Tag Archives: Omega Block

THIS WEEK’S WEATHER

Monday, May 29, 2023 9:24 am GFS

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

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—

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.)

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Friday, April 28, 2023 5:19 pm GFS

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Sunday Forecast Update

Update Sun 04/30 @ 4:24 PM — Areas of very heavy rain continue, especially in New Jersey, where large areas have received close to 5″ of rain since Friday. An additional inch or more is possible!

The latest HRRR shows some heavy downpours with thunderstorms possible between 8 PM and midnight ahead of a cold front passage.

The actual cold front will move through between 10 PM and midnight and periods of heavy rain will continue through the frontal passage.

Wind gusts may exceed 55 mph in some areas around midnight as a pressure gradient develops with the departing low pressure system.

Update Sun 04/30 @ 9:52 AM — The rain has moved in as expected and periods of rain, heavy at times, will fall throughout the day and evening hours. Total accumulations for the day will be in the 0.9″ to 1.5″ range with some locally higher amounts.

Each model has a somewhat different axis location for the heavy rain but the HRRR, which did best Friday night, now has a somewhat different heavy rain axis than it had last night—

Latest 12z HRRR total rain accumulation by midnight (Click on image for a larger view.)

A strong cold front will move through about 10 PM to midnight tonight. The HRRR and especially the RAP model are showing some low CAPE values (300 J/Kg).

Moderately high shear values (30 1/s) with that CAPE suggests the possibility of thunderstorm formation between 6 PM and 10 PM ahead of the front. Strong wind gusts of near 40 mph are possible. No tornadoes expected and it’s currently a low severity risk.

Forecast Update and the Broad Street Run

Update Sat 04/29 @ 10:25 PM — A quick update. The precipitation forecast from this afternoon remains intact. It showed fairly heavy rain through 2 PM.

The latest models tonight show heavier rain and possibly stormier weather during the afternoon and early evening hours on Sunday. As a cold front approaches Sunday evening, some thunderstorms can’t be ruled out.

Here’s the updated HRRR rain totals forecast through 8 PM Sunday evening—

Tonight’s 00z HRRR rain totals forecast through 8 PM Sunday. Many areas will see 1 to as much 1.5″ additional rain. (Click on image for a larger view.)

Update Sat 04/29 @ 4:51 PM — Looking back to yesterday’s model forecasts, yesterday’s 18z HRRR model was most accurate with its precipitation amounts and the axis of heavy rain in NJ.

Today’s 18z HRRR is now available and here’s its forecast amounts for the period now through 2 PM Sunday with special emphasis on the Broad Street Run location—

Today’s 18z HRRR model forecast total rainfall for Sunday at 2 PM. (Click on image for a larger view.)

Update Sat 04/29 @ 10:12 AM — The rain has tapered off a few hours earlier than forecast, but light drizzle remains. The latest 12z NBM still shows light drizzle through much of Saturday afternoon. The latest 12z RAP and HRRR also show a few very widely scattered showers into the evening.

The rain totals from yesterday’s NAM-NEST weren’t all that far off, but as is often the case, the exact location of the heavy rain axis was different than forecast, in this case further east. Here’s the MRMS based rain totals so far—

MRMS-based 24 hour estimated rain totals through 8 AM Saturday. (Click on image for a larger view.)

As for Sunday and the Broad Street Run, the current updated forecast is for the rain to move back in about 8-9 AM. Light and scattered at first, then moderate to heavy rain through much of the daytime. Another 0.30″ to 1.5″ expected.

If any consolation for the runners. the latest 12z HRRR shows only 0.33 inches of rain for Broad Street during the day, with the heaviest rain falling to the west and east—

Today’s12z HRRR accumulated rain forecast for Sunday. (Click on image for a larger view.)

As we saw today, the models aren’t always that good with forecasting the placement of the heaviest rain axis, so don’t hold me to that ‘lighter” 0.33 inches along Broad Street.

Update Fri 4/28 9:53 PM — It looks like the steady rain ends around noon on Saturday. Very widely scattered showers follow through the evening.

Previously Posted Fri 5:19 PM —

As outlined in several posts this past week, a blocked weather pattern will keep the major determinants of our weather relatively unchanged from today, Friday, through possibly next Thursday!

Blocked patterns are somewhat common in springtime and for this pattern, our area falls on the chilly and wet side of things —

Rainfall from Friday afternoon through Saturday afternoon will total 2-3.9 inches of rain according to the latest NAM-NEST—

Today’s 18z NAM-NEST total rainfall through Saturday evening. Additional rain on Sunday! (Click on image for a larger view.)

It should be noted that the NBM has much lower precip totals for Saturday, on the order of only 1.5″. The NBM shows about 3″ total for the weekend.

I should also note while the models have been consistent with this weekend’s rainy forecast, these type of weather scenarios driven by upper air low pressure systems often change dramatically in the short-range. Already, the rainfall for the first half of the weekend is forecast to be more substantial than the rainfall predicted for Sunday, a significant change. Stay tuned.

Saturday

Cloudy with continued rain. Rain may taper off far south and west of the city late afternoon, but continued spotty rain/showers are forecast for much of Saturday afternoon from the city, north and west. Windy and gusty.

High temperature 56.5º sd 2.5º ( NBM model location Blue Bell, PA)