A slow moving storm system will bring a prolonged period of unsettled weather to the NYC region this weekend. The upside of this storm is that it will usher in some warmer air from the south, giving us some above average temperatures for a brief spell. Behind this storm system a cold front is forecast to pass through early next week and return us to below normal cold for this time of year. I will be following up on this post with a detailed forecast for Saturday, as I attempt to keep my forecasting skills sharp.
Rest of today – overcast, with high temperatures around 50ºF. Increasing chances for rain going into this evening and overnight. Low temperatures in the mid-40s overnight.
Surface forecast for Saturday morning, 7AM
Saturday – similar day to today with a generally gloomy feel, overcast skies with a chance for rain, though looks like precipitation will be on the light side. High temperatures in the low-50s. Overnight lows into Sunday cooler than Friday, in the low-40s.
Sunday – yet more rain possible with the storm slowly making its way east and transitioning to a coastal storm. High temperatures are expected to be cooler because winds will shift onshore from the northeast – highs probably in the mid-40s. Overnight into Monday, temperatures should hold in the upper-30s with rain chances winding down.
Surface forecast for Sunday at 7AM
Monday – skies slowly clearing with high temperatures in the mid-40s.
I haven’t had time to post about the current week’s WxChallenge forecasts for Caribou, ME, which I’ll have a follow up post about Saturday or Sunday to go over my results. Instead, this week, I’m going to put the same forecast skills I’ve been applying to the upcoming nor’easter that’s going to impact NYC Saturday. I’ll provide a deterministic (exact number) forecast for high temperature, low temperature, maximum sustained wind speed, and total accumulated precipitation for the period starting 2AM Saturday and ending 2AM Sunday, with a less exact forecast for the rest of the weekend. In my later follow up post, I’ll verify my results and see just how close I got. For purposes of verification, my forecast will be for LGA. If you’re only reading this summary, just know that Saturday will be a windy, raw, rainy day, but that conditions will improve as the day goes on.
Friday – partly sunny skies with high temperatures in the low-50s. Light winds from the south-southeast.
Saturday – High temperature: 54°F. Low: 48°F. Maximum sustained winds (2-minute averaged): 34.5 mph from the east-northeast. Total precipitation: 1.25″. The bulk of the rain should end by the afternoon hours.
Sunday – mostly cloudy, high temperatures in the upper-50s with return flow from the west downsloping into the area.
Forecast Discussion
The nor’easter that’s coming to us this weekend has tropical origins as the remnants of Hurricane Willa, which originated in the Eastern Pacific and made landfall in Mexico as an intense Category 3 storm. This means that it will be able to tap to tropical moisture, bringing the potential for a heavy rainfall event. At the surface, a low pressure center will be intensifying as it slides up along the Mid-Atlantic coast. At the mid and upper levels of the atmosphere, the set up is favorable for robust divergence, which should result in to strong surface convergence and lift. At the 850 mb level, there are indications of a strong low-level jet with winds as high as 50 knots from the east to east-northeast during the Saturday morning hours ahead of the warm front attached to the nor’easter. This would enable highly efficient moisture transport and convergence in our area. The net result of this should be periods of heavy rain. This is what justifies rainfall totals over an inch, though because of the relatively fast forward speed of this storm, I don’t believe that totals above 1.50″ are likely.
These aren’t pictured below, but MOS (model output statistics), and forecast plumes (ensembles of many different iterations of the same forecast model) probabilities suggest a consensus range of 1.15″-1.24″. Short Range Ensemble Forecast (SREF) probabilities show a greater than 90% chance for rainfall totals greater than or equal to 0.50″, 70-90% chance of totals greater than or equal to 1.00″, but very low chances of totals exceed 2.00″. The record rainfall for Saturday at LGA is 1.76″, though there are higher record totals surrounding this date, so this record isn’t what we’d call particularly strong.
On the temperature front, the trajectory of the onshore flow accompanying this storm actually brings it over Atlantic waters that are warmer than the air temperatures have been this week. This should bring warmer air into the area, however, because it will also be raining and cloudy, evaporational cooling will be a concern (as it rains, some of the moisture from raindrops will evaporate, which takes heat input to achieve, lowering surrounding temperatures), and that will likely knock temperatures down a bit. Overnight lows should end up being fairly mild in comparison to previous nights because of cloud cover and the aforementioned winds over warmer waters.
