Mar 28, 2014: Heavy Rain Saturday Night

Forecast Highlights:

rad39Widespread areas of rain affected the region today, primarily north of the area, as high temperatures returned to more seasonable levels in the 50s and low 60s. Similar temperatures are expected early on Saturday before a slow moving low pressure affects the region with over 2 inches of rain through late Sunday night; tonight’s update analyses the development of the storm system on Saturday and Sunday and its impacts in the area, as well as the outlook for next week (image credit: PSU e-Wall).

 

 


 

Tonight – Monday: Heavy Rain, Over 2 Inches Expected

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Posted above from left to right are the latest available surface analysis and radar composite from WPC, and the GFS initialized 500 millibar heights and vorticity from NCEP MAG; both are valid at 18z (2pm). A surface low pressure associated with an upper level shortwave near southeastern Ontario aided in relatively strong warm air advection into the region with a strong south-SW flow earlier in the day, which supported development of moderate to heavy rain over Pennsylvania and New York states. The area was mostly south of the rain, with occasional showers observed in the morning and early afternoon hours mostly amounting to less than a tenth of an inch. With the strong push of warm air into the region, temperatures reached the mid 40s to low 50s over southern CT, most of Long Island and NW NJ, while the immediate NYC area warmed up into the upper 50s to low 60s, which is several degrees above average.

As the low pressure continues to track NE, its associated cold front will pass through the area tonight but will fail to progress far, stalling not far southeast of Long Island. Clearing skies are expected behind the cold front, with a short period of partly sunny skies expected to last through Saturday morning before cloud cover increases again. During this short break, temperatures will rise into the mid 40s-low 50s in Long Island and the low-mid 50s elsewhere, possibly reaching the upper 50s near NE NJ or southern CT depending on the duration of the break in cloud cover. Temperatures are then expected to fall back into the 40s by the later afternoon hours as light rain develops after at least 2-4pm from south to north, marking the beginning of a long duration rain event with over 2-3 inches expected.

 

0z run of the high-resolution 4k NAM at hour 30, valid at 6z Sunday (2am), showing 700mb heights (black), relative humidity (green), and vertical velocity (brown), where dark brown values indicate strong upward vertical motion. A broad area of strong mid level lifting extends from central NJ and further north, supporting heavy rainfall (image credit: NCEP MAG).

nam-hires_namer_030_700_rh_htImpact Analysis: Attention next turns to the southern US, where the surface map above depicts a low pressure near Texas. A mid level shortwave to its north, near Kansas, will continue to track east while amplifying, with the low pressure tracking northeast towards Virginia and later southern New Jersey as it catches up with the shortwave. The low pressure will gradually deepen on Saturday with the aid of positive vorticity advection ahead of the amplifying shortwave and upper level divergence near the right entrance quadrant of a strong jet streak, resulting in a tightening pressure gradient, with increasing southerly low level winds to its east advecting a warm and moist air mass into the region from the western Atlantic Ocean. The stronger southerly flow will act to push the stalled frontal boundary back north as a warm front, but the surface warm front will fail to progress much to the north as surface northeasterly winds to the north of the low pressure will keep colder, denser air near the surface, with the front likely to struggle reaching Long Island. The warm, moist and less dense air mass will then be forced to rise aloft, with the resulting strong low-mid level upward vertical motion supporting the development of widespread heavy rain on Saturday night, especially north of the surface warm front from central NJ into the NYC area, eastern NY state, and south-central New England.

rad39Heavy rain is expected to affect the area between about 6pm and 4am, with the heavier rainfall rates developing mainly after 9-11pm, as shown to the left from the simulated reflectivity from the 4k NAM (image credit: PSU e-Wall). With the surface warm front remaining south of the area, temperatures will generally remain in the upper 30s to mid 40s throughout the majority of the event, colder further inland and warmer near NYC and the coast, with a breezy northeasterly wind around 10-15 mph, gusting up to 25 mph near the coast. With the forecast dynamics supporting heavy precipitation, the latest forecast sides with the wetter end of the model guidance for this part of the event. As the system continues to track northeast and the strongest lifting shifts east of the area, along with mid level drier air and subsidence moving in behind the region of strong lifting, the heavy rain will mostly depart the area by 4-6am, with scattered light-moderate rain showers expected for Sunday morning.

