Unsettled weather takes hold during the mid-week period as a couple prefrontal disturbances affect the area. The focus for heavier rain will be later Wednesday as a cold front approaches. Behind this cold front, conditions should improve and result in a pleasant, dry weekend.
Rest of today – mostly cloudy with a chance of isolated showers and thunderstorms later in the day as a shortwave trough lingers around the area. Temperatures in the mid-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8PM TuesdayStorm Prediction Center severe weather outlook for Tuesday
Tuesday – shaping up to be an eventful day in regards to weather. Temperatures should rise into the low-80s with overnight lows in the low-70s. During the day, a mesoscale convective system (organized large cluster of thunderstorms) will be approaching the area from the Ohio Valley. Forecast is uncertain in terms of where the frontal boundaries in the vicinity of this MCS eventually set up. Current thinking is that we end up south of a warm front earlier in the day that will serve as the initial focus for shower and thunderstorm activity. This activity should continue and increase in intensity as the frontal boundary starts shifting south and encounters warm, moist and unstable air. There is potential for all severe hazards including hail, damaging winds, and an isolated tornado.
Wednesday – mostly cloudy with shower chances diminishing later in the day . High temperatures in the low-80s and lows in the upper-60s.
Thursday – still a possibility for lingering showers but overall a better day with mostly cloudy skies and high temperatures in the low-80s with overnight lows in the upper-60s.
During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. This post continues on this theme, adding a climatology for August.
City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)
Local Geography and Topography
Station Elevation: 10 feet above sea level.
Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.
KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view
Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.
On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.
National Weather Service – NYC Office
Wind Patterns
Below is a wind rose – you can read more about how to interpret this chart here.
Frequency (percentage) of the single most common wind direction: Due south (12.25%).
Directions that are most and least common: Most common wind directions include due southwest (10%), due northeast (9.75%), and south-southwest (8.75%). Least common wind directions are east-southeast (1%), due southeast (2.5%), and due east (2%).
Direction(s) most likely to produce the fastest winds: Winds of 16.5-21.4 knots are most frequently found coming from due northeast, and due south. It appears that due northeast also has some probability of exceeding 21.4 knots.
Direction(s) least likely to produce the fastest winds: As is the case with several other months, the least common wind directions of due east, east-southeast, and due southeast also rarely seen winds in excess of 16.4 knots.
Impacts of wind direction on local weather: August winds continue to exhibit a distinctive summer pattern, and share many of the characteristics of the wind profiles for June and July. Southerly and southwesterly winds are a touch less common in August versus July, while northwesterly winds increase in frequency slightly. Like in July, heat waves can accompany the clockwise return flow from the southwest when subtropical Bermuda highs set up southeast of the area. A slight down tick in the frequency of southerly winds may point to a decline in sea breezes, with sea surface temperatures continuing to warm and narrowing the gap with average air temperatures. Increases in northwesterly winds could be an indication of a stronger influence from Canadian high pressure.
Maximum observed two-minute wind speed for the month: 45 (52 mph)
Temperature and Precipitation Averages/Records
Temperature units are in Fahrenheit and precipitation is in inches.
Unsettled weather takes hold during the mid-week period as a couple prefrontal disturbances affect the area. The focus for heavier rain will be later Wednesday as a cold front approaches. Behind this cold front, conditions should improve and result in a pleasant, dry weekend.
Rest of today – mostly cloudy with a chance of isolated showers and thunderstorms later in the day as a shortwave trough lingers around the area. Temperatures in the mid-80s. Overnight lows in the low-70s.
Weather Prediction Center surface forecast for 8PM Wednesday
Wednesday – mostly cloudy with high temperatures in the mid-80s. Heavy rain possible especially later in the day with a cold front approaching. With a moist air mass in place, some thunderstorms that pop up could produce heavy downpours and localized flooding. Rain could continue overnight in more isolated form. Lows in the low-70s.
