Our extended period of calm weather comes to an abrupt end this weekend. A low pressure system with its attendant vigorous cold front sweeps through overnight bringing moderate to heavy rain. We could see as much as 1-1.5″ of rain with this event. Behind this front, windy conditions prevail Sunday as the low exits but continues to deepen. Next week, we’ll see a return to normal temps around 40ºF.
Rest of today – mostly cloudy with high temperatures in the mid-40s. Rain chances increase as the storm system approaches us. Warm air advection associated with the warm front of this storm will result in little movement on overnight lows, which will hover in the mid-40s. Moderate to heavy rain is likely after 9PM tonight and into the overnight hours.
High Resolution Rapid Refresh simulated radar for 1AM Saturday.
Saturday – as the storm pulls off to the east in the morning, rain chances should decrease, ending by noon. Isolated wrap around showers are possible. Temperatures will decrease during the day with cold air advection, dropping into the low-40s. Overnight lows in the mid-30s as skies clear.
Sunday – mostly sunny with high temperatures in the low-40s. Breezy with west winds in the 15-20 mph range and stronger gusts due to a large pressure gradient around the exiting and intensifying low. Overnight lows in the mid-30s.
Weather Prediction Center surface forecast for 7AM Sunday. The low affecting us this weekend is seen in the upper right corner, with a tight clustering of isobars around it, indicating a large pressure gradient.
Monday– an upper level low will affect the area pushing a cold front through our region, but this front should be moisture starved. Partly sunny with highs around 40ºF. Overnight lows around 30ºF.
I’m at my college for reunion weekend, and offered to do a detailed forecast for one of the days I’m here. Saturday is shaping up to be a bit of a mixed bag. Temperatures should be summer-like in the mid to upper-80s, but the downside is a risk for possibly strong to severe thunderstorms especially in the early evening hours. These thunderstorm chances are far from being certain though, and there’s a chance we may miss out entirely on any precipitation. In other words, it’s another good day for a precipitation forecast bust!
My Forecast High: 87°F | Low: 62°F | Max sustained winds: 21 mph | Total precipitation: 0.28″ – verification will come from METAR data for the period between 2AM Saturday and 2AM Sunday (06Z Saturday to 06Z Sunday) at LPR (Lorain County Regional Airport).
Verification High: 90°F | Low: 61°F | Max sustained winds: 25 mph | Total precipitation: 0.00″ – on high temperatures, hedging up towards the higher end of guidance turned out to be a good idea. It’s yet more evidence that MOS guidance tends to be too cool for breezy warm sectors. If anything, I could have been a touch more aggressive on the high here. Did well on the low temperature, and from what I could see, did decently on max winds too. Like I’d mentioned in the forecast, the bust potential for precipitation on this day was significant. That ended up being the case – KLPR and Oberlin missed out on the heaviest rain and more serious convective activity, leading to barely a trace of measurable precipitation. In retrospect, I probably could have gone even drier with my forecast, but I would still have not felt comfortable going with zeros.
Weather Prediction Center surface forecast for 2PM Saturday
Synoptic Set Up A stationary front eventually lifts through Ohio overnight as a warm front. We’ll spend the day in the warm sector of a surface low centered over central Ontario. At the 850 mb level, winds will not be strong enough to qualify as a low-level jet and relative humidity values likewise do not appear particularly high. At 500 mb, a robust shortwave trough is forecast to push through later in the day, bringing some decent vorticity and upper-level lift. Further up at the 300 mb level, there’s not much evidence of enhanced divergence and lift since we’ll be south of a 300 mb jet streak.
GFS model output for 500 mb vorticity and winds valid at 8PM Saturday. Note the strong cyclonic vorticity values over Northeastern Ohio at this time (bright oranges). This will provide some synoptic scale lift for initiating storms via upper-level divergence.
