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NYC Detailed Forecast for Saturday, Dec 15, 2018

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Saturday looks to be a dreary start to the weekend, however, it appears at this time that it’ll actually end up being the better half of the weekend. While rain chances exist for Saturday, the best conditions for heavier rain don’t look to materialize until Sunday. Even with clouds, rain, and winds shifting onshore from the east and northeast, Saturday afternoon may end up being the warmest day we’ve seen for quite some time. Below, I will provide a detailed forecast using KLGA (LaGuardia Airport) as a reference point. This is because KLGA will be easy to verify forecast outcomes for.

My Forecast
High: 51ºF | Low: 41ºF | Max sustained winds: 17 mph | Total precipitation: 0.13″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Saturday and 1AM Sunday (06Z Saturday to 06Z Sunday). Wind speed verification will draw on the daily climate summary.

Verification
High: 50ºF | Low: 45ºF | Max sustained winds: 24 mph | Total precipitation: 0.11″. Updated Sunday, December 16, 2018 1PM EST. Assessment: this ended up being a good forecast on precipitation and high temperature. However, I was too low on wind and low temperatures. Because of the light precipitation, little evaporational cooling occurred at the surface, where the air was nearly saturated most of the day, leading to warmer low temperatures. For winds, I may need to adjust my forecasts upwards for LGA when winds are out of the northeast because winds from this direction will flow over the open waters of the East River, with very little frictional drag.

Synoptic Set Up (The Big Picture)

A slow moving storm system with origins in the Deep South will make its way into the Northeast over the weekend. This storm will eventually transition to a coastal storm as it moves offshore of the Northeast. At the 500 mb level, this storm is driven by a closed low, which will almost nearly become completely cut off from the primary wind pattern (steering currents) at this level. This is what explains the slow forward progress of the storm. The result of this is that we’ll experience an extended period of cloudy and rainy weather this weekend. Low-level moisture appears to be maximized on Sunday, though, so Saturday may end up being a better weekend day.

Surface forecast from Weather Prediction Center, valid 7PM Saturday

High Temperature

MOS forecasts show high temperatures Saturday in the low-mid 50s, with NAM (North American Model) coming in at 53ºF and GFS (Global Forecast System) pointing to 56ºF. NBM (National Blend of Models) – a consensus based forecast model, showed highs of only 49ºF. I tend to side with high temperatures on the cooler side due to persistent low overcast clouds (discussed below), and an onshore northeasterly wind later in the day.

Low Temperature

The same MOS data above has lows on Saturday of 44ºF for NAM, 46ºF for GFS and 43ºF for NBM. I’m siding with the cooler side of the forecast envelope again due to the fact that precipitation will be ongoing overnight and there could be some evaporational cooling impacts to account for. Also, with the forecast period ending 06Z Sunday, I actually think the low for this period could be at the tail end, based on MOS. This would be after a prolonged period of onshore winds that I think could bring temperatures into the low-40s.

Max Sustained Winds

Although there will be precipitation during Saturday, as cited below, there looks to be a stable layer with a surface inversion during much of the day on Saturday (see the second Skew-T below). This will make downward momentum transfer during precipitation periods rather inefficient. The pressure gradient isn’t looking particularly strong either, since the low won’t be deepening until Sunday when it moves offshore. With these factors in mind, I think that a max sustained wind of 15 knots (17.25 mph) is reasonable.

Total Precipitation

This will be the trickiest part of the forecast. As is often the case with coastal storms, the gradient of precipitation totals is quite steep. There’s also considerable spread in ensemble models for total precipitation, leading to a low confidence in this forecast. A key factor here is that low-level moisture doesn’t look to be particularly robust as the day progresses (see graphic below). There’s also no clear lifting mechanism at the surface.

850 mb relative humidity, and wind at 1PM Saturday. Areas in blue are saturated, browns indicate very dry air, note how close that dry air is to NYC

While it’s clear that the day starts off with rain overnight, both GFS and NAM suggest that things will dry out a bit in the afternoon hours. That could translate to a lull in precipitation during that time frame.

NAM forecast sounding for 4AM at KLGA. Refer to my earlier post for a guide on how to interpret this Skew-T.
By 1PM, though, not that there’s a marked drying of the low levels above 850 mb. This could certainly cut into precipitation and result in light rain or even a lull in rainfall.

