Category Archives: Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#weather #paweather #wx #pawx #philadelphia

Updated Sunday Forecast

Update Sun 04/02 @ 10:08 AM

Winds subside during the day. It will be sunny and unseasonably cool.

High temperature 50.0º sd 1.4º ( NBM model location Blue Bell, PA)

12z NBM Wind meteogram forecast for KLOM Blue Bell PA (Click on image for a larger view.)

Update Sat 4/01 9:44 PM — Despite the tornadic radar ‘signatures’ seen on radar this evening (inferred from radar differential velocity), the NWS is currently not showing any actual tornado reports except off the coast, near Atlantic City—

Forecast Review

Update Sat 4/01 7:22 PM — Tonight’s storms were impressive, almost scary. Thunder/lightning was almost continuous. Small hail also thrown in for good measure. The ingredients were there. The models did well with the timing and the forecast of the severe weather possibility. The HRRR was particularly impressive, as it often is with this.

Radar with storms just entering Philadelphia 6:30 PM—

Here’s current NEXRAD radar (~ 7 PM) —

Areas of NJ will likely continue to encounter these severe storms over the next hour or so.

One more thing. The meteorologist on Channel 3 did an excellent job in covering the severe weather. Knowledgeable and impressive.


Storm Potential Update

Update Sat 04/01 @ 10:25 AM — Updated with the 12z NAM-NEST which shows a different potential location of severe weather at 6PM—

12z NAM NEST shows elevated CAPE and Helicity northwest of the city at 6 PM. (Click on image for a larger view.)

Update Sat 04/01 @ 9:42 AM — The important 12z models (those with direct upper air radiosonde/weather balloon measurements) are just becoming available. The HRRR and RAP have become available.

Specifically, the HRRR shows chance of an isolated severe storm in South Jersey as early as 5 PM. The main line of storms moves in between 6 and 9 PM.

Here’s how the ‘severity’ parameters stack up compared to the major storm outbreak of Sept 2021—

9/1/2021 Tornado Outbreak
(example of highly severe)
Todays’s Forecast
HRRR 12z
Comparative Impact
CAPE 3500-4200 Joules/kg2100
Helicity 1350 m^2/s^2655
Vertical Shear 40-4640-45
Precipitable Water 2.7″0.83”⇩⇩
Lifted Index minus 6º Kminus 9.3º
HRRR Hail Parameter 1.91.4
Peak Wind Gusts 40-5040-50
Aligned Storm Motion and Shear Vectors- ~ 90ºAlmost aligned
Jet stream level – wavyHighly cyclonic
Jet Stream 250 mb Speed – 63High 135
500mb – Highly CyclonicHighly cyclonic
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity
↓ indicates works against Severity ⇩ Significant, but less impact

Here’s where the greatest potential for severe weather appears to be at this time (this will likely change)—

Today’s 12z HRRR CAPE/Helicity/Radar forecast for 6 PM Saturday evening. Contours represent helicity. Max helicity South Jersey means some chance of tornadic activity. (Click on image for a larger view.)

Canadian RGEM Convective (thunderstorm) Precipitation Forecast 7 and 8 PM snapshots—


Update Sat 4/01 7:51 AM — Last night’s models continue with an increased possibility of severe weather this evening. The new 12z HRRR will become available about 9:35 AM. I’ll update before 10 AM.


Update Fri 3/31 10:49 PM — Tonight’s models are showing storm severity parameters that are more impressive for Saturday evening’s storms than previous model runs—

9/1/2021 Tornado Outbreak
(example of highly severe)
Tonight’s Forecast
HRRR 00z
Comparative Impact
CAPE 3500-4200 Joules/kg1900
Helicity 800-1100 m^2/s^2450
Vertical Shear 35-45 30
Precipitable Water 2.0″0.87”⇩⇩
Lifted Index (minus) 7-9 minus 8
Peak Wind Gusts 40-5040-50
Aligned Storm Motion and Shear Vectors- YESYes
Jet stream level – Highly CyclonicHighly cyclonic
Jet Stream Speed – HighHigh
500mb – Highly CyclonicHighly cyclonic
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity
↓ indicates works against Severity ⇩ Significant but less impact

Previously Posted Fri 8:52 PM —

Low pressure will move northeast of us on Saturday. Warm air will be brought up over us from the southwest with rain in the morning. A strong pressure gradient will bring strong and gusty winds. A dry slot from the low will bring some sunshine in the afternoon.

A fast moving cold front moves through early Saturday evening with showers and thunderstorms

Saturday

Rain in the morning, rapidly ending about 12-1 PM from west to east. Sunshine breaks out from the dry slot associated with the low pressure system in Canada.

A strong cold front moves through between 6 PM and 8 PM with showers and thunderstorms. Some storms. especially from Philadelphia and eastward into NJ, will be strong, although severe weather not expected. The big issue may be the winds.

The HRDPS shows a fast moving line of storms at 7 PM Saturday—

Today’s 18z HRDPS shows a line of heavy rain at 7 PM . This will be a fast -moving line. (Click on image for a larger view.)

High temperature 71.3º sd 2.7º ( NBM model location – Blue Bell, PA)

Wind Meteogram (19z NBM) for Blue Bell PA 40-53 mph winds possible (Click on image for a larger view.)