The wind speeds should be quite strong tomorrow. MOS is showing winds in the 24-26 knot range. However, given that 850 mb winds will be even strong, in the 50 knot range, and it will be precipitating, meaning downward transfer of momentum by raindrops should mix some of these stronger winds to the surface, which is why I think sustained winds could be as high as 35 mph at some point in the day at LGA (which is exposed near open water, so there’s less of an impact from friction over the land). I think gusts could easily top 40-45 mph tomorrow during the most intense rain squalls.
Surface forecast from the Weather Prediction Center valid for 8AM Saturday, showing the nor’easter in question
850 mb forecast for relative humidity and wind valid 8AM Saturday. The dark blues show saturated air mass, and the wind barbs show strong onshore winds
500 mb vorticity and heights, the yellow-orange-purple shading shows progressively stronger values of positive vorticity. Without getting too technical, positive vorticity is related to divergence aloft, which helps aid upwards motion (lift). Strong upwards motion can lead to clouds and precipitation.
Shaded areas indicate strong winds at the 300 mb level. These are referred to as jet streaks. Our region is forecast to lie within the left exit region of a jet streak, an area that favors divergence and enhanced upwards motion/lift
Not surprisingly, the SREF probability forecast shows a 70-90% probability of omega (a quantitative measure of lift) exceeding -9 microbars/sec, which is considered quite strong
Update: Verification
According to the KLGA Daily Climatological Report showed the following for Saturday, October 27, 2018:
High Temperature: 53°F
Low Temperature: 45°F
Max Wind: 41 mph
Max Gust: 50 mph
Total QPF (this is taken from METAR data): 0.98″
I’ll attempt to do a post-mortem on the winds and QPF forecast where I missed pretty significantly later in the week.
Cooler weather arrives just in time for the astronomical start of autumn. Cloudy conditions have been persistent, and will continue today. A chance for rain and scattered thunderstorms tonight going into Saturday from a passing cold front. These will not be drenching rains like with the remnants of Florence. High pressure returns Saturday, and should give us a dry weekend, though we may continue to see more clouds than sun.
Rest of today – cool, and cloudy, with high temperatures in the low-70s. Chance for showers and thunderstorms overnight with a passing cold front.
Saturday – sunnier with high temperatures in the mid-70s. The autumnal equinox will take place at 9:54PM EDT.
Sunday – partly sunny with high temperatures in the low-70s.
Monday (Mid-Autumn Festival) – partly sunny with high temperatures again in the low-70s.
After a week of gloomy, cloudy, and at times rainy weather, we will get a nice break this weekend as conditions improve. The same high pressure that’s caused Hurricane Florence to make landfall on the Carolinas by blocking its northward progress will bring us sunnier weather. Indeed, some of the cloud cover over our area is actually a result of outflow from Florence. Its remnants will impact us with rain at some point early next week.
Rest of today – mostly cloudy with high temperatures in the mid-70s as winds continue from the east. High surf with waves 8-12 feet are possible at the coast due to Florence.
Saturday – decreasing clouds as high pressure continues to build. High temperatures around 80ºF.
Sunday – sunny, high near 80ºF, high pressure remains in control.
Monday – clouds and the chance for rain returns. More moist air returns as the high pressure that gives us nice weather over the weekend slides east (and the flow on its western side starts to bring moisture from the south up). We’ll need to monitor the progress of the remnants of Florence as they are likely to bring us rain.
Cooler temperatures are finally upon us after a very hot and humid week. However, the cooler weather also brings a prolonged period of cloudy conditions, with chances for showers each day this weekend. Rain chances continue into the beginning of next week with a storm system approaching. Long-term, we look towards the tropics, as Tropical Storm Florence continues to have the potential for bringing some impacts to the area.
Rest of today – overcast, with much cooler high temperatures in the mid-70s. Chances for showers, especially later in the day.
Saturday – cloudy with high temperatures remain cool in the low-mid 70s with northeasterly onshore flow behind a cold front draped west-east south of us. This front (which passed through yesterday) will become stationary. Multiple impulses of energy will run along this boundary and allow for continued slight chances of showers.
Sunday – yet another cool, cloudy day with chances for rain. High temperatures around 70°F.
Monday – the stationary front stalled out south of us will return north as a warm front. As this happens, chances for rain increase, along with temperatures rising back into the mid-upper 70s.
Will Tropical Storm Florence Impact the East Coast?
