We're in the middle of the first heat wave of the year, but it won't last too much longer. Memorial Day Weekend will see a return of unseasonably cool temperatures, combined with clouds and some unsettled weather (e.g., light rain). In between now and then, we'll see a low risk of evening thunderstorms, which could be strong to severe.

A Heat Advisory is in effect until 8pm on Wednesday. A Heat Advisory is issued when the combination of heat and humidity is expected to make it feel like it is 95 to 99 degrees for two or more consecutive days, or 100 to 104 degrees for any length of time.

All of NYC is under a Heat Advisory until 8pm Wednesday night.

As I write this, temperatures have surpassed 90°F at Central Park and LaGuardia, with Newark Airport all the way up to 97°F! Southerly winds are keeping the south shore cool, leaving JFK at a balmy 77°F and temperatures in the mid-to-upper 80s further inland in Brooklyn and Queens.

NWS Forecast Temperatures (Actual), 11am Tues - 11pm Wed.

Tempratures will be marginally cooler tomorrow, but still dangerously hot, with high temperatures around 90°F. Heat indices (i.e., apparent temperature) will be in the mid-to-upper 90s. especially in the afternoon as the humidity begins to skyrocket.

NWS Forecast Apparent Temperatures (Heat Indices), 11am Tues - 11pm Wed.

These extraordinarily hot temperatures at the surface are creating low-level instability with steep lapse rates in the lower-to-mid levels of the troposphere, yielding between 500-1500 J/kG of CAPE (Convective Available Potential Energy) - more than enough juice to support thunderstorms.

HRRR model (18z May 19) skew-t showing a theoretical vertical profile of the atmosphere at 4pm on Wednesday.

Add in over 1.5 inches of precipitable water in the lower-to-mid levels, sandwiched between drier air aloft and at the surface, this could be recipe for downburst wind gusts or tiny hail. The official meteorologists at NOAA see this too: Today's Day 1 and Day 2 Convective Outlooks put NYC within the "marginal" risk zone for severe weather, namely for severe winds and hail.

NOAA/SPC Convective Outlooks for Tuesday (Day 1) and Wednesday (Day 2) show a marginal risk of severe weather in NYC driven by a 5% risk of severe wind and/or hail.

As far as timing and likelihood is concerned...

The risk for storms tonight is currently low (< 10%) but growing.

• Today (Tuesday): Models have been consistently trending towards no precipitation this evening, but in the past hour, we've gone from clear blue skies to increasing abundance of cumulus clouds. This tells me the atmosphere is mixing and taking advantage of the CAPE. So an isolated shower or thunderstorm can't be completely ruled out, but it is expected to remain dry as the best upper level support remains north of the area and subsidence helps to suppress convective development. Highest odds for rain this evening will be 7-9pm.
• Tomorrow (Wednesday): The risk for storms is meaningfully higher on - closer to 50% along with greater likelihood of severe storms. Based on what I'm seeing in mesoscale analysis, I think the best odds for severe weather will be in central/southern NJ and the Hudson Valley/CT/Long Island, but CAM guidance does support the possibility of thunderstorms in NYC. The best chance for showers and storms will be between 2pm-8pm, which could include isolated downpours, frequent lightning, strong gusty winds, and even tiny hail. However, these storms should be quick moving, which will mitigate the potential for any flooding.

HRRR model (18z May 19) reflectivity showing a theoretical look at the radar (3pm Tues - 2pm Thu)

Light rain showers may linger into Thursday morning's commute, but the day will dry out as high pressure takes control behind the cold front. Temperatures will be 20-30 degrees cooler with morning temperatures in the 50s and afternoon high temperatures expected to top out in the mid-to-upper 60s. Expect a similar story for Friday with high temperatures again only in the 60s.

By this weekend, the high pressure will move off the coast of Maine, creating easterly flow similar to what we saw throughout March and April. This easterly flow will lead persistent cool and unsettled weather conditions throughout the holiday weekend, with low clouds and ample moisture... plus a couple of weak shortwaves passing through the region. I'm not expecting a "wash out" for the weekend, but it will likely be cool, cloudy and drizzly on both Saturday and Sunday. Memorial Day Monday looks to be a little warmer - maybe breaking 70°F - but cloudy with some risk for light rain.

As of now, if you're looking to BBQ this weekend, Monday might be the best bet.

Also, don't expect a lot of heat waves this summer.

Expect a lot of clouds and rain this weekend and next week, with a well-timed reprieve leading to a generally pleasant day tomorrow for all the moms out there.

For Mother's Day, expect morning temperatures in the 60s with mostly cloudy skies giving way to a mix of sun and clouds by late morning as temperatures rise into the lower 70s. Clouds will begin to increase in the afternoon and there is a chance of a light rain shower around dinner time.

Odds of precipitation increase Sunday night into Monday, but that's getting ahead of ourselves because it's going to rain today (Saturday) too...

Sciencey Explanation

To make a long-story short to explain what is going on now and lately... La Niña está la muerta pero it is leaving behind some lingering impacts, and at the same time El Niño está despertando.

You've probably heard or read about the forecast for a strong El Niño to develop this year. I'll probably make a separate post to explain what that means for us, but suffice to say, it leads to more moisture in the sub-tropical jet stream, which eventually finds its way toward us. When the sub-tropical jet and the polar jet get close enough to interact, it leads to stronger low pressure systems with more rain along the frontal boundaries.

