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NYC (KLGA) Climatology for October

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During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. This post continues on this theme, adding a climatology for October.

Other Month’s Climatologies

January
February
March
April
May
June
July
August
September
November
December

Station Basic Information

City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)

Local Geography and Topography

Station Elevation: 10 feet above sea level.

Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.

KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view

Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.

Topographical map of New York State

Per the Local Climatological Data report from the National Weather Service:

On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.

National Weather Service – NYC Office

Wind Patterns

Below is a wind rose – you can read more about how to interpret this chart here.

Frequency (percentage) of the single most common wind direction: Due southwest (9%). Note: due northeast is basically co-equal most common wind direction, perhaps only off by a fraction of a percentage point.

Directions that are most and least common: Other most common wind directions include due northeast (9%), due northwest (8.75%), and west-northwest (8.25%). Least common wind directions are east-southeast (1.8%), due southeast (2.75%), and due east (3%).

Direction(s) most likely to produce the fastest winds: Winds of 16.5-21.4 knots are most frequently found coming from due northwest. West-northwest, north-northwest, due northeast, east-northeast, and due south directions can also see less frequent winds over 21.4 knots.

Direction(s) least likely to produce the fastest winds: As is the case with several other months, the least common wind directions of due east, east-southeast, and due southeast also rarely seen winds in excess of 16.4 knots.

Impacts of wind direction on local weather: October’s wind profile is a continuation on the trend established in September where wind frequencies begin to be distributed more evenly across the compass and less focused in the south. Winds from the northwest quadrant continue to increase in frequency, along with the frequency of the fastest winds coming from these directions. Northeasterly winds still figure prominently, though slightly less so than in September. Curiously, while southerly winds generally decreased in frequency, due southwest winds tick up slightly compared to September.

In general, northwesterly winds bring cooler, drier Canadian air into the region following cold fronts. Northwesterly winds will tend to warm slightly because of compressional warming as they downslope coming off the higher terrain of the Catskills, and Poconos outside of the city. This can sometimes lead to warmer temperatures than would normally be expected for this wind direction.

Northeasterly winds, on the other hand, are often related to backdoor cold fronts sweeping from the Canadian Maritimes, the onshore flow ahead of an advancing warm front, or a passing coastal storm to the south. Southwesterly winds are still capable of bringing in oppressive heat, as seen in the temperature section below.

Maximum observed two-minute wind speed for the month: 56 knots (64 mph) – this is the fastest recorded two-minute wind speed for any calendar month.

Temperature and Precipitation Averages/Records

Temperature units are in Fahrenheit and precipitation is in inches.

Worth noting: Even though average high temperatures in October fall into the 60s, it’s still possible to see summer-like warmth – you can see this from the record high of 95°F, set anew in 2019, and that record highs can reach into the mid-80s until late in the month. Overnight lows start to fall into the 50s and end up in the 40s by the end of the month!

DateNormal HighNormal LowRecord HighRecord LowRecord Lowest MaxRecord Highest MinNormal PrecipRecord Precip
17156873954730.132.24
27056954256720.121.15
37056854049690.121.07
47055863853730.121.05
56955933849690.131.58
66954924054700.121.69
76854854052720.123.44
86854894046710.133.51
96853854051710.132.00
106753893647710.132.26
116753863649710.132.59
126752864152680.124.39
136652863951700.121.63
146652824051700.131.66
156651833948680.132.55
166551823949630.121.71
176551833849660.120.95
186450823547680.121.33
196450863242680.123.62
206450793244650.122.36
216349823346640.121.81
226349833348650.132.15
236349863241670.122.47
246248803050670.121.43
256248783747610.121.52
266248763441650.123.30
276148793343660.111.76
286147803345630.122.84
296147773342630.112.78
306047813545650.111.18
316046813344620.123.30
Range60-7146-5676-9330-4241-5661-730.11-0.130.95-4.39



NYC Weather Update – Oct 1, 2019

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This week will see a wild temperature swing that will take us into the upper-80s, giving us a last taste of summer, followed up by a plunge into decidedly fall-like temperatures barely in the upper-60s. Friday night in particular looks like it could be the coldest yet this season, with overnight lows possibly dipping below 50°F. Temperatures start to recover over the weekend to around normal for this time of year around 70°F.

Rest of today – partly sunny with high temperatures warming into the upper-70s, near 80°F. A warm front is forecast to push through and winds will be coming from the southwest, ushering in some renewed warmth. Overnight lows around 70°F.

