Category Archives: Rain

NYC MLK Jr. Weekend Weather – Jan 18, 2019

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The dusting of snow we got last night into today is just a small preview of what lies ahead this weekend. A major storm, that first brought heavy rain to the west coast, has been tracking across the nation and will impact the region late Saturday through Sunday with potentially a mixed bag of everything from snow, sleet, freezing rain, rain, urban street flooding, coastal flooding, and strong winds that could result in power outages and tree damage (esp. if ice accumulates). Behind this storm, the coldest air of the new year surges in, bringing a bitterly cold start to next week. I’ll post an in-depth forecast of the incoming winter storm tomorrow.

Rest of today – the veritable calm before the storm. High pressure briefly establishes itself behind the weaker disturbance that brought us light snow overnight. Skies slowly becoming partly cloudy. High temperatures in the low-40s. Overnight lows colder around 30°F.

Saturday – should start off as a decent day with partly cloudy skies. Cloud cover increases through the day, and snow starts to spread from southwest to northeast during the late afternoon hours. High temperatures in the mid-30s. Overnight lows right around freezing will critically impact how much snow accumulation we get before what is widely expected to be a transition over to wintry mix of sleet and freezing rain, then finally bouts of heavy rain leading into Sunday. If a warmer scenario plays out, we’ll get less snow and ice, and more rain. Various forecast sources call for between 1-4″ of snow and sleet accumulations, with up to as much as 0.10″ of ice.

Weather Prediction Center surface forecast for 7AM Sunday. A real mixed bag of various precipitation types all along the Mid-Atlantic and Northeast

Sunday – high temperatures will continue to play a pivotal role Sunday in determining precipitation type. Some forecast sources suggest colder high temperatures in the mid-30s, while others remain warmer, in the upper-30s to even 40°F. There is broad agreement that the day starts off with plain rain but then transitions back to wintry mix of sleet, freezing rain, and finally snow. The differences in high temperature will be pivotal to the timing of this transition, with a warmer scenario keeping things all rain longer. As the center of the low responsible for the storm moves off to the north and east, it will continue intensifying and the pressure gradient around it will increase quite dramatically. Robust northwest winds are expected to develop, rapidly advecting cold, Arctic air into the area. The intensity of this cold advection will govern how quickly temperatures start to plummet during the second half of Sunday. It would appear that temperatures will be well below freezing by the late afternoon hours. This prompts concerns for a flash freeze of any standing water or untreated surfaces, leading to dangerous travel conditions. Temperatures will continue dropping overnight Sunday, with overnight lows going into Monday only around 10°F. Overall, this forecast shapes up to be a slushy mess, with a layer of snow coated by ice and sleet, melted by rain, but then re-frozen with an additional light layer of sleet and ice before all is said and done.

Weather Prediction Center’s plot of ensemble forecast low tracks/positions. Note that there’s still decent uncertainty even at this stage approaching the storm. The pink cluster denotes storm center positions by 7AM Sunday. Though they seem closely clustered, even a shift of the storm track 25-50 miles north or south could result in big differences in terms of whether locations at the coast receive more snow and frozen precipitation (southerly track) versus more of a rain event (northerly track).

Monday (MLK Day) – if you have the day off, count yourself lucky, because this is shaping up to be a brutally cold day where temperatures topping out in the upper-teens despite ample sun. Continued windy conditions will produce wind chill values below zero. The bitter cold continues into Tuesday morning with overnight lows again in the low-teens.

NYC Weather Update – Jan 7, 2019

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We get a quick passing storm tomorrow that will bring a shot of rain/snow mix. A couple windy days will follow this before our attention turns towards a possible weekend nor’easter. As of now, temperature profiles appear to support the possibility of an accumulating snow event, if the storm track supports precipitation. The picture will become more clear later in the week as more forecast data comes in.

Rest of today – increasing clouds with temperatures in the low-30s. Temperatures will increase overnight as a warm front attached to an incoming low pressure system approaches from the southwest. Moisture will also increase and precipitation could begin falling overnight. Forecast soundings suggest temperature profiles will support a period of snow/sleet when precipitation does fall.