Thursday – brief subsidence behind the cold front that passes through Thursday should result in calmer weather and warmer temperatures with high temperatures in the upper-80s. Before that happens though, rain and thunderstorms can still pop up. Overnight lows in the low-70s.
Climate Prediction Center 6-10 temperature outlook valid Aug 11 – Aug 15 (this Sunday into next week)
Friday – cooler, sunny, with high temperatures in the low-80s. Overnight lows cooling into the upper-60s as Canadian high pressure begins to build.
It looks like we’re going to get lucky for Independence Day this year in NYC. While many areas of the Eastern US may see a chance for rain, the Northeast should remain under the influence of strong enough high pressure that we stay dry. That means good news for all the outdoor activities folks have planned coming up! This will be my first detailed forecast that draws on the recently released GFS FV3 forecast model upgrade, so it will be interesting to see how this upgrade performs.
My Forecast High: 88°F | Low: 74°F | Max sustained winds: 15 mph | Total precipitation: 0.00″ – 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: 93°F | Low: 71°F | Max sustained winds: 16 mph | Total precipitation: 0.00″ – Spot on with the wind forecast for yesterday. It was a good call to bump that forecast up with possible sea breeze enhancement in the mix. Unfortunately, these winds didn’t do enough to cool temperatures off during the day, and I was off by 5°F for the high temperature. Skies turned out to be pretty much sunny the whole day and while statistical forecast models should factor in urban heat island effects, it would have been prudent to still have forecast at least 90°F in light of this. Also, given the 850 mb relative humidity profile, I should have probably sided with a less cloudy outlook that would have favored warmer high temperatures. Of note, the new GFS model had issues initially with a cold bias that should have been corrected. In this case, it was warm on the low, but too cool on the high. This is worth noting even though these two points of data aren’t necessarily indicative of a trend.
Weather Prediction Center surface forecast for 8PM Thursday
Synoptic Set Up A stationary front currently along the northern tier of Illinois, Indiana, Ohio, Pennsylvania and western New York will attempt to push northwards tomorrow as a warm front. However, with a 500 mb ridge aloft and an area of surface high pressure over the Northeast and translating over the Gulf of Maine, this frontal boundary is forecast to become stationary, holding just southwest of the NYC metro area. Moisture above the surface looks modest, and without any impulses of low pressure at the surface or troughs aloft, there doesn’t appear to be any support for precipitation.
NAM 500 mb heights and vorticity valid for 8PM Thursday. Note the presence of a ridge over the Northeast (upside-down U pattern of the black height lines)
High Temperatures GFS, NAM, and NBM agree that July 4th a warm day. All three statistical models come in at between 88-89ºF for high temperatures. EKDMOS (ensemble MOS) 50th percentile matches is very close, at 87-88ºF. The only hitch to warm temperatures is that NAM shows the possibility for low clouds and wind directions are forecast to veer from east-northeast to southeast. Cloud cover could eat into temperatures rising too much, and winds coming from the directions indicated would bring a cooling influence of marine air with sea surface temperatures still in the low-60s at this point in the season. With these factors in mind, but also keeping in mind the effects of the urban heat island, I’ll side with a middle of the road forecast of 88ºF.
Low Temperatures NAM and GFS MOS both show the chance that the low temperature during this forecast period may actually fall overnight going into Friday. Either way, both are very close showing lows ranging 73-75ºF. EKDMOS 50th percentile shows 74-75ºF for the low. Increasing cloud cover should stall any serious radiational cooling, and there’s little indication of any temperature advection either way with weak winds. I’m again going with the mean here and siding with a forecast of 74ºF for the low.
Max Sustained Winds NAM, GFS, and NBM average out to 8 knots for the max sustained wind tomorrow. EKDMOS is more aggressive with winds in the 50th percentile about 13-15 knots. While forecast soundings don’t show too much evidence to support a forecast above 10 knots since the entire wind profile looks remarkably calm for tomorrow, this same tendency to weak synoptic winds could allow for a fairly robust sea breeze to develop, and this could be the what allows sustained winds to top 10 knots. I’ll go with the low-end of the EKDMOS and call for 13 knots (15 mph).
