Tag Archives: Jet Stream

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Friday and the Weekend Outlook

Posted Thursday 05/14/26 @ 7:52 PM — As expected, the rather impressive line of showers Wednesday night fell apart as they approached an unfavorable upper air environment. Here’s the area rainfall—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

The upper low pressure system over us will gradually depart and a weak warm front will approach later on Saturday. We may see some clouds on Saturday but rain will fall apart as it approaches our area!

GFS forecast for Friday. Upper low over us will move off to the NE. Heat building in Texas will move up towards us over the weekend. (Click on image for a larger view.)

Much warmer on Sunday and into next week!

Wednesday Forecast Update

Posted Wednesday 05/13/26 @ 7:37 AM —The latest RRFS (06z) shows the approaching showers today moving in as two separate lobes; the first one, scattered and light swings through about 6-7 PM.

The second more organized line of showers moves through between 8 PM and 11 PM. The second line of showers is expected to weaken, for the reasons described below as it moves eastward. Total rainfall still expected to be light, although the latest RRFS shows 0.40″; most models still in the 0.25″ or less range.

06z RRFS forecast “simulated radar” at 6 PM. Two lobes of rain move through, the first very light and scattered.

An upper air low with a pool of cold air aloft will bring variable cloudiness with a chance of scattered instability showers during the afternoon hours on Thursday. Not all areas will see showers.

So Little Rain. Why?

Posted Tuesday 05/12/26 @ 8:02 PM — In recent months, we haven’t seen much in the way of typical Springtime thunderstorms. Most of the time, we’ve been an an anticyclonic upper air flow (clockwise curvature) which produces descending air flow.

Wednesday is another example where a large area of rain somewhat diminishes in intensity as it approaches.

This is a good example of another reason- our position in relation to the position of the jet stream (and more specifically, the constricted part of the wind stream resulting in higher speeds called the ‘jet streak’)

Our position on Wednesday evening will put us in what’s called the Left Entrance Region of the nearby jet streak. This region has vertical descending flow which works against precipitation.

GFS winds at 250 mb showing, the level of the jet stream. Philadelphia is in the Left Entrance region. The precipitation will reduce in intensity as it moves through this area. (Click on image for a larger view.)

A textbook representation is here—

The Left Entrance region has descending flow. (Click on image for a larger view.)

By the way, the jet in “jet stream” doesn’t get its name from being at the level that jet airplanes fly (34,000-38,000 feet).

The “jet” in jet stream refers to fluid dynamics terminology where constriction in flow causes an increase in velocity of flow. You’ve all seen a garden hose where the nozzle constricts flow causing a jet of water. Air is a fluid too. The upper air winds flow in the valley between air masses. Where that valley constricts causes the jet streaks.

Wednesday Forecast Update

Posted Tuesday 05/12/26 @ 5:46 PM —A cold front will approach and move through our area Wednesday evening. During the daytime hours, Wednesday will have mostly sunny skies through high cirrus clouds in the morning, then periods of clouds in the mid afternoon with the sun breaking out again about 4 PM. It will be windy and times.

Rain with this system will occur behind the front and wind shift. The wind shift occurs about 8 PM (Blue Bell) and the rain showers move in between 8 and 9 PM. Rainfall had originally looked to be impressive, now looks to be on the light side. Total rainfall about 0.20″ The rain is no longer expected to linger into Thursday.

18z RRFS Precipitation Rate at 9 PM Wednesday. (Click on image for a larger view.)

An upper level low will linger over our area Thursday and Friday. A mix of sun and clouds is expected, but with an upper low in May, one can’t rule out a scattered shower.

For weather nerds—

—”Effective on August 31, 2026, at 1200 Coordinated Universal Time (UTC), the National Centers for Environmental Prediction (NCEP) will discontinue the North American Mesoscale (NAM) Forecast System, the Short-Range Ensemble Forecast (SREF), the High-Resolution Ensemble Forecast (HREF), the High Resolution Window (HiresW), and the NAM Model Output Statistics(NAM MOS). These legacy systems are being replaced by the Rapid Refresh Forecast System (RRFS).

The RRFS and REFS production implementation and the retirement of the legacy models will occur on the same day.“

Tuesday-Wednesday Outlook

Posted Monday 05/11/26 @ 8:05 PM — We’ll finally get some sunny skies on Tuesday, as high pressure builds in behind the system that exited on Monday.

Yet another dip in the jet along with a surface low over the Great Lakes will push a cold front slowly through on Wednesday.

18z GFS forecast for Wednesday at 4 PM. Showers on the doorstep of our area ahead of a cold front. (Click on image for a larger view.)

The models continue to have some showers along this front linger into Thursday as the upper jet digs deeper towards the south. It look like this jet dip will have temperatures less chilly than the one with us on Monday.

Originally Posted Mon 9:40 AM —This week will be an active week weather-wise, as an atypical dip in the jet stream and cool air collides with May moisture and warmth from the south.

For today, the front that went through yesterday with the fast moving storms (we had some hail at the start) is still poised to bring some light rain to areas just east of I-95 today into the Jersey Shore.

Water Vapor image at 10 AM with superimposed RAP model MSL pressure (black contours), GFS/NAM potential vorticity contours (yellow & violet fine contours) with MRMS Radar and surface wind barbs, The dip in the jet stream is visible, as is the position of the rain over the coastal area. (Click on image for a larger view.)

Varying degrees of mid-level cloudiness is forecast for today but “mostly cloudy” captures it . There will be gradual clearing, perhaps about 5 PM, from the northwest. A few showers this evening are possible near Allentown as an upper air disturbance moves through.