Weather Prediction Center surface forecast for 8AM Wednesday

Wednesday – with a warm front passing Tuesday, and high pressure setting up over the Southeast US, warm air will be surging from the southwest towards the region. This should allow high temperatures to rise well into the upper-80s and possibly close to 90°F. A cold front will be approaching from the north Wednesday night, bringing along rain and thunderstorms. Temperatures will cool off quickly with the rain and the cold front, dropping into the upper-50s.

Thursday – much cooler with high temperatures only reaching into the low-60s. Rain chances linger during the day with the cold front stalling out south of us and another low pressure center pushing east. Overnight lows not much cooler than daytime highs, around 60°F as rain chances continue.

GFS model output for precipitation, 1000-500 mb thickness, valid for 10AM Friday. Note the relatively closely packed black lines (isobars) between the high pressure centered over Wisconsin and the Low off the coast of Nova Scotia. This pressure gradient will deliver some windy conditions Friday.

Friday – conditions should clear up as the low pressure responsible for rain on Thursday moves off to the east. A tight pressure gradient is forecast to form between this low and a high pressure center building over the Great Lakes. This should result in a windy day, with high temperatures bouncing back into the mid-60s. Overnight lows are forecast to drop to around 50°F with the influence of the much cooler continental polar airmass and north/northwesterly winds.

NYC Weather Update – Sept 24, 2019

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A warm start to fall yesterday gives way to cooler, drier weather that will actually feel like fall. Much as was the case last week, high pressure will be in control over the area. This should lead to generally fair weather. However, there is some chance for rain Thursday as a cold front pushes through.

Rest of today – mostly sunny with high temperatures in the upper-70s. Overnight lows in the low-60s.

Weather Prediction Center surface forecast for 8AM Wednesday

Wednesday – high temperatures again mild in the upper-70s with warming influence of downsloping northwest winds. Overnight lows in the mid-60s.

Thursday – increasing clouds with high temperatures in the low-80s and overnight lows in the upper-50s. A chance of showers later in the day as a cold front approaches.

GFS model output for 2 meter above ground level temperature and 10 meter above ground level winds at 8PM Friday

Friday – high temperatures in the mid-70s with sunny skies and overnight lows in the low-60s behind the cold front.

NYC Weather Update – Sept 16, 2019

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A weak frontal boundary passes through today. Behind this, high pressure will build and extend into the weekend. This persistent high pressure will result in mostly sunny conditions. Below normal temperatures in the low-70s will accompany northeasterly flow but a warming trend is forecast for the weekend as the center of high pressure shifts to the east.

Rest of today – mostly cloudy with a cold front pushing south then stalling out. Chances for light rain linger through the day. High temperatures in the upper-70s. Overnight lows in the low-60s.

High resolution rapid refresh simulated radar for 5PM today, showing some scattered light showers

Tuesday – high temperatures cool off into the low-70s behind the front. Winds turn to the northeast as high pressure builds over central Quebec. Overnight lows around 60°F but could dip into the upper-50s.

Wednesday – mostly sunny with high temperatures in the low-70s and overnight lows in the upper-50s.

GFS model output for surface pressure and winds valid at 2PM Thursday. The area of high pressure over the Northeast will serve to keep our weather fair, while pushing Hurricane Humberto safely out to sea.

Thursday – high temperatures remain in the low-70s, overnight lows in the upper-50s. It will definitely feel like autumn has arrived a few days early!

NYC Weather Update – Sept 10, 2019

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Temperatures are on an upward trend towards midweek. A cold front moving in from the north brings the chance for rain and thunderstorms tomorrow into Thursday. Behind the cold front, a brief cool down to below normal temperatures occurs before temperatures go back to above normal in the low-mid 80s over the weekend.

Rest of today – early clouds should give way to some partial clearing by the afternoon, high temperatures in the upper-70s. Overnight lows in the upper-60s.

GFS model output for 2 meter above ground level temperatures and 10 meter above ground winds at 11AM Wednesday. Noticeably warmer temperatures will accompany the shift to southwesterly winds.

Wednesday – high temperatures in the upper-80s, with a chance for scattered showers and thunderstorms later in the day as a prefrontal trough associated with a cold front approaches. Overnight lows in the low-70s with continued chances for showers. Overall, a much warmer and more humid feeling, summer-like day with southerly winds and the presence of a high pressure center south of the area.

Thursday – clearing skies with highs in the low-80s. Cooler and drier behind the passing cold front. Overnight lows in the low-60s.

