Bouts of heavy rain expected through Thursday

Although it has been quite the grey, dreary, chilly day, rainfall throughout the area has not been all that heavy yet. The main reason for this is because the surface pressures are still quite high, thanks to a large high pressure system north of Maine, which is wedging in very high pressures into the area. Higher surface pressure is an indicator of downward motion in the atmosphere, which is not conducive to precipitation.

This afternoon’s NAM valid for 5:00pm shows a large high pressure system to the north, with high pressures still wedged into our area. This is why a lot of the rain has broken up as it has headed east (weather.cod.edu).

The image above explains it well — note the blue isobars of higher pressures that are being “pushed” into our area. The lower pressures are relegated to the west, southwest, and northwest, while the higher surface pressures are to the north and northeast. The station pressure at Rutgers Gardens is 1027mb. That is certainly not conducive for heavy precipitation. This is further augmented by the fact that an associated warm front is still well to the south of the area. If the atmosphere is going to generate lift for precipitation with high surface pressures, we need forced ascent from a frontal boundary, which will finally be approaching the area on Wednesday.

Moving forward through 11:00pm tonight, the surface pressures remain high, and the warm frontal boundary is still well to the south. This most likely means the Yankees will play baseball tonight (weather.cod.edu).

However, over the next several hours, notice how the strongest temperature gradient associated with the warm front is still well to the south, and the high surface pressures are still being wedged in. This combination leads us to believe that any rain that falls this evening and tonight will be very light, and not enough to cancel the Yankee game. As time goes on, the warm front will slowly be approaching the area, and the easterly winds from the cold, yet moist ocean will finally be enough to moisten the atmosphere and generate the necessary lift for precipitation.

Additionally, as the warm front approaches, a jet stream in the low levels of the atmosphere will increase in strength — out of the south — which further helps to increase the moisture in the atmosphere and generate lift.

Today’s 12z NAM valid for 11:00am Wednesday morning shows a strong 925mb jet, which will help to advect moisture and increase our winds (weather.cod.edu).

Today’s NAM shows a period of heavy rain from 8:00am through 11:00am Wednesday morning, in association with the strengthening low-level jet and approaching warm front from the south (weather.cod.edu).

Not surprisingly, the strengthening low-level jet combined with the approaching warm front will generate heavy rain late in the overnight and on Wednesday morning. Since at this point we will still be to the north of the warm front, more stratiform, steady rain will be falling as opposed to the torrential, convective rains that happen in the warm sector. Still, though, a burst of rain Wednesday morning exceeding 1″ is certainly possible. Temperatures will remain in the upper 40s and low 50s, with strong easterly winds around 20mph, gusting to 30mph.

As the warm front gets even closer throughout Wednesday we will be in the brief lull between the stratiform rains, and the convective rains to the south of the warm front. Still, though, some periods of light to moderate rain can be expected. Eventually, the system will become more dynamic as a surface low moves northwest of our area. A warm front will begin to strengthen and push northward towards our area from the Mid Atlantic states on Wednesday. This assures us of a few things. First of all, another period of heavy rain with embedded thunderstorms is likely late Wednesday Night into early Thursday morning. This is likely to occur as the front nears the area, with enhanced lift and convergence along the warm front becoming a focal point for precipitation.

It is at this point when flooding concerns will be maximized. Forecast models indicate that 1-2″ of rain will likely have already fallen throughout the area, bringing many Central and Northern NJ rivers and creeks at or close to action stage. Additional heavy rainfall and thunderstorms could cause rivers to rise to Moderate flood stages. The Hydrometeorological Prediction Center’s outlooks feature several rivers in the area approaching flood stages. In addition, some coastal flooding concerns exist with the onshore winds and heavy rain. If you live in a flood prone area, be prepared for flooding potential.

The warm front will push through the region Wednesday night, which will actually warm our temperatures from the upper 40s and low 50s into the mid 60s. Heavy rain is also expected, with thunder also possible (weather.cod.edu).

Today’s NAM showing more heavy rain Wednesday night (weather.cod.edu).

Rainfall may actually be enhanced a bit along the hillier areas of W NJ, since the strong low-level jet will be blowing from the southeast and “bump” into those hills and mountains, leading to additional lift for precipitation. This leads to major flooding concerns for rivers in these areas. Additionally, the rain throughout the area could be heavy enough for flooding, as 2-3″ of rain can be expected Wednesday Night, on top of the 1″ of rain that should fall Wednesday morning. The organized lifting in association with the crossing of the warm front could even generate enough instability for extremely heavy downpours and embedded thunderstorms.

On a less serious note, the dynamic system approaching the area also enhances the likelihood of warm air surging into the area on Thursday afternoon, especially if any sunshine can break through behind the warm frontal boundary. Forecast models indicate the potential for temperatures to rise into the 70s for a period of time Thursday afternoon with our area nested in a “warm sector” — essentially, the warm front will have passed but the cold front will still be lingering back to our west.

Forecast models showing temperatures in the 70’s on Thursday.

As of now, Friday and the weekend look drier (at least compared to the steady rain during this week), but some instability may linger, leading to widespread cumulus clouds and a scattered showers during the afternoon. That being said, the widespread cumulus clouds will also be juxtaposed with periods of sunshine as well — so we may deviate between dark cumulus clouds shading the sun, and bright sunshine throughout the weekend — which will be quite pleasant compared to this week. Temperatures should rise into the mid and upper 60s during the weekend afternoons.

Keep it here over the next day or two for details on not only the incoming rain, but exact timing of the heavier rain bursts. In addition, we’ll have the latest on the river and coastal flooding situation and the low down on the potential warmup on Thursday.