During the overnight hours on Saturday into Sunday morning, the mid level trough will slow down as it becomes detached from the main flow, as southwesterly winds persist to its north over southeastern Canada. The cutoff low will gradually drift east off the coast of New Jersey, while a narrow band of moderate precipitation develops northwest of the vertically stacked low near the area towards Sunday afternoon and evening. With the low pressure to the east of the area, cold air advection is expected for Sunday into Sunday night as a northerly wind develops up to 15-20 mph, gusting up to 30-35 mph, with high temperatures peaking in the low-mid 40s inland and mid-upper 40s elsewhere before falling into the 30s by the evening. Since the storm will be detached from the main flow and thus cut off from the source of cold air to its north, temperatures are unlikely to be sufficiently cold enough to support a widespread changeover to snow, although interior locations may possibly change over to some wet snow with minimal accumulations, if any, towards the end of the event as the precipitation weakens and slowly departs to the east on early Monday morning.

 

Forecast for NYC Area: Increasing clouds are expected on Saturday in the mid-late morning hours as highs peak early in the afternoon in the low-mid 50s away from the coast before falling into the 40s by the later afternoon hours. Light rain will develop from south to north towards 2-4pm, becoming moderate by 5-7pm and heavy especially after 8-10pm. Periods of heavy rain will continue through the mid overnight hours, ending west of NYC by 1-3am and east of NYC by 3-5am, with lows falling into the upper 30s to low 40s. Drier conditions are expected through Sunday afternoon with scattered showers and highs in the low-mid 40s inland and mid-upper 40s elsewhere, with increasing northerly winds at 10-20 mph, gusting up to 30-35 mph, with another round of light to moderate rain from the late afternoon through the overnight hours as the storm gradually winds down. Some uncertainty remains regarding the exact ending time, with the latest forecast anticipating rain to end by Monday morning.

This will be a relatively long duration event compared with the progressive nature of most of this winter’s storms, with precipitation from Saturday afternoon through Monday morning. Heavy rainfall rates expected on Saturday night, along with another round of light to moderate rain on Sunday evening and night, which is generally expected to support rain totals around 2 to 3 inches across the area, with a higher probability of totals locally up to 4 inches towards SE NY and especially southern CT. These rain totals may lead to areas of flooding, and the National Weather Service has issued a Flood Watch north and west of NYC, where the higher rain totals are expected. This is the final forecast discussion for this storm; occasional storm updates will be posted on Saturday and Sunday, with the next full forecast discussion to be posted on Monday after the storm has departed the region.

 

Next Week: Warming Up

With the storm detached from a source of cold air, temperatures are anticipated to quickly warm behind the departing system as a low pressure tracks through the northern Great Lakes into southeastern Canada on Tuesday, advecting warmer temperatures into the region but with dry conditions expected. With the slow departure of the low on Monday, temperatures are likely to warm up into the 50s towards western NJ but struggle to climb above the mid-upper 40s near and east of NYC. More widespread warmth is expected on Tuesday with 50s likely reaching western Long Island and SW CT, with western NJ perhaps approaching 60 degrees, but with the more significant warmth expected on Wednesday with a southwesterly low level flow; some uncertainty remains regarding exactly how warm temperatures get and how far north the warmth extends, although at this time widespread highs in the low to mid 60s are likely, possibly warmer than currently forecast if there are no major changes in the anticipated setup.

Another trough is expected to enter the western US around Wednesday, gradually progressing eastward through the US, which will spread clouds and rain back into the region for the end of the week. The model guidance diverges with regards to the timing and setup of the storm, precluding a high confidence outlook on the specific details, although there is a high probability of rain between Thursday night and Saturday with temperatures likely to remain generally in the 50s for highs. Probability of snow does not appear high, as by this time period the strong cold air source which has frequently settled near southern Canada will have retreated back into northern Canada, another step marking the gradual demise of the long lasting winter.

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