High Temperatures GFS and NAM MOS and NBM all show Saturday to be a warm day. GFS is the warmest with 87ºF while NBM is coolest with 83ºF. EKDMOS (ensemble MOS) 50th percentile matches comes close to GFS MOS at 86ºF. Because forecast soundings show we’ll be in a well-mixed warm sector with decent though not ideal conditions for warm advection, I am tending towards thinking that a high of 87ºF is in fact possible. We could be even warmer if enough sun breaks out between possible earlier showers and rain later in the day.
GFS 2-meter temperature contours and winds at 2PM Saturday showing decent, though not ideal conditions for warm advection (southwesterly winds of 15 knots at a somewhat sharp angle across closely packed temperature contours). If surface winds were forecast to be stronger, we’d be looking at better chances for robust warm advection and hence warmer temperatures.
Low Temperatures NAM and GFS MOS are again in close agreement with 61ºF and 63ºF lows respectively. NBM is a touch cooler at 59ºF. Although both NAM and GFS show evidence of a nocturnal inversion forming, and calm winds at the surface, both models also show extensive cloudiness or even precipitation ongoing overnight. Because the boundary layer starts out with plenty of moisture, there shouldn’t be much in the way of evaporational cooling. Thus, clouds will actually serve to stave off any radiational cooling, which leads me to believe overnight lows will be on the warmer side of guidance. I’ll go with 62ºF here.
NAM MOS guidance for KLPR
GFS MOS guidance for KLPR
Max Sustained Winds NAM, GFS MOS have maximum synoptic winds averaging about 16 knots. I see no clear reasons to go much higher than 18 knots for a max sustained wind tomorrow, unless we happen have a thunderstorm roll through with sustained winds that are much higher. EKDMOS’ 50th percentile lies right around this figure. Even with well-mixed boundary layer forecast to form tomorrow afternoon, winds above the surface up to 850 mb do not appear to be that strong, only around 20-25 knots. Layer mean wind analysis of the NAM sounding only showed winds of about 16 knots.
GFS forecast sounding for 5PM Saturday
NAM forecast sounding for 5PM Saturday
Total Precipitation There’s a large spread between NAM and GFS on forecast totals tomorrow. This will be the trickiest part of the forecast because there’s clear signs that convective precipitation, with a couple rounds of possibly strong to severe thunderstorms could materialize. Any strong thunderstorm passing overhead could quickly dump a few tenths of an inch of rain. However, if we miss out on any significant thunderstorm action, we could see just a trace of rain instead. It’s still too early to tell the finer details of where storms will initiate at this point. Based on forecast soundings above, the best chance for thunderstorms to roll through would be during the late afternoon and early evening, maybe 5-8PM when moisture is best and instability is maximized from daytime heating. Strong, mostly unidirectional shear profiles with westerly winds increasing from 10 knots at the surface to 50 knots at 500 mb suggest the potential for damaging winds as the biggest severe weather threat.
NAM forecast for 850 mb winds and relative humidity at 2PM Saturday
GFS forecast for 850 mb winds and relative humidity at 2PM Saturday
Going against the potential for significant precipitation, neither GFS nor NAM really show strong signals of low-level jet support. Winds at the 850 mb level don’t look to be particularly strong, and moisture support isn’t looking great either. What’s more, while there’s mention in the local forecast office discussion of a right entrance region of an upper-level jet that would provide some dynamic lift, I’m not seeing that myself. They did also mention that a trough would pass through and provide a focus for some lift. This would be a necessary trigger since there’s no clear frontal boundaries that would provide the convergence and lift necessary to generate showers and thunderstorms. Given the hit or miss nature of thunderstorms, less than ideal conditions for heavy precipitation, and disagreement between GFS and NAM MOS at this time, I find it prudent to hedge down with a forecast for 0.28″ of total precipitation despite MOS guidance averages at 0.48″.
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.
A cool start to the weekend should give way to a nice couple of days before another storm moves in to start next week. During this period, high temperatures will start well below normal, then rise above average before finally settling around average. Looking ahead, another storm system appears to impact the area late next week, bringing a colder weekend next week in its wake.
