Nov 28

Nov 28, 2014 Brief Update

Notice

Blog updates will remain limited during the Thanksgiving period. Full blog updates, including the 8-day outlook, will resume on Monday.

Forecast Overview:

Following an active November pattern featuring numerous temperature swings and several light snow events, a major storm which produced significant interior snow accumulations but a wintry mix along the coast appears to have signaled the conclusion of this active pattern, as a generally warmer and drier than average pattern looks to dominate the 1-2 week range across most of the region.

In the short term, cold temperatures will continue tonight in the 10s inland of NYC, with mostly cloudy skies and isolated flurries returning on Saturday as temperatures struggle to rise above the low-mid 30s. A warmer air mass will briefly return for Sunday and Monday, however, bringing temperatures back into the upper 40s to mid 50s range along with a risk of isolated showers on Monday evening. A frigid air mass originating from Canada will make a quick swipe through the region on Tuesday, with temperatures cooling back down into the mid to upper 30s, but with the most significant cold remaining to the north of the area.

0z GFS valid at Wednesday morning, indicating the potential for light freezing drizzle northwest of NYC. Image from Tropical Tidbits.

gfs_mslp_pcpn_frzn_neus_18Tuesday – Beyond: As the cold air mass departs on Tuesday, temperatures aloft will warm due to a stronger mid-level southwesterly flow, but the high pressure will linger over the region, indicating a cold air damming signal as lighter winds near the surface fail to remove the shallow cold surface air mass as quickly. While coastal locations will be warm enough for isolated rain showers, the higher elevations towards northwest NJ and Orange county in NY may observe a period of light freezing rain or drizzle. Any frozen precipitation will change over to rain on Wednesday morning as temperatures surge into the 40s with scattered showers continuing through the day.

Beyond Wednesday, the model guidance significantly diverges with regards to the progression of Wednesday’s cold front, along with several other weak systems embedded in the progressive zonal flow aloft. The overall zonal upper level flow, however, indicates temperatures will generally remain near to above average through at least the second week of December. There are indications of possible increased storminess in the north central Pacific around the second week of December, however, which would amplify the upper level flow over North American and potentially enhance probabilities of colder temperatures and storminess in the region around or after the December 10 time frame.

Nov 26

Nov 26, 2014 Storm Updates

Occasional updates will be posted below on the snow, sleet and rain event affecting the area today. Radar images are from the Pennsylvania State University e-Wall.

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


 

11:45 PM: Snow Ending

11.27_0430As of 11pm, the low pressure was located just southeast of Cape Cod in SE Massachusetts. Minimum pressure is near 995 hPa, and the low is gradually progressing towards Atlantic Canada.

Sustained precipitation has gradually come to an end across the area as the low pressure continues to track away from the area, but with the mid level low having mostly moved off to the northeast as well, temperatures aloft have cooled down sufficiently to allow for a changeover back to snow across most of the area. As the latest radar to the left indicates, however, only scattered snow showers linger over the area, which will continue to subside over the next 1-2 hours.

The sharp snow gradient appears to have set up approximately along an axis between central Morris to western Bergen, northern Westchester, and northern Fairfield counties, along which snowfall totals sharply increased to the north over 4-6″ and decreased to the south below 2-3″. Widespread significant wet snow was recorded across interior portions of the area, with unofficial storm reports from the National Weather Service indicating 11″ accumulated in Highland Lakes, NJ (Sussex) and 9.6″ in West Milford, NJ (Passaic). Farther east, towards NYC, Long Island and coastal Connecticut, both the warmer than forecast low-level temperatures and the more significant than forecast inland extension of the warm layer aloft resulted in a primarily rain and sleet event with occasional snow mixing in at times, but with little to no accumulations for the main part.

This is the final storm update for tonight. A detailed post-storm analysis including a forecast verification and estimated snowfall accumulation maps will be posted on Thursday.

 

6:10 PM: Snow, Rain Winding Down

11.26_2300As of 5pm, the low pressure was located approximately east of Atlantic City, NJ and south of the eastern tip of Long Island. Minimum pressure is near 997 hPa, and the low continues to steadily track northeast.

Over the last several hours, southerly winds continued aloft, and the mid level warm layer spread through northeast NJ and southern parts of Rockland and Westchester counties, where precipitation type changed to sleet along with lighter precipitation rates as the strongest forcing shifted to the north of the area. Snow accumulations in the nearby suburbs of NYC are generally below 2 inches, increasing towards 4 inches inland of approximately the Garden State Parkway in NJ into Danbury, CT. The warm layer’s inland extent was most expansive over Connecticut, where widespread sleet was observed with lower than forecast accumulations along the coast. Northwest NJ and interior southeast NY continued to observe periods of moderate wet snow, with snow totals as high as 7.8″ of snow in West Milford, NJ and 9.0″ in Wantage, NJ, according to the National Weather Service’s compilation of unofficial storm snow reports (NWS Mt. Holly, Upton).