GFS forecast sounding for 1AM Tuesday morning, the atmosphere is beginning to saturate even at low levels. Temperature profiles remain below freezing through most of the atmosphere at this time, so if/when the atmosphere saturates, precipitation would be mostly frozen until the very near surface layer, suggesting sleet. See this post for how to read a Skew-T.

Tuesday – mixed precipitation could linger into the morning hours. Temperatures should continue to warm with southwest flow behind the warm front (warm sector) and ahead of an approaching cold front. High temperatures should reach into the upper-40s. Thing should dry out for a bit, but a second round of rain may accompany the passage of this trailing cold front. This cold frontal passage will be accompanied by strong lift, enough to even support a couple thunderstorms. Overnight lows will remain in the low-40s.

Weather Predicition Center surface forecast for 7PM Tuesday.

Wednesday – temperatures in the mid-40s. Winds will pick up into the 20-25 mph range with stronger gusts in the 30 mph range as the low pressure continuing to strengthen as it moves east. Partly sunny skies for the most part this day. Overnight lows will be much cooler, in the low-30s in the wake of the cold front passage.

GFS surface pressure and wind forecast for Thursday at 7AM. Note the dense packing of isobars (black lines indicating areas of the same pressure) over the northeast. This is due to the strengthening low exiting east over Nova Scotia and high pressure building from the west. The result will be strong winds impacting the area.

Thursday – windy conditions continue with high temperatures significantly cooler in the upper-30s under clearing skies.

NYC Weekend Weather – Jan 4, 2019

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The first weekend of 2019 holds a mixed bag for NYC. Saturday looks to be a rainy day, while Sunday should shape up to be a pleasant, milder day, though not nearly as warm as New Year’s Day. Going into next week, another storm system approaches and may bring precipitation as early as Monday evening. This weekend, I’ll have a follow up post regarding the high likelihood of El Nino developing in the Pacific this winter, and what implications this could have for tangible weather this season in our region.

Rest of today – Increasing clouds. High temperatures in the mid-40s. Overnight, rain starts to move in ahead of an approaching warm front associated with a surface low tracking east and offshore of the Delmarva Peninsula. Overnight lows around 40.

Weather Prediction Center surface forecast for 7AM Saturday. With a warm front positioned to our south, persistent northeasterly onshore winds will present a possible issue for coastal flooding, and keep temperatures suppressed.

Saturday – rain for the bulk of the morning and the afternoon hours. Cooler, with onshore flow north of the warm front mentioned above. Temperatures topping out in the low-40s. Rain tapering off towards the late evening hours, then skies steadily clearing with overnight lows in the mid-30s.

Sunday – bright, sunny day on tap behind the storm system Saturday day. Temperatures a touch warmer in the mid-40s. Overnight lows into Monday dropping into the low-30s as a cold front moves through. No precipitation anticipated with this cold front at this time.

GFS model output for Monday at 5PM. By this time, we’re forecast to be under an area of high pressure, but wedged between the departing weekend storm and the next storm approaching form our west.

Monday – increasing clouds ahead of the next storm system. Temperatures in the mid-30s with colder air in place behind the cold front mentioned above. Overnight lows holding steady, dropping a couple degrees maybe – another warm front will help stabilize temperatures as rain or a rain/snow mix develops. Exact precipitation type is difficult to determine at this time.

NYC New Year’s Day Detailed Forecast – Jan 1, 2019

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A wet, windy New Year’s Eve in NYC will give way to a very mild day to start 2019. However, for reasons outlined below, I don’t believe that record warm temperatures will be broken tomorrow. Strong winds will be a distinct possibility, though, and as such the National Weather Service has issued a wind advisory for the area. I think sustained winds could reach 30 mph, especially early in the day.