Rest of today – cool, cloudy with high temperatures only in the mid-40s. Increasing rain chances towards the evening and overnight hours. Rain should taper off overnight then we should see overnight lows stabilize in the low-40s as warm advection takes hold.
Saturday – clouds diminishing as the low responsible for the rain moves over the Atlantic to our south. High temperatures in the mid-60s, maybe a touch warmer if we see clouds break earlier than forecast. Overnight lows milder in the upper-40s.
Weather Prediction surface forecast for 8AM Sunday
Sunday – high pressure stays in control and gives us a pleasant, mild day with high temperatures in the low-60s. The high pressure and nice weather doesn’t stick around long though, with another storm system approaching during the overnight hours going into Monday. Overnight lows should hold around 50ºF as rain starts to fall.
Monday – despite clouds and rain sticking around, temperatures will surge to about 70°F with a warm front forecast to pass through.
A winter storm warning is in effect for NYC and the surrounding metropolitan region. This isn’t exactly how we’d all want to start March off! This storm is anticipated to bring some travel impacts to the area, however, for reasons discussed below, this won’t be a blockbuster snowmaker. Watch out for a slog of a morning commute tomorrow. This snow may also stick for a while – a frigid continental polar air mass from Canada will sweep in behind this storm bringing temperatures generally 10-15°F below normal for this time of year. High temperatures in the mid-30s should limit melting.
Headlines
Snowfall totals: I’m forecasting 4-6″ in parts of eastern Queens, southeastern Brooklyn, and lower totals further east into Long Island. Higher totals of 6-8″ are more likely to occur in Manhattan, the Bronx, and points further inland, particularly interior regions of Connecticut. Below are probabilistic forecast maps of various amounts of snow (>= 2″, >= 6″, and >= 8″).
Timing: Precipitation starting in earnest around 8PM. Starting out as a mix of rain/snow near the coast, but transitioning over to all snow later in the evening. The heaviest snow will happen overnight. Because of the fast-moving nature of this storm, precipitation is expected to end rather quickly between 4-6AM Monday morning in the city.
Uncertainties: There is still potential for a wobble in the storm track, further east and south would result in higher snow totals near the coast. Further west and closer to the coast would mean more mixing/rain at the coast and lower snow totals. There will be a rather sharp gradient of increasing snowfall totals spreading across the region (as seen in the previous probabilistic snowfall total forecasts). Mesoscale heavy snow bands will be difficult to pinpoint ahead of time. Some areas could see several inches more than neighboring areas just a few miles south and east.
Synoptic Set Up (The Big Picture)
A storm that’s currently unleashing severe storms with tornadoes across the Deep South now will slide up along the Mid-Atlantic and Northeastern coast of the US. As this storm progresses, it will move close to the 40°N/70°W benchmark, a spot that’s climatologically correlated to heavy snow events along the heavily populated I-95 corridor during the winter. This storm will continue to strengthen as it moves offshore. Snow is expected to develop ahead of the advancing warm front associated with this storm as its precipitation shield advances. Heavier snow is forecast to develop later on as strong isentropic lift associated with the warm front occurs, creating the potential for frontogenesis and some mesoscale bands of very heavy snow. The storm is expected to move quickly along the Northeast coast, such that the duration of precipitation in any one spot is expected to be less than 12 hours.
At the 500 mb level, a shortwave trough will provide positive vorticity and some additional lift/divergence, allowing the storm to continue strengthening. Finally, at the 300 mb level, the surface low will be close to the entrance region of a curved 300 mb jet streak. This will provide yet more divergence and lift, if only for a brief period.
Weather Prediction Center’s surface forecast for 1AM Monday morning
GFS model 500 mb vorticity and height for 1AM Monday
GFS 300 mb height, wind forecast for 1AM Monday, note the brighter red, orange, and white streak over the Northeast, indicating a jet streak.
Evolution of the Storm Track
Over the course of the last three days, forecast models have come into better agreement with this storm tracking close to the 40°N/70°W benchmark (circled in red in the images below). Note the increasingly tight clustering of storm center locations around the benchmark in progressive storm track forecasts from the Weather Prediction Center.
