Forecast Update

Update Wed 12/07 @ 8:22 PM — Latest GFS shows snow showers or flurries in our area after midnight Sunday into predawn Monday. No real accumulation expected except north of Allentown where it will be below 32º. The GFS model has a somewhat unusual looking forecast map, so I must confess that I have low confidence with its current forecast—

Today’s 18z GFS shows snow (lavender shading) falling at 1 AM Monday morning. (Click on image for a larger view.)

Forecast Updates

Update Wed 12/07 @ 5:43 PM see Sunday update highlighted below with possible snow showers Sunday evening.

Update Wed 12/07 @ 10:12 AM — Drier air will be gradually entering our area Wednesday afternoon, but today’s models show no sunshine, as the clouds are expected to hang tight. Clouds begin to break early this evening, but fog may form again towards midnight.

For Thursday, partly sunny, some clouds.

For Friday, things have finally clarified. Becoming partly to mostly sunny. No wintry mix expected in the evening as the ECMWF forecast has joined with the GFS; the cold high pressure will nose out the moisture to our south, keeping us dry even through early Saturday.

ECMWF forecast for early Saturday (10) shows high pressure nosing down into our area, suppressing any precipitation to our south. Notice the critical thickness lines are all to our south would have supported snow if any precip was falling. (Click on image for a larger view.)

For Saturday: mostly sunny. Increasing clouds in the late afternoon.

For Sunday: cloudy with rain most of the day. Update latest GFS and ECMWF show the possibility of light snow flurries or a coating Sunday night as low pressure develops off the coastline—

Today’s 18z GFS shows precipitation from low pressure developing off the coast with cold air aloft Sunday 6PM evening. Surface temperatures likely too warm for any accumulation, but precip may fall as snow. Unusual map appearance. Stay tuned. (Click on image for a larger view.)

Update Tue 12/06 @ 4:01 PM — The rain came in on schedule today, Tuesday. This weak but rainy system will exit Wednesday.

For Wednesday, we’ll be left with clouds, perhaps a few breaks of sun. High in the low 50s.

Commentary — Why do TV/radio weather people say “on YOUR Wednesday… “?

Is our Wednesday going to very different than their Wednesday?

For Thursday, partly sunny. High in the low 50s.

For Friday, another system approaches from the southwest. High pressure with cold air aloft nosing down from the northeast will either (1) suppress the moisture to the south, keeping us dry or (2) introduce enough cold air to allow a wintry, non-accumulating wintry mix to fall after midnight Friday-Saturday in areas especially west of the city.

Today’s GFS for 6 AM Saturday. The red, magenta and violet lines are ‘critical thickness’ lines which relate to the average temperature of certain layers (thicknesses) of the atmosphere. North of these lines, any precip will likely fall as snow. Notice that the GFS has enough cold air to suppress precipitation. The ECMWF, not shown, continues to show some precip as a wintry mix. (Click on image for a larger view.)

For Saturday, a northeast moist flow will send back a fair amount of cloudiness into our area.

Update Mon 12/05 8:53 PM — Regular visitors to this site know that over the past week, I’ve been watching for the possibility of a wintry mix to develop sometime over the Friday to Saturday timeframe. I nixed that possibility in last night’s outlook, based on the GFS and NAEFS. Today’s ECMWF has resurrected that possibility. The models have unusually low agreement in their forecasts for for later in the week. I’ll keep an eye on it.

Previously Posted Mon 11:04 AM —


A flat to slight ridge jet stream will influence our weather this week—

NAEFS Wednesday forecast for 250 mb winds (jet level). Included is the 540 thickness line. (Click on image for a larger view.)

The trend over the past few days is for the coldest air to remain further north of our area and for the flow to be more ridge-like with it just sinking southward by the weekend.

Moisture moving up from the southwest from the southern jet stream will bring a series of disturbances to our area with rain Tuesday into Wednesday and rain Friday into Saturday. With a flat to ridge-like jet flow, no major storms are forecast to develop, just these smaller disturbances.

NAEFS Surface forecast for Wednesday (Click on image for a larger view.)

By Saturday, the jet flow is flatter and further to our south, meaning colder air will move in——

NAEFS 250 mb forecast for Saturday. Includes 540 thickness line (red) (Click on image for a larger view.)

From the above map, we can see that some waves (note the wavy red 540 thickness line) are beginning to develop in Wyoming and Utah. This is the early stage of a large storm that will develop and affect us and much of the country next week.


Sunday and This Week’s Weather Outlook

Update Sun 12/04 @ 10:14 AM — Today will be mostly sunny. The ongoing forecast uncertainty about a period of cloudiness in the early afternoon still remains, as the HRRR and RAP show this cloudiness, the GFS and NAM do not. (The current versions of the HRRR and RAP have been overestimating cloud cover since their update last year.) Some high cirrus will also be visible in the southern parts of our region.

