Dec 16

Dec 16, 2014: Pattern Slowly Trending Active

Forecast Highlights:

gfs_mslp_pcpn_frzn_neus_21A temporary stretch of dry conditions following last week’s storm has ended with the arrival of light to moderate rain throughout the area this evening. This rain will mostly end by late tonight with less than 1/4 inch of rain, but with a breezy westerly wind persisting through Friday. Scattered interior snow and coastal rain showers are possible this weekend, followed by the potential for a more significant storm around the Christmas time frame.

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Dec 15

Dec 15, 2014 Brief Update


Tonight’s post is a brief update on the forecast for next weekend’s potential storm. A more detailed analysis will be posted on Tuesday morning.

Weekend Forecast Update: Stronger, Farther North Storm Possible

gfsp_mslp_pcpn_frzn_eus_26It has only been a week since the last major storm affected the region, and another case of model chaos is emerging for next weekend’s storm. Up until 1-2 days ago, there were more consistent signals regarding at least a moderate-impact low pressure system affecting the region on December 20-21, as a very moist low pressure system in association with the southern stream develops near Texas, producing heavy rainfall and potential thunderstorms, before continuing to track northeast towards the East Coast. Beyond this point, however, the model guidance diverges with the track and the intensity of the low pressure, with the operational GFS depicting a weak low pressure largely bypassing the region. The ECMWF depicts a stronger low pressure but with most of the snow south of the area, while the parallel-GFS, posted to the left from Tropical Tidbits, depicts a more significant system with heavy rain and snow affecting the region.

Some of these differences emerge from the handling of Tuesday’s rain event in the longer term, which after moving past the area strengthens and temporarily stalls near eastern Maine. The GFS has trended significantly more progressive with the departure of this system, depicting it near Greenland by Saturday morning while the ECMWF depicts it still affecting Nova Scotia in eastern Canada. Along with its depiction of a weaker southern stream shortwave trough which has less interaction with the northern stream, it results in a less amplified flow over the eastern US and an overall weaker and flatter low pressure system remaining mostly to the south. Considering that it barely detected last week’s storm several days in advance while the ECMWF was persistently showing a strong coastal low, however, confidence in its scenario is already starting off lower than average. Additionally, the ECMWF ensemble mean, typically considered to be fairly reliable along with its operational counterpart, is also more aggressive with a stronger and farther north low pressure with more widespread impact in the region.

Based on the overall data available at this time, confidence is high regarding a low pressure deepening as it passes south of the area, but its intensity and northward extent have yet to be determined. This would also influence how much precipitation extends into the area and accordingly snowfall amounts; as with the last two storms, only a stale, significantly moderated cool air mass will be in place initially, which decreases likelihood of snow along the coast, although a stronger and farther north low pressure would increase probability of accumulating snow, especially inland of NYC. More information will be posted with Tuesday’s full update.

Dec 13

Dec 13, 2014: Drier, Warmer Week Ahead

Note: The estimated snow map for the recent prolonged snow event is now available. Similar maps dating back to 2011 can be found here.

Forecast Highlights:

gfs_namer_168_850_temp_mslp_precipA prolonged low pressure system has finally departed the region, after producing widespread rain, snow and overcast conditions throughout the last 6 days. In its wake, a much drier and somewhat warmer pattern will return for the next week, with temperatures generally in the 40s along with the possibility of some showers on Tuesday. Stormier conditions are likely to return by next weekend, however, with the potential for rain and/or snow.

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Dec 09

Dec 9-11, 2014 Storm Updates

Occasional updates will be posted below on the prolonged rain, wind, flooding and snow event affecting the region this week. Radar images are from the Pennsylvania State University e-Wall.

Links: Yesterday’s Forecast | 8-Day Forecast | Twitter | Facebook


Wednesday, December 10

8:10 PM: Snow Winding Down

Note: The radar image posted to the left is from 2230 UTC (5:30pm EST), not the latest radar.

nerad16As of 7pm, the low pressure is situated just off the coast of Maine. Minimum pressure is near 996 hPa, and the low pressure is is steadily tracking west towards western Maine and northern VT/NH.

