Apr 25

April 25, 2015: Chilly Ending To April

Forecast Highlights:

na_f0A strong upper level low continues to persist over the region, accompanied by below normal temperatures and even some snow and graupel in interior locations on Thursday, but with temperatures slowly moderating as the upper level low gradually departs. Temperatures will return into the 60s and possibly 70s again by the mid week, but another approaching trough will keep any significant warmth suppressed to the south through at least next week.

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Apr 22

April 22, 2015 Brief Update

Afternoon Update: T-Storms, Gusty Winds Possible This Afternoon

As mentioned during the previous forecast update, a significant low pressure system developed on Sunday before affecting the tri-state area on Monday, producing a swath of very heavy rainfall on Monday morning which amounted to as much as 1.20 inch of rain in just 3 hours at Caldwell, NJ. Much of this heavy rain can be attributed to strong upward vertical motion aloft, particularly due to strong frontogenesis, related to the strengthening of the temperature gradient.

Drier conditions resumed on Tuesday, but with another rain event expected to affect the tri-state area today as a line of precipitation forced by a strong vorticity maximum rotating through New York state and Pennsylvania around the cutoff upper level low. This shortwave trough has noticeably trended slower over the last two days on the model guidance, allowing for an extended duration of sunshine in the morning hours resulting in warmer temperatures than expected, having surged into the upper 60s and low 70s in the immediate NYC area.

15z RAP Skew-T valid at 1800 UTC (2 PM EDT) this afternoon, depicting a vertical profile of the atmosphere with height in vertical coordinates, temperature in the red line, dewpoint in green, and the surface-based air parcel temperature in blue. Image from College of DuPage.

15_RAP_003_40.89,-74.1_skewt_SBThe warmer temperatures are only accompanied by marginal instability, however, with Convective Available Potential Energy (CAPE) values near 250-500 J/kg and Lifted Index values near -1 degree Celsius. A look at model-derived soundings, such as the 15z RAP sounding valid at 1800 UTC this afternoon posted to the left, from the College of DuPage, further supported by current surface observations and the SPC mesoanalysis, reveals a very deep mixed layer approaching the 750 hPa pressure level, as indicated by the nearly constant decrease of temperature with height following the dry-adiabatic lapse rate at -9.8 C/km. Such a deep mixed layer would also be expected to feature dry conditions near the surface, as is currently the case with dew points only in the 30s and relative humidity values below 35-40%. CAPE is defined as the region of positive buoyancy in the atmosphere, or where the temperature of an air parcel lifted from the surface is warmer than that of the environment; this is represented in the Skew-T to the left by areas where the blue line is to the right of the red line, with high CAPE where this difference is very large. There is a swath of positive CAPE, although very narrow, suggesting relatively minimal CAPE approximately near 500 J/kg which may be sufficient for some thunder but not conducive for a widespread severe thunderstorm event.

Latest regional radar, from the National Weather Service.

northeastOnce the rain to the west arrives into the area towards 3-6 PM from west to east, the boundary layer will moisten up with temperatures decreasing and dewpoints rising. This will also allow some of the stronger winds aloft to mix down to the surface, potentially allowing for wind gusts near or above 40 mph to accompany the heavier showers. Any stronger thunderstorm that develops may be capable of producing small hail, although at this time the risk of stronger wind gusts appears to be the highlight of today’s event, with a lack of sufficient moisture and forcing to produce heavy rain amounts over 1/4 inch for most locations. As synoptic forcing decreases to the east of NYC with the shortwave trough passing to the north of the area, along with a more stable air mass to the east, any thunderstorm is likely to weaken in intensity towards Long Island and Connecticut with a lower risk of strong wind gusts and likely lower rain totals.

Apr 19

April 19, 2015: Warm Weather Transitions to Stormy Pattern

Forecast Highlights:

nam4km_ref_neus_9The last two weeks have been relatively inactive, but featured a much warmer pattern with temperatures frequently in the 60s and 70s. Yesterday marked the peak of this warm pattern, however, as cooler temperatures return on Monday with a heavy rain event, while a return of high latitude blocking will result in a powerful upper level low stalling over the region with a return to cooler than average temperatures through the remainder of next week (Image from Tropical Tidbits).

