Rain to start this weekend leads to a dramatic pattern change with anomalous ridging taking place by late in the forecast period. Temperatures throughout the period range from a few degrees above normal in the mid-40s to the mid-50s by Monday. The warmth continues into the middle of next week.
Rest of today – mostly cloudy with high temperatures in the mid-40s. A relatively weak coastal low will develop and move towards our area overnight. The best chances for rain are in the early overnight hours. Overnight lows in the mid-30s.
Saturday – rain chances will diminish quite a bit as the core of the low continues exiting to the northeast. Mostly cloudy with high temperatures in the mid-40s. Overnight lows in the the mid-30s. Some snow could mix in overnight as lingering chances for precipitation continue.
Sunday – things will dry out as the low continues to distance itself from us. Mostly sunny with high temperatures in the mid-40s. Overnight lows in the upper-30s.
Monday– the synoptic scale pattern shifts as strong ridging at the mid-levels occurs. This will translate to warm high temperatures in the mid-50s on Monday with mostly sunny skies and southerly flow. Overnight lows in the mid-40s.
The weather story for this weekend in NYC will be the near record-breaking warmth we’ll have with temps at or above 60°F Saturday and Sunday. A storm system sweeps through overnight going into Sunday, and should clear by Sunday afternoon. Behind this, temperatures will drop back to the upper-40s, which is still almost 10ºF above normal for this time of the year! Enjoy this rare occurrence of near record warmth that isn’t accompanied by a washout.
Rest of today – mostly cloudy with high temperatures in the low-50s. A strong area of high pressure offshore southeast of our area will be steadily pumping warm, moist air into the region. Looking at the chart below, you’ll see southerly winds blowing at large angles across temperature contours that increase going south – this is an ideal set up for persistent warm air advection and will set the stage for the warmth we see this weekend. Overnight lows in the mid-40s. With moisture advecting into the area, we could see areas of fog develop overnight.
GFS 2-meter temperature with contours and 10-meter wind barbs.
Saturday – very mild with temperatures in the low-60s. Mostly cloudy with a southerly breeze and some fog possible. Overnight lows in the mid-50s. Rain chances increase particularly late overnight going into Sunday morning.
Sunday – elevated convection could bring a rumble of thunder Sunday morning, with rain chances peaking in the early morning hours. High temperatures should remain quite mild, in the mid-60s (and possibly higher with downsloping westerly winds for a period with clearing skies, following the passage of a prefrontal trough and before a cold front closes in). Overnight lows dropping into the low-40s – which is actually still above normal for what a daytime high temperature should be this time of year.
NAM high resolution model output of 1000-500 mb thickness, surface pressure, and precipitation valid for 5AM Sunday
Monday– high temperatures similar to Friday near 50ºF, partly sunny skies. Overnight lows in the low-30s.
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.
