Updated Thursday 12/21 @ 4:58 PM — Today’s models have backed off considerably on the chance of showers late Saturday evening and Sunday morning. In fact, at this time, most models with the exception of the ECMWF and ICON-EPS have any showers staying well west and far north of the immediate PHL areaon Sunday. Considerable cloudiness is still expected.
Posted Thursday 12/21 @ 9:55 AM — A back door cold front will slip through later today with colder air for Friday. The front will return as a warm front on Saturday into Sunday. I’ve summarized the situation in the graphic caption below—
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. A back door cold front will move through tonight. Colder weather for Friday. The front will return as a warm front through late Saturday into Sunday with scattered showers, mostly far north and west, but a shower here can’t be ruled out. Warmer weather for Sunday, but early morning showers are possible, along with a light shower later in the evening.
For much of Saturday and Sunday we’ll be in the transition zone between warmer and colder air masses and significant cloudiness is expected. Less certain is the amount of shower activity here.
Posted Wednesday 12/20 @ 8:25 AM — No, not what you think or want… it’s a warm front that will move through on Saturday. Past models had this frontal passage dry in our area, but the ECMWF and GFS are cranking out some light showers here Saturday night and Sunday morning into afternoon. Considerable cloudiness as well.
Wednesday’s 06z GFS shows light showers (3 hour precip ) at 1 PM on Sunday. Cloudy as well. Mild in the mid 40s. (Click on image for a larger view.)
Quiet and Cold
Posted Tuesday 12/19 @ 4:44 PM — A very quiet weather pattern will take over through Christmas day. We’ll be in an upper trough (cold) while much of the country will be under an upper ridge (warm). High pressure at the surface will dominate—
GEFS forecast for Saturday shows a huge upper level ridge with warmer than average air poking well into Canada. A trough over us will keep things somewhat on the cold side. While this upper ridge is expected to slide eastward towards us after this weekend, some of that strong warmer ridge will be flattened out as it makes it here. The low pressure system in southeastern Colorado is expected here next Tuesday into Wednesday. Colder and stormier the end of next week, but any snow still seems elusive. (Click on image for a larger view.)
So the this week is a good time for meteorologists in this area to take a vacation. Perhaps a few impulses will move through from the northwest, but nothing major forecast.
Light Snow Flurries Possible Tuesday Morning
Posted Monday 12/18 @ 7:47 PM — As low pressure continues to move away, a mid level disturbance will rotate through our area around daybreak Tuesday. Several models show scattered snow flurries but temperatures are expected to marginally at or just above freezing. The NBM puts the maximum probability at only 32% and while Canadian models (onboard for the last two flurry events) show nothing.
Here’s what the experimental RRFS is showing at 7 AM—
18z RRFS (experimental) – Precipitation type: lavendar (snow) green (rain) at 7 AM Tuesday. This does not show the amount, just the type and likely over-states the coverage. (Click on image for a larger view.)
Considerable low, instability cloudiness mid day and during the early afternoon.
High pressure builds in and fair skies and cold temperatures are expected for much of the rest of the week.
Posted Monday 12/18 @ 9:52 AM —This should be no news to anyone around here, but we had a generalized heavy 3″-4+” rainfall through this morning. It’s still raining, but I thought I’d post the current rainfall totals through 8 AM this morning—
MRMS rain gauge & radar-based estimate of rainfall received through 8AM today. Additional rainfall up to 0.10 inches possible. Color shading is in inches. Line contours are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)
I don’t think we had the high wind gusts forecast, but another period of windy conditions is expected later this morning into this afternoon, as temperatures drop off.
Posted Thursday 12/14 @ 8:08 PM — Significant changes in the model forecasts continue to occur with the speed, timing and path of the storm. Additionally, large changes in the amount of cold air moving southward have showed up in recent model runs.
Here’s the latest NAEFS forecast compared to yesterday’s NAEFS forecast for 7 AM Monday—
NAEFS forecasts for 7 AM Monday from yesterday and today combined into a gif. Same model, same time frame, very different placement. Notice the greatly amplified jet flow, as captured by the sharped dip in the 540 thickness line. (Click on image for a larger view.)
Meanwhile the latest GFS is more inland and much faster at 7 AM Monday with the system already in NY State—
Thursday’s 18z GFS shows the storm center in NY State at 7 AM Monda (Click on image for a larger view.)
So things are not nailed down with this system, especially since the GFS is so different than its related ensemble forecast.
One trend that is present is an earlier start of the rain on Sunday, now likely sometime in the morning. Increasing rainfall in the afternoon.
Friday and Saturday look great with temps in the low 50s. Some high clouds move in Saturday afternoon.
