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NYC Detailed Forecast for April 9, 2020

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It’s been nearly three months since my last detailed forecast, so forgive me if I’m a bit rusty. Thursday in NYC will be an active weather day. A rather complex picture will unfold, with a warm front passing early, then a triple point passing nearby, followed by a trailing cold front that could bring the potential for thunderstorms later in the afternoon. Strong winds will be in the picture, and these winds will persist (if not increasing in intensity) into Friday.

My Forecast
High: 61°F | Low: 49°F | Max sustained winds: 33 mph | Total precipitation: 0.24″ – verification will come from METAR data for the period between 2AM Thursday and 2AM Friday (06Z Thursday to 06Z Friday) at LGA (LaGuardia Airport), and the KLGA Daily Climate Report.

Verification
High: 58°F | Low: 44°F | Max sustained winds: 44 mph | Total precipitation: 0.26″ – this ended up being a decent forecast for the high temperature. The cooler scenario depicted by NAM ended up being correct, and siding with a more conservative high was the right call. Amazingly, I was only 0.02″ off from the precipitation total! However, in my focus on that aspect of the forecast, I neglected some basic signs about the winds and low temperatures. These two ended up being connected. Winds in the wake of the cold front were likely boosted by subsidence (sinking cold air), and these provided a boost to cold air advection, leading to the low actually occurring around midnight, as opposed to the overnight hours going into the day. This was actually apparent in the statistical guidance, but was something I failed to spot. Lesson learned: never forget to check the entire data set – and when cold fronts with convective activity swing through, consider being aggressive about the winds associated both with the storms themselves, and behind the front.

  • Weather Prediction Center surface forecast for 2AM Thursday
  • Weather Prediction Center surface forecast for 8AM Thursday
  • Weather Prediction Center surface forecast for 2PM Thursday
  • Weather Prediction Center surface forecast for 8PM Thursday

Synoptic Setup
At the surface, two lows will be influencing the picture, one centered further north over Ontario, and another tracking closer to the area. Early in the forecast period, a warm front associated with secondary low pushes through. This could lead to some lighter stratiform rain due to isentropic (overrunning) lift. After this, a triple point passes just north of the area in the afternoon, and this should provide a locus for some some enhanced lift. This will be followed shortly thereafter by a trailing cold front passing through in the late afternoon hours. This cold front could trigger some thunderstorms with NAM showing some instability, though GFS isn’t as aggressive.

GFS is less bullish on showing a saturated layer at 850 mb. NAM does have wetter conditions which explains why the QPF totals are higher with this model. One thing both models agree on is the presence of strong winds at this level, anywhere from 35-50 knots. Should moisture content be better upstream, we should see effective moisture transport from this low-leve jet.

Further up in the atmosphere at 500mb, a shortwave trough attached to a closed 500 mb low over Ontario/Quebec border is forecast pivot through the region. Models are suggesting very strong positive relative vorticity associated with this feature. This should lead to ample divergence aloft and enhanced lift potentially supportive of organized convective activity. Low-mid level lapse rates due to the presence of the closed low could also be decent enough to support convection.

Finally, at the 300 mb level, a jet streak with very fast core winds in excess of 140 knots looks to set up over the Ohio Valley. We will be in the favorable left exit region of this jet streak, a set up that further favors enhanced divergence aloft. This should likewise increase lift in the atmosphere. This is reflected by SREF that shows a decent probability of moderate to strong omega over the region during the passage of this storm.

  • NAM model 850 mb relative humidity and winds valid 2PM Thursday
  • GFS model 500 mb height and relative vorticity valid 2PM Thursday
  • GFS model 300 mb height and winds valid 2PM Thursday

High Temperatures
There was a large spread between NAM and GFS statistical guidance, with NAM much cooler and coming in only in the mid-50s, while GFS came in with low-60s. Given that there should be some warm air advection with southerly winds ahead of the cold front, I tend to buy more into the warmer end of the guidance envelope. However, both sets of statistical guidance also show a period of winds backing to the ESE in the afternoon which would bring a characteristically onshore flow and introduce a marine air mass. That might cut into overall high temperatures. Some limited evaporational cooling and clouds could also keep a cap on temperatures warming too much. With all this in mind, I’m going with a high temperature of 61ºF.

Low Temperatures
There is likewise a big spread between GFS and NAM on low temperatures. NAM is close to climatological averages for low temperatures in the low-40s while GFS is a full 10ºF warmer. Southerly winds are indicated during most of the overnight hours seem to suggest erring on the warmer end of guidance. I think that EKDMOS 50th percentile of 49ºF seems reasonable.

  • GFS MOS guidance for KLGA
  • NAM MOS guidance for KLGA

Max Sustained Winds
Both GFS and NAM indicate that a well-mixed layer will form by the late afternoon. As referenced above, winds aloft at 850 mb will be quite strong. This mixed layer should enable these strong winds to work their way down to the surface. This will be aided by any downward momentum transfer effected by falling rain. Layer mean wind analysis shows max wind speeds well above statistical guidance, and I few reasons to disagree with this, so I’m calling for max sustained winds of 29 knots, or 33 mph.