The weather for this week will be affected by a strong southwesterly jet flow aloft from the southern jet stream—

NAEFS mode shows strong southern jet stream (250 mb level winds) bringing moisture across the country. The red 540 thickness line can be seen as boundary between cold air northward and warm moist air southward. Areas of low pressure will develop along this boundary and affect our weather. (Click on image for a larger view.)

Rain will develop Tuesday into Wednesday and again Friday into Saturday.

NAEFS surface forecast for Wednesday at 1 PM. Notice the stream of low pressure systems moving into our area. (Click on image for a larger view.)

Laden with moisture from the Pacific, we’ll have several areas of low pressure form and affect us during the week.

BTW, for those following my postings from last week, I mentioned the possibility of a wintry mix on Friday. That speculation has fizzled out, as the boundary line between cold and warm air is now forecast to be north of our area.

Forecast Review: We had the sunshine about 3 PM and the cold front is coming through at 5 PM—

Forecast Update

Update Sat 12/03 @ 12:26 PM — This morning’s models are less clear-cut about any breaks of sun this afternoon. Still possible around 3 PM, according to the HRRR. Another line of showers about 5 PM.

For Sunday, once again the GFS has backed away from any instability cloudiness; the HRRR has some periods of clouds but not in all locations, so it looks like mostly sunny and cold for Sunday. Breezy but much less windy than Saturday.

Update Fri 12/02 @ 10:43 PM — Tonight’s models have clarified the earlier posted forecast for Saturday. Rain with the warm front moves through by 12-1 PM. The actual cold front moves through about 4-6 PM with additional showers.

Sunshine should break out for a period of time in the afternoon. It will be mild but very windy.

Here’s the latest HREF model forecast for 1 PM showing the back end of the rain moving into NJ—

Tonight’s 00z HREF simulated radar forecast for Saturday at 1 PM. Notice the break in the clouds after the rain moves east. (Click on image for a larger view.)

Previously Posted Fri 3:34 PM —

A warm front associated with deep low pressure in Canada will pass through our area on Saturday, followed by a cold front. Rain and very WINDY conditions will develop

High pressure builds in for Sunday.

This morning’s water vapor image shows both warm fronts and cold fronts associated with the low pressure system in Canada—


The warm front approaches about 7 AM-8 AM from the west with rain and very WINDY conditions. The GFS shows gusts to 50 mph (although we saw with a similar condition on Wednesday that wind gust forecasts can be an inexact science.) Rainfall is only expected to be about 0.35 inches. Warm temperatures on Saturday, but WINDY. Becoming colder in the late afternoon.

High temperature 57.6º sd 0.9º NBM model Blue Bell, PA

NBM Wind Meteogram for the weekend, for Blue Bell PA. High winds mid-day on Saturday following warm front. Cold front moves through about 5 PM.

The rain ends around noontime in the city and later in NJ. Considerable cloudiness is forecast for Saturday afternoon, although some breaks in the cloud cover are possible. The main cold front moves through about 5 PM with a quick shower or sprinkle, then temperatures drop.

The GFS shows the expected scenario well—

Today’s 12z GFS forecast for 1 PM Saturday. The bulk of the rain has moved into NJ. The main cold front in central PA will move into our area by 5 PM with some additional light showers. Cold windy conditions follow. (Click on image for a larger view.)


High pressure builds in for Sunday. Colder. Sunny in the morning. The models have been on and off with a potential upper air disturbance that may bring a period of clouds in the early afternoon Sunday (1 PM- 4 PM). The latest GFS shows this period of cloudiness again—

GFS relative humidity at levels for cloud formation along with contours of vorticity at 500 mb. (Click on image for a larger view.)

It should be noted that the NBM doesn’t show cloudiness for Sunday.

High temperature 43.3º sd 1.7º NBM model Blue Bell, PA


Any snow on the horizon for us?

Update Fri 12/02 @ 8:24 AM — The latest NAEFS continues to show a warm front type snow situation, now for early NEXT Friday morning. Still a long ways off in the future, but it is potentially our first wintry possibility for this season.

This morning’s NAEFS forecast for NEXT Friday morning—

Today’s NAEFS forecast for next Friday 4 AM. Critical temperature 32º lines (blue, magenta) and surface 32º line (white). Red is 540 thickness line. This forecast suggests some snow will fall above the red thickness line, but too warm for any accumulation right around the city. (Click on image for a larger view.)

