the day should actually turn out to be decent, as a weak area of ridging will move in between the departing storm system, and another storm system coming for Thursday morning. It will be a bit cooler and drier than today, and skies should remain partly sunny, with temperatures in the upper 70s.

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Doug Simonian

Showers and thunderstorms possible Tuesday through Thursday

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Another gorgeous afternoon is on tap for today, as a ridge of high pressure remains in place. Temperatures will generally be in the upper 70s and lower 80s, though a developing sea breeze will lead to gusty winds and cooler temperatures along the shore.

Tuesday: Changes are in store for the rest of the week, however. A blocking pattern remains in-place to our northeast, which will cause a strong ocean low pressure system to retrograde to the west, towards our coast. This will weaken the ridge of high pressure, and allow a trough of low pressure to move through the Great Lakes. When a trough of low pressure interacts with a warm airmass and weakens a ridge, it is often a good recipe for shower and thunderstorm development.

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Today’s NAM model at 500mb (middle of the atmosphere) valid for tomorrow afternoon shows a large ocean system retrograding towards our coast, weakening the ridge of high pressure. At the same time, a trough will be moving into the Great Lakes. (weather.cod.edu)

One fortunate aspect is that the blocking pattern will make the trough slow to move. This means that we may still be able to salvage the entire morning and some of the afternoon tomorrow with sunshine, though clouds will be on the increase throughout the afternoon. Temperatures should rise into the upper 70s and lower 80s, with somewhat humid conditions. The threat for rain will start during the late afternoon and early evening from west to east.

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Doug Simonian

Beautiful weekend, potential showers and thunderstorms next week

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Most of us are currently enjoying a beautiful Friday afternoon, with scattered clouds and sun, and temperatures in the mid to upper 70s, as well as low humidity. Some of these scattered clouds, however, are of the cumulus variety, which are very well illustrated by the current satellite. Given our relatively unstable airmass from an approaching cold front, there is the chance that some of these cumulus clouds will turn into a few scattered showers and thunderstorms for this evening. Fortunately, most of us will still have a couple more hours of dry weather before having to worry about any of this, and a lot of areas may not see any rain at all, but it may be a good idea to have an umbrella if you are going out tonight.

Rest of tonight and Saturday: A cold frontal boundary that is helping to trigger these showers will eventually move through tonight, giving us an area of high pressure, and dropping our temperatures into the mid 50s. It will also dry out the weather even more, as dewpoints will fall into the 40s. For tomorrow, we can expect sunny skies, and very low humidity values, and temperatures in the low to mid 70s. There will be a bit of a stiff northerly breeze at times, so it may be tough to play outdoor basketball or keep a blanket sturdy for a picnic, but overall the weather looks great. A few cumulus clouds may develop in the afternoon, which may make it feel a tad cooler at times during the afternoon.

Saturday night and Sunday: As high pressure continues to build in, winds will lessen, and clouds will continue to clear. Given the already very dry airmass, this will allow temperatures to rapidly fall, via ideal radiational cooling conditions. Temperatures will be falling into the 40s throughout much of the interior; otherwise, lows in the low 50s can be expected. Temperatures should rebound beautifully during the day, however, as a ridge axis builds in from the west, with high temperatures in the upper 70s. The airmass will remain dry — with dewpoints in the low 40s, relative humidity values may fall below 20%. Definitely a 10/10 day. Another cool night can be expected on Sunday night, but not as chilly as Saturday night.

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