An arctic chill with bitterly cold winds kicks off the weekend as strong high pressure moves in. Overnight lows in the teens and high temperatures in the 20s will range 10-15ºF below normal for this time of year. This cold air could set the stage for a nor’easter bringing significant snowfall to the region, with totals > 6″ possible Monday into Tuesday. As usual, track details on this storm will be pivotal, and are not clear at this time.
Rest of today – mostly clear, cold, and windy with high temperatures in the low-20s. A strong arctic high and a departing low will create a tight pressure gradient that ushers in stiff northwesterly winds 20-25 mph. Wind chills will hover around 0ºF. Overnight temperatures drop into the mid-teens with wind chills likewise dropping below zero.
Weather Prediction Center surface forecast for 7AM Saturday. A strong area of high pressure is parked over much of the Eastern US.
Saturday – winds abate with high pressure moving more directly overhead. Sunny, but still cold with high temperatures around 30ºF. Overnight lows will be in the upper-teens under increasing cloud cover.
Sunday – mostly cloudy as the next storm system starts to work in. High temperatures around 30ºF. Overnight lows in the upper-20s with the possibility of snow developing overnight into Monday morning.
GFS model total snowfall accumulation as of 7AM Tuesday
ECMWF model total snowfall accumulation as of 7AM Tuesday
UKMET model total snowfall accumulation as of 7AM Tuesday
Monday– stiff northeasterly winds developing along with snow. High temperatures in the low-30s. The storm possibly bringing us significant snow came ashore over the west coast yesterday and will be transiting the country the next couple days. It is eventually forecast to spawn a coastal low, bringing a nor’easter into our region. Even though cold air will precede this storm, offshore waters are still mild, in the low 40s-50s, and with the storm likely to bring southerly/northeasterly flow at the outset, we could see enough warming to mix to rain at the coast. Storm track and strength will be vital to the eventual snowfall totals. Models seem to signal a very tight banding feature bringing heavy snow – you’ve heard this before but these bands are notoriously difficult to pinpoint even a day ahead of the event. If the optimal scenario plays out, we could see a storm with similar or possibly bigger totals than the one in December. Overnight lows Monday shouldn’t budge much, in the low-30s, with some warm advection associated with portions of the storm.
A cold and breezy start to the weekend with a cold front ushering in below average temperatures mostly in the mid-30s. High pressure builds and keeps things dry during the weekend. However, the upper-level pattern shifts to allow a shortwave trough to push through early next week, leading to a storm that could bring some accumulating snow overnight Monday into Tuesday.
Rest of today – partly cloudy with high temperatures in the low-40s. A moisture-starved cold front will work its way through the region. The main impact of this front will be to induce a shift in winds to the northwest, and trigger some decent cold air advection. This will lead to overnight temperatures plummeting into the mid-upper 20s.
Weather Prediction Center surface forecast for 7AM Saturday
Saturday – high pressure builds in behind the cold front, giving us a mostly sunny day. This high pressure and a low off to the northeast over the Canadian Maritimes will lead to a fairly tight pressure gradient, and the result should be breezy winds from the northwest, sustained in the range of 15-20 mph at least. So despite the sun, the cold start to the day will lead to temperatures in the low-30s and a much colder feel with wind chills around 20ºF. Overnight lows will be the coldest in quite some time, in the low-20s.
Sunday – windy conditions continue at least part of the day with high temperatures in the mid-30s. Once the center of the high moves over us, winds should subside. Overnight lows in the mid-20s with clouds building in.
GFS model 500 mb height and vorticity animated loop valid 7PM Sunday through 7PM Monday, showing a de-amplifying shortwave trough ejecting from the southwest towards our region.
Monday– as the high pressure eventually moves off, a shortwave trough will be progressing towards our region. The accompanying surface low will bring mostly cloudy skies. Temperatures should peak in the mid-30s. Overnight, the storm is positioned to possibly bring us some light accumulating snow. Currently, it’s hard to pin down a total, but the following images from GFS and ECMWF models suggest something on the order of 3-5″ is possible. Low temperatures in the upper-20s should support all snow during this event, so storm track will be key here.
GFS model 24 hour snowfall total ending 1AM Wednesday
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
We start the weekend with hot temperatures but some showers and thunderstorms Saturday and Sunday should cool things off back to near-normal warmth. In general, there’s going to be enough sun this weekend to enjoy some outdoor activities. Just be prepared for a quick moving afternoon thunderstorm. Saturday’s storms could edge into the severe range.
Rest of today – hot, sunny, with high temperatures ranging in the upper-80s to low-90s. Overnight lows in the low-70s.
Storm Prediction Center convective outlook for Saturday, showing slight risk for severe thunderstorms in NYC
Weather Prediction Center surface forecast for 8PM Saturday
Saturday – partly sunny with increasing clouds later in the day. Showers and thunderstorms possible especially in the afternoon and early evening hours when solar heating is maximized and a cold front/shortwave trough moves through the area. There is some potential for the storms to reach severe limits, with the primary threat being damaging winds. High temperatures near 90°F, with overnight lows in the low-70s.
Sunday – cooler, with high temperatures in the mid-80s but mostly sunny skies. There’s also a chance for showers and thunderstorms with a second cold front pushing through during the day, but coverage will be more scattered and storms are expected to be weaker if they form due to lack of upper-air support and less favorable conditions for instability. Overnight lows noticeably cooler, in the mid-60s.
GFS model output for 1000-500 mb thickness, surface pressure, precipitation for 8AM Monday
Monday – a gem of a day with mostly sunny skies and temperatures in the low-80s. Overnight lows back into the upper-60s.