Interesting Weather Sunday -Tuesday
Posted Thursday 12/14 @ 9:41 AM — A very active southern jet stream will spawn low pressure over Florida.
Here’s the current setup as seen via satellite with superimposed RAP model —
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. Note the violet contours showing potential vorticity areas, areas where low pressure will develop. (Click on image for a larger view.)
There’s been uncertainty with the development and track of the low. What had been expected to be a coastal low now appears to be more of an inland storm, with plenty of rain and wind, beginning here Sunday.
For several days, the low was expected to be coastal and linger. Now, it’s expected to be inland, move north faster, and then a second low will develop along the coast. —
Three frames of the NAEFS assembled as a gif, shows a likely path and development of this storm. Note the last frame shows a second low off the North Carolina coast. The red 540 thickness line captures the influx of cold air behind the first system. (Click on image for a larger view.)
With a faster moving system, rain will begin around noontime on Sunday and continue through much of Monday. More showers (or even snow showers) possible Tuesday.
The trends shows the influx of cold air behind the first system, a new trend with this storm. There won’t be enough cold air with the main system for snow here, but snow showers possible with the second system on Tuesday.
This is a complex system and the final forecast is not complete. (is it ever?) Stay tuned.
Coastal Storm Early Next Week
Posted Wednesday 12/13 @ 10:21 AM — As discussed yesterday, we’ll be under the influence of a cold upper trough Thursday that transitions to a warm upper ridge as early as Friday into Saturday.
An interesting setup for late Sunday through at least Tuesday as a southern jet stream spawns a low over Florida that ends up somewhere in the western Atlantic and possibly moves up the coast (GFS). Other models have different tracks and speeds with this system. The NAEFS has the storm lingering through much of early next week, which, while possible, is unlikely.
The current setup for next week depicts a pattern that does NOT favor snow. In colder winters this would be different.
Todays’s 06z NAEFS forecast for next Monday morning, showing a deep low off the SC coast. There is high uncertainty with the path and speed of this system, as well as the amount of cold air that will move either south or southeast. The color shaded areas are jet stream level wind speeds, the red contour is the 500-1000mb 540 thickness line and the black contours are surface pressure isobars. (Click on image for a larger view.)
With the (red) 540 thickness line so far to our north, snow isn’t possible in our area from this storm. Should high pressure push further southward from Canada, things might change. No model is showing that now.
An interesting situation. Stay tuned.
Philly Weather Outlook
Posted Tuesday 12/12 @ 5:51 PM — High pressure will continue to provide fair skies and cold temperatures through Saturday. A weak front will replenish the cold air Thursday.
The jet stream configuration continues to be strange for December, with the northern jet well into Canada. My discussion of what’s happening is in the caption of the graphic below—
Tuesday’s 12z NAEFS forecast for late Thursday shows the jet winds (shaded by speed) well into Canada, instead of dropping down into the US as is the old normal. The trough (dip in the jet) over the Northeastern US will continue to filter some cold air into our area. The southern jet is crossing Florida and is expected to spawn a major rainstorm (Blue L) in Florida that may become a Nor’easter. There’s quite a bit of spread (uncertainty) with the path of the potential nor’easter, but even if it stays close to the coast (GFS), there is not enough cold air here for snow; it will be rain late Sunday or Monday.
“There’s not enough cold air here for snow.”
-Likely to become the defining phrase of this winter.
Previously Posted Mon 11:17 AM —
We’ll start with a forecast review:
Mon 9:47 AM Forecast Review — We had the rain, as forecast, but the very strong winds forecast never really materialized. Here’s the MRMS estimate of the rainfall we received—
MRMS based total rainfall received (radar and measured rain gauge based) (Click on image for a larger view.)
If we go back to my previous postings for the weekend, the general range of rainfall, 2-3+ inches is correct, but no model consistently nailed the exact placement of the heaviest rain; most had the heaviest rain in NJ, but those that did missed the other band of heavy rain just west of the city.
This is entirely analogous to why it’s impossible to nail down the heaviest banding of snow storms; it’s similar model precipitation physics but snow leaves its forensic remains in plain sight; with rain, few people know if they received 1.8 inches or 3. Not so with snow. Not to mention the multiplier factor of typically 1:11 for snow.
The Coming Week
As for the week’s weather, look for dry conditions through the week (and likely through the weekend and much of next week)! Not much happening. As mentioned in my updated Winter Outlook, I don’t see any snow here for Christmas at this time.
I can’t think of the last time I saw the jet stream so far north into Canada in December—
Today’s 06z NAEFS jet stream winds (with red 540 thickness line) Thursday morning. Large warm ridge in much of the country. We’ll be in a somewhat colder trough area. (Click on image for a larger view.)