Tropical Storm Florence has been active over the eastern and central North Atlantic for over a week now. Within the last few days, Florence underwent a rapid intensification that brought it up to Category 3 status, making it the first major hurricane of the 2018 Atlantic hurricane season. Subsequently, it has weakened to a tropical storm under relentless strong shear. However, it is expected to reintensify into a major hurricane once shear subsides. Worrying trends in the track forecast for Florence continue to allow for the possibility of landfall on the East Coast, or at the very least, substantial coastal impacts like dangerous surf and beach erosion. However, the National Hurricane Center continues to note that there is considerable uncertainty to the track forecast for Florence, such that it is still too early to determine what effects it will have on the East Coast. This storm will warrant careful observation over the coming days and I will have subsequent updates on it.
A brief break in the heat over the weekend has led back into another warm spell to begin this week. This latest blast of heat will be short-lived as a backdoor cold front sweeps through tomorrow, followed by another, stronger cold front later this week. Temperatures for the upcoming weekend will once again be comfortable, seasonable to below average. In the tropics, things are starting to get busy with tropical storms Florence and Gordon active and another tropical wave poised to become a tropical cyclone off the coast of Africa. Gordon will make landfall overnight on the Gulf Coast east of New Orleans.
Rest of today – hot and humid with temperatures in the low-90s, mostly sunny skies.
Wednesday – cooler, though still warm with high temperatures in the upper-80s and partly sunny skies. A backdoor cold front will bring cooling onshore southeasterly breezes that help keep a lid on temperatures.
Thursday – temperatures rise back a couple degrees into the low-90s ahead of an approaching cold front from the northwest. The warm air ahead of the cold front could help set the stage for a few scattered strong thunderstorms as the front passes through.
Friday – temperatures cool off quite a bit into the low-80s under mostly cloudy skies. Chance of showers during the day with the cold front lingering south of the area.
Tropics Heating Up
We have 2 active tropical cyclones (Florence, Gordon), and another incipient one in the Atlantic Basin. Tropical Storm Gordon could strengthen into a hurricane before landfall on the Gulf Coast between Mississippi/Alabama overnight into Wednesday. Heavy rain could induce flash flooding in southern portions of these states. The immediate coastline will be at risk of storm surge flooding. Florence doesn’t pose an threat to land. The disturbance shown below that’s furthest east is a tropical wave that’s just emerged off the coast of Africa. This is worth watching as it has a high chance of becoming a tropical cyclone and its eventual track could pose a threat to portions of the Lesser Antilles.
After a week of sweltering temperatures, the weekend brings some welcome relief from the heat. The downside is that these cooler temperatures come with mostly cloudy conditions and chances for rain. Labor Day marks the traditional end of summer, and it will feel like that this weekend, but temperatures will rise right back into the upper-80s and low-90s next week.
Rest of today – mostly cloudy with onshore easterly flow keeping temperatures suppressed in the mid-70s. This easterly onshore flow is happening on the back side of a cold front that passed through yesterday. This front will stall out and become stationary south of us over the Delmarva Peninsula. As impulses of energy move along this front, there will be chances for rain. However, it does appear the best chances for rain stay south of us.
Saturday – mostly cloudy still with slightly warmer temperatures in the upper-70s. Easterly onshore winds continue along with some chances for rain in the afternoon and more chances for showers and thunderstorms overnight.
Sunday – warming up into the mid-80s with scattered chances for showers and thunderstorms. Partly sunny skies otherwise.
Monday (Labor Day) – conditions improve just in time for the West Indian Day Parade festivities with high temperatures in the upper-80s and partly sunny skies.
Enjoy the pleasant weather today if you can, because conditions will deteriorate going into the weekend. Wet weather will impact the NYC region starting tonight, lingering well into early next week. Rain could be heavy at times, and flash flooding is a possibility. Temperatures during this time will be considerably cooler than this past week with the influence of clouds and rain. We will be stuck under the influence of multiple shortwaves moving through at the upper level during this time. Starting next week, forecast models suggest that a closed 500 mb low will form, then linger in the area due to strong downstream ridging (high pressure) over the western North Atlantic. This cut-off upper level low will provide for persistent unstable conditions conducive for multiple rounds of rain and thunderstorms. I will be off the next two weeks on vacation and will not be posting updates unless there’s severe weather forecast for the area.
Rest of today – increasing clouds as a cold front approaches the area from the northwest but stalls out and dissipates overnight. High temperatures in the upper-80s. Chances for showers and thunderstorms increase overnight.
Saturday – multiple chances for showers and thunderstorms, starting early in the day, then again in the afternoon and evening hours. Much cooler with mostly cloudy skies and high temperatures only in the mid-70s. The cold front that passed through yesterday will return north as a warm front. Ahead of this, winds will be easterly and onshore, bringing the cool, damp influence of the Atlantic over the area.