The regional radar composite above indicates that the heaviest precipitation has already moved off to the north of the area, with only some additional moderate snow lingering over eastern Pennsylvania. The high-resolution HRRR model indicates that some weakening of this band is likely as it pivots through the area between at least 7 and 10 PM, and no additional significant snow accumulations are expected, although with the 700 hPa low passing near the area, temperatures aloft will begin to cool down over the next 1-2 hours with precipitation type expected to change back to snow in the immediate north and west suburbs of NYC. Precipitation is mostly expected to taper off by 10 PM, perhaps lasting a bit longer east of NYC.

 

1:30 PM Update: Thundersnow was reported across Bergen, Rockland and Westchester counties around 1:18 PM EST. The lightning was centered near the NY/NJ border.

 

1:00 PM: Heavy Wet Snow Inland of NYC

11.26_1745As of 12pm, the low pressure was located just northeast of North Carolina. Minimum pressure is near 1003 hPa, and the low continues to steadily track northeast.

Since the previous update, temperatures aloft at 850 and 700 hPa slightly cooled down, with snow and sleet mixing with the rain over parts of New York City into northern Nassau county, while locations north and west of NYC into Connecticut away from the coast continued to observe moderate to occasionally heavy wet snow. The highest accumulations so far are inland of the area, although snow totals up to 2-4 inches have been recorded in northwest NJ and Orange and Putnam counties in New York.

The latest SPC Mesoanalysis indicates the 700 hPa freezing line has began to progress north again as strong southerly winds continue at that pressure level, and currently extends from about central New Jersey into eastern Long Island. The boundary layer remains warm with low-level temperatures at 925 hPa above freezing from NYC and farther east, where rain is expected to continue with occasional snow and sleet mixing in at times. Farther inland, there are some disagreements between the higher-resolution RAP and HRRR short range models, although the timing of the warming aloft would support a changeover to sleet along and east of the Garden State Parkway in NE NJ into southwest CT and southern Rockland/Westchester counties towards 2-4 PM, which would slightly tamper accumulations while interior locations continue to observe heavy snow. The rest of the forecast at this time is still unchanged.

 

10:25 AM: Snow Forecast Lowered

11.26_1500As of 9am, the low pressure was located over eastern North Carolina. Minimum pressure is 1006 hPa, and the low pressure is quickly tracking NNE. The low pressure is forecast to track well within the 40N/70W “benchmark” position for major snowstorms, towards east of central NJ and south of eastern Long Island by later this afternoon.

As of yesterday’s forecast update, the model guidance had been slowly but gradually shifting towards a slightly more amplified system, leading to a deeper low pressure but also with a farther west track, accompanied by a stronger surge of warm air aloft transporting above freezing temperatures farther inland across the region. This trend was most noticeable with the NAM; even though this model is typically over-amplified and occasionally too warm aloft, in this case it appears to have correctly identified this trend, which is resulting in less snow than forecast from northeast NJ and farther east into southern Connecticut.

 

fgenLatest Observations: As of 10 AM, heavy precipitation has spread into most of the area, which allowed the boundary layer, essentially the lower levels of the atmosphere close to the surface, to cool towards the wetbulb temperature as surface temperatures fell and dewpoints went up. From NYC and farther east, however, precipitation type is still predominantly rain with occasional sleet mixed in. Temperatures last night struggled to cool below the 40s, but dewpoints were still near or just above freezing, indicating that once precipitation would begin to fall, temperatures would still remain above freezing and likely too warm to support snow. Farther inland, however, this was less of an issue, with the rain/snow line approximately along the I-95 corridor with moderate wet snow falling inland of I-95. There are widespread reports of accumulations inland of NYC, generally still around a coating in lower elevations but increasing to an inch towards the higher elevations of NW NJ.

Posted to the left is the latest SPC Mesoanalysis of the 700 hPa pressure level, depicting height and frontogenesis, of the strengthening of the temperature gradient. As previous forecast discussions have noted, as the low pressure system deepens, southerly winds aloft will strengthen as well, transporting warmer temperatures aloft into the region and enhancing the temperature gradient. Strong frontogenesis values, as seen to the left by the purple contours highlighting most of the area, indicate upward vertical motion and accordingly heavy precipitation, matching up with current observations of moderate to heavy snow northwest of NYC. As the strong warm air advection continues aloft, snow is expected to continue through at least 12-1 PM north and west of NYC into interior southern Connecticut, heavy at times, with occasional bursts of heavy wet snow in New York City, coastal Connecticut and western Long Island, but otherwise with rain mixing with snow and sleet.