Sunday

Following Saturday’s cold front, winds subside during the day. It will be sunny and unseasonably cool.

High temperature 50.6º sd 2.2º ( NBM model location Blue Bell, PA)



WEEKEND WEATHER FORECAST

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Forecast Update

Update Sat 03/25 @ 4:38 PM — The models have backed away from thunderstorms developing in the immediate PHL area. The area of development may be South Jersey, about 10 PM

This afternoon’s 18z HRRR shows thunderstorms developing about 10-11 PM in South Jersey. Little to no activity in the immediate PHL area. (Click on image for a larger view.)

Sunday looks suuny. A bit windy. Temps upper 50s to near 60.


Update Sat 03/25 @ 10:33 AM — There have been some changes in the forecast.

The rain will be moving in over the next hour or so. Rain ends about 5-6 PM. Total rainfall about 0.2-0.5 inches. A second batch of rain and thunderstorms about 9-11:30 PM

Here’s the current water vapor image. The first boxed area (1) is the area of upward motion bringing the rain in the next few hours. The second boxed area (2) may bring some thunderstorms between 9-11:30 PM, (maybe missing the immediate PHL area, maybe not.) Some model uncertainty in the placement of the second batch of rain/thunderstorms.


Update Fri 3/24 11:20 PM — Tonight’s models show breaks in the rain Saturday with total rainfall on the light side. The warm front gets further north and temperatures rise to upper 50s late afternoon.


Update Fri 03/24 @ 7:59 PM — Minor update to include the mention of thunderstorms mid-day with the rain. The latest models have very high levels of helicity and vertical shear, greater than that which spawned Thursday evening’s surprise thunderstorms . Nothing severe expected because CAPE values are expected to be low.


The cold front that moved through Friday night has stalled to our south and will try to return as a warm front later Friday night as a cold front moves in from the northwest later on Saturday.

It appears that the warm front won’t make it north of our area, as low pressure is expected to form near the Delmarva coast.

Water Vapor image at 4 PM shows large storm in Mississippi Valley and significant water vapor flow into our area. This will affect us on Saturday. (Click on image for a larger view.)

The warm front, the low pressure system and the cold front will result in periods of rain for much of Saturday. Thunderstorms appear possible Saturday evening, especially north and west of the city.

Saturday

Cloudy with rain becoming heavier late morning and continuing on and off during the day. Some thunderstorms mixed in with the rain possible mid day. Becoming very WINDY and gusty. Scattered thunderstorms again possible between 6 and 10 PM, mostly northwest of the city. Clearing after midnight.

High temperature 53.5º sd 2.4º ( NBM model location – Blue Bell, PA)

Sunday

Sunny and mild. Still quite windy.

High temperature 60.3º sd 1.7º ( NBM model location Blue Bell, PA)

Winds
Strong wind gusts about noon-2 PM Saturday. Large wind shift about 10 PM signifies cold front passage. (Click on image for a larger view.)

WEEKEND WEATHER FORECAST

#weather #paweather #wx #pawx #philadelphia

Sun 4:44PM Forecast Review — Well, we had little cloudiness, despite the GFS, HRRR and RAP model forecasts of cloud cover of 100%.

The NBM graphic I used with mean cloud cover captured it well.

In the fall of 2021, these models were upgraded with new cloud parameterization physics packages that have often over-forecast cloud cover. Until this issue is addressed, it’s something to keep in mind.

Sunday Forecast Update

Update Sat 03/18 @ 9:50 PM —We had more sunshine and less high, thin cloudiness from the city and westward Saturday than forecast.

Sunday starts sunny. There’s still uncertainty with the cloud cover early afternoon. Today’s GFS, HRRR and RAP still maintain a period of considerable low cloudiness Sunday from about noon until about 4 PM. More cloudiness northwest of the city. The HREF has minimal clouds.

There’s little disagreement about the very cold and the windy conditions for Sunday.

The NBM shows the wide “spread” (uncertainty) in the cloudiness forecast—

Today’s NBM shows uncertainty (red-violet contours) in the forecast cloudiness extending into Philadelphia from the northwest in the early afternoon. The red-violet represent 30- 40% cloud cover uncertainty. The models are split on this. I guess we’ll see.

Previously Posted Fri 5:22 PM —

A cold front will move through this evening (Friday). Some showers expected tonight about 10PM- midnight, with most of the activity just south of the city.

A second cold front will move through Saturday night and lingering upper air disturbance may affect us Sunday morning with clouds.

There’s uncertainty regarding the amount of cloud cover this weekend.

Average high temperature is 53-54º for the coming week.

Saturday

Some cloudiness in the morning behind the departing cold front may last until noon. High pressure builds in for sunshine through high, thin cloudiness in the afternoon.

High temperature 50.1º sd 1.1º ( NBM model location- Blue Bell, PA)

Sunday

Today’s GFS shows some upper air disturbance with moisture lingering Sunday morning. So some cloudiness is expected Sunday mid day. The NBM is more optimistic about sunshine for the day except areas far northwest of the city. It will be cold and windy.

High temperature 39.6º sd 2.4º ( NBM model location- Blue Bell, PA)

Winds
Today’s 19z NBM wind meteogram for location Wings Field Blue Bell PA (Click on image for a larger view.)