Updated Fri 8/05 9:52 AM —We had some showers Thursday evening, more widespread than had been forecast by the models.
For today, Friday, there’s a range of forecasts, with many models showing little in the way of rainfall to as much as an inch. The trend is for not much. Timing varies from as early as mid to late afternoon, to storms moving through late evening and even after midnight.
I’m curious whether the new, experimental limited area model clarifies the forecast. It has maintained a forecast of a moderate cluster of storms early evening.
Updated Wed 9:38 PM — The forecast is unchanged for Thursday. Very hot and humid. A few scattered thunderstorms my pop up after 5 PM, but with low vertical shear, they won’t last long or cover much territory.
A cold front approaches on Friday. A large cluster of thunderstorms will develop in western suburbs and move into the city between 6 and 8 PM. Some heavy rain possible with these storms. Shear values will again be low, so we’ll have to see if the models are getting this right.
Update Tue 08/02 @ 8:20 PM — Not much happening except increasing heat. A weak front moved through about 1 PM today. A few isolated showers broke out with the frontal passage (not predicted by the models.)
The increasing temperatures for Wednesday and Thursday are well-advertised by the regular radio and TV media.
Here’s some specifics info for Thursday based on the current NBM model —
19z NBM Temperature Dew point meteogram for Blue Bell PA for Wednesday through noon Saturday. (Philadelphia tends to run a bit hotter by 2-3º) (Click on image for a larger view.)
High Temperatures Thursday—
NBM forecast high temperatures for Thursday. The “±” temp is the standard deviation of the over 20 models that comprise the NBM. Most areas are over 95-100º (Click on image for a larger view.)
Here’s the NBM forecast Heat Index (“apparent temperature”)—
NBM forecast max HEAT INDEX (apparent temperature) for Thursday about 2:55 PM. Most areas over 100º (Click on image for a larger view.)
Next chance for showers/storms: possibly Thursday night, but most likely Friday afternoon and or evening.
Update Mon 08/01 @ 1:21 PM — We finally had some large scale rain in the area last night, but amounts were overall only in the 0.4-0.6 inch range with some localized higher amounts and some areas with little—
MRMS 24 hour rain accumulation through noon Monday. (Click on image for a larger view.)
A weak front moving through Pennsylvania today (Monday) will fall apart as it moves towards the Delaware Valley with all showers associated with the front forecast forecast to die out.
For Tuesday and Wednesday: Temperatures will approach and exceed 90º but the dew points on Tuesday will drop during the afternoon and remain in the tolerable 60s on Wednesday.
The severe heat and humidity return on Thursday before a cold front slowly sinks south Thursday night with showers and thunderstorms possible. It’s possible that we’ll again see some anemic rainfall with this front.
Previously Posted Sun 5:45 PM —
The large hot air mass in Texas and the Midwest that has been making headlines for the past several weeks will be shifting northward and eastward and influencing our weather this coming week.
A warm front will move northward tonight (Sunday) and bring some showers and possibly thunderstorms after midnight and into the early morning hours of Monday—
Water Vapor with drawn warm front Sunday 5:40 PM (Click on image for a larger view.)Today’s 18zHRRR precip forecast by 5 AM Monday morning. It would be great if we get this much rain. The model forecasts regarding amounts and location can’t every be taken literally. (Click on image for a larger view.)
We’ve been in a mini-drought situation and past precipitation forecasts have fizzled. Unlike recent cold front passages, this will be a warm frontal passage that may not succumb to the issues that’s been reducing rainfall here recently.
While the HRRR shows some nice precipitation amounts, the experimental FV3-LAMDAX model (under development) shows somewhat lower amounts.
(With the recent weather pattern we’ve been in, the HRRR and the LAMDAX have been the best performing models according the NOAA’s Model Verification Web site.)
The rest of the week, we’ll be under the increasing influence of upper level high pressure—
GEFS forecast for Thursday showing jet stream winds located north in Canada with the 576 (500-1000mb) “thickness”(orange) contour showing the outline of the hot air mass extending eastward. Notice this contour encompasses most of the country! It will be extending well into Maine and into Montana and Dakotas. (Click on image for a larger view.)