My Forecast

High: 57°F | Low: 36°F | Max sustained winds: 30 mph | Total precipitation: 0.04″ –
verification for temperatures and precipitation will come from METAR data for the period between 1AM Tuesday and 1AM Tuesday (06Z Tuesday to 06Z Wednesday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification

High: 60°F | Low: 40°F | Max sustained winds: 36 mph | Total precipitation: 0.02″ – this wasn’t a great forecast, aside from the precipitation. I was once again overly conservative on max wind speeds. Following layer mean wind analysis results would have yielded an almost perfect forecast here. For temperatures, I think the main issue was that the low level clouds I thought would materialize based on the forecast soundings available the day before just never came to be. This allowed high temperatures (and hence low temperatures) to become warmer than I forecast. In this case, perhaps it would have been prudent to consider that winds would be downsloping from the northwest, hence drying out somewhat at lower levels. This wind direction also favors clearing conditions following a cold front passage from the west.

Weather Prediction Center surface forecast for 7AM Tuesday

Synoptic Set Up

NYC will start the day off in the warm sector of a low pressure center tracking northeast through New England. This low will continue to deepen as it progresses east. The tightening pressure gradient will induce strong winds, and a trailing cold front will cross the region bringing about much colder air in its wake. Temperatures will peak early in the day and drop steadily through the day. The low temperature will occur late in the forecast period.

High Temperatures

Steady warm advection overnight with a warm front and warm sector associated with the parent low above will mean that temperatures increase to the mid-50s by day break. Forecast soundings from both GFS and MOS indicate, persistent, low overcast that I believe will be pivotal to keeping temperatures from going above 60°F. These low overcast clouds should block out enough sun to keep things a bit cooler than otherwise would be. Forecast soundings also show some upper level clouds that would help block additional sun. For these reasons, I’m siding with cooler NAM MOS guidance that calls for 55°F, and bumping up a couple degrees. Cold advection should take hold quickly in the afternoon in the wake of the trailing cold front. This will usher in westerly and northwesterly winds, which will downslope and warm a touch but will still overall bring colder air into the area. Once this cold advection takes place, temperatures should start to fall.

GFS forecast sounding for 9AM Tuesday, showing a well mixed layer up to about 825 mb and low clouds around 800 mb.

Low Temperatures

Because of the factors above, the low temperature should end up coming in late in the evening/overnight period going into Wednesday. I believe there’s not too much reason to deviate from MOS guidance which averages out to be about 35°F.

Max Sustained Winds

A well-mixed boundary layer is forecast to develop with pretty strong winds aloft in the late morning hours. A period of efficient downward momentum transfer should allow fast winds from above to mix down to the surface. Layer mean wind analysis suggests that winds could be as strong as 35 mph, however, I’m going a bit lower with sustained winds topping out at 30 mph since neither set of MOS guidance goes over 20 mph.

Total Precipitation

It’s clear at this point that the bulk of precipitation will fall before the beginning of the forecast period. However, both sets of MOS guidance, SREF and GEFS means all suggest some precipitation early in the overnight period. I’m going with a composite average of various sources at 0.04″.

NYC New Year’s Eve Weather – Dec 31, 2018

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New Year’s Eve this year looks like a washout, but temperatures will be very mild. In fact temperatures will maintain a non-diurnal trend, warming through the night. New Year’s Day could see the possibility of record warmth before a return to more normal conditions mid-week. Another storm system approaches for the end of the week.

Rest of today – cloudy with high temperatures in the upper-40s. A warm front approaching from the south and west will start to bring steady rain to the area by late afternoon. A favorable low-level jet, along with robust isentropic lift (overrunning of warm air above cold air along the warm front) should enable plenty of moisture and enough instability to trigger bouts of heavy rain later in the evening and overnight. Unfortunately, the bulk of rain looks to fall right during periods when revelers will be out on the town celebrating the new year. Winds will pick up as well with the pressure gradient tightening and efficient downward momentum transfer of low-level jet winds during periods of precipitation. As mentioned above, temperatures will follow a non-diurnal trend, with the warm front and warm sector of this storm advecting milder air from the southeast. Temperatures will actually rise into the mid-50s overnight.

