The active weather pattern we’ve seen so far this week with alternating days of sun and rain continues with two more storms coming for the end of the week into the weekend. This time around, the rainy days will outnumber the sunny ones. We will start off with a beautiful Thursday before a weaker storm impacts the area tomorrow. A more significant system will move through later Saturday into Sunday. Temperatures during this time will range around normal to above normal in the 50s.
Rest of today – high pressure will be in control of the weather today, yielding a sunny day with temperatures steadily rising into the upper-50s. Overnight lows should be around the mid-40s.
Weather Prediction Center surface forecast for 8AM Friday
Friday – clouds will build overnight into the day and by the morning hours a weak area of low pressure will be moving through the area. We can expect some light rain with this system in the morning, rather than a full day washout. Clouds will remain through the rest of the day even after the system clears and rain stops. High temperatures around 60°F. Overnight lows will be in the low-40s as a result of radiational cooling as high pressure briefly builds.
Saturday – with high pressure positioned to our north to start, northeasterly flow will keep temperatures cool in the low-50s. A stronger storm system with origins in the lee of the Rockies will be moving towards us throughout the day. A warm front will slowly move up from the south bringing a steady rain ahead of it. Overnight lows will be in the mid-40s as rain continues.
GFS 500 mb height and relative vorticity valid 8PM Sunday. Note the presence of a closed 500 mb low over Michigan and southern Ontario generating lots of positive voriticity around it. This will provide divergence and lift at the upper levels to support the storm hitting us Sunday.
Sunday– With the warm front making its way through and flow turning towards the south/southwest, temperatures should rise to around 60°F. However, rain chances are expected to continue throughout the course of the day before tapering off overnight. Overnight lows will warm into the upper-40s.
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.