GFS model output for surface pressure and 10 meter above ground level winds on 2AM Friday. Winds by this time are forecast to have shifted to the northeast and east. Onshore flow will keep temperatures cool on Friday.

Friday – high temperatures remain in the mid-70s, overnight lows in the mid-60s as high pressure builds in briefly to the north. This will bring a cooler, northeasterly onshore wind to the area.

NYC Detailed Forecast for September 6, 2019

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The forecast for tomorrow in NYC hinges largely on what happens with Hurricane Dorian’s track. Although the center of this still powerful storm is expected to pass well south of the region, its influence on wind patterns, and its outer bands of clouds and rain could still impact the city. In essence, as has been alluded to by others, Dorian’s impact on NYC will be not unlike that of a Nor’easter. This forecast, as a result, carries a fair deal of bust potential especially as it relates to precipitation totals and to some extent with the maximum sustained winds.

My Forecast
High: 71°F | Low: 63°F | Max sustained winds: 24 mph | Total precipitation: 0.02″ – verification will come from METAR data for the period between 2AM Friday and 2AM Saturday (06Z Friday to 06Z Saturday) at LGA (LaGuardia Airport), and the KLGA Daily Climate Report.

Verification
High: 69°F | Low: 60°F | Max sustained winds: 32 mph | Total precipitation: 0.03″
Surprisingly, despite personally witnessing some moderate rain bands move over Midtown on Friday, KLGA itself only picked up 0.03″ of precipitation from the outer rain bands of Dorian that hit the city. That means my precipitation forecast was almost spot-on in this respect. I was 2-3°F too warm on both high and low temperatures. Statistical models weren’t really expecting much in the way of rain. When rain did occur temperatures cooled somewhat via evaporational cooling since the atmosphere did start off on the dry side Friday and there was room for this cooling to take place. The forecast bust, as it turns out, was with the maximum sustained wind, where I fell short by 8 mph. Despite low rainfall totals, even a small bout of rain could have kicked up winds via downward momentum transfer of somewhat faster moving winds aloft. The other thing to remember is that northeast winds are climatologically most likely to produce the fastest winds in September. This wind direction also happens to have a long fetch over open water for many north-facing shores in NYC, including KLGA. That long fetch over open water would tend to reduce surface friction since water is smoother than land on average, and allow for faster winds.

  • Weather Prediction Center surface forecast for 2AM Friday at the beginning of the forecast period
  • Weather Prediction Center surface forecast for 8PM Friday
  • Current GOES satellite image of Hurricane Dorian – it’s cirrus shield has already reached NYC

Synoptic Set Up
Hurricane Dorian will be approaching from the south and the storm’s center will accelerate northeast well offshore of the NYC area during the forecast period. At the surface, high pressure will be in place early in the forecast period, but will weaken quickly. A surface trough is forecast to form and travel east while a second surface trough associated with Dorian may form late in the forecast period.

At the 500 mb level, a shortwave trough will be moving east. This will assist in continuing to accelerate Dorian off to the northeast. This interaction with Dorian will also bring some relatively strong divergence across the area later in the day, providing some enhanced lift that could be conducive to rain. Finally, at the 300 mb level, there is some evidence that we could be in a favorable exit region of a jet streak. This will server to further enhance upper level divergence and lift at these levels.

North American Model (NAM) forecast for 500 mb height and vorticity at 11PM Friday. The intense vorticity associated with Dorian is clearly evident in the bottom right, along with a fairly potent shortwave trough approaching from the west

High Temperatures
GFS, NAM, and NBM are in consensus that Friday will be a much below average day in terms of high temperatures. All three statistical models come in at 71ºF or 72ºF for high temperatures, which is about 8-9ºF below normal for this time of year. The synoptic setup for tomorrow favors northeasterly to east-northeasterly winds given the counter-clockwise flow around Dorian’s center, and similar to the situation with a passing Nor’easter. These wind directions favor onshore flow, bringing in a cooler, moist air mass off the Atlantic into the area. With sea surface temperatures hovering right around 70ºF, there’s not too much more room for cooler temperatures. This set up also favors low clouds, and this is indicated in the statistical models as well. I don’t see too many “gotchas” so I’m going with 71ºF.

Low Temperatures
Statistical models show the chance that the low temperature during this forecast period may actually occur overnight going into Saturday. The fact that GFS, NAM and NBM all converge on 63ºF as the low gives me increased confidence that this is a fine temperature to settle with. There aren’t any signs of strong temperature advection or changes in airmass to warrant a big deviation from this.

