Update Thu 8/17 11:06 PM — Despite the jet streak explanation below (supported by tonight’s HRRR), tonight’s HIRESW closely matches current radar. It has a line of showers moving through around 5 AM Friday morning with some gusty winds.
Updated Thu 08/17 @ 5:32 PM —A cold front will move through about 4 AM -7 AM Friday morning.
As showers with this front approach from the north and west, they are expected to reduce in intensity or even fall apart as they approach the city and move into NJ.
Why will they reduce in intensity? We’ll be in the ‘left entrance region’ of a jet streak, which has descending motion—
ECMWF jet stream wind forecast for early Friday morning shows the Philadelphia area will be what’s referred to as the “left entrance region” of a jet streak (white box area) , associated with descending motion aloft and unfavorable for precipitation. (Ascending motion is needed for showers.) (Click on image for a larger view.)
Clouds clear during the morning and it will be sunny by afternoon. It will become WINDY. Highs 81º Blue Bell, 84º Philadelphia.
After another delightful day on Saturday, the heat dome affecting the central US will attempt to move into our area for Sunday.
Next week will have several unsettled days as disturbances move around the heat dome affecting our area. Several models are hinting at the beginnings of several tropical storms.
Thursday Forecast & Outlook
Updated Thu 08/17 @ 8:35 AM —Another sunny day! (A few pop up thundershowers possible about 4-6 PM. Most places will not see anything.) Another front will move through tonight (Thursday night) into Friday morning. Some showers and thunderstorms are expected to develop ahead of this front around daybreak Friday, but the models have them losing intensity as they approach the I-95 corridor. Clearing Friday for a very nice weekend.
This summer’s “heat dome” has moved eastward and is now expected to affect the central US. We’ll again be protected from the heat by a cyclonic flow from Canada.
The temperatures in the Central US will be intense by Sunday and Monday—
GEFS forecast for Monday. These are actual forecast high temps, not heat indices. (Click on image for a larger view.)
Updated Wed 08/16 @ 10:25 AM — The front that slowly passed through last night is lingering off of the coastline. Moisture will continue to move up along this front, affecting central NJ and the Jersey shore this afternoon. From Philadelphia westward, we’ll have partly to mostly sunny conditions. (Some of the HIRESW models have some widely scattered popup showers in our area, but these models can often overdo this.) See the image caption below for more details.
06zGFS forecast for Wednesday. Moisture continues to move northward along the coast near the stalled frontal boundary, affecting NJ today as high pressure builds in from the west. Notice the orange ‘thickness line ‘that winds down near Georgia and Arkansas and then up near Wisconsin. This is the boundary of the “heat dome” that will move closer to our area by Sunday and Monday. Before that time, a sharpening upper level trough will swing a cold front through here later Thursday through early Friday. (Click on image for a larger view.)Today’s NAM-NEST shows showers at 4 PM (Click on image for a larger view.)
Tuesday
Updated Tue 08/15 @ 9:20 AM — We’ll try again for today’s forecast. A cold front moves through later this afternoon. CAPE values are more elevated today than yesterday. The HRRR shows showers and thunderstorms forming in a line from Philadelphia and moving into NJ. (The HRDPS has the line forming a bit further eastward, excluding the immediate Philadelphia area.) PWAT is still in the 2 inch range, so some quick heavy downpours possible. Current time frame is about 4-6 PM near Philadelphia and then moving through Southern NJ.
06z HRRR shows hourly Precipitation rate and cloud cover at 6 PM. (Click on image for a larger view.)
Tue 9:07 AM Forecast Review — As suspected last night, the forecast for heavy rain last was a bust. Very little rain, certainly not heavy rain, fell across the Philadelphia region. Here’s the MRMS estimate of the rainfall we received—
MRMS rain-gauge/radar estimate of last night’s rainfall. Few areas received even 0.25 inches of rain. (Click on image for a larger view.)
The missing ingredient was elevated CAPE in our area, specifically something called ‘surface CAPE’.
As mentioned, the model guidance had shifted away from the heavy rainfall forecast during the afternoon model runs and had clearly shifted away (with the exception of the HRDPS and a few others) in the evening model runs. With other parameters elevated, it was difficult not to leave the original heavy rain forecast intact. That’s my excuse. I can’t account for the TV/radio forecasts that were still pushing the severe forecast at 11 PM.
