Tag Archives: warm air advection

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 Weekend Weather – Jan 31, 2020

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Rain to start this weekend leads to a dramatic pattern change with anomalous ridging taking place by late in the forecast period. Temperatures throughout the period range from a few degrees above normal in the mid-40s to the mid-50s by Monday. The warmth continues into the middle of next week.

Rest of today – mostly cloudy with high temperatures in the mid-40s. A relatively weak coastal low will develop and move towards our area overnight. The best chances for rain are in the early overnight hours. Overnight lows in the mid-30s.

Saturday – rain chances will diminish quite a bit as the core of the low continues exiting to the northeast. Mostly cloudy with high temperatures in the mid-40s. Overnight lows in the the mid-30s. Some snow could mix in overnight as lingering chances for precipitation continue.

Sunday – things will dry out as the low continues to distance itself from us. Mostly sunny with high temperatures in the mid-40s. Overnight lows in the upper-30s.

Monday – the synoptic scale pattern shifts as strong ridging at the mid-levels occurs. This will translate to warm high temperatures in the mid-50s on Monday with mostly sunny skies and southerly flow. Overnight lows in the mid-40s.

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 – Jan 10, 2020

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The weather story for this weekend in NYC will be the near record-breaking warmth we’ll have with temps at or above 60°F Saturday and Sunday. A storm system sweeps through overnight going into Sunday, and should clear by Sunday afternoon. Behind this, temperatures will drop back to the upper-40s, which is still almost 10ºF above normal for this time of the year! Enjoy this rare occurrence of near record warmth that isn’t accompanied by a washout.

Rest of today – mostly cloudy with high temperatures in the low-50s. A strong area of high pressure offshore southeast of our area will be steadily pumping warm, moist air into the region. Looking at the chart below, you’ll see southerly winds blowing at large angles across temperature contours that increase going south – this is an ideal set up for persistent warm air advection and will set the stage for the warmth we see this weekend. Overnight lows in the mid-40s. With moisture advecting into the area, we could see areas of fog develop overnight.

GFS 2-meter temperature with contours and 10-meter wind barbs.

Saturday – very mild with temperatures in the low-60s. Mostly cloudy with a southerly breeze and some fog possible. Overnight lows in the mid-50s. Rain chances increase particularly late overnight going into Sunday morning.

Sunday – elevated convection could bring a rumble of thunder Sunday morning, with rain chances peaking in the early morning hours. High temperatures should remain quite mild, in the mid-60s (and possibly higher with downsloping westerly winds for a period with clearing skies, following the passage of a prefrontal trough and before a cold front closes in). Overnight lows dropping into the low-40s – which is actually still above normal for what a daytime high temperature should be this time of year.

NAM high resolution model output of 1000-500 mb thickness, surface pressure, and precipitation valid for 5AM Sunday

Monday – high temperatures similar to Friday near 50ºF, partly sunny skies. Overnight lows in the low-30s.

NYC Detailed Forecast for November 21, 2019

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Thursday will probably end up being the best day of the week. High pressure will be in control from most of the period allowing for a sunny day with seasonable temperatures. Winds may be stiff overnight as well as late Thursday night into Friday. Rain associated with a low pressure system moving in will hold off until Friday.

My Forecast
High: 50°F | Low: 38°F | Max sustained winds: 20 mph | Total precipitation: 0.00″ – verification will come from METAR data for the period between 1AM Thursday and 1AM Friday (06Z Thursday to 06Z Friday) at LGA (LaGuardia Airport), and the KLGA Daily Climate Report.

Verification
High: 53°F | Low: 39°F | Max sustained winds: 14 mph | Total precipitation: 0.00″ – high temperatures ended up being a bit warmer than expected, more towards the EKDMOS 90th percentile range. I think this had to do with the fact that overnight lows were warmer (did a good job edging up on that), and because northwesterly winds backing to the west introduced an element of downsloping. Compressional warming from this was not offset by any other temperature advection, and allowed for temperatures to hit exactly the average high for this time of year. The other aspect I missed on was max wind speed. Here, looking at surface analyses from yesterday compared to the forecast models, there was a larger distance between the center of high pressure and the departing coastal low, leading to a weaker than expected pressure gradient early. The fastest winds ended up being clocked from the southwest as a result, ahead of the approaching cold front.