SREF probabilities of total precipitation and the best lift show that these remain south of the area, over the open waters of the Atlantic. However, the gradient is quite sharp for this, so any deviation northward on the storm track could throw a lot more rain our way.

SREF (Short Range Ensemble Forecast) forecast of strong lift (omega >= -9 microbar/sec). Lift is a critical ingredient in producing precipitation. I’ll explain in a later post 🙂


NYC Weekend Weather – Dec 14, 2018

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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.

NYC Weather Update – Weekend Nor’easter – Oct 26, 2018

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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.

NYC Weather Update – Sept 26, 2018

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Severe thunderstorms could impact the area later today ahead of an approaching cold front. Following the passage of this cold front, conditions will begin to improve, setting us up for a weekend with pleasant, cooler, classic autumn weather.

Rest of today – Storm Prediction Center in Norman, OK has placed parts of the NYC Tri-State (the city, north and west) under a slight risk of severe weather today. Warm, moist air (read: unstable, theta-e rich) is in place with the area in the warm sector of an advancing low pressure center located in Ontario/Quebec. Some rain/thunderstorm activity could take place along a weak warm front this morning. The main focus of activity will occur later today with a cold front approaching from the west.

This cold front will be accompanied by a streak of strong upper level winds (40-50 knots), which will be capable of providing the magnitude of vertical wind shear necessary for some strong to severe thunderstorms to organize. Given the direction of the winds at upper levels being largely parallel to the advancing frontal boundary, expect that the primary storm format will be a QLCS (quasi-lineary convective system) – a line or broken line of storms. High Resolution Rapid Refresh models depict the bulk of storm activity approaching the city around 8PM this evening. Given this storm format and synoptic set up, the most likely severe threat will be damaging wind gusts. Heavy rain could trigger flash flooding as well, since the ground is still quite saturated from yesterday’s rain.

The biggest limiting factor to storm strength today will be overcast skies limiting daytime destabilization from solar heating. As is often the case, I would expect the storms to be weakening somewhat as they approach a more stable marine air layer near NYC. Overall, expect a cloudy day with high temperatures in the low-80s and a muggy feel.

Storm Prediction Center Day 1 Convective Outlook

Weather Prediction Center forecast surface conditions at 8PM EDT

High Resolution Rapid Refresh simulated 1-km radar for 8PM EDT

NAM forecast for 500 mb winds and height valid 8PM EDT

Thursday – the cold front that brings us this possibly severe today will slow down as it pushes south of us, such that it remains close enough that a weak disturbance moving along it could bring us some additional showers late in the day. Skies will be partly sunny with much cooler high temperatures in the upper-60s.

Friday – easterly onshore flow on the back side of this cold front forecast to be to our south will keep skies cloudy with high temperatures around 70°F – chance for showers lingers.

Saturday – high pressure finally starts to take control of sensible weather and should produce a mostly sunny day with crisp conditions and high temperatures in the low-70s.

NYC Weekend Weather – Sept 21, 2018

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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.

NYC Weather Update – Sept 17, 2018

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We had some great late-summer weather this past weekend with high temperatures in the low-80s and lots of sun. Most of this week will be quite nice as well, with the notable exception of Tuesday, when remnants of Florence impact the region and bring around a day of heavy rain. Fortunately, this wet weather will be short-lived, and the remainder of the week should see a return to mostly sunny conditions.

Rest of today – increasing clouds and humidity as remnants of Florence approach from the south. High temperature around 80°F.

Tuesday – periods of rain, heavy at time, with scattered thunderstorms possible. Otherwise cloudy, with high temperatures around 80°F again.

Wednesday – conditions improve as the remnants of Florence quicly move off to the east. A return to mostly sunny weather with cooler highs in the upper-70s as the back side of the cyclonic rotation of the remnants of Florence brings a cold front through.

Thursday – another mostly sunny day with high temperatures in the low-70s, high pressure building should continue to keep things dry for the region.

NYC Weather + Special Tropical Update – Sept 10, 2018

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This week kicks off with a prolonged period of unsettled weather as a warm front and a cold front both impact the area. Chances for rain will be with us through Wednesday. Conditions finally improve late in the week. We luckily will avoid direct impacts (aside from high surf) from what will be an extremely dangerous Hurricane Florence, which looks set to make landfall on North Carolina late this week. Aside from Florence, two other storms pose threats to US territories this week.