GFS forecast sounding for 8PM Thursday. Indications of high clouds are shown, but low levels remain dry. Wind profile in this forecast looks weak with no winds above 20 knots forecast anywhere in the atmospheric column.
Total Precipitation High pressure is forecast to be in control, with a 500 mb ridge in place. There aren’t any frontal boundaries moving through to create lift and even if there were, there simply isn’t enough moisture to support precipitation. Both the forecast sounding above and 850 mb forecast for relative humidity and winds below show moisture values that are insufficient to bring about any precipitation.
NAM forecast for 850 mb winds and relative humidity at 8PM Thursday
During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. This post continues on this theme, adding a climatology for July.
City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)
Local Geography and Topography
Station Elevation: 10 feet above sea level.
Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.
KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view
Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.
On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.
National Weather Service – NYC Office
Wind Patterns
Below is a wind rose – you can read more about how to interpret this chart here.
Frequency (percentage) of the single most common wind direction: Due south (13.5%).
Directions that are most and least common: Most common wind directions include due southwest (9.5%), due northeast (9.25%), and due northwest (8.5%). Least common wind directions are east-southeast (1%), due east (2%), and due southeast (2.5%).
Direction(s) most likely to produce the fastest winds: Winds of 16.5-21.4 knots are most frequently found coming from due northwest, and due south.
Direction(s) least likely to produce the fastest winds: As is the case with several other months, the least common wind directions of due east, east-southeast, and due southeast also rarely seen winds in excess of 16.4 knots.
Impacts of wind direction on local weather: July winds are remarkably similar to June. Southerly and southwesterly winds are a bit more common in July than in June, and northwesterly winds a little less so. This pattern of winds continues to suggest the influence of mid-latitude lows tracking inland north of the area, bringing more southerly warm sector flow. As with June, persistent, subtropical high pressures continue to be a factor at times in July over the western North Atlantic. The clockwise flow around these high pressure centers would result in more southerly and southwesterly winds, which can lead to the onset and maintenance of heat waves. Southerly winds (along with easterly and some northeasterly winds) would bring cooler, and more stable marine air, sometimes in the form of sea breezes. Sea breezes can become commonplace during this month whenever there’s light synoptic winds. At this point in the year, the average max temperatures well exceed the threshold of 5-7°F warmer than sea surface temperatures (which are slowly climbing through the mid-60s) required for sea breezes.
Maximum observed two-minute wind speed for the month: 46 (53 mph)
Temperature and Precipitation Averages/Records
Temperature units are in Fahrenheit and precipitation is in inches.
Worth Noting
July is the month when average temperatures peak at 86ºF. However, sweltering heat is possible well beyond this point, and record highs in this month are frequently in the upper-90s. Meanwhile, record highest low temperatures can easily be in the upper-70s and low-80s – making for very uncomfortable nights. July is also on average the wettest month of the year, which makes sense given the possibility for thunderstorms and a connection to tropical moisture.
During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. This post continues on this theme, adding a climatology for June.
City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)
Local Geography and Topography
Station Elevation: 10 feet above sea level.
Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.
KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view
Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.
On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.
National Weather Service – NYC Office
Wind Patterns
Below is a wind rose – you can read more about how to interpret this chart here.
Frequency (percentage) of the single most common wind direction: Due south (13.5%).
Directions that are most and least common: Most common wind directions include due northwest (9.5%), due northeast (9.25%), and due southwest (just below 9%). Least common wind directions are east-southeast (1%), due east (1.5%), and due southeast (2.5%).
Direction(s) most likely to produce the fastest winds: Winds of 16.5-21.4 knots are most frequently found coming from due northwest, north-northwest, and due south.