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Apr 12

Blog Statement on the Recent Lack of Updates

April 12, 2015 Blog Statement:

Over the last several months, updates to the blog have been increasingly sporadic. This is due to a multitude of factors, including an increasingly busy schedule restricting the hours I can dedicate to providing a high-quality, detailed analysis of the forecast on a daily basis, as well as work on various projects which in the long term would serve to benefit the blog with a larger quantity and improved quality of products, such as model guidance plots, which are well under way at this time. I will not be able to resume daily updates immediately, although updates will be posted as frequently as possible, especially during any major weather event; a forecast update for this evening is currently in progress. I apologize for any inconvenience resulting from the current lack of updates, as progress continues on long term projects for the blog with a focus on continuing to provide high-quality forecast analyses.


Apr 06

April 6, 2015: Rain, Cooler Temperatures Upcoming

Forecast Highlights:

temp36A frontal boundary became stationed over the region yesterday, accompanied by a wave of rain and high elevation snow remaining north of the area. This front will remain to the north again today with mild temperatures, but will gradually push southward with occasional waves of rain through the rest of the week, potentially ending by Saturday as a stronger cold front approaches with more seasonable temperatures possible.

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Apr 05

April 5, 2015: Cloudy, Wet Week Ahead

Forecast Highlights:

nam4km_T2m_neus_2Mild temperatures returned into the region on Thursday and Friday, surging into the 60s for many locations. A wave of low pressure on Friday night ushered in colder temperatures yesterday, but with a baroclinic zone becoming stationed over the region serving as the focus for numerous waves of low pressure to progress through the region over the next week, bringing frequent periods of overcast and rain with frequent temperature swings.

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Mar 31

Mar 31, 2015: March Begins, Ends With Snow

Forecast Highlights:

wrf-nmm_ref_frzn_neus_23March has been a notably snowy month in the tri-state area, currently ranking as the 7th snowiest March on record in Central Park, accompanied by below normal temperatures but to a much lesser extent than February. The month is set to end on the same note as it began for parts of the region, with a low pressure producing mostly rain for eastern regions but heavy wet snow over Pennsylvania into northwest NJ.

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Mar 27

Mar 27, 2015: 60s Yesterday, Snow Tomorrow

Forecast Highlights:

wrf-nmm_ref_frzn_neus_26The major temperature swings typical of spring are once again occurring, as following a warmth spike yesterday with temperatures surging into the 60s in parts of the area, a cold air mass will overlap with a developing low pressure offshore to produce widespread snow over southern New England into Long Island on Saturday. A gradual warming trend is likely into next week, building up to a potentially significant warmth surge ahead of a cold front next Friday. (Image from Tropical Tidbits)

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Mar 25

Mar 25, 2015: Rain, T-Storms Likely Tomorrow

Forecast Update: Rain, T-Storms Likely on Thursday

18z NAM hour 24, valid at 1800 UTC Thursday, 3/26 (2 PM EDT), depicting scattered heavy showers and thunderstorms over the tri-state area with an approaching low pressure to the west. Image from Tropical Tidbits.

nam4km_ref_neus_8After a relatively inactive week, with a period of cold temperatures, rain and possibly snow return into the forecast for the late week into Saturday, possibly accompanied by warm temperatures for southern parts of the area. A developing low pressure over Oklahoma, currently triggering the first major severe weather outbreak of the month after a nearly month-long stretch of no tornado reports in the US, will continue to quickly progress northeast, remaining relatively weak due to a lack of broad synoptic forcing but nonetheless producing widespread precipitation north of a strong baroclinic zone.

Thursday Afternoon: The low pressure is expected to track through Pennsylvania on Thursday afternoon with its warm front slowly crawling north through central NJ, with a very warm air mass to its south as temperatures surge into the 60s and 70s. To the north of the warm front, over the tri-state area, a shallow cold air mass will linger near the surface, with temperatures remaining in the 40s and low 50s through the early afternoon, but with a strong southwesterly low-level jet streak advecting a warm and moist air mass aloft, where sufficient elevated instability is modeled to result in elevated convection, and accordingly locally heavy rain and thunderstorms, despite the colder surface temperatures. This activity is likely to be centered towards the 12 to 5 PM time frame, with the heaviest rain likely towards Long Island where over 1/2 to 1 inch of rain is possible by the evening hours.

