Monthly Archives: April 2023

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#weather #paweather #wx #pawx #philadelphia #phillywx

Weekend Outlook Update

Update Fri 4/14 7:30 AM — Upper low pressure, part of the Rex Block pair, will move up over us on Saturday and spawn a weak surface low pressure system off the coast. Showers and thundershowers are expected early Saturday morning and will increase in coverage during the afternoon.

The weak low pressure system is expected to linger Sunday. It should be dry but the GFS is forecasting considerable cloudiness on Sunday. I’m thinking that Sunday will be have more sunshine than currently forecast.

I’ll be posting my regular Weekend Weather Forecast later today where I’ll try to nail down the the thunderstorm potential for Saturday and the Sunday’s weather.

Previously Posted Wed 5:56 PM —

Weekend Outlook

The Rex Block that has given us this extended period of beautiful weather has moved into its full position as forecast earlier this week—

Water Vapor image Wednesday afternoon (with superimposed RAP model 250mb wind streams shows the upper level High (H) and the upper level low (L). Exceptionally dry weather in this area, while the southeastern US had plenty of clouds and rain. The upper low over Louisiana will move up towards us on Saturday. (Click on image for a larger view.)

The upper level low approaches from the southwest on Friday. The upper winds shift from westerly to southwesterly and we’ll have an increase in upper level moisture.

There will be a noticeable increase in high cirrus cloudiness during the day on Friday.

The upper low moves up over us early Saturday morning and a warm front is expected to move to our north with an increase in dew points and humidity. (Dew points are currently in the dry 40s. On Saturday, they’ll be in the low 60s.

Showers and even some thunderstorms are looking likely on Saturday.

There are model differences in the forecast position of the upper low on Saturday. Here’s the current ECMWF foreast at 2 PM—

Wednesday’s 12z ECMWF 3 hour accumulated precipitation forecast for Saturday at 2 PM. (Click on image for a larger view.)

A cold front approaches later on Sunday. The forecast timing has been changing with this front and it looks like it will move through closer to Sunday night into Monday.

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#weather #paweather #wx #pawx #philadelphia #phillywx

Rex Block Takes Shape

Update Tue 04/11 @ 4:37 PM — We had a layer of high cirrus clouds move in today and they should move off to our northeast on Wednesday.

The Rex block is taking shape in the middle of the country. Here’s the current Water Vapor Satellite image —

Water Vapor mid level image at 4:30 PM. The high level cloudiness we saw today was from a disturbance to our north (Blue arrow) . This will move eastward. The upper level high (H) should become north of a closed off upper low (L) on Wednesday, forming a Rex Block. This will keep our weather delightful through late Friday. The upper low will slowly move up to the Carolinas on Saturday, affecting our weather. (Click on image for a larger view.)

Update Mon 04/10 @ 8:04 PM — Continued beautiful weather through Friday. Very warm but low humidity.

NBM High-low temperature forecast (see below for interpretation)—

NBM Text products Temperatures for Wings Field Blue Bell PA (KLOM) (Click on image for a larger view.)

A warm front moves through Saturday with some showers, clouds and an increase in humidity. A cold front moves through Sunday with thunderstorms.

Previously Posted Sun 5:06 PM —

“Rex Block” Will Keep Things Quiet This Week

A recognized upper air pattern for Spring (and Fall), is setting up for this coming week and it will keep our weather quiet for all of the week with increasingly warm temperatures.

A “Rex Block” (named in the 1950s for a meteorologist, Daniel Rex) will develop in the middle of the country and the jet stream will be sending any disturbances far to our north. The Rex block, with an upper level high just north of an upper level low, tends to be a stable configuration and blocks any west to east movement of the usual dips in the jet stream.

Here’s the GEFS model forecast for Wednesday afternoon of winds at the 250 mb level (jet stream level). The upper circulation is notated with red arrows, while the main jet flow (white arrows) is well to our north—

This week’s GEFS jet stream wind forecast (250 mb level) for Wednesday afternoon. The Omega Block is shown- A stable upper levle configuration that blocks the usual progression of dips in the jet stream flow. We’ll be in the increasingly warm ridge for this week.
Temperatures- NBM Model Text Output Products

Here are the high/low temperatures expected this week from the NBM model. You can use this link to get your own data— NBM Model Text Output—

The NBM model output text for KLOM (BLUE BELL) is a bit confusing unless you understand the meaning of the columns and how they define the high temperature period. TXN is the max and minimum temperature row. The HIGH temperature is is in the 00z column of each day. Example: For Tuesday’s high temperature— Since the high temperature is for the 12 hours preceding 00z on Wednesday, the 71º indicated on Wednesday’s 00 column really refers to TUESDAY’S HIGH TEMPERATURE. ( 00z Wednesday is at 8 PM EDT Tuesday evening . ) Similarly, the 82º high temp in the Friday 00 column is the high temperature for THURSDAY afternoon, since 00z Friday is at 8 PM EDT Thursday. Strictly speaking, it makes sense, but it isn’t at all intuitive. The XND is the standard deviation, rounded. The FHR is the Forecast Hour.