At the same time as the hot air is moving towards us, a series of weak fronts associated with low pressure systems in Canada will affect our weather, especially late Thursday and Friday. More about that later in the week.
Update Fri 07/29 @ 7:44 AM — Last night, our area received varying amounts of much-needed rain. Here’s the amounts we received Thursday evening according to the MRMS—
MRMS accumulated 12 hour precipitation (Click on image for a larger view.)
For this evening (Fri), the models are less enthusiastic about additional rain. The HRRR has much of the area with just spotty amounts, many areas with no rain. The HREF forecasts some rain, but scattered with light amounts. Not a drought-buster this evening.
The HREF has any showers/thunderstorms beginning this evening around 5-6 PM (a few isolated showers before then) and continuing through 11 PM or so.
HREF shows light, scattered showers—
HREF 1 hour precipitation at 8 PM this evening. Exact placement should not be taken literally with weather models. This is a 1 hour snapshot. Additional rain through 11 PM. (Click on image for a larger view.)
My Weekend Weather Forecast will be posted this evening.
Thu @ 8:50 PM —Forecast Review— The storms tonight came in fairly close to the time forecast of 6-9 PM. (This morning’s forecast had been based on the HREF and the 06z HRRR.)
Much more [needed] rain than had been forecast and some of the storms are blossoming in Philadelphia. The activity is still expected to dye off as it moves into NJ as vertical shear values drop off significantly.
MRMS live capture at 8:40 PM (Click on image for a larger view.)
Update Thu @ 9:14 AM — We did have some showers shear off to the northwest last night, as some models had forecast—
MRMS Precip totals show about the showers just brushed the northwest corner of Philadelphia early this morning. Far northwest areas enjoyed some much needed rainfall (Click on image for a larger view.)
The models are advertising another round of showers and storms Thursday evening (tonight) about 6 PM- 9 PM. These showers are more likely to make it through more of Philadelphia before they diminish over NJ.
Latest HREF forecast for 7 PM this evening—
HREF simulated Radar and accumulated rain (contours) forecast for 7 PM this evening. (Click on image for a larger view.)
I am a bit more confident that some accumulating rain will make it into Philadelphia and immediate areas, although amounts may be small, variable and very non-uniform.
Why am I more confident? First, several models have this forecast, including the model blend (NBM). A new model under development (FV3 LAMDAX) correctly predicted last night’s showers and it is predicting the showers making it into Philadelphia. Predicted vertical shear values are somewhat higher tonight in our immediate area as well.
Finally, the 576 thickness line is flat and is no longer anticyclonically curved in our area as has been the case for several weeks now —
RAP forecast 576 thickness line shows flat contour instead of the anticyclonic curvature of the past several weeks.
Working against the chance of rain is the continued low soil moisture and the relatively weak vertical forcing of the trigger for tonight.
Update Wed @ 8:13 PM — Not much happening. The dry spell continues and the models are over-forecasting rain in western PA as I write this.
Several models have some showers developing in our area around and after midnight. Current water vapor imagery shows weak disturbances to our west—
Water Vapor Satellite image shows some weak disturbances in western PA expected by some models to move in between midnight and 5 AM with light showers. (Click on image for a larger view.)
While some models have showers for us between midnight and 5 AM, the HRRR has them falling apart as they approach Philadelphia. I wish I could be confident in forecasting another 0.10 inches of rain tonight, but I’m not.
The dry spell (perhaps “mini drought”) has played havoc with the model forecasts over past weeks. Rainfall has been consistently over-forecast by the models.
There’s a front moving through Friday afternoon to clear out the weekend. The latest GFS has little to no rain for the Philadelphia area, while forecasting some for western areas and parts of NJ.
Updated Tue 9:46 PM — Tonight’s models keep any light showers well to our south. The dryness continues.
Revised Update Tue @ 8:29 PM — A weak wave in West Virginia along the stalled front will give us a chance of very light scattered showers towards daybreak Wednesday especially south of the city—
WV image with GFS contours Tuesday evening (8 PM) shows weak wave that may give us light showers towards daybreak. The model seems a bit fast compared to the water vapor, so any light precip may linger into Wednesday morning. (Click on image for a larger view.)NBM accumulated rain forecast for 5 AM Wednesday. The NBM may be overdoing the precip.