  • NAM MOS forecast
  • GFS MOS forecast

Max Sustained Winds
Even though the center of Dorian will pass well offshore of NYC, the large radius of its wind field will still result in fairly robust winds across the area. The average of GFS and NAM MOS is about 18 knots. Climatologically speaking, northeasterly winds are actually the most likely to produce winds exceeding 21 knots during September. I think it’s possible that if a rain storm related to Dorian moves through, we could see sustained winds up to about 25 mph with some downward momentum transfer possible.

Total Precipitation
This is a tricky precipitation forecast because forecast models like SREF place the probability for precipitation over 0.10″ not that far away from the city, and the gradient of probabilities increases quickly the further east you go. This makes sense, since the best conditions for rainfall will be closer to the core of Dorian. Given that the track forecast for Dorian seems to be shifting further offshore, though, it doesn’t make sense to bank on a big rainfall total from this event. The GEFS and SREF QPF plumes average is only 0.06″, hardly a big rainfall total, though the spread is anywhere from nothing to about 0.20″. The atmosphere starts of relatively dry tomorrow, and 850 mb forecasts don’t suggest any low-level jet conveying deep moisture (though Dorian can definitely provide this too). Furthermore, forecast soundings show a pretty persistent dry layer in the mid-levels of the atmosphere. So despite the fact there’s going to be some enhanced lift from divergence at the 500 mb and 300 mb levels tomorrow, I’m erring on the side of this passing storm only dealing a glancing blow. I don’t think it’ll be completely dry, though, like GFS and NAM MOS, so I’ve gone ahead with 0.02″ as a forecast for total rainfall.

  • NAM forecast for 850 mb relative humidity and winds at 11PM Fridayat
  • GFS forecast for 850 mb relative humidity and winds at 11PM Fridayat

NYC (KLGA) Climatology for September

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During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. This post continues on this theme, adding a climatology for September.

Other Month’s Climatologies

January
February
March
April
May
June
July
August
October
November
December

Station Basic Information

City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)

Local Geography and Topography

Station Elevation: 10 feet above sea level.

Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.

KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view

Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.

Topographical map of New York State

Per the Local Climatological Data report from the National Weather Service:

On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.

National Weather Service – NYC Office

Wind Patterns

Below is a wind rose – you can read more about how to interpret this chart here.

Frequency (percentage) of the single most common wind direction: Due northeast (9.5%).

Directions that are most and least common: Most common wind directions include due south (9%), due southwest (8.5%), and northwest (8%). Least common wind directions are east-southeast (1.5%), due southeast (2.5%), and due east (3.5%).

Direction(s) most likely to produce the fastest winds: Winds of 16.5-21.4 knots are most frequently found coming from due northeast, and due northeast. North-northwest, east-northeast, and due south directions can also see less frequent winds over 21.4 knots.

Direction(s) least likely to produce the fastest winds: As is the case with several other months, the least common wind directions of due east, east-southeast, and due southeast also rarely seen winds in excess of 16.4 knots.

Impacts of wind direction on local weather: September’s wind profile sees a shift away from the general pattern of the summer months preceding it (June, July, August) during which southerly winds are predominant, and during which due south is the single most common wind direction. April is the last calendar month when due south isn’t the single most common wind direction, so it takes quite a bit of time in order for winds to shift off this summer pattern.

The most common wind directions in September are almost evenly split between northeasterly and northwesterly and southerly and southwesterly winds in percentage terms of frequency. Northwesterly winds are notably more frequent, and faster northeasterly and northwesterly winds start to appear in September as opposed to the summer months. Northwesterly winds in general will bring cooler, drier Canadian air into the region following cold fronts. Northeasterly winds, on the other hand, are related to backdoor cold fronts sweeping from the Canadian Maritimes, the onshore flow ahead of an advancing warm front, or a passing coastal storm.

The continued downward trend in southerly winds is likely a reflection of the diminishing influence of sea breezes as average land temperatures cool while average sea surface temperatures are still close to their peak (though also cooling). The narrowing gap between these two will tend to reduce the potential for sea breeze circulations to set up. However, these sea breezes can still exert an influence on local temperature and can still provide boundaries for convection. Southwesterly winds are also similarly capable of bringing in oppressive heat, as seen in the temperature section below.

Maximum observed two-minute wind speed for the month: 44 knots (51 mph)

Temperature and Precipitation Averages/Records

Temperature units are in Fahrenheit and precipitation is in inches.