Direction(s) least likely to produce the fastest winds: As is the case with several other spring months, the least common wind directions of due east, east-southeast, and due southeast also rarely seen winds in excess of 16.4 knots.
Impacts of wind direction on local weather: Wind patterns in June present a similar pattern to those in May, but with a notable increase in the frequency of winds from the southwestern quadrant, and a decrease in winds from the northeastern quadrant. This could be a reflection of shifting storm patterns, with coastal storms and backdoor cold fronts from the northeast becoming less common while more mid-latitude lows track inland north of the area, bringing more southerly warm sector flow. Persistent Bermuda highs can sometimes also develop in June over the western North Atlantic. The clockwise flow around these high pressure centers would result in more southerly and southwesterly winds. During this time of year, southwesterly winds should generally advect warmer, more humid air from the Southeastern US into the region. When this pattern becomes persistent, it can cause oppressive heat waves. Meanwhile, southerly winds (along with easterly and some northeasterly winds) would bring cooler, and more stable marine air, sometimes in the form of sea breezes. In the wake of passing cold fronts associated with mid-latitude storms, northwesterly and northerly winds, though decreasing in frequency compared to May, should still be effective at transporting cooler, drier Canadian (continential Arctic) airmasses into the area.
Maximum observed two-minute wind speed for the month: 36 (41 mph)
Temperature and Precipitation Averages/Records
Temperature units are in Fahrenheit and precipitation is in inches.
Worth Noting June is the first month of summer, and not surprisingly is also the first month of the season where record highs have exceeded 100ºF.
During my time taking classes as part of Penn State University’s Undergraduate Certificate in Weather Forecasting, we were taught that understanding the climatology of the location you are interested in is an important prerequisite for making accurate forecasts. This post continues on this theme, adding a climatology for May.
City Name / Station ID: New York, NY (LaGuardia Airport – KLGA)
Local Geography and Topography
Station Elevation: 10 feet above sea level.
Station Location: LaGuardia Airport (KLGA) is situated on the north shore of Queens along the East River, approximately 6 miles east-northeast of Midtown Manhattan.
KLGA’s location within the broader NYC area, as seen in a Google Maps terrain view
Important Topographical Features: New York City is located in the extreme southeastern corner of New York State, bordering suburban New Jersey and Connecticut. These suburban regions combined with those in Long Island comprise the Greater New York City Metropolitan Area, which is the most populous urban agglomeration in the United States and one of the populous urbanized areas in the world with an estimated population of 18 million. New York City itself sprawls across the coastal plain around the Hudson River estuary. The terminal moraine formed by glaciers of the last Ice Age result in a ridge of higher terrain that cuts a swath from southwest to northeast across the boroughs from northern Staten Island, northern Brooklyn, southwestern through central and northeastern Queens. Otherwise, the city itself is low lying. This ridge varies in height between 200-400 feet, rising sharply from south to north, but tapering more gently north. North and west of the city (about 30-50 miles away), lie significant elevations of the Catskills (north), Poconos (west), Taconics that are part of the broader Appalachian Mountain Range. The elevations of the lower foothills can range from 1000-1500 feet. Some of the elevations in the Poconos and Catskills, west and north of KLGA respectively, peak between 2000-3000 feet. The open expanse of the Atlantic Ocean lies south of KLGA and New York City. Long Island Sound also lies east-northeast. The vast urbanized area of the NYC metropolitan region has significant effects on local microclimates via differential heating (urban heat island effect). KLGA is in a low-lying area sensitive to UHI effects and marine influences.
On winter mornings, ocean temperatures which are warm relative to the land reinforce the effect of the city heat island and low temperatures are often 10-20 degrees lower in the inland suburbs than in the central city. The relatively warm water temperatures also delay the advent of winter snows. Conversely, the lag in warming of water temperatures keeps spring temperatures relatively cool. One year-round measure of the ocean influence is the small average daily variation in temperature.
National Weather Service – NYC Office
Wind Patterns
Below is a wind rose – you can read more about how to interpret this chart here.