Sunday – warmer with high temperatures in the low-80s but still mostly cloudy. Chances for rain throughout the day as the frontal boundary above remains in the area.
Monday – yet another impulse of shortwave energy aloft brings a surface low towards the area, so rain chances persist with a shot for thunderstorms as well. Mostly cloudy with highs in the low-80s.
A wet start to the weekend will then give way to much warmer weather. We’ll get a break from the rain Sunday and Monday before the next shot for rain. Of note, a possible tornado may have impacted parts of northern Queens last night.
Rest of today – increasing chances for showers and thunderstorms especially in the early afternoon. Otherwise, mostly cloudy with high temperatures in the mid-80s.
Saturday – more widespread showers and thunderstorms in the morning, with some concern for flash flooding. The set up also could produce some isolated severe weather as a weak impulse of energy travels up the upper trough that’s been locked in over the Eastern US this week. Improving conditions later with highs in the mid-80s.
Sunday – the upper trough that’s been causing this week’s wet weather finally exits east. High pressure begins to build south of us, bringing warmer temperatures with highs in the low-90s and lots of sun.
Monday – more of the same as Sunday, sunny, hot and humid with highs in the low-90s.
Possible Tornado in Northern Queens Last Night
At 10:18 last night, residents in northern Queens, the eastern Bronx and northern Nassau received alerts from our local National Weather Service forecast office of a tornado warning. The damage was evident this morning, with many downed trees in College Point. A team from the NWS is assessing the damage to determine whether what hit this area last night was really a tornado, or if it was some other non-tornadic wind phenomena.
I was in a neighborhood that received this warning, but at first, I almost thought it was in error because there were no signs outside of anything awry, no thunder, no rain. However, a quick look at the base radial velocity imagery on the JFK high-definition terminal Doppler radar was revealing – there certainly was a storm with very strong rotation, hence why the warning notes “radar indicated tornado”. In the annotated base radial velocity image below, the white circle indicates a tight mesocyclonic circulation.
Without going too much into technical detail, the greens indicate inbound velocities relative to the radar station (air was flowing towards the radar), while oranges and reds indicate outbound velocities (where air was flowing away from the radar). When forecasters see a “velocity couplet” like this where inbound and outbound velocities are close together, they know this is a signature of strong storm rotation within a mesocyclone. If, within this couplet, you get a “gate to gate” (i.e. adjacent pixels) where the difference between inbound and outbound velocities exceed 90 knots, this is referred to as a “tornado vortex signature”. In this image, if you do the math, you get that the max difference between inbound and outbound velocities was 86 knots, and you an see what appears to be a tornado vortex signature.
We’ll see what the NWS’ assessment says later today, but it appears to me that there was a tornado at some point over northern Queens last night!
UPDATE (1:46PM August 3, 2018): The National Weather Service issued a report confirming that an EF0 tornado touched down in portions of College Point, Queens last night.
Thunderstorms roll through later this afternoon and into the overnight hours. The cold front bringing these storms stalls out before finally pushing through Sunday. A welcome break from this muggy weather follows, with dew points finally dropping out of the upper-60s into more comfortable ranges. The break is short lived, as the cold front will return mid-week next week as a warm front, bringing another round of warm and humid weather with chances for rain.
Rest of today – hot and humid today with high temperatures approaching 90°F and plenty of moisture still lingering. Late in the day, clouds will increase as a cold front approaches from the west. Thunderstorms are expected to develop along a pre-frontal trough. Atmospheric dynamics favor some possibility for storms to develop damaging wind gusts. There will be modest instability with little convective inhibition by later in the day with plenty of solar heating and humidity in place. Wind shear, though weak in the lower layers of the atmosphere, increases substantially aloft, and so storms do have a chance to become strong. Any storm could bring about heavy rain and isolated minor flash flooding. The best timing of the onset of the rain appears to be between 7-9PM as of recent High Resolution Rapid Refresh model runs. The Storm Prediction Center has placed the western part of the region (including parts of NYC) under a slight risk of severe weather. By the time the storms hit the city, given the current timing, they should be weakening due to day time heating ceasing.
Saturday – only slightly cooler with high temperatures in the mid-80s. Still quite muggy. Spot showers are possible due to the cold front lingering in the area.
Sunday – finally, we get a refreshing day off from the humidity with dew points dropping into the low-60s and high temperatures in the mid-80s with lots of sun. The cold front finally pushes through with high pressure behind it bringing cooler, drier Canadian air our way.
Monday – should be another comfortable day with high temperatures in the mid-80s and relatively less humid conditions.
