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!
Easterly onshore winds will keep things cool to start the weekend. Strong high pressure slowly gives way to a frontal system late tomorrow, bringing a chance for rain. Behind this system, high pressure should build in again and persist, giving us some nice weather Sunday into next week.
Rest of today – mostly sunny with strong high pressure north of the area. Cool with highs in the low-70s under the influence of easterly onshore winds. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Saturday
Saturday – clouds build ahead of an advancing warm front and the day should end up being mostly cloudy. High temperatures a bit warmer in the mid-70s with winds shifting to the south ahead of the warm front. Rain chances increase towards the evening and overnight hours, with lows in the upper-60s.
Sunday – a trailing cold front will pass through and conditions should improve with rain ending early and gradual clearing. Highs in the low-80s, with some assist from downsloping winds from the west and northwest. Overnight lows in the nid-60s.
GFS model output for 2 meter above ground level temperature and 10 meter above ground winds at 2PM Sunday
Monday – high temperatures in the low-80s with mostly sunny skies as high pressure continues to build. Overnight lows in the mid-60s.
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.
Hurricane Dorian’s center passes well southeast of the city to start the weekend. Because of its large and expanding size, it will still influence our weather. Once Dorian passes, we’ll see improving conditions though cooler temperatures will be the rule with Canadian high pressure taking over.
Rest of today – mostly cloudy with isolated showers, more likely during the late afternoon and evening hours. It’s difficult to say even now whether the outermost rain bands from Dorian actually manage to hit the city, and even if they do, it’s also unclear how intense they will be. High temperatures in the low-70s. Overnight lows in the low-60s with cool, northeasterly onshore flow due to the cyclonic circulation around Dorian. For more info, see this detailed forecast.
GOES satellite imagery of Hurricane Dorian
Saturday – much improved conditions as drier air works in behind Dorian. Mostly sunny with high temperatures in the upper-70s. Overnight lows in the low-60s.
Sunday – a touch cooler though still mostly sunny with high temperatures in the mid-70s as winds turn more to the north and Canadian high pressure builds. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Sunday
Monday – high temperatures continue a 5-6°F below average trend, with highs forecast to be in the low-70s and mostly sunny skies. It will definitely feel like fall. Overnight lows in the mid-60s.
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
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!
Labor Day weekend is shaping up to be a good one overall in terms of sensible weather. A dry cold front passage today sets the stage for high pressure to take control over the remainder of the weekend. The only wrinkle is a warm front forecast to pass Monday that could bring some rain. Looking ahead into next week, a warm up should precede another cold front mid-week. The big weather story this weekend will be much further south of us, as Hurricane Dorian takes aim at Florida as a dangerous Category 4 storm.
Rest of today – sunny with high temperatures in the mid-upper 80s. Overnight lows in the upper-60s.
Saturday – high pressure starts to build from the west, which will result in a cooler weather with northerly winds. High temperatures topping out in the low-80s with lots of sun and overnight lows in the mid-60s.
Sunday – high pressure will progress east, giving us another pleasant day with lots of sun and comfortable temperatures in the low-80s. Overnight lows warm up in the upper-60s with a warm front approaching from the southwest.
Monday (Labor Day) – chance for rain during the day as a warm front, then a weakening cold front approach. High temperatures in the low-80s with mostly cloudy skies. Overnight lows around 70ºF.
Hurricane Dorian Heading Towards Florida
Over the course of the last few days, Hurricane Dorian made a pass over the Northern Lesser Antilles, hitting the Virgin Islands, and Martinique, but sparing Puerto Rico. This was a blessing for an island still recovering from the devastation of Hurricane Maria. However, because Dorian avoided the disruptive influence of high terrain in Puerto Rico, it entered the open waters of the Western Atlantic north of the island ready for continued intensification. Unfortunately, various forecast models have been consistently depicting Dorian making landfall somewhere in Florida early next week as a dangerous Category 4 hurricane packing max sustained winds of 140 mph.
Latest GOES visible satellite imagery of Hurricane Dorian
Overnight going into today, Dorian’s been steadily strengthening and is now showing a markedly more symmetrical structure than before, along with an eye developing in visible satellite imagery. This suggests that the southerly shear and dry air that was afflicting the storm yesterday has abated somewhat. At this point, there is nothing really stopping Dorian from continuing to strengthen to a major hurricane.
National Hurricane Center official forecast for Dorian as of 11AM Friday
Forecasters continue to have high confidence that Dorian will make a turn towards a more westerly direction as high pressure builds over Bermuda and keeps building to the west. This will block Dorian from taking a northerly route away from Florida. Worryingly, forecast models have trended towards a slower and more southerly track overnight, then a turn north after landfall. On this current track, a large part of Florida would be affected by the storm for a considerable amount of time, increasing the risk of damaging storm surge and inland flooding from prolonged heavy rain.
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
After a week of hot, humid weather and several bouts of strong to severe thunderstorms, we will finally get a break from both the heat and the rain just in time to enjoy the weekend. Behind a cold front that’s slowly pushing south, high pressure will build in and usher in a Canadian polar airmass that will bring a noticeable change to high temperatures – 10ºF cooler in most cases. Rain chances return late in the weekend period.
Rest of today – showers associated with a wave of low pressure moving along the cold front that passed last night should end by noon. Mostly cloudy with gradual clearing. High temperatures in the upper-70s and overnight lows in the mid-60s.
NAM (North American Model) forecast for 2 meter above ground level temperature Saturday at 2PM
Saturday – mostly sunny, high temperatures in the upper-70s, noticeably drier feel with dew points much lower than the past few days with Canadian high pressure taking over. Overnight lows in the mid-60s.
Sunday – below average temperatures continue with high temperatures in the mid-70s. By this time, the high pressure center will have shifted to the northeast and will thus enable cooler, more moist onshore flow to influence the weather. We should see more clouds and could even see some scattered rain showers. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Sunday
Monday – high temperatures in the upper-70s with partly sunny or mostly cloudy skies. An area of low pressure (which could become a tropical depression this weekend) off of Florida will track along the East Coast and should at the very least bring cloudier weather. Overnight lows in the mid-60s.
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.