Frequency (percentage) of the single most common wind direction: The most common wind direction for this month by frequency of occurrence is due south (~13%).
Directions that are most and least common: Due northeast (11.5%) winds present a second maximum of most frequent winds. The least common wind direction is east-southeast (~2%), followed by due east (~2.25%).
Direction(s) most likely to produce the fastest winds: Due northwest winds have a slim possibility of generating winds over 21.5 knots. No other wind direction seems to have a measurable occurrence of winds exceeding this speed. This is a marked difference from the previous two months when wind speeds of this magnitude were much more common. Winds of 16.5-21.4 knots are most frequently found coming from the west-northwest, due northwest, north-northwest, and due south.
Direction(s) least likely to produce the fastest winds: Due east, east-southeast, and due southeast winds rarely exceed 16.4 knots.
Impacts of wind direction on local weather: The most common wind directions in May differ from the previous two months, with northwesterly winds becoming less frequent, while southerly and northeasterly winds become more prominent. In fact, winds from the southern semicircle are generally more common than previous months. Cold air advection from northwest winds continue to influence temperatures in the wake of cold fronts approaching from the west, however, the effect should be dramatic than in prior spring months. Winds from the northeast can be associated to backdoor cold fronts arriving from the Canadian Maritimes, bringing a moist, cool maritime polar air mass; the advance of coastal Nor’easter type storms; or a warm front approaching from the south. The difference between sea surface temperatures and temperatures over land widens during May as daily normal highs increase. This increases the chances for sea breezes during periods of overall weak synoptic winds, though generally, southerly winds would allow for advection of warmer, more moist airmasses into the area. This would often be the case when the NYC region is in the warm sector of mid-latitude lows, preceding the passage of a typical cold front attached such lows.
Maximum observed two-minute wind speed for the month: 36 (41 mph)
Temperature and Precipitation Averages/Records
Temperature units are in Fahrenheit and precipitation is in inches.
Worth Noting: Average temperatures climb above 70ºF in May for the first time during the spring months. In 2019, new record lows were set for the 13th and 14th of May, at 42ºF and 44ºF respectively. In 2020, an all-time monthly record low of 36ºF was set on May 9th. In 2021, record lowest max temperatures were set for the 29th and 30th (52ºF, brrr), and record lows were set for the 29th-31st (48ºF, 48ºF, and 50ºF respectively)!
For reference, here’s the post that triggered the following forecast post-mortem analysis. To start, here’s my forecast and the verified totals.
My Forecast High: 70°F | Low: 58°F | Max sustained winds: 30 mph | Total precipitation: 0.10″
Verification High: 76°F | Low: 56°F | Max sustained winds: 24 mph | Total precipitation: 0.16″
Since I did decently at forecasting the low temperature and total precipitation, this analysis will focus primarily on why I missed the mark on the high temperature and maximum sustained winds.
How I Verify Forecasts For an explanation of how I verify my own forecasts, see the methodology outlined in this prior post-mortem post
Post-Mortem Analysis On this forecast, I ended up handling total precipitation and low temperatures but missed the mark on the high temperature badly and to some extent the max sustained winds. Let’s look at what happened here.
High Temperature When I was putting together my forecast on the high temperature, the MOS output for both primary models (GFS and NAM) suggested the day would be largely overcast. Forecast soundings also suggested a deck of low clouds would be present for nearly the entirety of the day before the onset of an initial round of precipitation in the late evening/early overnight hours. Indeed, morning soundings at 8AM seemed to bear out what forecast models were calling for.
Sounding at 8AM from Brookhaven National Laboratory in Long Island, the site of the local National Weather Service forecast office for NYC. Note that the lower atmosphere, below 850 mb, is largely saturated, indicating low clouds and fog
Above the low level clouds, though, there was a noticeable dry layer up to about 500 mb, where high clouds were present. For temperatures, the biggest impact that’s hardest to pick up in advance is a low, thin, but optically thick layer of overcast clouds. Such a layer would suppress temperatures, whereas the lack of such a layer would allow for higher temperatures.