The second part of the storm verified with more significant snow than expected across parts of the area into New York state, as wraparound snow bands rotating around the low aloft stalled near the area and produced persistent periods of light snow. A full list of snow accumulations is not available yet, although LaGuardia Airport has recorded 1 inch of snow so far. Most of the snow fell between 1pm and 7pm, but largely began to accumulate towards the later end of this period as snow rates intensified and temperatures gradually cooled down. Local radar imagery indicates that a stationary band of moderate snow set up over Bergen/Westchester counties into the Bronx and north Nassau county, where accumulations over 1-2 inches may have been observed, with accumulations otherwise generally around or less than 1 to 1.5 inch.

This second round of the system has generally produced more precipitation than expected across the region, and has been a relatively recent change to the forecast as the model guidance has struggled to handle this system correctly over the last few days; for instance, the low pressure from yesterday was forecast to stall near Long Island or off the coast of New Jersey, but in reality stalled near Rhode Island, and the secondary low pressure near Maine was stronger and wetter than initially expected. As the low pressure continues to progress westward into New England, drier conditions are expected tonight, but with wraparound bands over northern Pennsylvania perhaps rotating around the low and extending into parts of the area towards Thursday morning. Any additional accumulations are likely to remain below 1 inch, although periods of light snow through the morning hours would result in areas of lower visibility and additional minor accumulations.

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Dec 08

Dec 8, 2014: Prolonged Coastal Storm Begins Tomorrow

Forecast Highlights:

rgem_newFollowing a cold morning inland of NYC, temperatures have struggled to significantly warm up today, with some ocean effect snow and rain observed in parts of Long Island and New Jersey. A strong coastal low pressure will approach the area before stalling nearby, affecting the region with widespread impacts throughout most of the week, including heavy rain, strong wind gusts and flooding near the coastal plain, as well as heavy wet snow and ice inland, with precipitation perhaps lingering as late as Friday or even Saturday.

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Dec 07

Dec 7, 2014 Brief Update

Mid-Late Week Forecast Update: Heavy Rain, Wind, Flooding Expected

As of this evening, the model guidance is slowly coming into a better consensus regarding the upcoming long duration coastal storm, as a deepening low pressure system tracks northward just off the Mid Atlantic coast before likely occluding and stalling near NYC on Tuesday and Wednesday, and gradually weakening while drifting northeast towards the late week period. Strong upper-level forcing is expected to produce heavy rainfall across the area, especially on Tuesday, but with a lack of sufficient cold air aloft to produce snow except for parts of western New Jersey, which may change over to snow on Tuesday night into Wednesday with minor accumulations possible.

rgemSome uncertainties still persist, however, such as the exact track of the low pressure, which will influence the inland extent of the dry slot expected to spread into the area following the initial surge of rain on Tuesday morning, as well as the eastward extent of the changeover to snow overnight. The latest model guidance ranges from a track over New Jersey as depicted by the high-resolution NAM and RGEM models, to a low pressure near central-eastern Long Island as depicted by the GFS model. The GFS has been slow to catch up with this system over the last several days, however, and just 2-3 days ago hardly even depicted any coastal low pressure or any precipitation, and at this time a compromise of the ECMWF and UKMET appears most suitable with a low pressure tracking near or just east of NYC. The heaviest rainfall is currently expected to occur between at least 8 AM and 2 PM on Tuesday, generally amounting to 1.5 to 2 inches of rain, as depicted to the left by the high-res RGEM model from Environment Canada, accompanied by strong wind gusts as well especially near and east of NYC as a strong low level jet with 925 hPa winds over 60 knots may mix down wind gusts over 40 mph to the surface.

Another area of uncertainty is the potential for a second round of precipitation on Wednesday night into Thursday, which the ECMWF and UKMET have been indicating as a second low pressure develops off the coast of New England before tracking west into NY state, which would lead to additional light to moderate rain and/or snow in the area, especially north of NYC, which the GFS does not depict. These models also side with a slower departure of the system, as showers may potentially linger into Friday or even Saturday. These differences will be narrowed down with the next full forecast analysis to be posted on Monday.


A full forecast package, including a forecast discussion, 8-day outlook, impact/snow maps, and new features including hourly forecasts for NYC and 3-hour forecast maps for the area, will be posted on Monday in the late afternoon or evening.

Dec 06

Dec 6, 2014: Heavy Rain, Wind, Flooding Possible This Week

Forecast Highlights:

ecmThe rain event which has affected the area over the last day will begin to gradually wind down, with drier conditions but accompanied by much colder temperatures returning for Sunday into Monday. A strong low pressure system is expected to affect the region in the mid-week period, but with a lack of sufficient cold air aloft indicating that heavy rain, wind and coastal flooding will be the primary risks, along with a potential for freezing rain inland of NYC on Monday night. Image from Tropical Tidbits of 0z ECMWF.