Monday Storm Update
Update Mon 8/14 11:48 PM —Tonight’s HRDPS, just available, shows some intense storms developing across the immediate Philadelphia area about 3AM.
Updated Mon 08/14 @ 8:46 PM — As highlighted in my previous update, the immediate Philadelphia area may escape heavy rainfall and severe weather.
The latest HRRR and RAP model runs (these models come out hourly) are conspicuously missing any heavy rainfall and severe weather signatures in our area.
I could be fooled by something called ‘model spin-up’ errors. Spin-up errors occur because models take time to approximate a realistic forecast, often several hours. Since the event is occurring imminently, the heavy rain and severe weather signatures may not be showing with the latest models. (But what is also perplexing is the lack of activity on radar and satellite, except in central PA. Will that move eastward or more likely northeastward, missing us?)
It is possible the severe weather forecast for regions just outside of the immediate Philadelphia area tonight is a bust. I guess we’ll know tomorrow.
Monday Storm Potential Update
Revised Update Mon 08/14 @ 7:08 PM — Showers and thunderstorms will move in about 9 8 PM.
Today’s models continue with very high PWAT (2.3″ to 2.4″) and moderately elevated CAPE. Heavy rain potential (localized) is likely in in our area; Severe weather may occur later, after midnight, in some locations (see below). The immediate Philadelphia area seems to escape the heavy rainand maybe the severe weather.
Areas to our west, immediate south and immediate east may have the stronger to severe storms.
The HRRR model total rainfall this evening gives an idea of where the most significant storms will be—
Today’s 18z HRRR showing total rainfall prediction for this evening. Red Circle is Philadelphia, not in an area of likely severe storms. Delaware remains an area of likely significant heavy rainfall. Most models show this fork in the precipitation. Unfortunately, we’ve seen that the models are limited when it comes to accurately predicting the exact locations of heavy rainfall.
Some of the severe weather may develop in northwest suburbs after midnight. The HRRR shows very high vertical shear about 2-3 AM in our northwest suburbs—
HRRR shows highly elevated vertical shear at 3 AM in northwestern suburbs. Coincides with an increase in CAPE (Click on image for a larger view.)
Tornadoes are possible after midnight. Very elevated helicity is forecast to move up east of us in NJ.
HRRR forecast Helicity at midnight. This area in NJ may move northeastward. Helicity values near 800 /sec are impressive and tornadic activity is possible. . (Click on image for a larger view.)
Monday Update
Updated Mon 08/14 @ 9:49 AM — There’s been a shift towards a later onset of showers/thunderstorms Monday evening. It now appears that showers and storms will move in about 9PM to 11 PM and continue through 4AM. Unchanged is the biggest threat: heavy rain. CAPE values suggest that storms could be strong but not necessarily severe in our immediate area. The heaviest rain/activity in northern Delaware and near the Jersey Shore.
Previously Posted Sun 7:54 PM —
A warm front will approach on Monday and move through our area Monday night. A mix of sun and clouds on Monday, increasingly cloudy towards evening. The warm front and associated cold front are part of two rapidly moving systems that are currently in South Dakota and Kentucky right now—
The showers/thunderstroms associated with the warm front will move in between 7PM and 9 PM. The main feature of this system will be a very high precipitable water (PWAT). Many models are showing PWATs of 2.3″-2.4″ which translates into a very heavy rainfall potential.
Here’s the current HRRR forecast rainfall for Monday night—
Today’s 18z HRRR shows areas of heavy rain 1-3+” for Monday night.
As for severe weather, predicted CAPE values are in the moderate range and the the greatest likelihood for severe weather is northern Delaware and South Jersey. Nonetheless, the HRRR is showing a few areas of highly elevated helicity nearer the city, so some severe weather here can’t be ruled out.
(We’ll have to wait for tomorrow’s 06z HRRR to get a better handle on things.) Check back tomorrow morning.
A cold front will move through on Tuesday morning, possibly with some more showers, although they’re not looking that impressive right now.