NAM (North American Model) forecast of 850 mb relative humidity and winds for 10PM tonight. The blue hues indicate saturated air, while we can see some wind barbs showing winds > 60 knots. These are classic signs of a low-level jet that is often implicated in producing heavy precipitation events.
Weather Prediction Center surface forecast for 7AM Tuesday. Note the tight packing of isobars (black lines) around the parent low that’s going to bring us rain. This indicates a tight pressure gradient that will drive strong winds. This surface forecast also suggests that our time in the warm sector of this low (outlined in light orange above) will be brief. Warm advection (transport of warm air) won’t last long into Tuesday.

Tuesday (New Year’s Day) – I’ll follow up with a detailed forecast, the start of 2019 will be quite interesting for a couple reasons, one being possible record warmth and the other being strong, gusty winds. Temperatures will be rising throughout the night into Tuesday such that high temperatures will peak early in the day. Depending on exactly when this happens, temperatures could push above 60ºF, which would be nearing or exceeding records. The record high for January 1st at Central Park is 62ºF set in 1966. A lot will hinge on whether clouds clear up early enough to allow for a few hours of solid sunshine. This warmth will be short-lived though, because we won’t spend that much time in the warm sector of this parent low. As a trailing cold front moves through, winds will shift to the northwest and result in cold advection. As a result, temperatures will be falling during the day into the upper-40s by sunset. Temperatures are forecast to continue dropping overnight into Wednesday with lows bottoming out around freezing in the city.

Wednesday – a rude awakening to normal temperatures for this time of year. There will be a cold start to the day with high temperatures 20ºF cooler than Tuesday, in the upper-30s. Partly cloudy skies. Overnight lows in the mid-30s.

Thursday – warmer than Wednesday with another storm system approaching. So far, looking at a mostly cloudy day with highs in the low-40s. Rain may start as early as the overnight hours. A lot of uncertainty still at this time with the timing, scope, and precipitation type of this next storm though.

NYC Weekend Weather – Dec 28, 2018

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The last weekend of the year gets off to a mild, rainy start as a storm system moves through. Things cool off and dry out the remainder of the weekend but it looks like another rain event and warm spell hits for New Year’s Eve.

Rest of today – rain through most of the day. Gusty winds around 30 mph possible. High temperatures well above normal in the mid-50s due to a surge of warm advection accompanying the warm front initially responsible for widespread rain. At the 850 mb level, a low-level jet develops, helping enhance moisture convergence and allow for heavier rains. A flood watch is actually in effect for areas of New Jersey in the metro NYC region. Lows going into Saturday will be quite warm in the mid-40s.

Storm Prediction Center mesoanalysis of 850 mb moisture transport. The maroon vectors indicate the magnitude and direction of moisture transport. The long vectors over the NYC region show efficient moisture transport due fast moving winds of a low-level jet at the 850 mb level. The shaded contours show values of theta-e (equivalent potential temperature), while the details of this are quite complex, the easiest way to understand theta-e is that high values indicate very moist airmasses that can be prime for development of convective activity, as an example. In this case, we can tell that the potential for heavy rain exists within areas of high theta-e (dark-green to red shaded areas).

Saturday – a strong cold front will move through during the afternoon which should result in high temperatures peaking early in the day around 50°F, even though it will remain mostly sunny all day. Behind this cold front, Canadian high pressure builds yielding much colder overnight lows into Sunday in the low-30s.

Weather Prediction Center surface forecast for Sunday at 7AM EST.

Sunday – much colder day on tap with Canadian high pressure in control. High temperatures around normal for this time of year near 40ºF even with plenty of sun. Overnight lows into Monday are forecast in the mid-30s.

Monday (New Year’s Eve) – clockwise return flow from the southwest on the western side of the exiting high pressure warms temperatures up into the upper-40s ahead of the next storm. The timing of this storm looks like it will result in a rainy New Year’s Eve in the city. This may sound less than ideal, but recall that we rang 2018 in with near record cold. It’ll be downright balmy in comparison this time with overnight lows forecast to barely budge in the mid-40s.