GFS forecast sounding for KLGA valid 2PM Thursday. The presence of a well mixed layer is apparent from the surface up to just above 850 mb. This should allow for strong winds at this layer to mix down to the surface, something that would be enhanced by precipitation drag effects from rain falling.

Total Precipitation
There is high bust potential for total precipitation forecasts here with convective precipitation in the picture. The earlier light stratiform precipitation with the warm front probably will not result in too much overall precipitation. However, just one strong thunderstorm/heavy downpour could wreck any forecast here. The blended GFS/NAM mean is about 0.25″ and SREF probabilities for precipitation greater than this are not a lock, despite the overall synoptic picture showing decent support for good lift. SREF gradients are quite sharp going south. In addition, onshore winds that are possible could help stabilize the area by bringing in a cooler marine layer. I don’t feel confident that we’d see more than 0.25″ unless a convective burst hits KLGA head on, so I am going with 0.24″ as a conservative play.

NYC Detailed Forecast for January 25, 2020 (Lunar New Year)

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Lunar New Year day in New York city will see the passage of a relatively robust storm system. Ahead of this storm, persistently easterly onshore winds could result in some minor coastal flooding. A band of moderate to heavy rain looks like it will sweep through in the afternoon hours. Rain should clear up quickly by the early evening. Temperatures should be above normal with some warm air advection.

My Forecast
High: 49°F | Low: 38°F | Max sustained winds: 26 mph | Total precipitation: 0.85″ – verification will come from METAR data for the period between 1AM Saturday and 1AM Sunday (06Z Saturday to 06Z Sunday) at LGA (LaGuardia Airport), and the KLGA Daily Climate Report.

Verification
High: 52°F | Low: 38°F | Max sustained winds: 30 mph | Total precipitation: 0.78″ – all around this wasn’t too bad of a forecast. Wind speeds were slightly higher, and coming from the southeast. This could help explain why temperatures broke into the low-50s as this set up would have produced better warm air advection. Got the low temperature spot on. As for precipitation, at first I thought this was going to be a bust because the storm seemed to be moving faster than model data suggested. However, even with rain stopping by around 4pm, enough moderate to heavy rain fell that we went above 0.75″, though not too much over. I’m glad I didn’t buy into a higher precipitation total than 0.85″!

Weather Prediction Center surface forecast for 7AM Saturday

Synoptic Set Up
A primary occluded low tracking over the Great Lakes will spawn a secondary coastal low at the triple point between its occluded, warm and cold fronts. The primary low should weaken as it becomes vertically stacked with a 500 mb closed low (depicted below) and slowly consolidate with the secondary low. During the day, the secondary low is forecast to track northeast with the triple point tracking almost directly over NYC by around 7PM.

Ahead of this, we could see some light rain due to isentropic lift north of the surface warm front. Strong lift is expected as the low/triple point tracks closer, and lift will be enhanced by favorable conditions at 300 mb and 500 mb levels. At the 500 mb level, the aforementioned closed low is forecast to cause ample downstream positive vorticity resulting in increased divergence. Further up at the 300 mb level, it appears the left exit region of a powerful jet streak over the Southeastern US will lie south of us, and we’ll be under the right entrance region of a smaller, weaker jet streak. Both these regions will produce increased divergence and enhance lift. SREF probabilities for moderate to strong omega > -6 mb/sec were in the 50%-70% range.

High resolution models suggest a narrow band of moderate to heavy rain during the afternoon hours. A significant low-level jet at 850 mb with winds 45-55 knots and will aid with transporting subtropical moisture northwards. A dry slot works in quickly behind the primary slug of moisture by 7PM – this should cut off precipitation for the most part.

  • GFS forecast 850 mb winds and relative humidity valid 1PM Saturday
  • NAM 500 mb heights and vorticity valid 4PM Saturday
  • NAM forecast 300 mb heights and wind valid 1PM Saturday

High Temperatures
Statistical models were in decent agreement, with NAM being the coolest coming in at 46°F. GFS was closer to 50°F and EKDMOS 50th percnetile was 49°F. Erring on the side of being cautious about going warmer than 50°F because of of the proximity of the triple point. If that tracks offshore more, we’d see a switch to westerly winds sooner and would end up being in a cooler airmass. Rain and clouds will also keep things in check. There’s some warm air advection indicated, but also need to keep in mind that sea surface temperatures are only in low-40s offshore around NYC. For these reasons, went the EKDMOS 50th percentile at 49°F.

Low Temperatures
The surface and near surface layer start off pretty dry. Evaporational cooling as the column saturates could have potential to keep temps on the cool side. EKDMOS 50th percentile was steady across two runs at 39°F. Statistical models had a spread from NBM at 36°F to GFS with 42°F. I’m going with 38°F because I think evaporational cooling will be a factor overnight.

  • NAM MOS forecast for KLGA
  • GFS MOS forecast for KLGA

Max Sustained Winds
Seems like winds will mostly driven by synoptic factors, since I don’t see signs of a well-mixed surface layer. However, downard momentum transfer with heavy rains could help bring some of the very fast 850 mb winds to the surface. It’s not hard to imagine sustained winds in excess of 20 knots. Winds coming from the east-northeast and east will be off the water towards LGA, with decent fetch over open water, so edging towards a higher max wind of 26 mph. This matches EKDMOS 50th percentile and is higher than GFS and NAM statistical guidance of 16 and 19 knots respectively.

