Oct 9, 2013: Coastal Low Brings Rain Thu-Fri

Forecast Highlights:

nam-hires_namer_021_sim_reflectivityA coastal low developed off the southeast US coast today, slowly tracking north while rain spread north into Virginia and Maryland. The storm will continue to spread north, with rain developing across the area late tonight and continuing through at least Friday night or Saturday. Temperatures will remain chilly with breezy coastal winds, with a brief break likely around the weekend or early week before rain returns again.


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Tuesday, October 8 Observations:

10.8.13The cold front exited the area on Monday night, with a cooler air mass returning for Tuesday as a high pressure built into the region with more widespread sunshine observed. The front, however, failed to push the remnants of tropical storm Karen offshore, which stalled off the southeast US coast and gradually began to track north. High temperatures reached the mid 60s to low 70s across the area. The highest temperature was 71 degrees in Teterboro, NJ, and the coolest high temperature was 64 degrees in Yorktown Heights, NY.



Tonight – Saturday: Rain, Wind, Chilly Temps From Coastal Low

LateWeekStormPattern Analysis: The latest synoptic pattern consists of a strong upper level low diving into the southwestern US, with widespread ridging covering the eastern half of the US. Ridging is displaced north of the region, however, with energy that previously split from the trough that brought Monday’s storms across the region and an EF1 tornado in Paramus, NJ currently near eastern North Carolina. The current trough in the western US will lift north into Canada by Friday, which will lead to amplification of the ridge north of the region, but with another trough entering the western US; as a result, ridging will remain nearly stationary north of the area through at least the weekend, keeping the upper level low trapped over the region throughout this time frame.

Focusing on the lower levels, a surface high pressure currently centered near New England will gradually shift east, allowing a relatively weak low pressure (about 1010 mb) to slowly shift north, reaching its northernmost extent on Friday before a stronger high pressure from Canada builds into New England, forcing the low to retreat southwards. The coastal low will produce widespread steady moderate rain, locally heavy at times, focusing towards the Delmarva Peninsula, Maryland, Pennsylvania, and New Jersey. The main uncertainty remains how far north the rain spreads, and despite the storm being one day away, there remains a fair amount of spread between the models regarding the exact northern boundary, and the axis of heaviest rainfall; solutions range from 1/2 inch of rain in the area with a sharp gradient to the south, to the gradient north of the area with widespread 2-4 inch rain totals. As the storm is nearly stationary, rain totals will gradually add up largely due to the duration rather than the intensity of rainfall, although locally heavier rain bands will enhance totals, with localized totals near or over 4-5 inches not out of the question especially south of the area.

Despite the rain less than a day away from starting, confidence on the exact rain totals is lower than average, although the latest model runs are showing somewhat less differences. At this time, the forecast is for rain to spread across the area between tonight and Friday night before retreating to the south, with at least 1/2 to 1 inch of rain for SE NY and southern CT and 1 to 2 inches of rain in northern NJ, NYC and Long Island. Locally higher totals up to 3 or more inches may be possible near or south of NYC. There remains some uncertainty, however, and should the wetter case scenario verify, most of the area would observe at least 2-3 inches of rain with totals locally up to 4-5 inches possible.


Forecast for NYC Area: Showers are expected to gradually spread into the area late tonight into Thursday morning, gradually intensifying into occasional periods of light to moderate rain from Thursday afternoon through Friday afternoon. The rain is expected to continue into Friday night before gradually tapering off, with mostly cloudy skies and isolated showers possible for the weekend. During Thursday and Friday, a breezy northeast wind of 10-15 mph is expected for most of the area, up to 20 mph in Long Island with gusts up to 30-35 mph possible. Due to the cloud cover and rain, high temperatures will struggle to warm up beyond the upper 50s inland and low 60s near the coast. Slightly warmer temperatures are expected for the weekend with at least mid 60s, possibly reaching the upper 60s north and west of NYC.

As previously mentioned, the rain outlook is still somewhat uncertain; at this time, the latest expectation is for 1/2 to 1 inch of rain in SE NY and southern CT, and 1 to 2 inches in northern NJ, NYC and Long Island, with locally higher totals up to 3 inches possible. The forecast is subject to some changes, however, and storm updates will occasionally be posted on Thursday and Friday as the storm affects the area. In the wetter case scenario, which remains a possibility in which case moderate to heavy rain bands focus on the area, rain totals of 2-3 inches would be observed, locally up to or over 4 inches.

This is the final forecast discussion before the storm; occasional storm updates will be posted on Thursday and Friday. The next full forecast discussion will be posted on Saturday morning.


Sunday – Next Week: Break, Then Rain Returns

As previously mentioned, a stronger high pressure will build into New England, which is likely to suppress rain to the south of the area for Sunday and Monday, although isolated showers can’t be ruled out. Meanwhile, temperatures are likely to slightly warm up as the upper level low moves out and ridging shifts closer to the area, with highs at least in the mid 60s to near 70 degrees expected at this time. Rain is likely to return by next Thursday and Friday, however, as the next cold front approaches the region associated with a low pressure developing in the central US and tracking north of the area. While there is some uncertainty with the setup as this is still a week away, there are signs of possible ridging towards western North America and the northeastern Pacific, which would support a stronger trough focusing towards the central US by late next week and next weekend; while the most significant cooler than average temperature departures are likely to stay west of the region, temperatures may fall back to near or below average behind the cold front, with highs in the lower half of the 60s and possibly 50s likely to return.

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