A mild start to this weekend will transition quickly to colder temperatures more typical of this the of year. A windy day in store on Saturday in the wake of a cold front bringing us rain today. cold temperatures remain in place into the beginning of next week. No more 60°F+ days in store for us!
Rest of today – rain tapering off by around lunchtime. Mild temperatures with highs in the mid-50s with clearing conditions. Once the cold front sweeps through, winds will pick up from west into the 15-20 mph range. This will advect cold air into the area and overnight lows are forecast in the mid-20s. By midnight, temperatures will be around the freezing mark.
Weather Prediction Center’s surface forecast for 7PM tonight
Saturday – a windy day on tap with much colder high temperatures in the mid-30s. Winds expected in the 15-20 mph range with strong gusts around 35 mph. Overnight lows again in the mid-20s.
Sunday – winds abate as the low responsible for the rain today moves further off. High temperatures in the mid-30s with mostly sunny skies. Overnight low around 30°F, with a small chance of snow showers.
Monday – increasing clouds ahead of the next storm system that could bring us some wintry weather Tuesday. High temperatures in the low-40s. Overnight lows in the upper-30s.
The weekend (and ironically February) brings welcome relief from the blast of Arctic cold that gripped a large swathe of the Midwest and Northeast the second half of this week. This record-breaking cold resulted in multiple deaths from people literally freezing to death to being involved in weather-related accidents. Not only does the cold snap end, but there will be a gradual warming trend each day such that by Tuesday, we could see high temperatures near 60°F. The overall pattern of upper level flow across the continent looks to be flattening into a zonal and progressive one, versus the extreme ridging/troughing that brought this polar vortex on us.
Rest of today – high clouds build in as a disturbance to our south moves offshore. Not anticipating any precipitation from this. Warmer than yesterday though still cold with highs in the low-20s. Overnight lows also remaining cold in the mid-teens
Saturday – slow clearing from partly sunny skies. High temperatures in the mid-30s. Overnight lows not much colder around 30°F.
Weather Prediction Center’s surface forecast for 7AM Saturday
Sunday – as a center of high pressure moves offshore to the east, clockwise return flow around the western periphery of this high will bring much warmer air with it, bringing high temperatures into the mid-40s with partly cloudy skies. Overnight lows in the mid-30s may equal the high temperature for Saturday
Monday – we’ll be in the warm sector of an approaching low, and that’ll bring high temperatures into the low-50s with mostly sunny skies. Again, overnight lows look to be similar to the previous day’s highs, in the low-40s.
A quiet, and mostly dry weekend is on tap after a wet and windy day yesterday. This will be a welcome change of pace from the active weather pattern we’ve seen on prior weekends. There isn’t any precipitation expected and temperatures are forecast to alternate between the low-40s and low-30s. The next chance for meaningful precipitation comes Tuesday.
Rest of today – high temperatures near 40ºF, partly sunny with winds picking up in the afternoon. Overnight lows will be much cooler in the wake of a dry cold front passing through around 20ºF.
Saturday – high pressure will be in control for the day, but it’ll be a cold day with high temperatures topping out in the low-30s under partly sunny skies. Overnight lows warmer in the upper-20s as the high pressure center moves off to the east and winds shift to the south.
Sunday – temperatures warm into the low-40s with return flow from the south on the west side of the departing high. Overnight lows will be similar to Saturday in the upper-20s.
Weather Prediction Center surface forecast for 7AM Sunday
Monday – things turn colder again in the wake of another cold front passing through with high temperatures in the low-30s with partly cloudy skies.
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.
We’ll have a cold weekend ahead, however, we will dodge a bullet with a major winter storm tracking to our south. The northern fringes of this system could still bring some light snow to the city, though. The cold temperatures stay in place through the weekend and into early next week. Looking ahead, we should see a pattern change that leads to some warmer temperatures by mid-week next week
Rest of today – mostly sunny, cold, with high temperatures hovering around 30°F. Gusty northwest winds steadily subsiding as a strong Arctic high pressure center moves closer to us. Clear, crisp night with overnight lows around 20°F.
Saturday – partly sunny, high temperatures topping out around the freezing mark. Snow flurries are possible overnight into Sunday. Overnight lows in the mid-20s.
Weather Prediction Center surface forecast for Saturday at 7AM. Note the strong Arctic high pressure to our north, 1040 mb, which will effectively block the winter storm to the south from heavily impacting this region.
Sunday – partly sunny with clouds possibly breaking from north to south later in the day. Some flurries still possible early in the day. High temperatures similar to Saturday around freezing. Overnight lows in the low-20s.
Monday – mostly sunny, with high pressure building again, temperatures still cold in the mid-30s.
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.
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.
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.
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/s
Speed in knots
997
8.20
15.94
975
12.90
25.08
950
14.60
28.38
925
15.90
30.91
900
17.10
33.24
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.
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.
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.
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 🙂