Update Thu 12/01 @ 8:10 PM — I’ve had my eye on the long-range forecasts. Over the past week, there has been a consistent signal for a warm-front type winter mix for NEXT Friday evening (12/9). I get a kick out of trying to spot these events. (Usually these extended range forecasts fall apart with time. )

Here’s the possibility as shown in the NAEFS—

Friday Dec 9th forecast— near freezing temperatures with warm front type snow possible west of the city, possibly also affecting our immediate area. White wavy line is 32º line. Red line is 540 thickness line. Way too early to really take this forecast literally, but the NAEFS has been showing this possibility for the past week. (Click on image for a larger view.)

Sunday now looking sunny

Update Thu 12/01 @ 8:02 PM — This afternoon’s models have backed away from the instability cloudiness previously forecast for Sunday. Sunday now looks to be mostly sunny but cold.

Weekend Weather Outlook

Update Thu 12/01 @ 11:41 AM — A strong cold front moved through as forecast yesterday. Another very strong cold front, associated with deep low pressure in Canada, will move through Saturday morning with additional rain and strong winds.

Saturday morning—

Thu 12z NAM forecast for 10 AM Saturday showing cold front, rain/snow. High winds expected again with this frontal passage. There may be clearing by afternoon. (Click on image for a larger view.)

Most models have the rain ending by 12PM -1 PM Saturday with gusty winds continuing into the afternoon and some clearing and sun possible by 3 PM. This morning’s GFS shows a secondary cold front moving through about 5 PM Saturday with a few sprinkles.

Sunday looks to have considerable instability cloudiness and windy conditions.

Another front expected Tuesday and the general trend is for cold weather the end of next week.

My 2022-2023 Winter Climate Snow Outlook Posted

Wednesday Forecast Update

Update Wed 11/30 @ 8:04 AM — Last night’s models show little change in the forecast. The cold front comes through about 3 PM with strong wind gusts starting around noontime. Rain this morning precedes the frontal passage.

Latest RAP model (09z) meteogram for location Blue Bell, PA shows barometric change at 3 PM, indicating frontal passage. Wind gust near 45 mph at Blue Bell. Some higher gusts predicted in western suburbs. (Click on image for a larger view.)

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Wednesday Forecast Update

Update Tue 11/29 @ 5:41 PM — We’re back to a frontal passage between 2 and 4 PM with rainfall about 0.5 inches. Rain starts in the morning Wednesday, about 8-9 AM.

Winds increase early afternoon with many models exceeding 40 mph wind gusts and with some approaching 50 mph.

Here’s the latest GFS (18z) wind gust forecast for 3 PM Wednesday—

18z GFS wind gusts and wind direction at 3 PM Wednesday. (color coding is wind gust speed) Notice the change in streamline direction (white dashed line) that occurs at 3 PM, indicating the winds are shifting from the SW to the WNW with the frontal passage. (Click on image for a larger view.)

Here’s the latest RAP model (21z) wind gust forecast for 3 PM—

21z RAP model Wind gust forecast for 3 PM Wednesday (Click on image for a larger view.)

Update Tue 11/29 12:22 PM — The latest models show the front moving through here somewhat earlier on Wednesday than previously forecast. Current timing for the frontal passage about 12-2 PM with the highest wind gusts (40-50 mph) around that time. Total precipitation in our immediate area is now forecast to be only about 0.40-0.50 inches.

Update Mon 11/28 @ 8:22 PM — Heavy rain and high winds Wednesday afternoon between 2 and 6 PM. The strong cold front mentioned in the “week overview” below looks like it might pack a punch as it moves through between 2 and 6 PM (peak probability 3-5 PM).

Rainfall 1.0 inches (CMC-GEM) to 1.4 inches (NBM) are possible in a few hour period. Wind gusts 40-50 mph possible.

Previously Posted Mon 5:52 PM —

Week Overview

Two weather events for this coming week, currently timed for Wednesday and Saturday. There’s good agreement in the models for Wednesday’s rain. There’s less agreement in the timing for Saturday’s rain.

The jet stream will be making some significant changes over the week as cold high pressure pushes the jet flow down into our area on Wednesday with milder temperatures becoming colder by Thursday as a strong cold front moves through—

NAEFS forecast (250 mb winds) for Wednesday 10 AM shows a dip in the jet flow in response to cold air moving down from Canada. (Click on image for a larger view.)
NAEFS forecast for Wednesday shows a cold front in western PA and rain in our area. (red line is the 540 thickness line ) (Click on image for a larger view.)

Here’s what the surface map is forecast to be Wednesday at 1 PM with the cold front approaching the Delaware Valley—

High pressure builds in for Thursday and Friday. Another dip in the jet flow will bring another cold front through on Saturday—

Split Jet flow on Friday 1 PM with a ridge trying to develop, but the split southern stream jet flow will bring rain by Saturday.

Surface forecast for Saturday—

NAEFS forecast for Saturday. Another cold front moves through. (red line is the 540 thickness line )

Weather… and Other Things 'Up in the Air'

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