Rest of today – cloudy, with periods of rain, heavy at times, especially later in the day towards the overnight hours. High temperatures will be in the upper-60s with cool, persistent onshore flow from the east. Ahead of an approaching warm front.

Tuesday – chances for thunderstorms will persist into Tuesday, with this slow-moving warm front finally pushing through. There could be thunderstorms overnight. Then, later in the day, as the trailing cold front moves through. Once the warm front passes over, we’ll be within the warm sector of the parent low pressure system, which will allow temperatures to climb into the low-80s with a marked increase in humidity.

Wednesday – the cold front mentioned above will slow down as it approaches us and dissipate. This will lead to continuing chances for rain, and thunderstorms with temperatures again in the low-80s.

Thursday – high pressure will finally begin to build in to our north by Thursday behind the cold front above. This will lead to better conditions, with partly sunny skies and temperatures in the low-80s.

Friday – partly sunny skies with high temperatures in the low-80s again with high pressure to our north.

Florence, Isaac, and Olivia All Pose a Threat to the US

These three storms have the potential to impact the Southeastern US, Puerto Rico, and Hawaii respectively within the next week. Among these, Florence is by far the most dangerous, as it could make landfall on North Carolina as a Category 4 storm with maximum sustained winds of 145 mph.

Hurricane Florence

Over the past day or so, the picture for Florence has become much clearer. Various forecast models have come into remarkable alignment on both the overall track and intensity of Florence over the next 5 days, which means bad news for the Carolinas. The picture looks grim, with Florence set to make landfall as a very strong Category 4 storm packing sustained winds of 145 mph. Florence has nothing but warm water ahead of it, and there should be minimal impact ot this storm from vertical wind shear. Even if the storm weakens due to eyewall replacement cycles prior to landfall, that would only serve to increase the radius of its damaging winds. No matter how you slice it, Florence is likely to bring deadly storm surge and risk of life-threatening flash flooding and inland flooding. The fact that its forecast track slows to a crawl after landfall makes the flooding risk inland especially serious.

Hurricane Isaac

Isaac is a compact storm, which is leading to some uncertainty regarding both its track and intensity forecast. Small tropical cyclones are subject to wild swings in intensity both ways, which can be unpredictable at times. As of now, Isaac is forecast to cross the Windward Islands of the Lesser Antilles as a Category 1 storm, then continue into the Caribbean where hostile shear conditions should cause it to weaken. It is worth noting, despite the UKMET model being an outlier, that there are some signals that the storm could turn to the northwest later in the period, which could mean a threat to Hispaniola and Puerto Rico.

Hurricane Olivia

This storm continues to track towards Hawaii. Although weakening due to the influence of moderate wind shear, it is still set to hit Hawaii later this week as a strong tropical storm. The primary threat to Hawaii from Olivia will be in the form of heavy rain and flash flooding regardless of whether it makes direct landfall on one of the main islands.

NYC Weather Update – Sep 4, 2018

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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.

NYC Labor Day Weekend Weather – Aug 31, 2018

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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.

NYC Weather Update – Aug 27, 2018

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The dog days of summer arrive in full force this week before we close out the month of August. With a strong Bermuda high in place well southeast and secondary high pressure in the area, we will see a heat wave take place from today into Thursday. Several days of hot, humid weather are on tap until a cold front moves through Thursday, bringing the potential for strong thunderstorms and much cooler air in its wake.

Rest of today – hot, though not quite as humid as it will be later this week. High temperatures in the low-90s with mostly sunny skies expected.

Tuesday – even hotter, and more humid than today with high temperatures in the mid-90s with sunny skies.

Wednesday – yet another hot, oppressively humid day with high temperatures in the mid-90s. Both Tuesday and Wednesday should see heat index values above 100°F due to high dew point values as well.

Thursday – a cold front is forecast to push through the area Thursday. Along and ahead of this front, we are expecting the chance for some scattered strong to severe thunderstorms given the ample moisture available and hot temperatures. Temperatures will still be hot, in the low-90s, before this frontal boundary passes through. Behind this cold front, temperatures will be 10-15°F cooler Friday into the weekend.