Update Sun @ 9:48 PM — Latest radar and latest RAP and HRRR have the storms not making it into our area tonight. I can’t account for the GFS and HREF which had forecast this disturbance to move in.

Update Sun @ 8:41 PM — The line of storms expected earlier this evening did fall apart, as many models (but not all) had predicted. Looking closely, the shear vectors and storm motion vectors were at 90º to each other.

There is another strong upper air disturbance that is very much blossoming on water vapor imagery this evening (see the live water vapor image on this site).

Water vapor image. The yellow ‘river’ of air in southwest PA is warm and unstable as it moves against colder air (blue) (Click on image for a larger view.)

Here’s a current superimposed radar with water vapor image at 8:40 PM —

Current Radar and Water Vapor image 8:40 PM (Click on image for a larger view.)

Storm motion and shear vectors align better about 11 PM. These storms may make it into Philadelphia between 10PM to 1AM. It will be interesting to see if it happens, as it is forecast by the latest GFS.

Monday will be an active day here weather-wise as a strong cold front moves past. Isolated storms may break out in the morning. Available CAPE values are expected to be very high, exceeding 2500 joules/kg. Heavier storms in the 3-6PM time frame. More on that tomorrow.

Update Sun @ 6:11 PM —More models show some storms moving into the Philadelphia area between 9 PM and 1AM. Monday looks to be an active weather day. Stay tuned.

Update Sun @ 11:37 AM — Last night’s models had any rain dissipating before making into our area this evening. However, some of this morning’s models show the showers and thunderstorms making it into the Philadelphia area this evening, between 6 PM and 11 PM. (Not all are on-board: the HRRR has them missing us; the Canadian RGEM has them falling apart just west of us.)

Here’s the latest high resolution HIRESW forecast for 7 PM—

12z HIRESW-2P5 (ultra high resolution) simulated radar/accumulated rain/clouds (black) forecast for 7 PM Sunday. (Click on image for a larger view.)

Updated Sat 10:41 PM — Sunday Forecast below unchanged. Fog lifts and clouds break for sunshine around noon. Showers and thunderstorms look less likely late Sunday and Sunday evening; they will weaken as they approach Philadelphia from the west. Western suburbs may see some activity after 6 PM.

Monday may be an active day for afternoon thunderstorms.

Update Sat @ 9:42 AM — The latest HRRR radar/rain/clouds forecast for 2 PM Saturday—

Today’s 12z HRRR with radar/rain (green contours) and clouds (black/grey) forecast for 2 PM Saturday. The arrow shows the direction of movement of these showers, from southeast to northwest. (Click on image for a larger view.)

Updated Sat 7:34 AM — Last night’s models have come together with the forecast of showers moving in from the southeast as early as 11- 1 PM Saturday. About 0.2-0.3 inches of rain expected.

For Sunday, the forecast is mostly the same- cloudy in the morning, becoming sunny in the afternoon. Chances of thunderstorms moving in, especially western suburbs late afternoon Sunday.

Read More


Cloudy early, then by late morning sunny, hazy warm, and a bit humid with dew points in low 60s. Showers move in late in the afternoon. Thunderstorms possible late afternoon and evening, especially western areas. High temp 80.0º ± 3.0º NBM model, Blue Bell.


Early Edition

The persistent closed upper low that is forecast to “open up” early Saturday has presented some forecast challenges: few of the models from last night are accurately forecasting current conditions —

  • Current Water Vapor and Radar

The above HREF radar/rain forecast was the best match to current radar and water vapor imagery. Even then, the match is poor. All models fail to predict the area of rain in southern NJ. Even the models just coming in (HRRR, RAP, NAM and NAM-NEST) fail to capture what’s on radar currently.

As is always the case, when the models are not capturing “reality” in the short term, it’s unlikely they’re going to forecast correctly for a day or more going forward.

With the HREF being the best match, I’ll use it going forward with this weekend’s forecast.

(For those of you who wonder how/why I select a specific model for a forecast, it’s a comparison and match to current conditions that guides the process, in addition to the model’s conformity to the forecasts of the larger group of models. At times, I’ll select based on past experience with the model’s handling similar conditions.)

For today, Friday, the HREF is consistent with partial clearing and no showers in the afternoon.

NAM forecast for Sunday at 11 AM.