NAM forecast sounding for KLGA valid 1PM Saturday. The atmospheric column is saturated throughout by this point (dew point temperature in green and environmental temperature in red overlap), meaning precipitation is ongoing. Also note how strong winds are throughout the column.

Total Precipitation
Strong lift will accompany the triple point as stated above. A low-level jet is also forecast to set up, creating conditions conducive for heavy rain. However, development of a dry slot is apparent in the early evening – this would put the brakes on precipitation. If that dry slot moves through earlier, bust potential for higher end total precipitation amounts is there. Statistical guidance ranges 0.60″-0.75″ on the low end, up to 1.23″-1.48″ (which would be record-breaking) on the high end. Various ensemble means range from 0.77″-0.94″, which is on the low end of the statistical guidance. Ensembles also showed a fairly tight spread, indicating decent confidence. The ensemble plume mean is close to the average mean of statistical guidance around 0.85″. This seems reasonable as it allows for possibility of heavy rain while still falling squarely within predicted ranges for both statistical and ensemble guidance. SREF probabilities for > 1.00″ of total precipitation aren’t a lock (10%-50%) with a very tight gradient southeast.

NYC Weekend Weather – Apr 26, 2019

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A wet start to the weekend this Friday with multiple rounds of rain moving through. Saturday shapes up to be a better day at least in terms of being dry. A second disturbance/low moves through Sunday bringing yet more chances for rain before skies clear up to start next week. Temperatures behind this second storm/cold front will be below normal for this time of year.

Rest of today – periods of rain during the afternoon as a warm front works its way across the area. Some thunderstorms possible. Second round of more focused/heavier rain later in the evening around 7-8PM as a trailing cold front pushes through. This front will have decent upper level support with some indications of a vorticity maxima at 500 mb and us being in the exit region of a 300 mb jet streak. High temperatures today topping out around the upper-50s even with the rain as we’re expected some time in the warm sector of the low bringing us this stormy weather. If the warm front below fails to push north through here though, we may only see temps in the mid-50s. Overnight lows should bottom out around 50°F.

Weather Prediction Center’s most recent surface analysis, showing a marked warm front south of NYC. Some thunderstorms and showers have popped up ahead of this frontal boundary due to isentropic lift (overrunning) as warm air from the south slides over colder air at the surface further nort
High resolution rapid refresh simulated radar for 8PM tonight showing a line of showers with some thunderstorms possible.

Saturday – with the trailing cold front clearing the area tonight, precipitation should end and Saturday should end up being a decent day though with more clouds than sun. High temperatures should be able to get into the low-60s, edging into the mid-60s if more sun breaks out. Overnight lows noticeably cooler with a colder, drier airmass behind today’s cold front, in the mid-40s.

Sunday – a second low approaches from the Ohio Valley, again bringing some potential for rain. This rain is expected to be on the lighter side. Temperatures should reach into the upper-50s. Overnight lows in the low-40s.

Weather Prediction Center’s surface forecast for 8AM Sunday

Monnday – start of next week should be sunny, with Sunday’s storm system moving offshore. High temperatures will be below normal though, with the cold front associated with Sunday’s low bringing a reinforcing shot of cooler air. High temperatures probably topping out in the upper-50s. Overnight lows 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 Weekend Weather – Dec 8, 2017

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The weather headline for this weekend will be our first accumulating snow of the season! Aside from this, below average and cold temperatures will be in place throughout the weekend and well into next week. This persistent cold spell has a direct link to high pressure over the western US that has resulted in fires in California. Another chance for additional accumulating snow is possible early next week.

Rest of today – high temperatures in the low-40s with mostly cloudy skies. Snow beginning in the overnight hours into tomorrow.

Saturday – snow throughout the day, accumulations of largely between 2-4″ during the day expected in the city, but with higher totals moving east, in the 4-6″ range for parts of Queens, and Brooklyn. High temperatures in the low-30s.

A coastal storm will develop and strengthen off the Southeast coast as an upper level shortwave and a jet streak work to enhance it. The jet streak in particular will provide strong lift for the development of snow showers. Thermal profiles throughout the atmosphere in our region will support all snow. Model trends seem to continue nudging the track westward over the past couple of days. Further shifts west would result in higher snowfall totals, which are suggested by the GEFs plumes in the last image in this section that point a range of 6-8″ of snow for LGA.

Weather Prediction Center surface low tracks and clusters. Focus on the coastal low track. The mean track takes this storm very close to the 40°N 70°W benchmark, a location for coastal low centers that generally translates to robust snowfall for winter storms.

Sunday – an additional accumulation of an inch or so is forecast overnight Saturday as snow begins tapering off going into Sunday. As the low pulls away, winds will turn to the west and strengthen. High temperatures will actually be somewhat warmer in the upper-30s with mostly sunny skies.

Monday – mostly sunny skies again with high temperatures around 40°F.