Light Snow Possible before Daybreak Wednesday

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.

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—

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

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—

By Wednesday, things begin to change. Here’s the NAEFS model showing more detail at jet stream wind level as it transitions on Wednesday —

By Friday, we’re under an upper level trough with cold air—

No large storms expected to form here this week and no snowstorms expected here within in the next 10 days.

Thursday

Update Wed 01/18 @ 6:19 PM — Low pressure will move in Thursday morning with rain starting between 7 and 10 AM—

Rain through the day. Increasingly windy during the afternoon. Total rainfall is still expected to be from 0.5 -0.8″ rain —

Friday looks fairly cloudy. Windy!

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.

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.

Tuesday Update

Update Tue 1/17 10:59 AM — So much for the showers dissipating.

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 —

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.

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.

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—

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—

Wednesday: A mix of clouds and sun. Mild in the 50s

Thursday: A more robust system moves through on Thursday with rain.

Friday: Clearing windy and colder.

Weekend Outlook

Update Thu 01/05 @ 6:01 PM — A quick note that the latest GFS just available shows some light snow possible, western and southern sections late Sunday evening. Very light and not quite a storm. Stay tuned.

Thursday

Update Thu 01/05 @ 5:36 PM — Some interesting weather sights from the Schuylkill trail today-

First, several areas of sprinkles and showers did develop during the afternoon, as forecast by the HRRR and NAM-NEST. Some unusual virga seen from mid-level clouds—

Another sight was a somewhat rare wintertime rainbow in the afternoon.—

Update Thu 01/05 @ 9:36 AM — This morning’s HRRR and NAM-NEST show much more cloudiness this afternoon than previously forecast (previous forecast was “high uncertainty”). Even a few widely scattered sprinkles possible.

Update Wed 01/04 @ 8:38 PM — A cold front will move through this evening and temperatures over the next 24 hours will begin a descent closer to normal by late Friday.

Thursday will feature a mix of sun and clouds. High uncertainty about the amount of cloudiness due to increasing upper trough and a storm southeast off the coast.

An upper air disturbance and upper trough will bring some showers later Thursday evening and night.

The weekend looks closer to ‘normal’ temperatures. No snow on the horizon. Slight chance at the end of next week. Increasing chances of snow towards the third week in January when colder air might return.

Wednesday

Update Wed 1/04 8:08 AM — Clouds move in about 11 AM. Showers move in 2-4 PM, earlier western Montco and southern Chester counties. Rainfall amounts will be on the light side, 0.10 inches. Highs near 65º!

Update Tue 01/03 @ 8:03 PM — A cold front approaches. High cloudiness, perhaps with some breaks of sun early, then thickening and lowering clouds. Rain moves in between 2 and 3 PM (earlier west of the city.)

Increasing winds during the evening, ahead of the cold front that moves through about 11 PM.

Yet Another Wrong Model Forecast

Tue 11:04 AM Forecast Review — I didn’t have time last night to post a Tuesday forecast, but I can tell you that the models had only light showers barely making it into northwest sections of Philadelphia late morning and then exiting to our northeast. This morning’s models were similar.

12z NBM forecast for noon showing precip coverage barely making it into Philadelphia —

Current Radar at 11 AM —

Some of the rain coverage error can be explained by the resolution of the models, but not all of it.

I’m usually in awe about the weather models, but I can’t explain why there have been two consecutive days where the model forecasts have been substantially wrong.

Mon 5:23 PM Forecast Review — Today’s forecast was one of those where meteorologists just have to scratch their head, wonder what happened to the sunshine with 60º temperatures, and admit a failed model forecast.

Eating lunch outdoors today, I was wondering where the sunshine was. When it started raining about 1:30 PM, well…

I’ve gone back and reviewed the morning models. I’ve looked over every one of the 14 different models that automatically download each day to my own ‘little weather data server’, and every one of them had little to no clouds at noon and no rain during the mid-day hours.

What I did find: the HIRESW and HRRR models showed areas of strong vorticity and a strong negative/positive vorticity couplet in the mid levels of the atmosphere (500mb) at 1-2 PM —

So with the models getting the dynamics correct, clearly there was an input error in the available moisture. There was much more moisture and clouds than forecast. (You can’t get rain when there are no clouds/moisture, despite the dynamics being present. ) At least we know what likely happened with today’s forecast.

MONDAY

Update Mon 01/02 @ 9:56 AM —The cloudiness this morning was expected. We should see more sun especially from Blue Bell and eastward towards noon, BUT the latest NBM shows advancing cloudiness by 2-3PM with more uncertainty elsewhere—

LACK OF COLD AIR

Update Mon 01/02 @ 9:44 AM — I mentioned yesterday that following the extreme cold of last week, there’s a relative lack of cold air in Canada right now. There’s no extremely cold, dense air mass to push the jet stream to our south.

Here’s the forecast temperatures from last year, (January 21st) in comparison to today —

While the temperatures change day to day, and the above maps are not a direct calendar day-to-day comparison, the temperatures in Canada this week are ‘milder’ than last January (and last January wasn’t a particularly cold season.)

For snow lovers, we won’t see much of anything until things chill down to our north.

MILD AND WET

Previously Posted Sun 8:12 PM —

This week’s weather will be influenced by a warm moist flow with the jet stream to our north. (There’s a lack of extremely cold air in Canada and Greenland after the main blob came down last weekend. It will take a week or two for it be replenished. )

Monday will be the nicest day of the week. We may has some low clouds and fog early Monday, but the fog will break for sunshine. Temperatures in the upper 50s to near 60º!

The current water vapor image shows the moisture stream that will affect us later Tuesday and Wednesday and again on Friday.

The water vapor image (above) corresponds to the current GEFS forecast jet stream flow for the present time—

Tuesday and Wednesday; Temperatures will be very mild, but we’ll have clouds on Tuesday and clouds with rain on Wednesday.

By Friday, some cold air associated with Canadian high pressure will push the jet into a moderate trough over the eastern US with low pressure spawned off the coast—

With temperatures above freezing on Friday, we’ll have rain, but not far to our north, there will be some snow, despite temps above freezing.