Very light precip (0.10 inches or less) is expected with this disturbance. Again, most of the rain will be south of us. Another may affect us early Thursday morning.
Update Tue @ 9:30 AM —There’s a stalled (stationary) front to our south. Low pressure waves will develop in the Midwest will move along it to our south. Precipitation from one wave will mostly pass to our south early Wednesday morning—
Current Water Vapor Image from 9:16 AM with superimposed RAP model contours. Low pressure along the stationary front will move eastward late tonight, mostly missing our area. (Click on image for a larger view.)Latest 12z HRRR forecast for rain early Wednesday morning. Much will pass to our south. (Click on image for a larger view.)
Update Mon @ 8:37 PM —The front that passed through Monday afternoon will stall to our south. Low pressure is expected to develop along this front and bring a chance of rain late Wednesday evening into Thursday morning. Not a sure thing, since the forecast for this time period has changed several times.
GFS forecast for Thursday at 1 AM Low pressure develops and moves along the stalled front to our south. Rain is predicted by both the GFS and ECMWF at this time. Most of the rain will fall as shown, just south of our area. Still, we’ll take what we can get. (Click on image for a larger view.)
Update Mon @ 5:48 PM — Most areas did not get very much rain today. In this pattern, “dry begets dry”. The current MRMS shows the actual rainfall we received in the area—
MRMS total precipitation past 12 hours through 5 PM. (Click on image for a larger view.)
It will take a large storm (hurricane or tropical storm) to change the current pattern and break us out of this dry spell.
Revised Update Mon @ 11:20 AM — Latest NAM-NEST has a line of storms moving through 3:30-5 PM. Cranks out 0.8+ inches of rain in some areas. Wouldn’t that be good!
The HIRESW has the line moving through as early as 2 PM
NAM-NEST simulated radar and accumulated rain (contours) at 4 PM. (Click on image for a larger view.)
Update Mon @ 9:30 AM — Based on the latest ECMWF model just available, I’m concerned that we will be disappointed with the amount of rain we get today, despite the forecast for heavy thunderstorms. The ECMWF has only 0.2 inches of rain falling (Blue Bell)
06z ECMWF 3 hour precipitation forecast at 8 PM. Notice the orange “thickness line” representing the edge of a mound of dry stable air that the storms will need to work against. Some areas north and south may get more rain, but I’m feeling that the rain amounts will be disappointing. Usually this pattern needs a major storm (e.g. hurricane) to break the pattern. (Click on image for a larger view.)
It’s rare that I hope I’m wrong about a forecast. This is one of those times.
Updated Mon 8:45 AM —Last night’s models from 2 AM (06z) explain last night’s uncertainties and differences in the timing of the showers and storms today. The showers/storms (if any) are forecast to come in two batches.
The first batch (as forecast by last night’s RAP and ECMWF models) will come through scattered at 1-2 PM.
The front is expected to slow down and eventually become stationary after it slowly moves through.
The second batch will result in weak low pressure developing along the slowing front, with more showers and storms between 7 PM and 11 PM.
The 06z NAM-NEST shows rainfall amounts as little as 0.4 inches of rain and as much as 1.5 inches of rain (in some lucky areas).
This type of two batch forecast occurred with a frontal passage in June, but the second batch was a dud, leaving us wanting for more rain.
Updated Sun 11:02 PM —Tonight’s models are having a rough time with tomorrow’s forecast. The RAP and ECMWF have showers as early as 1 PM. The NAM-NEST has showers and thunderstorms as late as 7-8 PM. The three versions of the HIRESW are all different; the HIRESW-FV3 has no rain for immediate Philadelphia area, with all showers passing to our north and south.
I’m leaning towards showers/thunderstorms between 2 and 6 PM with rainfall amounts on the low side. A low confidence forecast leaning towards the HRRR and HIRESW-FV3. We need the rain. I hope I’m wrong.
Previously Posted Sun 8:24 PM —
For several weeks now, I’ve noticed a trend towards very dry conditions here. I first discussed it here and then put together a more complete discussion here.
