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.)
Posted Friday 12/08 @ 4:46 PM — I’m preparing the Weekend Weather Forecast, but I wanted to post a big change in the forecast for Saturday ASAP. The warm front is now expected to bring some showers Saturday morning with clearing after 1-2 PM. That’s a significant change than what had previously been the model guidance for just clouds and fog.
Posted Friday 12/08 @ 9:10 AM — A quick update. A few changes in the forecast outlined in my Weekend Preview below—
There will be heavy fog very early Saturday morning and especially Saturday night.
Rain ahead of the cold front on Sunday will begin with spotty showers in the morning. Rain and winds pick up after 2 PM (earlier far western suburbs). Rain becomes quite heavy Sunday evening and overnight with high winds expected.
Latest GFS (06z) total rainfall forecast for Sunday into Monday. (Click on image for a larger view.)
Check back for my Weekend Weather Forecast later this afternoon or early evening.
Friday and Weekend Preview
A warm front will try to move through tonight (Thursday night) and Friday will start cloudy. Clouds should break for some sunshine through high clouds by late morning or early afternoon. Highs should reach 48-49º Friday afternoon.
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. Warm front in Ohio will gradually move north of our area Friday morning and some sunshine is expected with moderating temperatures.A second warm front now in Missouri will move through on Saturday with an increase clouds. (Click on image for a larger view.)
A second warm front will slowly move through on Saturday. Saturday will be milder with highs 50º-53º but will become considerably cloudy in the afternoon.
The second warm front will be north of us Sunday and highs will reach the mid 60s!
A strong cold front approaches Sunday night. Heavy rain, high winds (45-50 mph) Sunday night. Thunderstorms possible.
NBM shows widespread heavy rainfall for Sunday into predawn Monday. (Click on image for a larger view.) NBM Meteogram for KLOM Blue Bell shows wind gusts 40-50 mph Sunday evening (red box). The NBM tends to be conservative with wind gust forecasts. (Click on image for a larger view.)
Thu 11:25 AM Forecast Review — The snow flurries came and the surface temperatures rose above freezing early in the event, so little to no accumulation occurred in the immediate Philadelphia area and adjacent suburbs. Interestingly, the heavier band of precipitation occurred south of the city, not northwest of the city as forecast.
RADAR image 11:23 AM showing temperature line (32º) well north or our area and heaviest band of precip consistently to our south. (Click on image for a larger view.)
Light Snow Update
Posted Wed 12/06 @ 11:11 PM — Tonight’s models are suggesting that the light, accumulating snow will stay north of Doylestown. Only flurries closer to the city.
Light Snow Thursday Morning
Posted Wednesday 12/06 @ 5:17 PM — Today’s models are in fairly good agreement that a clipper type disturbance will move down in the jet flow Thursday morning, giving us some light snow, possibly a coating, dusting or up to 0.5 inches in the far northwestern suburbs—
Latest (18z) GFS shows the clipper like disturbance that will bring light snow Thursday morning. (Click on image for a larger view.)
Here’s the expected “snowfall totals” (I’m using quotes for a reason) —
18z HRDPS forecast accumulation of snow by noon. (Click on image for a larger view.)
These totals are in the range of model noise and anything less than 0.5 inches should be viewed somewhat skeptically. That said, I think these forecast amounts are possible, considering the consistency of the several models.
Temperatures are heading upwards and anything that sticks will likely melt by the end of the day.
Wednesday Forecast Update
Posted Wednesday 12/06 @ 10:38 AM — The snow flurry forecast for Wednesday has been on-track and additional snow flurries (non-accumulating) are likely early this afternoon.
10:30 AM: Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. An area of snow showers in the northwest suburbs will move in. The clipper type disturbance for Thursday is shown (white arrow) The yellow contours are thickness lines and the yellow arrows capture the increase in thickness expected as the airmass moves to the northeast on Thursday. (Click on image for a larger view.)
