Tag Archives: Rex Block


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Shower Forecast Update

Update Fri 06/02 @ 9:10 AM — Despite reasonably high CAPE and thermal instability, the lack of soil moisture feedback and marginal available precipitable water (PWAT= 1.3″) will result in a low degree of shower activity Friday afternoon.

Very widely isolated/scattered activity as early as 3-4 PM lasting until about 2 AM Saturday morning.

Here’s the HRRR total accumulation forecast—

HRRR total precipitation by 2 AM Saturday. Totals are in the range of 0.00- 0.16 inches (Click on image for a larger view.)

Outlook for Saturday— periods of clouds with periods of sunshine. Low probability of any showers. Cooler.

Friday Forecast Update

Update Thu 6/01 @ 10:38 PM —Tonight’s models just coming in show widely scattered showers and thunderstorms moving down from the north Friday afternoon. Peak time about 5-6 PM but possible as early as 3 PM. Widely scattered is the operative word. There’s enough CAPE and thermal instability, but not very high available moisture. Minimal shear and little jet stream support. Not a drought buster.

Update Thu 06/01 @ 4:30 PM — Most models are forecasting high temperatures around 92º-93º on Friday afternoon.

That said, the ECMWF, German ICON and Canadian HRDPS are showing some suburbs of Philadelphia reaching 95º-96º.

The NBM which is designed to iron out differences in the model forecasts is showing 92º but with a high standard deviation of ± 3.2º.

Interestingly, the dew point temperatures will be low, so the heat index (“apparent temperatures”) will likely be lower than the air temperature.

NBM Apparent Temperatures at 3 PM Friday (HEAT INDEX) (Click on image for a larger view.)

It appears that a lack of soil moisture and precipitable water may significantly reduce thunderstorm development and coverage Friday evening. Much needed rain doesn’t currently appear to be a likely possibility in many areas.

Possibly some thunderstorms early Friday evening?

Update Wed 05/31 @ 5:35 PM — The incredible weather we’ve been having is really quite perfect, BUT there’s been a significant deficit in rainfall in may areas, especially from Philadelphia and the suburbs immediately westward.

The backdoor cold front expected Saturday could produce some light showers.

Today’s higher resolution models are suggesting the possibility of some thunderstorms late Friday afternoon or early evening in advance of the front.

(There will be plenty of heat and CAPE available for thunderstorm formation on Friday. Precipitable water is expected to increase to 1.7″. Missing will be soil moisture, which tends to enhance convective activity.)

The NAM-NEST model is showing a few scattered storms early Friday evening, mostly to our far north and more later in the evening.

Just out of curiosity, I took a look at the experimental RRFS (Rapid Refresh Forecast System) model under development and scheduled for release the end of this year.

Here’s a comparison of the NAM-NEST and the experimental RRFS at 6 PM Friday—

The RRFS is showing a thunderstorm cluster over Philadelphia Friday at 6 PM. It will be interesting to see if this model forecast holds. (Click on image for a larger view.)

Ninety Degrees Plus on Friday. Light showers possible Saturday

Update Wed 05/31 @ 9:39 AM — The upper level ridge will move eastward enough on Friday to give us some true summer temperatures. There’s a range of forecast high temperatures, but here’s the latest ECMWF and NBM to give a sense of what’s expected—

Lastest ECMWF high temperatures forecast for Friday (Click on image for a larger view.)

Here’s the NBM model version 4.1 forecast which is a weighted average of over 40 models, weighted based on the model’s most recent 6 hour performance—

Today’s 12z NBM model high temperature forecast with standard deviation shown as ±. (Click on image for a larger view.)

Dew point temperatures will be relatively low (60º or less) so the actual “apparent temperature” (“heat index”) will not be that high (less than 90º in most areas.)

For Saturday, a back door cold front moves through. Most models are showing light showers mid-day with about 0.2-0.3″ of rain. Not very much, but we’ll take it. With soil moisture so low, we’ll see if we even get that much.

Blocking Pattern Retrogrades. Expect 90º on Friday

Update Tue 05/30 @ 11:50 AM — Not only is the blocking pattern expected to persist this week, but the latest GFS has joined the ECMWF in showing a deepening of the upper low over the Newfoundland and a retrograde of the entire block westward—

ECMWF jet stream forecast for Saturday shows a strengthening of the upper level low near Newfoundland over with it extending further southwestward (retrograde) with the entire block circulation retrograding westward.

Following 90+º temperatures here on Friday, the retrograde will cause a backdoor cold front to move across us Saturday. The last two days of the ECMWF model shows a bullseye of rain with this backdoor cold front over us on Saturday. The GFS has joined the ECMWF with this forecast, but has significantly less rain for us.

Following the backdoor cold front, we cool down significantly for Sunday.

Previously Posted Mon 9:24 AM —

Blocking Pattern Entrenches

It’s been over three weeks since the immediate Philadelphia area has had any significant rain.

While the term “pattern” is sometimes loosely used regarding weather changes, this is beginning to look like a true persistent pattern for our area, all the result of a relatively unchanged, blocked upper air configuration.

Over past weeks, I’ve used the terms Omega Block and Rex Block to describe the upper air configuration that is resistant to change.

The latest ICON ensemble (ICON-EPS) clearly shows we’re heading into a combined Rex block and large scale Omega block for the end of this week—

ICON-EPS (ensemble) mean 250 mb Heights (jet stream summer level winds) forecast for this Friday. (Click on image for a larger view.)

The GFS ensemble (GEFS) shows a similar Omega block configuration without the Rex Block—

GEFS 250 mb mean Wind forecast (summer jet stream level) (Click on image for a larger view.)

What does this all mean? Well, what we have weather-wise is essentially what we will be getting through at least the next week or so— Beautiful, cooler to near average weather with little-to-no chance of rain.

The center of the US will be in the very warm (hot) sector of the block, but we’ll be in the cooler part of the block, influenced by the upper low pressure system.

As I’ve expressed over the past week or two, I’m concerned that this may progress into a significant dry period (short term drought) for the immediate PHL region.

Patterns change, often without advance notice, but for now this is what we’ll see. The NAEFS suggests we may get some light rain the end of NEXT week (June 7-10th time frame.)


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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.