Thursday Evening-Night: Following this activity, the warm front associated with the approaching surface low pressure is expected to progress north into the area, but the northward extent is somewhat uncertain as model guidance typically exaggerates the northward extent of warm fronts in similar synoptic setups in the spring. Unlike the GFS and ECMWF, which depict widespread temperatures in the upper 50s and 60s up through the central Hudson Valley region, the NAM model depicts the warm front struggling to expand north of NYC until about 5-7 PM, when temperatures briefly rise into the mid 50s to low 60s throughout the area ahead of the cold frontal passage. This scenario is more typical of past analogs and appears to be more reasonable at this time. The cold front is likely to move through between 8 PM to 12 AM from west to east, with temperatures sharply dropping into the 30s to low 40s and scattered showers and thunderstorms continuing through about 12-2 AM.

Friday & Beyond: Another trough is expected to amplify over the region by the weekend, bringing colder temperatures back into the picture with high temperatures struggling past the upper 30s on Saturday and Sunday. While significant cyclogenesis will occur too far offshore for significant impact with the trough axis too far east, some models are hinting at a potential inverted trough setup which may produce some light snow in eastern parts of the area on Friday night into Saturday. A more detailed update will be posted on Thursday morning.

Mar 20

Mar 20, 2015 Storm Updates

Occasional updates will be posted below on the snow event affecting the region today. The latest updates, along with the latest available snow reports, will be at the top of this post. Radar images are from the National Weather Service.

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


Saturday, March 21
10:30 AM: Snow Tapers Off in Connecticut, Long Island

northeast (2)As of 8 AM, the coastal low pressure which developed offshore last night continues to slowly deepen with a minimum pressure near 1004 hPa. The low is located well southeast of Cape Cod and will continue to deepen over the next day as it tracks towards Nova Scotia.

Following yesterday afternoon’s update, snow intensified on the back end of the system with widespread moderate to heavy snow for most places, but generally tapered off by 8-10 PM with scattered freezing drizzle and snow showers persisting east of NYC overnight. A separate vorticity maximum approaching from the Great Lakes is beginning to interact with this system offshore, which will eventually initiate rapid deepening of the low pressure offshore, although the redeveloped precipitation shield is more robust than modeled, with widespread light to moderate snow persisting over Long Island and Connecticut. This snow will generally taper off towards 11 AM to 12 PM, although may not accumulate to much as temperatures continue to rise and snowfall rates may not be intense enough to offset the melting rate.

Based on preliminary storm reports, snow totals so far are within the 2 to 4 inch range over southeast NY, eastern CT and interior CT, which is near to slightly below the forecast amounts; snow totals were higher than forecast over central NJ into Long Island and coastal Fairfield county, where snow accumulations were generally in the 4.5 to 7 inch range. Totals were near the forecast amounts over northern NJ and NYC, with 3 to 5 inches.


Friday, March 20
3:45 PM: Snow Lighter Than Expected for Some

northeastAstronomical spring is starting across the region with yet another snowstorm, but with less snow for parts of the area than others. The latest regional radar imagery continues to depict widespread snow across the region, but with lighter snow rates persisting over northern NJ north of the I-80 corridor into southeast NY and southern CT.

The lighter snow rates over most of the area are partly associated with the stronger synoptic forcing and upward motion focused over central NJ, where heavy snow continues to fall with over 3-4 inches in some locations as of 3 PM, with subsidence, or sinking air, to the north of the heavier snow over most of the tri-state area resulting in lighter snow rates and smaller flakes. Another issue, however, is the stubbornly dry air mass, with relative humidity only at 19% in Central Park last night as dew points remained in the single digits. The atmosphere took longer to moisten up than anticipated, with a longer period of snow evaporating before reaching the ground. These factors contributed to result in lighter snow rates, which struggled to accumulate at the onset given the warm surface temperatures coming off of the recent relatively warm pattern, and in some locations snow has yet to accumulate on paved surfaces.

Snow will continue to gradually wind down over the next several hours as a coastal low pressure quickly forms offshore with enhanced subsidence over the tri-state area, mostly tapering off by 8-11 PM. Last night’s forecast update noted a risk of less snow than forecast north of NYC; this appears to be the case for some locations, and parts of northern NJ, NYC and eastern Long Island may struggle to surpass 2-3 inches for a storm total. Farther south, steady moderate to heavy snow continues over central NJ into parts of NYC and western Long Island, where 3 to 5 inches of snow are still on track.

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