Worth noting: September can still offer up oppressive heat, especially in the first half of the month. The monthly all-time record high of 102°F is also the second highest record temperature recorded for the year at KLGA (August has the hottest record high: 104°F). September’s monthly precipitation record of 4.63″ in one day is also has the 3rd highest single-day precipitation record after April and August. While it can get quite hot in September, it’s also possible for temperatures to get cold too, with the record low for the month in the low-40s! Average high temperatures dip back below 80°F and average lows drop below 60°F again in September after the summer months, marking a definite fall feel.

‘DateNormal HighNormal LowRecord HighRecord LowRecord Lowest MaxRecord Highest MinNormal PrecipRecord Precip
18167965567780.111.20
281661025665810.122.88
38066955468770.122.33
48066935466800.123.12
58066935467790.121.00
68065965362790.123.21
77965905366740.122.37
87965955465760.123.85
97964935365770.110.56
107864955067760.121.47
117864965164780.122.93
127863945268750.123.63
137763935061750.132.93
147763924863740.132.93
157762914959760.130.82
167662954862730.124.63
177662954861790.121.65
187661894864720.131.99
197561924961720.131.62
207560904661740.131.72
217560894359730.133.02
227460934756730.121.00
237459934257770.122.14
247359914452730.121.34
257359904257720.132.84
267358904457720.123.19
277258904356730.132.72
287257844256720.143.27
297157864358720.141.85
307157884356710.141.80
Range71-8157-6784-10242-5652-6871-810.11-0.140.56-4.63



Upper Level Divergence on Display – Aug 21, 2019 Thunderstorms

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Last Thursday, we saw a round of severe thunderstorms develop and roll through the NYC area in the afternoon hours. My instructor (Steve Corfidi) and TA (Phil Lutzak) from my Penn State World Campus Weather Forecasting Certificate program noticed an noteworthy feature in satellite images of the event.

GOES archived visible imagery satellite loop from 1:16 PM through 5:11 PM Thursday, Aug 21, 2019.

You can see that there’s an arcing, convex, wave-like feature oriented southwest-northeast that sweeps east across the Northeast in the visible satellite loop above. If you’re having trouble spotting it check out the series of annotated images below that marks the leading edge of this feature in different points along its progression.

Professor Corfidi noted that this feature seemed to line up well with an area of drier air at the mid-upper levels of the atmosphere, which he picked up in the infrared Channel 8 satellite images. For reference, I’ve superimposed the IR channel on the visible satellite channel from the same time, which is also the 3rd frame of the gallery above.

  • GOES East IR Channel 8 image at 3:36PM Thursday, Aug 21, 2019
  • GOES visible satellite image from the same time, annotated to indicate the position of the leading edge of the wave-like feature
  • GOES IR Channel 8 superimposed on the visible satellite image taken at the same time (3:36PM Thursday, Aug 21, 2019)

What’s more, referring back to the visible satellite loop above, it’s evident that this feature was also partially responsible for firing up strong to severe thunderstorms along the NJ/PA border that eventually tracked east over the NYC area. Storm reports from the day indicate that several of these storms produced damaging wind gusts.

  • Archived radar image from 3:35PM, Thursday, Aug 21, 2019. Note the positioning of the strong radar echoes and the leading edge of the wave-like feature at this same time.

It’s evident there’s some causative relationship between this wave-like feature and the eruption of afternoon thunderstorms along its leading edge, and this all raises the question: what was this phenomenon? I did some investigation of various upper air analyses from the Storm Prediction Center and found that this phenomenon correlated well with two features at the upper levels of the atmosphere.

  • Storm Prediction Center analysis of 300 mb divergence valid for 4PM Thursday, Aug 21, 2019
  • Storm Prediction Center analysis of 400-250 mb potential vorticity valid for 2PM Thursday, Aug 21, 2019

First, we can see that there’s a swath of increased divergence noted at 300 mb (areas outlined in pink) that correlates somewhat with this area of drier mid-upper level air. The second image is perhaps even more convincingly linked to this phenomenon – showing an area of increased potential vorticity. But what does potential vorticity indicate about the atmosphere? In this case, potential vorticity indicates a lowering of the local tropopause – the boundary between the troposphere, where all our weather takes place, and the stratosphere above it. The stratosphere, relative to the troposphere is much drier, and this explains the source of the clear drier region picked up in the GOES Channel 8 infrared images.

