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.
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.
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!
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.
GOES East satellite image of Tropical Depression Six
GOES East satellite image of Tropical Storm Dorian
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.
A stationary front stalled out southeast of the city over the Mid-Atlantic will be the primary agent responsible for an extended period of humid, cloudy, and unstable weather over the weekend. As was the case last weekend, temperatures will be on an upward trend from the low-80s into the low-90s by the start of next week.
Rest of today – mostly cloudy with isolated showers and thunderstorms particularly closer to higher terrain north and west of the city. High temperatures in the low-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – more clouds than sun with the stationary front just south of the area. Continued onshore easterly flow behind north of this stationary front will keep things cooler and humid. High temperatures in the low-80s. Chance for showers and thunderstorms to develop again in the afternoon hours with an unstable airmass (due to the high moisture content caused by the onshore flow) as heating increases. Overnight lows in the low-70s.
Sunday – temperatures begin to warm up into the upper-80s as winds shift towards the south. The stationary front attempts to move north as a warm front but high pressure is forecast to develop west of us and block this northwards advance. Scattered showers and thunderstorms are again possible in the afternoon hours. Overnight lows in the mid-70s.
GFS model 2 meter above ground level temperature, 10 meter above ground level wind forecast for 2PM Monday
Monday – with winds shifting to towards the southwest, much warmer air will advect into the region. High temperatures expected to top out in the low-90s with overnight lows in the upper-70s. Mostly sunny skies forecast with the high pressure building.
Unsettled weather takes hold during the mid-week period as a couple prefrontal disturbances affect the area. The focus for heavier rain will be later Wednesday as a cold front approaches. Behind this cold front, conditions should improve and result in a pleasant, dry weekend.
Rest of today – mostly cloudy with a chance of isolated showers and thunderstorms later in the day as a shortwave trough lingers around the area. Temperatures in the mid-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8PM TuesdayStorm Prediction Center severe weather outlook for Tuesday
Tuesday – shaping up to be an eventful day in regards to weather. Temperatures should rise into the low-80s with overnight lows in the low-70s. During the day, a mesoscale convective system (organized large cluster of thunderstorms) will be approaching the area from the Ohio Valley. Forecast is uncertain in terms of where the frontal boundaries in the vicinity of this MCS eventually set up. Current thinking is that we end up south of a warm front earlier in the day that will serve as the initial focus for shower and thunderstorm activity. This activity should continue and increase in intensity as the frontal boundary starts shifting south and encounters warm, moist and unstable air. There is potential for all severe hazards including hail, damaging winds, and an isolated tornado.
Wednesday – mostly cloudy with shower chances diminishing later in the day . High temperatures in the low-80s and lows in the upper-60s.
Thursday – still a possibility for lingering showers but overall a better day with mostly cloudy skies and high temperatures in the low-80s with overnight lows in the upper-60s.
A couple stormy nights in the city the last couple days will give way to a superb weekend of mostly sunny skies and warm, but comfortable temperatures. High pressure will remain in control over the region’s weather going into next week. The next chance for significant precipitation won’t enter the picture until mid-week next week.
Rest of today – mostly sunny with high temperatures ranging 84-87°F. There is a slight chance for an isolated shower or thunderstorm later this afternoon with a weak disturbance pivoting through at the upper-levels. Forecasters cite the lack of any significant convective inhibition so storms could pretty easily get going, however, moisture content isn’t all that great. Overnight lows should be mild in upper-60s.
HRRR model simulated radar for 4:45PM today
Saturday – with a cold front passing today, we’ll have cooler temperatures tomorrow, high temperatures reaching only into the low-80s under mostly sunny skies. Overnight lows cooling into the mid-60s.
Weather Prediction Center surface forecast for 8PM Saturday
Sunday – just another beautiful day with mostly sunny skies and high temperatures in the low-80s. Overnight lows starting to warm back up into the upper-60s.
Monday – temperatures will warm back up as the high pressure center shifts east, and we get the warmer southerly, and westerly return flow. We should see high temperatures in upper-80s and overnight lows in the low-70s.
Unsettled weather takes hold during the mid-week period as a couple prefrontal disturbances affect the area. The focus for heavier rain will be later Wednesday as a cold front approaches. Behind this cold front, conditions should improve and result in a pleasant, dry weekend.
Rest of today – mostly cloudy with a chance of isolated showers and thunderstorms later in the day as a shortwave trough lingers around the area. Temperatures in the mid-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8PM Wednesday
Wednesday – mostly cloudy with high temperatures in the mid-80s. Heavy rain possible especially later in the day with a cold front approaching. With a moist air mass in place, some thunderstorms that pop up could produce heavy downpours and localized flooding. Rain could continue overnight in more isolated form. Lows in the low-70s.
Thursday – brief subsidence behind the cold front that passes through Thursday should result in calmer weather and warmer temperatures with high temperatures in the upper-80s. Before that happens though, rain and thunderstorms can still pop up. Overnight lows in the low-70s.
Climate Prediction Center 6-10 temperature outlook valid Aug 11 – Aug 15 (this Sunday into next week)
Friday – cooler, sunny, with high temperatures in the low-80s. Overnight lows cooling into the upper-60s as Canadian high pressure begins to build.
As with last week, high pressure will play a big role in sensible weather this week. Rain chances are going to be limited to later this week when a frontal system moves across the area. Temperatures will be warm to start in the upper-80s then cool off later in the week back into the mid-80s.
Rest of today – early showers move through then we’ll see gradual clearing. Temperatures in the low-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8PM Tuesday
Tuesday – much warmer with temperatures in the upper-80s, and lots of sun with high pressure building and taking over. Overnight lows in the low-70s.
Wednesday – continued warmth with highs again in the upper-80s and mostly sunny skies. Overnight lows in the low-70s.
GFS model output for 1000-500 mb thickness, surface pressure, and precipitation valid for Thursday 8PM
Thursday – clouds increasing late with high temperatures in the mid-80s. Chance for showers and thunderstorms late and going into the overnight hours. Overnight lows in the low-70s.
This weekend starts off with warm temperatures, and then will get punctuated by the passage of a cold front bringing showers and thunderstorms later tomorrow afternoon into the evening hours. Behind this front, a cooler, drier Canadian air mass moves in with high pressure building to start next week.
Rest of today – warm, with mostly cloudy skies and high temperatures in the upper-80s. Overnight lows in the mid-70s. Slight chance for showers and thunderstorms going into Saturday.
Weather Prediction Center surface forecast for 8PM Saturday
Saturday – mostly cloudy with chances for showers and thunderstorms peaking in the late afternoon and early evening hours. High temperatures in the upper-80s. Shower and thunderstorm chances will peak late with instability and lift maximized from daytime heating and an approaching. Some of these storms could be strong in nature and with plenty of moisture available, these could bring heavy downpours. Overnight lows in the mid-70s.
Sunday – generally improving conditions with decreasing clouds and high temperatures in the mid-80s. Some showers still possible early. Overnight lows into Monday in the low-70s.
GFS Model for 1000-500 mb thickness, surface pressure, and precipitation at 2AM Monday
Monday – a nice day with high temperatures in the low-mid 80s and sunny skies as high pressure takes control. Overnight lows in the upper-60s.