 

rapNE_700_temp_010Afternoon Forecast: By later this afternoon, however, the strong southerly flow aloft will have advected above freezing temperatures into the region in the mid levels of the atmosphere, indicating that snow will at least partially melt as it falls through this layer, before either refreezing into sleet inland of NYC with colder surface temperatures, or fully melting into rain from NYC and farther east due to warmer low level temperatures. Posted to the left is the RAP forecast for the 700 hPa heights, temperatures and wind valid at 5 PM this afternoon, from the College of DuPage. The 700 hPa low pressure, indicated by the lowest heights and the center of the counter-clockwise flow, is over eastern Pennsylvania, well to the west of NYC. This is an aspect of the storm that was not forecast well, as model guidance yesterday indicated the 700 hPa low would set up near or just west of NYC; notably, the NAM model, which is occasionally over-amplified or too warm aloft in its shorter range, correctly identified this trend before most of the short-range model guidance. The positioning of this 700 hPa low this far west of the area allows the southerly flow to continue transporting above-freezing temperatures inland, and as of the frame posted to the left, only northwest NJ and Orange County would remain with plain snow, while northeast NJ and interior Connecticut change to sleet, with rain elsewhere.

Latest observations indicate that the 700 hPa freezing line has temporarily regressed to the New Jersey coast as heavier precipitation continues to fall. As the low pressure deepens and the warm air advection continues to strengthen the temperature gradient, however, the 700 hPa low will continue to deepen as well as it tracks northeast, leading to the surge of warm air aloft progressing into the area towards the early afternoon. This would support a changeover to plain rain from NYC and farther east by 2 PM, and to sleet in northeast NJ and southwestern CT by at least 2-4 PM. Precipitation rates are also expected to lighten up as the strongest frontogenesis and accordingly the heaviest snow shift north of the area. As the mid level low progresses to the northeast, temperatures aloft will cool down, with a 1-3 hour window of light to moderate wet snow possible this evening as far southeast as NYC before precipitation mostly ends by 10 PM.

 

updated_map

Nov 25

Nov 25, 2014: Major Early-Season Snowstorm Tomorrow

Forecast Highlights:

rgemFollowing yesterday’s unseasonable warmth with temperatures having peaked in the 70s west of NYC, a gradual cooling trend has commenced with temperatures generally in the 50s as of this afternoon. More significant cooling is expected into the overnight hours, however, as a moisture-loaded low pressure develops near the southeast US and quickly tracks up the East Coast, producing a major early season snowstorm before the beginning of meteorological winter and during one of the busiest travel days of the year.

Read the rest of this entry »

Nov 25

Nov 25, 2014 Early Noon Update

Notice

The 8-Day Forecast has been updated as of 1 PM. Today’s full forecast analysis will be posted by 3:30 PM.

12:10 Forecast Update: Heavy Snow Inland of NYC; Sleet, Rain Likely NYC & East

12z RGEM valid at 1pm Wednesday, depicting heavy snow north and west of NYC, while the inland extent of sleet and rain is along NYC into coastal Connecticut. Image from Environment Canada (Link to EC home page).

rgemAs last night’s update mentioned, the model guidance has not been consistent with the handling of the fine details of Wednesday’s low pressure. After an initial trend towards a more amplified low pressure on Sunday, yesterday’s model guidance trended towards increased separation between the two main shortwave disturbances aiding in the development of the low pressure, having resulted in a trend towards a farther east but also colder low pressure favoring a snowier outcome over most of the area. Beginning with the 0z model guidance last night as the forecast discussion was in progress, however, the high-resolution model guidance, which typically extend to about 48-60 hours and are often more reliable than the global GFS and CMC models in the short range, began reversing the eastward trend with a slightly deeper and more amplified low pressure.

The trend towards increased amplification will mostly lead to more snow over northern NJ and southeast NY, with an initial period of heavy snow likely in NYC and the north/west suburbs as well. The main concern with this trend towards more amplification, however, is a mid-level warm layer aloft which is able to penetrate farther inland due to the west trend relative to yesterday, which would result in a changeover to sleet. The inland extent of this warm layer is not completely agreed upon by the model guidance, although coastal Connecticut and Nassau county into parts of New York City are at highest risk of changing over to sleet, especially after the early afternoon hours. Southwestern Connecticut, Rockland and Westchester counties, New York City away from the coast, and northeast New Jersey along the I-95 corridor are more uncertain, however, with the model guidance indicating this warm layer stops extending inland just south of this aforementioned axis. This placement would support snow as the primary precipitation type in this axis with at least moderate snow accumulations, but a warming trend in this mid-level warm layer would increase the probability of sleet and lower accumulations.