Colorado State University depiction of the relationship between the stratosphere and troposphere when there’s an increase in potential vorticity

In fact, there’s a known relationship between potential vorticity and water vapor satellite imagery:

There is a clear relation between PV (potential vorticity) and water vapour imagery. A low tropopause can be identified in the WV imagery as a dark zone. As a first approximation, the tropopause can be regarded as a layer with high relative humidity, whereas the stratosphere is very dry, with low values of relative humidity. The measured radiation temperature will increase if the tropopause lowers. This is because of the fact that the radiation, which is measured by the satellite, comes as a first approximation from the top of the moist troposphere. High radiation temperatures will result in dark areas in the WV imagery.

Colorado State University

Potential vorticity in this case was an indicator of increased divergence at upper levels, and this helps explain why severe thunderstorms initiated on the afternoon of Aug 21, 2019, despite the lack of a strong surface boundary providing convergence. This is because divergence and vorticity aloft helps induce convergence at the surface (and may have helped generate a prefrontal trough that day). Divergence aloft is essentially removing air from the top of the column, and since the atmospheric system always attempts to maintain a balance in terms of conservation of mass, momentum, etc, this air leaving the top of the column gets replaced by air flowing in at the surface. This is inflow of air results in convergence, and enhanced lift, as this air rises to replace the air that continues to be evacuated aloft. A source of lift is always a critical ingredient to any severe thunderstorm!

NYC Weather Update – Aug 27, 2019

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A mid-week cold front brings a chance for rain as temperatures warm towards the end of the week. Tropical Depression Six (which could become Tropical Storm Erin) could impact local beaches with high surf for Labor Day weekend.

Rest of today – partly sunny with high temperatures in the mid-70s. Overnight lows in the mid-60s. Some chance for light rain in the overnight hours.

Weather Prediction Center surface forecast for 8PM Wednesday

Wednesday – high temperatures in the low-80s, with a chance for scattered showers and thunderstorms later in the day as a cold front approaches. Overnight lows in the upper-60s with continued chances for showers and thunderstorms.

Thursday – clearing skies with highs in the mid-80s. Overnight lows in the upper-60s.

Friday – high temperatures remain in the mid-80s, overnight lows in the upper-60s as high pressure builds in briefly.

A Note on the Tropics

A week ago, there was talk about how this month could play out to be one of the few Augusts on record that had no tropical cyclones form in the Atlantic. That calm has been quickly shattered by the development of two tropical systems: Tropical Storm Dorian and Tropical Depression Six, which may become Tropical Storm Erin. While TD Six won’t impact the US directly, it could bring large swells and waves to beaches along the Northeast this weekend. Dorian, on the other hand, is an imminent threat to Puerto Rico, Dominican Republic, and could cause problems for Florida this weekend into early next week.

  • tropical depression six 2019
    GOES East satellite image of Tropical Depression Six
  • GOES East satellite image of Tropical Storm Dorian

NYC Weather Update – Aug 19, 2019

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We begin this week with some typical “dog days” of summer heat. Shower and thunderstorm chances stick around for the first half of the week with the heat and humidity providing necessary instability. A more organized threat for possibly severe thunderstorms emerges Wednesday into Thursday with a pronounced cold front forecast to move through. Behind this frontal boundary, a refreshingly cool and dry air mass is expected to take hold for the weekend.

Rest of today – hot, humid, with high temperatures in the low-90s. Heat index values in the upper-90s. Partly cloudy with a chance for scattered showers and thunderstorms later this afternoon around 5-6PM as a prefrontal trough nears. Some storms could produce heavy rain and gusty winds. Overnight lows in the mid-70s.

Tuesday – still hot but a touch drier, as a cold front pushes south overnight into Tuesday. High temperatures around 90°F. Could maybe see a scattered thunderstorm still pop up but chances are lower relative to today with less humidity and instability to work with. Overnight lows in the mid-70s.

Wednesday – the cold front passing through Tuesday lifts back north as a warm front on Wednesday. Temperatures remain warm in the upper-80s. Chance for strong to severe thunderstorms possible late in the day as a cold front pushes through. If this front pushes through faster than forecast, in the late afternoon or early evening, there’s better chances for strong storms. Otherwise, showers and thunderstorms could continue overnight with lows in the mid-70s.

Thursday – should be the last day of this spell of summer heat with high temperatures in the upper-80s possible before the cold front above clears the area. Showers and thunderstorms may linger until the front clears. Overnight lows noticeably cooler in the upper-60s behind the cold front as a cooler Canadian air mass moves through.