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Dec 05

Dec 5, 2014 Brief Update

Forecast Overview

Rainy Saturday, Cold Sunday Expected

nerad25Posted to the left is the latest regional radar, from the Pennsylvania State University e-Wall. Somewhat similarly to Tuesday earlier this week, strong warm air advection is occurring aloft, but with a strong high pressure situated east of Maine indicating that surface temperatures will take longer to warm up, with another round of freezing rain over parts of Sussex and Orange counties and light rain elsewhere. As the low pressure system passes through the region on Saturday, some breaks in the rain are expected through the early afternoon hours as the warm front approaches NYC, with temperatures climbing into the low 50s south of I-80 and holding steady in the low 40s to the north. Another round of moderate to heavy rain is expected from about 2-4 PM to 12-2 AM, with at least 1/2 to 1 inch of rain likely to fall.

Yet another strong high pressure will build into Maine by Sunday, the third such occurrence over the last week, which will suppress the low pressure well to the south while transporting a much colder and drier air mass into the region. Temperatures on Sunday will accordingly struggle to rise into the low-mid 30s before crashing into the 10s inland and 20s elsewhere overnight. This high pressure will remain transient as well, however, with a more significant storm system likely to follow.


Significant Storm Possible Next Week

Several days ago, a significant storm system produced heavy rainfall across most of California, delivering much needed rainfall to locations under a persisting extreme drought but also resulting in flooding and landslides. A wetter pattern is anticipated to continue over the West Coast as two distinct shortwave energies, one over Washington and another near California, land onshore on Saturday afternoon. These shortwaves will then converge near the northeast US coast, facilitating the development of a low pressure near the region, but the model guidance continues to differ with the amplitude, timing and extent of interaction of these shortwaves, leading to high uncertainty and a wide variety of outcomes for this time frame. To start off, the GFS remains the most progressive model, having only indicated the development of a coastal low pressure with widespread precipitation for the first time with its latest 18z run. Considering that yesterday it hardly had any evidence of a coastal low pressure and depicted only a weak frontal passage, and especially with recent seasonal trends towards more amplified systems, today’s forecast updates still considers the GFS to be an outlier scenario, and leans more heavily towards the remainder of the model guidance which indicates a more significant storm system.

ecmwf_T850_us_6Among the significant differences in the model guidance are the track and intensity of the low pressure system, and accordingly the amount of precipitation and southward extent of accumulating snow. The antecedent cold air mass from Sunday departs quickly, and marginal cold air aloft and near the surface complicates the forecast for precipitation type inland of NYC, while confidence is higher for a largely rain event from the immediate NYC area and farther south and east at this time. The 12z ECMWF valid at Wednesday, from Tropical Tidbits), is the most aggressive with the amplification of this system, with a strong low pressure system looping near the area before continuing northeast, with heavy precipitation mostly in the form of rain except for interior locations. As previously stated, uncertainty remains high due to the continued differences in the model guidance, although confidence is increasing for a significant precipitation event with at least near or over 1 inch of liquid-equivalent precipitation, along with the potential for snow at least for interior parts of the area but with low confidence regarding precipitation types.

The next full forecast analysis, along with an updated 8-day forecast, will be posted on Saturday.

Dec 02

Dec 2, 2014: Light Ice Inland; Little Snow Expected

Forecast Highlights:

wrf-nmm_ref_frzn_neus_25Following yesterday’s significant warmth near and south/east of NYC, much colder temperatures will return today along with a risk of interior light freezing rain this evening. Temperatures will return again into the 50s on Wednesday, followed by overall warmer than average temperatures with widespread cloud cover and a risk of showers likely throughout much of the weekend into early next week.

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Dec 01

Dec 1, 2014: Light Snow, Ice Possible on Tuesday

Forecast Highlights:

I_nw_r1_EST_2014120100_048The cold and snowy pattern that marked the end of November has ended with the onset of meteorological winter, with generally above average temperatures expected through most of the next 1-2 weeks. In the short term, however, significant variations in temperatures will continue, with highs in the 50s today falling into the 30s by Tuesday with a risk of light snow and ice inland of NYC on Tuesday, warming back up into the 50s by Wednesday followed by another risk of showers by next weekend.

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