NYC Christmas Weather Update – Dec 24, 2018

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We won’t have a white Christmas in NYC this year, but it will feel like winter. The weather this week is shaping up to be pretty average. Like last week, a notable warm up appears to be on the way for the end of the week, which will also be accompanied by rain.

Rest of today – light rain should be ending within the next hour or so in NYC. After that, expect mostly cloudy conditions this Christmas Eve with high temperatures in the mid-40s. Lows going into Christmas will be cold, around freezing.

Tuesday (Christmas Day) – decent day with increasing clouds as a cold front draws near but dissipates. High temperatures in the low-40s. Lows overnight will once again be around freezing.

Wednesday – high pressure will build in and provide for a mostly sunny day, with temperatures remaining in the low-40s. Cold overnight lows around 30°F.

Thursday – the high pressure center mentioned above continues to build, and is forecast to be quite strong at 1040 mb by Thursday. That’ll all but ensure sunny skies with high temperatures around 40°F.

NYC Detailed Forecast for Sunday Dec 23, 2018

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On Friday, I forecast that Sunday and Monday would be mostly sunny and that we wouldn’t see any precipitation on either day. Updated forecast data shows that there is potential for some light mixed precipitation overnight Sunday into Monday. Below, I’ll provide a detailed forecast update for Sunday

My Forecast
High: 42ºF | Low: 33ºF | Max sustained winds: 25 mph | Total precipitation: 0.02″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Saturday and 1AM Sunday (06Z Sunday to 06Z Monday). Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification

High: 44ºF | Low: 36ºF | Max sustained winds: 24 mph | Total precipitation: 0.00″ – this was a decent forecast in terms of wind speed, and total precipitation. Precipitation didn’t start falling until late in the forecast period, so it ended up being a good call to hedge down below some of the more aggressive model output that suggested earlier/heavier precipitation. Layer mean wind analysis aided my forecast, though again, noting a possible small surface inversion in the NAM forecast sounding led me to make a wise decision to hedge down on max sustained winds. On temperatures, the overnight lows going into Sunday ended up being on the warmer side of the forecast envelope. Here, I mixing and downsloping played a bigger role than I anticipated.

Synoptic Set Up (The Big Picture)

An occluded low pressure center tracking over the Great Lakes will bring a weak cold front through the area Sunday night into Monday. Above the surface, a limiting factor will be the lack of deep moisture or strong winds at the 850 mb level. At the 500 mb level, a negatively tilted shortwave trough will supply good positive vorticity and divergence. However, there isn’t a clear signal for favorable entrance/exit regions of a jet streak at the 300 mb level to provide additional lift.

High Temperature

MOS forecasts show high temperatures Sunday in the mid-40s, with NAM (North American Model) coming in at 45ºF and GFS (Global Forecast System) pointing to 46ºF. NBM (National Blend of Models) – a consensus based forecast model, showed cooler highs at 42ºF. I tend to side with high temperatures on the cooler side due to persistent low overcast clouds during much of the day. Low overcast clouds are a known bias for statistical forecast models like NAM and GFS because they can effectively block out solar radiation, keeping things cooler than otherwise expected.

NAM forecast sounding valid 1PM Sunday, showing multiple layers in the atmosphere where the environmental temperature profile (red) and dew point (green) approach each other, which would suggest multiple layers of clouds hindering solar heating.

Low Temperature

The same MOS data above has lows on Saturday of 33ºF for NAM, 34ºF for GFS and 32ºF for NBM. I’ve gone with the middle of the road, in line with NAM at 33ºF because despite relatively clear skies overnight, strong winds are forecast to be blowing from the west to west-northwest, resulting in some downsloping and staving off any radiational cooling.

