Update Tue 01/31 @ 4:32 PM — Just a quick update…latest ECMWF and NAM-NEST are forecasting very light snow between 3 AM and 6 AM Wednesday morning. A light dusting possible less than 0.20 inches, especially in South Jersey.
NAM-NEST snow-rain at 6 AM (Click on image for a larger view.)
Who Predicted This Wet Snow?
Update Tue 01/31 @ 3:20 PM — So, I’ve been “off-duty” the past two days and I haven’t done a daily forecast, just a jet stream over-view yesterday. But I’m pretty sure I heard the forecast on the radio— ”some showers possible in the morning’. That was about it.
We’ve had very light, wet, non-accumulating snow falling for the past 4-5 hours.
So folks, this is going to have to be a Retroactive Forensic Forecast to answer the question, “what the heck is going on??”
Basically a cold front went through overnight, as had been forecast. Looking back at last night’s models, several were hinting for the front to stall just to our south and several waves of low pressure have apparently formed along the stalling front. That said, this morning’s HRRR had light rain, not light wet snow, and ending earlier than it has.
This morning’s GFS was not capturing this afternoon’s wet snow in its forecast, although the wavy nature of the thermal contours is apparent—
Today’s 12z GFS shows the 800 and 900mb freezing temperatures (blue and violet) are quite wavy, suggesting the weak waves of low pressure. The isobars just south of us are all “baggy” which also shows the very weak low pressure waves. Also notice the 32º surface temperature line (faint white) is north of us. It doesn’t have to be freezing outside to have snow. (Click on image for a larger view.)
I’ll have to check to see if there are any more surprises in store for us. Stay tuned.
Transitioning Jet Flow
#weather #paweather #wx #pawx #philadelphia
Useful Weather Link from the NWS—
The NWS has an Impact Based Decision Support Services(IDSS) websitethat provides detailed and up to date hour-by-hour weather information and forecasts. There’s a button that allows you to save a specific location URL to your web browser Favorites or Bookmarks.
Excellent! Better than most weather mobile apps!
A split jet flow, flat in our area, will transition to a trough by Friday. Most systems will pass by to our south and to our north through Friday.
Current Water Vapor Image shows jet positions—
Monday Morning Satellite Water Vapor image (mid level) shows two jet streams and a flat configuration over our area on Monday morning. (Click on image for a larger view.)
By Wednesday, things begin to change. Here’s the NAEFS model showing more detail at jet stream wind level as it transitions on Wednesday —
NAEFS Wednesday 250 mb (jet stream level) winds Strong jet streak over Pennsylvania (white color code) (Click on image for a larger view.)
By Friday, we’re under an upper level trough with cold air—
NAEFS 250 mb wind Friday forecast shows full transition into trough over our area. (Click on image for a larger view.)
No large storms expected to form here this week and no snowstorms expected here within in the next 10 days.
Update Wed 01/18 @ 6:19 PM — Low pressure will move in Thursday morning with rain starting between 7 and 10 AM—
Satellite Water Vapor image at 5 PM Wednesday with superimposed radar. (Click on image for a larger view.)
Rain through the day. Increasingly windy during the afternoon. Total rainfall is still expected to be from 0.5 -0.8″ rain —
Today’s HREF total precip by 2 AM Friday morning. Heavier rainfall to our north. (Click on image for a larger view.)
Friday looks fairly cloudy. Windy!
Today’s New York Times has an interesting article: “Can we talk about how we talk about the weather”. It addresses the increasing hype about storms and the use of exaggerated weather forecast descriptive terms. It touches on the new [absurd] trend in naming ordinary storms and pseudo-scientific, non-specific approach to what are “bomb cyclones” and “atmospheric rivers” and the “polar vortex”. And my favorite: media hype.
Long-time followers of this blog know of my disdain for all of this. I often see this blog as my crusade to cut through this on a regional level.
This blog has a small regular daily following, fairly large when a winter/summer storms loom.
As many of you regulars know, I’ve always geared what you read here towards authenticity, not the most re-tweets [or ‘re-toots’ in the Mastodon world].
Wednesday
Update Tue 1/17 9:39 PM — Tonight’s models are showing both periods of sun and periods of clouds for Wednesday. More sun than previously forecast.
