June 12, 2013: Heavy Rain, Flooding Tomorrow

Forecast Highlights:

Mostly to partly sunny skies were observed across the area today with highs in the mid 70s to near 80 degrees. A developing low pressure in the central US currently producing a significant severe weather outbreak in the Great Lakes region will continue to intensify and track into the region on Thursday, becoming an unusually strong non-tropical low pressure for this time of the year while producing unseasonably cold temperatures, heavy rain with over 2-3 inches likely, and windy conditions with gusts up to 40-45 mph possible. With another rain event next week, this month is likely to become the wettest June on record in NYC.

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Thursday - Friday: Heavy Rain, Flooding, Wind

Earlier this afternoon and evening, a developing low pressure in the north central US produced a widespread severe thunderstorm outbreak, with thunderstorms still ongoing over Indiana and Michigan. This low pressure will continue to intensify while tracking east into the region on Thursday, becoming an unusually strong low pressure for this time of the year while producing significant impacts across the region, ranging from unseasonably cold temperatures, heavy rain over 2-3 inches and flooding to the north, and hot temperatures with severe weather to its south.

Setup Overview: As mentioned last night and this morning, this is a complicated forecast, and even though the storm is less than 12 hours away, the model guidance is still struggling to narrow down the specifics, and has been fluctuating for the majority of the day. For example, yesterday the GFS was one of the northernmost models. Today's 12z run was much weaker and further south, showing a 1000mb low sliding southeast into Virginia with less than 1.5 inch of rain in the area, decreasing to the north. The 18z and 0z runs, however, trended significantly to the north, with the GFS now showing the heavy rain axis north of the I-90 corridor in central New England, with 1-2" of rain in NYC and temps warming into the upper 60s-low 70s with marginal instability for some thunderstorms, possibly strong. Even the ECM, which is usually a consistent model especially in the short range, somewhat fluctuated today, showing a further south and faster scenario with its 12z run. This morning's updated noted the south trend that took place with the 12z model suite; since the models have since backed away from this trend, and the 12z model guidance is already different from the latest surface observations with both the orientation of the low pressure and the location of the Great Lakes storms, the 12z guidance is being considered as an outlier set of models.

What is likely at this time is that plenty of moisture will be available for significant rain totals, likely around 2-4 inches and locally higher, north of the low pressure, along with windy conditions with gusts up to 30-45 mph and unseasonably cold temperatures in the 50s. The heavy rain axis looks to be moderately displaced to the north of the low pressure track. To the south of the low pressure and warm front, hot temperatures are expected with severe thunderstorms likely in the afternoon and evening. The main uncertainty in the forecast is regarding the track of the low pressure and the location of the impact zones mentioned above.

For tonight's forecast, I am siding with a low pressure track through central Pennsylvania and New Jersey, with the NYC area in the southern end of the heavy rain axis. The warm front is likely to reach at least central New Jersey, with thunderstorms likely in NYC around the evening hours before the low pressure slides to the south of the area; while the storms could be strong, instability is likely to be minimal, with the highest risk of severe weather likely to be from central New Jersey/SE Pennsylvania and further south. Going into the overnight hours, as the low pressure tracks offshore, widespread moderate to heavy rain is likely in southern New England with temperatures falling into the upper 40s-mid 50s; some models show this back end moving over the area, which would significantly increase rain totals, while others keep it just to the north, with light rain showers overnight. Based on the aforementioned track scenario, the area would be under the south end of the back-end rainfall, with lighter rain from NYC and further south and heavy rain north of NYC. The timing of the storm is also somewhat uncertain, although at this time I am siding with the moderately slow scenario, with rain mostly ending by early Friday morning except for eastern areas, where rain is likely to continue into Friday morning. It is important to note that confidence in this forecast is lower than average for a short range outlook, and the possibility is there for minor changes in the track, intensity and/or timing of the low pressure that would result in a slightly different outcome.


Forecast for NYC Area: Based on the previously mentioned scenario, showers are expected to develop late tonight, with a steady moderate rain by the early-mid morning hours. Going into the afternoon hours, heavy rain will develop north of NYC and in southern CT, with the southern half of the area seeing lighter rainfall rates. Temporary breaks in the rain may not be out of the question as well. As the warm front makes its closest approach in the evening, the southern half of the area is likely to reach the high temperatures in the mid to upper 60s, while northern areas steadily drop into the mid 50s to low 60s. Heavy rain and thunderstorms are expected, which may be strong near or south of NYC with strong wind gusts. In the late evening and early overnight hours, temperatures will steadily fall into the low to mid 50s while rain continues, heavy at times north of NYC into southern CT, while winds increase as well, changing direction from the east to the north at 10-25 mph, gusting up to 30-45 mph. Rain is likely to gradually taper off from west to east in early Friday morning except for the eastern half of the area, where light rain is likely to continue into the morning hours.

Significant rain totals are likely out of this storm. As previously mentioned, there is still some uncertainty regarding the placement of the heavy rain axis and thus exactly how much rain falls in the area, although at this time, 1 to 3 inches of rain are likely in the southern half of the area, with 2 to 4 inches north of NYC into southern Connecticut. Locally higher totals are possible, especially north of NYC. After the recent heavy rainfall, flooding is likely again across parts of the area. So far, Central Park is just under 8 inches of rain for the month; if this storm produces at least 2.30 inches of rain, this would become the wettest June on record, surpassing the current record of 10.27" from 2003. Even if this storm fails to produce this much rain, with another half month left for additional rain events, including another storm on Monday and Tuesday as will be mentioned in the next section, there's high probability that this month becomes the wettest June on record, and perhaps one of the 10 wettest months on record. In addition to the rainfall, windy conditions are expected with an east wind on Thursday becoming north overnight at 10-25 mph, gusting to 30-45 mph. Depending on how far north the warm front advances, strong storms may be possible from NYC and further south.


Uncertainty: As previously mentioned, there is still some uncertainty in the forecast, and confidence is lower than average for a short term outlook. There remains uncertainty regarding the exact track of the low pressure, as well as the placement of the heavy rain axis, how far north the warm front advances, and how much, if any strong thunderstorms affect the area. Storm updates will be posted throughout the day tomorrow, and the track, intensity and timing of the low pressure will be monitored for any possible changes that may have to be made to the forecast.


Longer Range: Record Wet June Continues

Behind the low pressure, a mostly sunny and dry weekend is expected, with temperatures warming back up to slightly cooler than average levels, peaking in the mid 70s to low 80s across the area for highs. Meanwhile, another low pressure will approach the region, with showers and thunderstorms likely once again for Monday and Tuesday. There is some uncertainty regarding whether these storms are scattered in nature, or if a more organized low pressure takes shape with another round of heavy rain as the GFS and CMC indicate, although any rain that falls in this time frame will only add onto what is already a very wet month. More information will be posted on this storm in the next few days. Uncertainty increases beyond the early week, although overall thinking is for a drier mid-late week period with temperatures remaining close to average.