The greatest chance for snow flurries today is where POPs (Probability of Precipitation) are highest this afternoon—
HRDPS POPs (probability of precipitation) at 2 PM Wednesday. While 56% may not seem high, on an hourly basis, anything greater than 40% is a high. (Click on image for a larger view.)
Changing Focus to Thursday
Posted Tuesday 12/05 @ 7:55 PM — So there’s a slight chance of some flurries Wednesday. Of greater interest is a disturbance that moves down from the northwest while cold air is in place Thursday morning, at the same time the upper half of the atmosphere is warming. Some additional snow showers are seeming more likely, possibly with a dusting type accumulation in far northern areas.
ECMWF forecast for 7 AM Thursday morning. Very cold air at the surface with moisture associated with a clipper type disturbance will bring the chance of some additional snow flurries/showers Thursday morning. Temperatures are expected to warm during the day Thursday to well above freezing. Any (light) accumulation, if any, should melt. Areas far north of the city (far northern Bucks County) may see a light accumulation before it melts later in the day.
This clipper type disturbance is looking more pronounced in today’s models, than it had previously. I’ll keep an eye on it.
Snow Flurries Wednesday
Posted Tuesday 12/05 @ 5:03 PM — A system to our south combined with disturbances rotating around an upper trough will result in some light precipitation Wednesday, likely snow flurries or snow flurries mixed with light rain showers. No accumulation for most of the area; a dusting possible north of Doylestown on grassy surfaces.
The latest NBM shows extremely light flurries possible during Wednesday afternoon—
18z NBM shows extremely light blue (snow flurries) and light green (rain showers) at 2 PM. Compare to the previous forecast below (Click on image for a larger view.)
The latest HRDPS shows an area of light precipitation moving through about 1 PM. The 36.5º contour is highlighted; it’s the temperature where there’s a 50% chance of either rain or snow (an overly-simplistic, back of the envelope methodology).
18z HRDPS forecast at 1 PM Wednesday. (Click on image for a larger view.)
Additional snow showers possible on Thursday as warmer air returns aloft!
Update
Posted Monday 12/04 @ 5:42 PM —A cold front moved through and we have had some colder temperatures today. These slightly below seasonable temperatures will be with us through Thursday. (Average high is about 47º Blue Bell)
Latest NBM 18z temperature meteogram for Blue Bell, PA (Click on image for a larger view.)
Despite a weak disturbance moving near us early Wednesday morning, no snow or even flurries are expected here.
NBM conditional precipitation type for 7 AM Wednesday morning. 32º line is drawn as white. Blue is light snow, green light rain. (Click on image for a larger view.)
A warm up is expected Friday through Sunday, but it will be accompanied by rain late Saturday into Sunday.
Today’s 12z NAEFS forecast for Friday at 4 PM. Large upper ridge over us, but low pressure in the Texas panhandle will move towards us bringing rain late Saturday into Sunday. More cold air dropping down in Montana. (Click on image for a larger view.)
Previously Posted Sun 9:21 PM —
A changeable week, weather-wise, as an upper cold trough moves in by Wednesday, then replaced by an upper (warmer) ridge into the weekend.
ECMWF forecast for Wednesday afternoon shows a cold upper trough over us. A large ridge of milder weather to our west is poised to move in late in the week and into the weekend. That ridge will be less pronounced when it gets here, as some colder air is pushing southward from Canada. With the milder weather next weekend, expect some rain, likely Sunday. (Click on image for a larger view.)
For Monday, we’ll have considerable mid level cloudiness develop that will be with us for most of the day. Highs 47º-49º which is actually near our seasonal average temperature.
Our coldest day will be Wednesday.
Milder air by Friday and especially Saturday. Rain expected on Sunday.
I don’t see any pattern for snow in the coming week or two. As was the ‘diagnosis’ from last year’s winter, “there’s just not enough cold air”/