Update Tue 01/17 @ 5:17 PM — A strong upper air flow (jet streak) directly over us on Wednesday with low pressure in eastern Canada will cause mild but cloudy conditions for us on Wednesday. Some strong mid-level disturbances with a strong lower level jet will mix down some of these higher winds.
NBM wind meteogram for Blue Bell PA (Click on image for a larger view.)
So Wednesday looks to be fairly cloudy (with some sun early and some occasional breaks in the clouds.) (More sun east of the city into NJ. ) It will be WINDY. Highs 50º sd 1.2º Blue Bell.
Thursday looks quite rainy. 0.5″- 1″ of rain expected.
No snow on the horizon, but several coastal storms are forecast— next Monday and next Wednesday/Thursday. There’s just not enough cold air for snow here.
Nonetheless, the weather pattern has changed a bit over the past few days and during a more ‘normal’ winter, this pattern would bring us a good chance of snow.
Tuesday Update
Update Tue 1/17 10:59 AM — So much for the showers dissipating.
Radar at 11 AM. More showers than forecast.
Follow up –The showers turned out to be minimal. A few sprinkles.
Update Tue 01/17 @ 10:00 AM — As expected, the radar echos have arrived earlier than forecast. Latest radar, supported by current models, has the showers dissipating as they move towards the immediate PHL area and will pass mostly north and south of us —
RADAR 9:45 AM. Most activity will dissipate as it moves eastward. (Click on image for a larger view.)
Monday -Tuesday Update
Update Mon 01/16 @ 9:45 PM — Tonight’s models are leaning towards very light precip staying to our south. A few sprinkles in the morning possible northern sections.
Tonight’s 00z NAM-NEST accumulated precipitation by 7 PM. Barely anything north of the city. (Click on image for a larger view.)
Update Mon 01/16 @ 5:10 PM — This afternoon’s models continue with the forecast of clouds for Tuesday with some very light rain (0.01-0.02″) moving in between noon and 3 PM. A comparison of the GFS and NAM models with current water vapor imagery suggests the models may be a bit slow with the light showers, so it wouldn’t surprise me if we see some very light showers as early as 11 AM.
Water Vapor image with superimposed NAM potential vorticity (PV) contours (yellow) shows the actual image is more northeastward than the models, suggesting the system is moving somewhat faster than the model forecasts. (Click on image for a larger view.)
I should add that this afternoon’s NAM-NEST has no rain for our immediate area, having the light precip move to our north and south. The German ICON and Canadian RGEM models are similar. So there’s some uncertainty whether our immediate area will get any precip.
Update Mon 01/16 @ 9:05 AM — It’s still breezy/windy Monday and the winds will make it feel colder than the thermometer reading.
As for Tuesday, there’s some uncertainty in the amount (if any) of rain we’ll get in our immediate area. The low resolution models have the rain missing us, moving to the north and south of us.
The higher resolution models do crank out a small total amount of scattered showers (0.01- 0.02″) as captured by the latest NBMv4.1 model—
Today’s NBM 12z 12 hour accumulated rainfall through Tuesday. Very light amounts. (Click on image for a larger view.)
Previously Posted Sun 8:16 PM —
Our snow-less winter will likely continue for this coming week into this weekend. At one point last week, I had posted that enough cold air was building in Canada in the coming 7-10 days that we could could see a change in the pattern with the cold air sagging down into an upper trough.
There is no sign of that happening in the current medium range model forecasts.
This week will continue the pattern of milder than average temperatures, punctuated by low pressure systems that track up to the Great Lakes with their attendant cold fronts moving through each time.
Monday: Fair skies. Still breezy. Temps in the mid 40s.
Tuesday: The first example of this repeating pattern will be on Tuesday with showers moving through. The showers lose upper air support as they pass through our immediate area mid day Tuesday, and it’s uncertain if we’ll get much more than a few sprinkles if anything.
Here’s the current RAP model forecast for Tuesday—
RAP model simulated composite radar for 1 PM Tuesday. The rain is moving into an upper ridge and falls apart. (Click on image for a larger view.)
Wednesday: A mix of clouds and sun. Mild in the 50s
Thursday: A more robust system moves through on Thursday with rain.
GFS model forecast for Thursday at 1 PM. (Click on image for a larger view.)