Tuesday looks quite nice!

As for rainfall, here’s what the area received on Sunday evening/night. The rain was very localized- There were sharp gradients with areas receiving 0.25″ and a mile or so away, the totals were 0.75″—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)
Commentary

THOUGHTS: WARM WEATHER IN DECEMBER & JET STREAMS

Updated with graphic below.

This week’s weather is expected to be influenced by a flat to ridge-shaped jet flow.

I know the TV weather people like to talk about the jet stream as something that has a mind of its own. They talk about changes in the jet stream as something that just changes, without any apparent reason.

Frankly, I don’t think that is all that accurate nor is it conceptually useful.

What makes more sense to me is that the jet stream winds flow confined to the three dimensional valley created between the bulges of the two major air masses. The position of the jet stream and its configuration takes on the shape of the edge of the air masses and the valley between them.

GEFS forecast jet wind speed and position (300 mb level) for this Thursday. Air mass 1 and Air mass 2 have their own circulations. Between the two air masses bulges is a valley where the the jet stream flows. Conceptually, it makes more sense to think of the jet flow shape and position as influenced by the size of the relative air masses and their individual shape. Speed ups (“jet streaks”) and slow downs in the jet flow are result of constrictions and widening between the air masses. I have intentionally drawn air mass 1 smaller than the tropical air mass 2.

Localized increases in speed of the jet stream (called ‘jet streaks’) are related to narrowing of the valleys between the air masses, just as a garden hose nozzle creates a water jet of increased velocity.

(Indeed, the the name jet stream has little to do with jet airplanes. The stream of air comprising the jet stream displays the physics of “jets” in the field of fluid mechanics, as in the garden hose reference above.)

The movements, size, temperature and densities of the main air masses on the spinning earth determine the changes in the jet wind shape and position. While there certainly is a feedback mechanism where the jet flow reciprocally affects the air mass shape, conceptually, I feel this leads to a poorer grasp of the main determinants of jet stream position and shape.

So why am I talking about this today?

Well, the position of the jet flow this week with its northern position (for December) and bulge northward is the result of the the tropical warm air mass (2 above) being larger than the current colder denser polar air mass to the north (1 above.) Simplistically, there isn’t a larger or dense enough cold air mass to displace the very large warm tropical air mass to the south.

Why is the tropical air mass so warm? Well maybe the ocean surface temperatures, warmed by the atmosphere are maintaining their heat.

Global Sea Surface Temperature anomalies. Things are looking pretty warm out there! (Click on image for a larger view.)

People talk about climate change and global warming. This is one of the ways it manifests itself.

Eventually, very dense cold air to our north will accumulate enough that it will bulge downward and push the jet stream position to our south. How the cold air slides down and its shape will determine the new valley position and the new jet stream configuration.

There is some evidence that a pattern change may be setting up, with cold air pooling in far northwestern Canada by Christmas eve. Following a very mild upcoming Christmas weekend, the final week of this year and the first week in January may be positioning itself for a pattern change

ICON model forecast for this Friday. Cold air pooling in Canada suggests a movement southeastward, pushing the jet into a more winter-like position by the very end of December. (Click on image for a larger view.)

Philadelphia Weather Outlook

SO WHERE’S THE SNOW?

I should preface this post by saying that I enjoy trying to predict snow rather than dealing with it after it has fallen. Pretty when it falls, a pain afterwards.

That said, I’m being asked, ‘ so where’s the snow?’

I’ve mentioned since the end of November that my sense of the weather pattern so far this winter is what I like to call “a lack of very cold air” to our north.

Here’s the current GFS statistical ensemble forecast (GEFS average) expected minimum temperatures for next week—

GEFS statistical average minimum temperature forecast for next Friday.  (Click on image for a larger view.)

Well, maybe you’re thinking, -24ºF seems awfully cold to me? The truth is, when things get going, temperatures are as low as -40º to -50º in the same areas above, especially in Greenland. (Greenland is looking awfully warm this year.)

Contrary to what many weather entertainers on TV seem to suggest, the jet stream doesn’t just do it’s own, arbitrary thing. The jet stream, a fast flow of winds at about 35,000 to 40,000 feet in winter, is a river of air actually flowing in the three dimensional “valley” between air masses.

It is the density and shape of the cold air mass that determines the shape of the valley and of the jet, not the other way around. In essence, the jet flow shape in winter depends on the “sag” of very cold moving southward, as depicted below with the white line—

GEFS forecast Jet Flow (300 mb winds) next Friday (Click on image for a larger view.)


Notice that the white line is tilted slightly eastward instead of straight southward.  The relatively “not so cold air” up north doesn’t have the mass density to dive straight south.     The jet stream flow (red arrows) isn’t suppressed directly southward, but rather southeastward.  

There also hasn’t been much of a blocking effect of a large cold air mass over Greenland and the Northern Atlantic to cause the same jet flow to bend back north up the coastline. 

As a result the two “jet streak” impulses (numbers 1 and 2 above) develop low pressure systems, but these move out to sea.  

So we need colder air in Canada, and colder air in Greenland and the North Atlantic.

The time period depicted above (January 15th-20th)  does show some positive trends towards snow here:

  • Temperatures are colder in Canada than they’ve been.
  • The jet flow next week will be “less split” than it’s been.   It does appear that some phasing may occur between the northern jet flow shown (disturbance #1 above) and the southern jet flow (just visible on the map from Mexico and near #1)

So, some colder temperatures and some action is possible January 15th – January 20th, although not currently captured on the models.  Stay tuned