Weather Prediction Surface forecast for 7AM Thursday

Synoptic Set Up
The forecast period starts with a north-south elongated high pressure over much of the Eastern US, centered over the Ohio Valley. The coastal low that impacted the area Monday is still forecast to linger southeast of Nova Scotia. A decent pressure gradient will be in place at the beginning of the forecast period as a result (high pressure measuring ~1025 mb, low ~998 mb). An approaching low pressure center will travel northeast from the Midwest across the Great Lakes into Southwestern Quebec during the forecast period. The low will continue intensifying and will erode the northern part of the high pressure center. Precipitation associated with this low won’t reach NYC until Friday, though. The primary influence of this low for the forecast period will be in shifting winds from the northwest to the southwest.

Above the surface, at 850 mb winds are forecast to start off northerly, between 20-25 knots. Wind speeds ease as they continue backing from north to west, then pick up in intensity to 30 knots from the southwest. Dry air, though moistening, will prevail at the 850 mb level throughout the forecast period. At 500 mb, slight ridging takes place during the day, then increased vorticity starts to pivot through ahead of the primary shortwave axis associated with the maturing surface low over the Great Lakes. Finally, at 300 mb we’ll start in the entrance region of a jet streak, then followed by a period of calm before the exit region of another jet streak approaches from the west.

  • GFS 2-meter (surface temperatures) in contours, and 10-meter wind barbs, valid 1AM Thursday. This shows a decent environment for cold air advection with winds at almost perpendicular angles to the temperature contours, pointing from colder to warmer temperatures. However, wind speeds are modest so the strength of an CAA would not be strong.
  • By 1AM Friday, GFS 850 mb height, moisture, and winds show depict winds shifting to the southwest, but the air above the surface will still be quite dry.

High Temperatures
Statistical models (GFS, NAM, NBM) are good agreement, within a degree or two of 50°F. EKDMOS at 12Z similarly showed a tight band around 50°F, though this widened a bit at 18Z. 50°F is just a touch below climatological norms. I’m not seeing too many surprises here. Moisture is lacking for clouds to form until late in the period after the sun goes down. As winds shift to the southwest later in the day, they will largely parallel local isotherms so there won’t be appreciable warm air advection to look at either. I think 50°F is a good bet here.

Low Temperatures
Stiff winds from the northwest will offset relatively clear conditions in terms of radiational cooling. Cold air advection (CAA) is looking modest. While the wind direction is looking to cut at almost perpendicularly across temperature contours from cold to warmer temperatures, wind speeds aren’t going to be sustained at strong speeds for too long. I think 38°F is reasonable because statistical guidance is only a degree cooler or so, with EKDMOS showing this temperature at about the 50th percentile.

  • GFS MOS forecast
  • NAM MOS forecast

Max Sustained Winds
850 mb winds have two peaks and so do surface winds in EKDMOS: one early, then one late in the forecast period. I tend to concur the fastest winds will come early in the forecast period, since northwest winds are climatologically favored to produce fast high speeds. However, I also don’t see evidence to support mixing of strong winds aloft down to the surface. One wrinkle is if the pressure gradient ends up being tighter between the lows and the high pressure, as that would drive stronger than anticipated winds. 20 mph is above statistical guidance, but not that much higher.

NAM forecast sounding for KLGA valid 7PM Thursday. Distinct moistening of the mid and upper levels is evident with the dew points (green line) nearly overlapping the environmental temperature (red line). Lower levels are still markedly drier.

Total Precipitation
This is the easiest part of the forecast as strong high pressure and dry air throughout most of the atmospheric column for most of the day makes it nearly impossible for precipitation to materialize.