Water Vapor Satellite Imagery - morning of Saturday May 9

Cue La Niña. The lingering effects of La Niña are causing the -NAO/+TNH weather pattern we've been experiencing for the past month. This features a persistent low pressure over northeastern Canada and mid-latitude blocking near the Canadian Maritimes. This has led to persistent troughing across the Great Lakes, which results in jet stream interaction over the Mississippi/Tennessee/Ohio River Valleys.

https://preview.redd.it/ans5tcugv40h1.png?width=3072&format=png&auto=webp&s=397ac0303f0316c0b06f2c6588d23a0cb6fd402d

If this were winter, we'd be getting hit by a lot of storms, and probably getting a lot of snow (aka like last winter). But because of the angle of the sun vis-a-vis the tilt of the earth (i.e., seasons), the polar jet isn't as amplified or strong, and instead has been more progressive, allowing troughs and fronts to move quickly across the east.

La Niña also leads to high pressure ridging over the Bahamas, producing a faux Bermuda High, and this is what is to blame for the cooler and dryer weather we have experienced lately. This high pressure has been producing southerly winds, but before those winds reach NYC and Long Island, they have to pass over coastal waters that are still quite cold - around 50-55°F.

Sea Surface Temperature Anomalies show NY coastal waters several degrees below normal.

This injection of cooler air at the surface creates a layer of stable, well-mixed air near the surface. When troughs approach the region with saturated air in the mid-levels, much of the precipitation evaporates as it falls through this layer of stable, dryer air. This is what happened on Friday - some areas saw a brief light rain shower, but nothing as robust as was suggested on the radar at the time.

As it happens, NYC is currently rated D0-D1 on the drought index, i.e., abnormally dry to moderate drought conditions. And La Niña conditions during the summer often correlate with drought conditions, so that is not totally surprising.

But as El Niño gains strength, we will see more and more tropical moisture injected into the southern US, and as we approach summer (and also as our atmospheric angular momentum changes), we will see more and more storm tracks that bring that wet weather directly into our region. It will be a humid, muggy and wet this summer, but at least it shouldn't be too hot.

Saturday Forecast

A broad upper-level trough in Canada is moving eastward from Ontario into Quebec today, and the associated cold front will swing through the Great Lakes and Mid-Atlantic region, leading to rain showers and possibly thunderstorms today.

Convective-allowing model guidance is in agreement with strong lift at all heights with model skew-Ts suggesting some elevated instability of around 250-500 J/kG, but only around 1" of precipitable water.

HRRR (13z May 9) Model Skew T for Saturday evening shows strong lift and marginal elevated CAPE (200-370 J/kg) - above a temperature inversion - with just 1.01\" of precipitable water.

This indicates to me that rain today will be mostly light to moderate, but with a slight chance of a downpour or thunderstorm. The strongest divergence and diffluence aloft is south of us, meaning the greatest likelihood of thunderstorms will be over NJ and the DelMarVa Peninsula. The NOAA/NWS Storm Prediction Center agrees based on their Day 1 Convective/Severe Outlook issued this morning. The local NWS office (OKX) also agrees with the possibility of some mostly weak thunderstorms across NYC and Long Island this afternoon, with the forecast discussion highlighting the possibility of low rumbles of thunder. Still, I can't rule out some strong-to-severe wind gusts or even tiny hail.

NOAA/NWS Storm Prediction Center Day 1 Severe Outlook for Saturday May 9.

As far as timing, models suggest light rain could begin as early as 1 or 2pm, but it actually started raining as I was writing this up before noon. There is some uncertainty as far as when exactly the band of thunderstorms and strongest rain will arrive - anywhere between 3pm and 8pm it seems - and rain could linger past midnight.

HRRR (13z May 9) model reflectivity through 3am Sunday.

Above, the 13z run of the HRRR model favors heavy rain and/or thunderstorms in NYC around 6-8pm. Below, the 12z run of the NAM 3km model suggests the heaviest precipitation will be around 3-5pm:

NAM 3km (12z May 9) model reflectivity through 2am Sunday.

This rain is expected to be more beneficial for plans than impactful to people and infrastructure. Only one-quarter (1/4) to one-half (1/2) inch of rain is expected in all today.

Official NWS (OKX) Quantitative Precipitation Forecast for Saturday

Cloudy skies and breezy southerly winds coming off the cold ocean will keep temperatures on the cooler side with high temperatures in the lower-to-mid 60s, about 5-10 degrees below normal.

Sunday Night and Next Week

Another frontal system approaches late in the weekend. There remains some uncertainty as far as when the front will arrive and how quickly it progresses, but the official NWS thinking is that any chance of rain showers will not be until at least Sunday evening, with high confidence of rain Sunday night into Monday morning. The NWS is describing this second front as "another non impactful, but beneficial rainfall for the region."

NAM 3km (12z May 9) reflectivity showing what the radar might look like 8am Sun - 8am Mon.

The NAM 3km model reflectivity (above) shows isolated downpours and thunderstorms possible Sunday evening followed by brief, light rain Sunday night, but synoptic models (e.g., ECMWF) are more aggressive with overnight rainfall. The official NWS (OKX) foreast calls for less than one-quarter (1/4) inch of rain Sunday night into Monday morning.

Temperatures on Monday will be well below average following the passage of the cold front, with highs struggling to break 60. Tuesday and Wednesday will be marginally warmer, but still below-average.

Official NWS (OKX) forecast high & low temperatures, Saturday - Wednesday.

Another frontal system will dig through through the Great Lakes on Wednesday, and is expected to stall out over New England, leading to more rain likely on Wednesday and through the end of the week.

I probably won't provide an update for Sunday night's rain, but I will provide an update later in the week if it looks like there will be any significant daytime/commuting hour precipitation.