GOES East 1km resolution visible satellite image at 2:56 PM Friday, April 19. Note the relatively small pocket of clear skies right over NYC, western Long Island and Southwestern Connecticut.
In this event, where we were already in the warm sector of a low, any breaks in the clouds allowed for a dramatically warmer high temperature. As it turns out, there was a period of a couple hours in the afternoon during peak heating hours for this time of year that the skies were basically clear over NYC. In the spring, as the angle of the sun’s rays becomes ever more direct, even a small span of time like this with clear skies can result in significantly warmer temperatures. The small geographic scale of this pocket of clear skies, which was largely surrounded by cloudy conditions, shows you how difficult it is to predict something on a small scale like this, even just a day before the event itself!
Max Sustained Winds On max sustained winds, I was off by 6 mph. That’s not too bad in the grand scheme of things. However, looking back at it, I should have realized that my own forecast calling for a small potential for precipitation would likewise mean a lower possibility for strong winds mixing down from 850 mb to the surface.
The slow-moving storm system that brought us heavy rain overnight last Friday into Saturday continues to linger off the coast of the Northeast, finally exiting by tomorrow. Rain chances will exist today, but give way to better conditions tomorrow. More rain is possible overnight into Wednesday, before drier weather works its way in on Thursday.
Rest of today – cloudy, with an increasing chance for showers in the afternoon and evening hours. High temperatures in the mid-60s. Overnight lows in the mid-50s.
High resolution rapid refresh model simulated radar for 4:45 PM today
Tuesday – with this low finally exiting, we should see a sunny day with temperatures warming into the low-70s. The sunny weather doesn’t last, as clouds increase overnight and rain is possible. Lows in the mid-50s.
Wednesday – conditions should improve after the morning hours, when the cold front that produces the rain chances overnight into Wednesday moves offshore. High temperatures should rebound nicely into the upper-60s. Overnight lows will be cooler in the low-50s behind this cold front.
Weather Prediction Center surface forecast for 8AM Wednesday
Thursday – high pressure should take over and result in a sunny day with high temperatures in the low-60s. Overnight lows going into Friday in the low-50s.
It’s been four months (!!) since I’ve done one of these detailed forecasts. I’m a bit out of practice but trying to get back into the groove. Lucky for me, I picked a day to forecast that has a decent bust potential for precipitation. I’ll discuss that below.
Friday is going to start off decently enough. It should end up being the warmest day of the week. However, it’s also going to be the start of a protracted period of unsettled weather that will affect the weekend. A flash flood watch is in effect late overnight Friday going into Saturday for this reason.
My Forecast High: 70°F | Low: 58°F | Max sustained winds: 30 mph | Total precipitation: 0.10″ – verification for temperatures and precipitation will come from METAR data for the period between 2AM Friday and 2AM Saturday (06Z Friday to 06Z Saturday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.
Verification High: 76°F | Low: 56°F | Max sustained winds: 24 mph | Total precipitation: 0.16″ – I did decently forecasting the low, and the total precipitation. Hedging way down on precipitation totals towards the NAM solution on a slower moving storm was a good idea. On the low temperature, I think a 2°F margin is acceptable here. Being off by 6°F on the high temperature here constitutes a forecast bust that I’ll put a post-mortem together for – as it turns out the bust happened here and not with precipitation. Winds were also not as strong as anticipated. I should have hedged a bit down too with less intense precipitation anticipated.
Weather Prediction Center surface forecast for 8PM Friday
Synoptic Set Up We begin the period in the warm sector of a low with a center forecast to sit over southwestern Quebec. A secondary surface low is forecast to form over the OH valley and strengthens as it briefly moves northeast. This low is then forecast to stall out and retrograde northwest as it matures and occludes. This will lead to a protracted period of precipitation, with the primary focus being along a cold front moving through Sat morning. This may result in the best chance for heavy rain being outside the forecast period.