WPC storm track forecast issued 7AM Saturday, the brown spots indicate forecast storm center locations around the benchmark
WPC storm track forecast issued 7AM Sunday, the green spots indicate forecast storm center locations around the benchmark
The tightening clustering of these forecast storm center locations lends greater confidence to the idea that the storm will track very close to the benchmark.
Ensemble Snowfall Totals
The two primary model ensembles (GEFS and SREF) have been edging ever so slightly upwards in their forecast mean snowfall totals, while the model spread has decreased over the weekend
Latest GEFS ensemble plumes for snowfall totals at KLGA
Latest SREF ensemble plumes for snowfall totals at KLGA
These means/spreads were part of what informed my own forecast snowfall totals at the top of this post.
Factors Supporting Heavy Snow
Storm track over or very near the benchmark
Strong isentropic lift and possible frontogenesis (see images below). Strong lift is a critical ingredient for generating heavy precipitation
Possibility of mesoscale bands as a result of this lift, generating heavy snowfall rates
Temperatures probably supporting frozen precipitation through the atmosphere
GFS forecast for 850 mb frontogenesis
NAM 12km forecast for 850 mb frontogenesis and temperature advection (red shades indicate warm advection)
NAM forecast sounding for 1AM Monday
GFS sounding for 1AM Monday
Factors Suggesting Lower Snow Totals
Possibility still remains for storm track to shift further inland, introducing more warm air off the ocean, more rain than snow at the coast
Warm advection associated with the storm’s warm front possibly also affecting snow development. Note how close the overlapping temperature and dew point profiles in the forecast soundings above are to the freezing mark, the dashed blue line the middle of the image that is angled to the right at 45°. Evaporational cooling should help somewhat in staving off warming but if temperatures warm more than forecast, we could see more mixing
Mesoscale bands of heavy snow may not push far enough onshore
Surface temperatures ahead of the storm in the upper-30s near the coast, urban heat island effect could retard snow accumulation
Fast moving nature of the storm, total precipitation window only 12 hours
Small window for the best moisture support at the 850 mb level. No real evidence to suggest a low-level jet carrying a ton of moisture into the region.
NAM model forecast of 850 mb relative humidity and winds. There’s not too big of an area of completely saturated air at this level, and winds are not strong at this level either.
Cold start to this weekend gives way to a milder feel but a soaker of a day Sunday. Going into next week, much of the week will see above average temperatures.
Rest of today – increasing low-level clouds associated with warm air advecting from the south ahead of a warm front. Temperatures still on the cool side during the day with high temperatures in the mid-30s.
Saturday – overnight temperatures actually start to rise as warm advection really takes hold. High temperatures will substantially warmer near 50°F. Cloudy with increasing chances for rain as a cold front approaches from the west.
Sunday – the cold front above will stall in our vicinity, with tropical moisture surging along this frontal boundary. This will result in multiple rounds of moderate to heavy rain. Temperatures will be in the low-50s.
Monday – cooler but still decent with decreasing clouds and high temperatures in the mid-40s.
Temperatures this weekend will be largely around normal levels for this time of year, with the exception of Saturday. Warm advection will precede the passage of cold front. This frontal boundary does not look like it will have much moisture to work with. Travel conditions are expected to be trouble-free for those returning from the Thanksgiving break.
Rest of today – sunny with high temperatures around 50°F. Light winds shifting from west to south.
Saturday – temperatures warming into the upper-50s as a result of continuing warm winds from the southwest. Increasing clouds with the approach of a cold front. The best chance for precipitation with this front will be late in the day or in the overnight hours. Limited moisture will mean that this is not a washout.
Sunday – behind the cold front, winds once again shift back to the northwest, resulting in cooler temperatures in the 50°F. Sunny and breezy otherwise.
Monday – on the cool side again with high temperatures in the upper-40s and mostly sunny skies.