 

To summarize, heavy snowfall is still expected to affect locations north and west of NYC, with the heaviest snow between about 10 AM and 4 PM, resulting in at least 6 to 10 inches of snow in northwest NJ, southeast NY and interior Connecticut. NYC and its immediate suburbs will also likely observe a period of heavy wet snow, but with a possible transition to sleet in the afternoon potentially limiting accumulations. This is more likely to be the case in NYC into Long Island, with at least 2 to 5 inches of snow likely in NYC and up to 2 inches in Nassau county, while the northwest suburbs of NYC into southern Connecticut are forecast to receive at least 4 to 8 inches of snow unless sleet expands farther inland than currently expected, which would slightly decrease accumulations.

Stay tuned for a full forecast analysis to be posted by 3:30 PM, with a final set of updated snow maps and a localized breakdown of forecast snow accumulations and timing.

Nov 24

Nov 24, 2014: From 70s To Heavy Snow on Wednesday

Notice

Tonight’s post reviews the latest trends in the forecast, with an update on the expected conditions. Two updates will be posted on Tuesday; a brief morning update with an updated 8-day outlook and any changes in the forecast, and an afternoon update with the final storm forecast and snow map.

Forecast Highlights:

rad42Significant warmth was observed across the region today following heavy rainfall last night, with temperatures having surged into the 70s west of NYC. This warmth will be short lasting, however, as a cold front moves through the area tonight leading to gradual cooling, with a low pressure developing to the south and tracking northeast up the coast, producing significant snow accumulations across the region, including the area, during the day on Wednesday on one of the busiest travel days of the year.

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Nov 24

Nov 24, 2014 Evening Update

9 PM Update:

11.24.14_snow_1As of this evening, there have been several changes in the forecast, which will be analyzed more in-depth later tonight. First, the low pressure has trended slightly east and weaker relative to yesterday, indicating a larger portion of the event remaining as snow from NYC and farther north/west, although mixing with rain is still expected over parts of NYC and Long Island, while parts of Connecticut may mix with sleet. The second change has been to speed up the timing of the storm, with the heaviest snowfall now expected from the mid-morning hours through the early to mid afternoon hours, especially north and west of NYC.

The full blog update will be posted by 11 PM, analyzing the forecast in greater detail. In the meantime, the 8-Day Outlook has been updated, and the latest snow map reflecting the aforementioned changes has been posted to the left.

Nov 23

Nov 23, 2014: Nor’easter To Produce Snow, Rain on Wednesday

Blog Update: A new algorithm has been implemented into the 8-Day Outlook to identify and split rain and snow outlooks by storm, meaning forecast rain and snow totals are now being listed separately for each individual storm.

Forecast Highlights:

gfs_namer_078_850_temp_mslp_precipThe forecast period will begin on a warm note, as periods of heavy rain tonight will give way to a much warmer Monday as temperature surge to near 70 degrees. A gradual cool down will then follow behind the cold front as a nor’easter develops to the south, quickly tracking northeast along the coast and producing significant snow and rain across the region on Wednesday during one of the busiest travel periods of the year leading up to Thanksgiving. Image from NCEP MAG.

Read the rest of this entry »

Nov 22

Nov 22, 2014: Warmth Monday, Snow Wednesday?

Forecast Highlights:

gfs_namer_108_850_temp_mslp_precipThe cold pattern which has brought well below normal temperatures over the last week is finally breaking down, with a trend towards warmer temperatures peaking in the 50s on Sunday and the 60s to even 70 degrees on Monday. A cold front will slowly move through overnight but will later stall offshore, when a nor’easter may develop along the frontal boundary and potentially affect the region with rain and snow on Wednesday. Image from NCEP MAG.

Read the rest of this entry »

Nov 21

New 8-Day Forecast Available

As of November 21, 2014, the 8-Day Forecast page has been upgraded to a more visual and interactive format. As before, forecasts are still generated by county, with the exception of Passaic county which is split to west and east. This upgrade, however, adds several new features to the page, with more to come in the near future. The new features and enhancements to the page currently available are listed below:

 

1. Point-and-click format – In the top of this page, the “View Mode” section allows toggling between the standard city/zip code format and the point-and-click format, which displays the forecast for the selected county.