Read More


Updated Thu 9:55 PM — Here are the forecast trends for Friday through Sunday—

Light scattered showers/sprinkles early Friday especially southeast of the city, low clouds. Much of the day cloudy but dry. There may be some breaks in the cloud cover especially west of the city. Updated Thu 10:43 PM —Tonight’s high resolution models have some sunny breaks in the afternoon along with some widely scattered showers.

For Saturday, the models (with the exception of the ECMWF) have further reduced the chance of showers except far western suburbs. Brightening skies and some sun expected. Increased chance of some showers late afternoon and evening. Possibly some thunder. Updated Thu 10:54 PM — Tonight’s. NAM-NEST shows a fairly nice day for Saturday

Sunday should be a lot like Saturday, but with more sunshine and warmer. Some showers possible late in the day or evening.

Update Thu @ 9:43 AM —The GFS and other models are in agreement with clouds and showers on Friday as a jet streak associated the closed upper level low affects us.

3 PM Friday—

GFS Jet stream forecast (250mb level) for Friday 3 PM. Notice the closed low is in a somewhat different position than had been forecast earlier this week. A counter-clockwise circulation around the upper low brings a jet streak (1) up the coast. Our area’s position in relation to the jet streak will be s in what’s called the “left exit region” – an area of upward vertical motion (white rectangle). With the upward vertical motion and moisture from the southerly flow, we’ll have showers on Friday. (Click on image for a larger view.)

3 PM Saturday—

GFS Jet stream forecast 3 PM Saturday. By Saturday, the previously closed low will have “opened up” in Virginia and the jet streak position is forecast to be somewhat north of our area. Our area will be in what’s called the “right entrance region” of the the jet streak (white rectangle). The right entrance region has less upward vertical motion. Showers should be less numerous on Saturday with a mix of clouds and even breaks of sunshine. (Click on image for a larger view.)

The latest ECMWF model, just available, has a different jet flow and forecast than the GFS. The ECMWF continues with a wetter Friday and Saturday.

Updated Tue 11:15 PM — Tonight’s models are showing the chance of clouds and scattered showers as soon as Thursday, with moisture brought inland from the retrograding coastal low.

Updated Tue 7:15 PM — Looking towards the weekend, the same low that gave us the rain last weekend, threatens to rotate back into our area by Friday (see image below).

For Wednesday and Thursday, the same cloud deck that moved in late Tuesday may affect our area at times. Temps in the 70s

As for Friday, there are differences between the ECMWF and the GFS, with the ECMWF showing showers moving in during the day, while the GFS keeps us just with some clouds.

Saturday looks unsettled with clouds and occasional scattered showers but not looking like a rain-out. Again, the ECMWF shows more rain than the GFS. Highs mid to upper 70s.

Sunday may feature warm temperatures and breaks of sunshine ahead of an advancing cold front. Highs may reach near 80º.

Updated Tue 6:53 PM — Well, the clouds from the retrograding coastal low moved in eventually, about 7 hours later than had been forecast last night.

Visible Geocolor Satellite Image 6 PM

Updated Tue 8:43 AM — The 06z models backed away from the clouds making it into Philadelphia today.

Updated Mon 10:47 PM — Tonight’s models are suggesting a slight forecast change for Tuesday. The retrograding low pressure system off the coast may throw back some low level cloudiness far west enough to make it into the immediate Philadelphia area.

Originally Posted Sun 7:33 PM

This coming week’s weather will feature a blocked pattern, the other side of the coin of this past weekend’s cold, cloudy, rainy and slow moving upper air low pressure system.

Indeed, the same upper low that gave us the rain will retrograde to our southeast instead of the usual path moving off to the northeast.

ECMWF forecast upper air forecast for Tuesday morning showing the upper low in the Atlantic and its current forecast path (white arrows). This is the same upper air low that gave us the rain Friday through Sunday morning! Ordinarily, it would move off to the northeast (Path 1) , but its path (Path 2) is the result of a block in the Atlantic. This low may circle up to affect us again next weekend! (Click on image for a larger view.)

This week, upper air winds will come from the north and northeast. We should have dry conditions through Friday.

Meanwhile the same blocked pattern will allow temperatures to build to record levels in the mid section of the country.

GEFS mode Temperature forecast with upper air contours for Wednesday 5 PM. Notice the same upper low off the Carolina Coast and heat building in the mid section of the country. Temperatures here will approach 80º later in the week. (Click on image for a larger view.)

Incredibly, the same low is forecast to retrograde westward over Florida and then move back over our area for next weekend with more rain.

Weather… and Other Things 'Up in the Air'

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