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
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.