2. New Visual Format – When viewing the forecast for a location, the names of the city/town and county are now displayed in large font in the top of the forecast. The date the forecast is valid through is displayed as well.

3. Hazardous Weather Headline – Based on the forecast for each location, any potential hazards within the 5-day range, their timing and probability are displayed above the forecast.

4. 7-Day Forecast Graphic – In addition to plain text, a 7-day forecast graphic is now displayed for every location in the area above the forecast text.

5. Forecast Summary – The bottom of the page displays a broad overview of the forecast, including the warmest and coldest days of the week, as well as projected rain, snow and ice totals.

 

When opening the page, it should automatically refresh to load the latest available forecast, which will be indicated by an “/#updated” appearing at the end of the address bar. If this does not occur, a manual refresh of the page may be needed in order to view the latest forecast. Work on this page remains in progress, and additional features will be added in the short term. Any comments, suggestions, or error reports can be posted in the comments section below, or e-mailed to weatherinnyc@gmail.com. Included below is a preview of the latest forecast generated for Manhattan.

 

fcst_preview

Nov 19

Nov 19, 2014: Historic Lake Effect Snow in W NY

Notice

This post is an overview of the significant lake effect snow affecting western NY. For the latest NYC metro-centered forecast, please refer to yesterday’s forecast discussion.

Historic Lake Effect Snow Continues in West NY

While significant cold air continues to affect the region, with temperatures this morning in the 10s inland of NYC and even as cold as 22 degrees in Central Park, the main current highlight in the region is the excessive lake effect snow accompanying the strong cold blast, with snow totals over 5 feet over portions of western NY state.

 

les_setup

Graphic illustrating the typical development of lake-effect snow bands. Image URL from Pennsylvania State University, image credit to NWS Buffalo.

Posted above is a simple illustration of a typical lake effect snow setup; in this example, a shallow cold air mass advances over the warm Great Lakes into the northeast US, with moisture from the relatively warm lakes rising and condensing into clouds, which then advance over land and produce precipitation, in the form of snow if temperatures are cold enough. The setup and intensity of lake effect snow bands, among many other variables, also depend on the direction of the wind relative to the lake axis; if the wind in the lower atmosphere is nearly perpendicular to the lake axis, multiple narrow lake effect snow bands may develop, or simply no bands at all. If the lower level wind is parallel to the lake axis, however, development of a single, intense snow band is often favored, producing heavy snowfall rates just downwind of the lake.

 

nam_wind

3-hour 18z NAM forecast from yesterday, valid at 4 PM EST Tuesday (11/18), depicting surface dewpoints in colors and wind barbs in black. Wind barbs indicate the direction the wind is coming from; in this case, winds are coming from the southwest over Lakes Erie and Ontario, and from the west over the NYC area. Image from Tropical Tidbits.

Yesterday’s approximate surface temperatures and winds are illustrated above from the short term NAM forecast. Even though the map above shows dewpoints, the overall idea of cold temperatures over land and warmer lake temperatures can still be inferred from the map. Additionally, the wind direction was oriented nearly parallel to the axes of Lake Erie and Lake Ontario, conditions which are both indicative of the potential for significant lake effect snowfall. Ultimately, a single lake effect snow band developed just east of Lake Erie, remaining stationary over the same locations throughout the majority of the day while consistently producing heavy snow, resulting in excessive snow totals over western New York. Instability was also sufficient to result in thundersnow and snowfall rates over 4 inches per hour under this band earlier in the day yesterday.

 

11.18_1800

Regional radar imagery as of yesterday afternoon, depicting the heavy snow band south of Buffalo just east of Lake Erie, and another band over northwest NY east of Lake Ontario. This band was nearly stationary throughout most of the day, producing excessive snow totals. Image from PSU e-Wall.

Even though locations surrounding the Great Lakes frequently receive lake effect snowfall, especially early in the winter season before the lakes begin to freeze, snowfall of this magnitude is quite anomalous. According to the latest snowfall reports from the National Weather Service in Buffalo, as of 11 AM, many locations near Lake Erie recorded over 40 inches of snow, including a report of 63 inches in Lancaster, NY. Due to the persistence of the snow band over the same locations, however, these significant snow totals are also very localized; less than 10 miles to the north, Buffalo airport only recorded 6 inches of snow. While the lake effect snow will gradually decrease in intensity, snow continues to fall, and is approaching record values in some locations; according to the National Climatic Data Center (NCDC), the maximum 24-hour snowfall record in the US is 75.8 inches in Silver Lake, Colorado.

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