I’m hoping I’m wrong about this trend. Tomorrow, Monday, may put it to the test.
A cold front will move through between 2 and 7 PM Monday.
Water Vapor image at 4 PM Sunday showing cold front approaching NW Pennsylvania. This front will move south of our area Monday afternoon. (Click on image for a larger view.)
Based on the usual parameters of thunderstorm development, tomorrow’s setup would ordinarily provide some heavy thunderstorms and heavy rain. We have an approaching cold front, high CAPE values (~2800-3000 j/kg), high PWAT values (2.1″) and decent areas of vertical lift.
What’s missing is medium to high shear and reasonable soil moisture values (which are thought to provide positive feedback for thunderstorms.)
The models are predicting unimpressive accumulated rain for this frontal passage. The forecast average quantity of precipitation falling is only about 0.3-0.6 inches of rain. No torrents of rain here tomorrow.
Even more concerning…we may have even less rain that that. This afternoon’s HRRR and Canadian models have very little rain falling in the immediate PHL area—
Today’s 18z HRRR shows rain encountering that pesky ridge of high pressure as revealed by the 576 “thickness line” (orange contour.) This tongue of warm stable air jutting up into our area may further reduce the amount of rainfall we get tomorrow. This has been a recurrent theme for our area since the beginning of the summer. (Click on image for a larger view.)
Some models have the ridge eroding enough for showers to pick up in NJ. Some have the entire area of rain diminishing as it moves into Philadelphia and eastward. I guess we’ll see tomorrow
Update Thu @ 7:49 AM — A front will move through between 2 PM and 5 PM today. The front is clearly visible on current satellite water vapor imagery—
Water Vapor at 7:41 AM shows distinct front moving through central PA this morning. This front will move through here later this afternoon. The front represents more of a dew point change/wind shift than a temperature change. (Click on image for a larger view.)
Some models have a few very widely scattered showers and thunderstorms between 2-3 PM around Philadelphia and about 5 PM into NJ. Little in the way of accumulating rain is expected.
Today’s HREF forecast for rain accumulation 2-3 PM Thursday. This may be over-predicting the amount of showers. (0.02-0.20 inches of rain) (Click on image for a larger view.)
The frontal passage will bring lower dew points (less humidity) later this afternoon and evening. Higher humidity gradually returns by Sunday. The heat continues.
NBM meteogram for Blue Bell, PA temperature dew point and “heat index” (apparent temperature) through Sunday. Note that the dew points are lower Friday and Saturday. (Click on image for a larger view.)
Update Wed @ 8:09 AM — Temperature- dew point meteogram says it all—
NBM meteogram (temperature dew point, max temp and heat index (apparent temp) with standard deviations for Blue Bell PA (Click on image for a larger view.)07z NBM forecast Apparent Temperature (“heat index”) for Thursday at 2:55 PM. (Click on image for a larger view.)
The extended 90º+ stretch we’re entering into has been well-advertised by the media.
Current satellite water vapor image shows plenty of hot air streaming up from the southwest—
Hot air (orange-red) “conveyor belt” with heat and humidity streaming up from the southwest. Stalled frontal boundary (1) just to our south (Keep in mind that water vapor imagery is NOT showing reflected light from clouds. Water Vapor shows the temperature (and therefore the height) of the very top layer of substantial water vapor. In effect, it shows a three dimensional view of the atmosphere. Cold temperatures, shown in blue and green, indicate rising air — vertical motion. (Click on image for a larger view.)
This week’s jet stream is typical for a summer pattern, albeit the temperatures are well above seasonal averages. Our area has been in a consistent, slight upper trough since May and this trough and the current pattern is forecast to remain unchanged for the next 10 days—
GEFS 250 mb jet stream winds on Thursday. Slight upper trough over the Northeast and the hot dry H over the southwest has been with us for a few months/
The big item for us will be the heat—
NBM meteogram temperature/dew point/heat index for Blue Bell PA. Philadelphia is generally 2s-3º hotter. “Apparent Temperature” is the blue line (heat index) Dew points (magenta lines) are in the upper 60s to near 70. (Click on image for a larger view.)
There’s a chance of thunderstorms Thursday evening as a weak front move through.