At the 850 mb level, a strong low-level jet will provide efficient moisture transport. At 500 mb we’ll be downstream of a longwave trough, with an embedded closed low that becomes cutoff (this mid-level cutoff low will be tied to why the secondary surface low retrogrades). The best vorticity and divergence (lift) associated with this upper low seems to move through after forecast period according to NAM forecast model. GFS has a bit earlier timing, and hence brings precipitation in earlier Friday. Finally, at the 300 mb, there’s not much evidence of enhanced divergence and lift at this level since, we’ll be east of any favorable entrance/exit regions of a 300 mb jet streak.
NAM forecast model’s depiction of the 500 mb layer at 11PM Friday. The cut off low referenced above is circled in green. The shades of yellow-purple indicate increasingly strong vorticity, which is linked to divergence and enhanced lift.
High Temperatures GFS and NAM MOS are in pretty close agreement here. It should be warmer than average, since we’ll be in the breezy warm sector of a low as mentioned above. Limitations on warmth will be evaporational cooling due to precipitation, but this doesn’t appear to be a factor until late in the forecast period, past normal times for peak temperatures. The other factor is thick cloud cover. However, temperatures will largely be driven by warm advection tomorrow and not necessarily solar heating. EKDMOS suggests a temperature over 70ºF. In deference to this ensemble model, I’ve sided with the warmer GFS and EKDMOS (as well as National Blend of Models) and call for a high temperature of 70ºF.
GFS MOS output
NAM MOS output
Low Temperatures NAM and GFS MOS are again in close agreement. With warm advection happening during the course of the overnight hours (strong winds at large angles to isotherms, southerly flow), have no reason to disagree that temperatures could actually go up early then back down a touch before rising again with daybreak, and I’m going with an overnight “low” of 58ºF that is well above average for this time of year.
NAM showing temperature contours and wind direction/speed for 2AM Friday. Note the direction of the wind barbs (pointing from south) is crossing at large angles of relatively closely packed isotherms. This is a set up favorable for warm advection (transport of warm air).
Max Sustained Winds NAM is lower on winds than GFS, but the ensemble EKDMOS skews towards faster GFS solution. This makes sense with heavy rain possible and downward momentum transfer. Winds are forecast to back from south-southwest to south-southeast during the period. These directions don’t typically produce the strongest winds climatologically. However, a strong 45-60 knot low-level jet aloft mixing down with precipitation could easily top 20 knots, especially if elevated convection and thunderstorms materialize. I’m going with the EKDMOS 50th percentile range and going with 25 knots.
The NAM forecast sounding at 5PM tomorrow shows considerably drier low-mid levels.
In contrast, the GFS forecast sounding shows a deep saturated layer indicative of ongoing precipitation by 5PM tomorrow.
Total Precipitation There’s a large spread between NAM and GFS on forecast totals (more than 0.50″) because the GFS is coming in with precipitation starting much earlier in the forecast period than the NAM. The local forecast office seems to buy more into the NAM solution due to its consistency with slower onset of heaviest precipitation. There is some indication as well of MUCAPE present in SREF plumes, could result in some elevated convection that would boost precipitation totals. Any faster timing and/or burst of elevated convection could cause heavier rain to fall earlier, resulting in a forecast bust.
Counting against that, I don’t see any strong signal for frontogenesis at the 850 mb level enhancing lift. SREF doesn’t show too much in the way of strong lift either. Still, the fact that SREF ensemble shows much heavier precipitation totals are not far off to the west during this forecast period makes me a bit nervous.
Lastly, moisture isn’t a total lock, and there’s some evidence of drier air offshore and pockets of dry air moving through from the west at times. NAM is notably drier than GFS at both outset of the forecast and throughout the forecast period, as seen in the comparison forecast soundings above. Training of storm cells possible with steering currents set up parallel to the forecast orientation of the cold front.