Max Sustained Winds

Overnight, forecast soundings indicate the presence of a well-mixed near surface layer. A well-mixed layer allows for faster winds aloft to transfer down to the surface relatively efficiently, and as such, the layer mean wind method can be applied to forecast maximum sustained winds in these cases. We can calculate layer mean by averaging wind speeds at every pressure level within the well-mixed layer shown in forecast soundings that are available in forecast model output. Performing a layer mean wind analysis for NAM and GFS produces an average of about 28 mph. However, noticing that there is a possibility for a small surface based inversion in the NAM forecast sounding, I’ve cut this down to 25 mph. In the chart below, the layer mean wind ends up being 26.71 knots (31 mph).

Pressure (mb)Speed in m/sSpeed in knots
9978.2015.94
97512.9025.08
95014.6028.38
92515.9030.91
90017.1033.24
GFS forecast sounding valid for 1AM Sunday, the black circled section shows a well mixed layer where the environmental temperature profile (red line) largely parallels the nearest dry adiabatic lapse rate line (light blue). Note, at the top of this circled area, the environmental temperature profile almost touches the dew point temperature, indicating a layer of thin clouds.

Total Precipitation

An approaching cold front will provide convergence and lift at the surface. However, at the 850 mb level, conditions look pretty dry through much of the day Sunday. 500 mb level divergence and lift looks decent, but the 300 mb level support doesn’t look great. Precipitation, if it occurs, won’t happen until the overnight hours going into Monday, and if it does happen, signs point to no more than a light event. I’m forecasting 0.02″, or just a trace of precipitation by 1AM Monday. Temperature profiles aloft support snow, though it appears that near the surface, temperatures will be above freezing, so it’s more likely that we’ll see a mix of rain and snow.

GFS forecast for 500 mb vorticity and height valid 10PM Sunday. The light blue line shows the axis of a negatively tilted shortwave trough west of the NYC area. This supports divergence downstream (east) of the axis which induces surface pressure drops and growing storms.
GFS 850 mb forecast for relative humidity and wind. The NYC area at this time is far from being saturated at this level.
By 1AM Sunday, the atmosphere starts to become saturated to basically the surface, indicating ongoing precipitation. Most of the precipitation looks to fall in a zone of the atmosphere that’s below freezing, but temperatures at the surface are above freezing.


NYC Weekend Weather – Dec 21, 2018

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The official start of winter hits at 5:23PM EST, though you’d be forgiven for thinking it were a spring day instead, with temperatures well above normal and a driving rainstorm to end the week. The storm tapers off rather quickly. Cold air wrapping around the exiting storm will drop temperatures right back into normal ranges for the rest of the weekend and going into the week of Christmas.

Rest of today – bouts of heavy rain, which have already induced flash flood and flood warnings, will continue through the early afternoon hours. There should be a lull in activity as the first cold front that’s been bringing this heavy rain exits east. More rain is possible with a secondary cold front set to pass through later this evening and tonight. Strong winds are set to continue throughout the day as two areas of low pressure responsible for this storm consolidate and continue to deepen (hence inducing a large pressure gradient). High temperatures likely have already occurred today in the low-60s, and temperatures will slowly drop into the upper-50s. Overnight lows should still be quite warm, in the upper-40s.

High Resolution Rapid Refresh model simulated radar for 3PM

Saturday – the first full day of winter will see a return to more seasonable temperatures with highs in the mid-40s under mostly cloudy skies. Clouds will be slowly dissipating as high pressure builds in behind the exiting storm. Overnight lows will get back into the mid-30s.

Weather Prediction Center forecast surface map for 7AM Saturday

Sunday – mostly sunny day with high temperatures in the low-40s and overnight lows into Monday in the mid-30s.

Monday – another mostly sunny day anticipated with a high in the low-40s.

NYC Detailed Forecast for Saturday, Dec 15, 2018

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

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

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

Synoptic Set Up (The Big Picture)

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

Surface forecast from Weather Prediction Center, valid 7PM Saturday

High Temperature

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

Low Temperature

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

Max Sustained Winds

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

Total Precipitation

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

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

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

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

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

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