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Drier weather settles into the region
as we approach the Autumnal Equinox
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Long Range Outlook

Important information:

  • Ongoing discussions and comprehensive model analysis ahead of any storm threat is conducted exclusively in the EPAWA Premium Forum, and not publicly until a threat is imminent. To become part of the advanced discussion, and to get long range updates on Mondays and Wednesdays in addition to this Friday public update, visit http://epawaweather.com/my-pocket-meteorologist/ and select “Premium Weather Forum”.
  • The EPAWA app is available and is completely FREE and does not require a download from the app store. This is a progressive web app that replaced the old app in December 2017. From a mobile device, simply open your browser and go to https://m.epawaweather.com/ and be sure to accept notifications and allow the app to detect your location for full functionality of the app. You then have the ability to save the PWA to your home screen, and it will work similar to a native app.
Screenshot (1)
DatesThreatsSignificanceConfidence
9/15-9/19TemperatureSlightly above averageVery high
9/20TemperatureNear averageVery high
9/21-9/22TemperatureSlightly above averageHigh
9/23-9/26TemperatureNear to slightly below averageModerately high
9/27-9/30TemperatureNear averageModerate
10/1-10/7TemperatureSlightly below averageModerate
10/8-10/14TemperatureNear averageModerately low
9/17-9/18Florence remnantsMultiple guidance suggests the remnants of Florence, at that time post-tropical, will move through the area from late Monday through Tuesday. The most aggressive ECMWF brings a large swath of 2-4"+ rain as the low tracks over PA and Northern NJ, and the GFS tracks it a bit farther north and weaker, bringing more of a 0.5-1.5" rainfall to the region.Moderately high
9/22Cold frontA cold front ahead of a trailing trough will move through the region next Saturday, and could be accompanied by scattered showers and thunderstorms. Timing is still variable.Moderate
September as a wholeTermperatureSlightly above average (+2.0°F to +4.0°F)Moderately high
September as a wholePrecipitationSlightly above average (+1.0" to +2.0")Moderately high
October as a wholeTemperatureSlightly above average (+1.0°F to +2.0°F)Moderate
October as a wholePrecipitationNear average (-0.5" to +0.5")Moderate

Outlook table last updated: Friday September 14th, 11:15am. Next scheduled update: Friday September 21st.

This is a weekly updated public long range guidance product from EPAWA. For daily long range updates and more detailed updates M-F, please join the EPAWA forum. More information/sign-up at: http://epawaweather.com/my-pocket-meteorologist/ 

*Indications of above or below average temperatures in the table above are relative to what is considered “normal” using data collected over the long term for a particular date. This is collected and maintained by the National Climatic Data Center in conjunction with the National Weather Service actual data from previous years collected at official ASOS/climatology stations across our coverage area. Also note that as time moves forward into a different period as shown above, average temperatures for those dates will also change. See the example below using Philadelphia, PA as the climo station:

DateAverage Hi/LoEPAWA projection
September 15th78°F/61°FSlightly above average
September 22nd76°F/58°FSlightly above average
September 29th73°F/55°FNear average
October 6th70°F/52°FSlightly below average

The departure from normal uses the average temperature for the date, averaging temps over 24 hours for any given location, using both high and low temperatures hourly during any particular day. This outlook determines warm vs. cool periods relative to normal temperatures.

Long range analysis: Technical discussion is below for advanced readers:

Technical discussion below will feature two (2) subcategories: Precipitation, and Pattern Discussion. Storm possibilities will be discussed exclusively in the Premium Forum with intense model analysis leading up to any major rain or severe events, not publicly. To join this discussion and hear our updated thoughts from our team, visit The My Pocket Meteorologist Page by clicking HERE and select the “Premium Weather Forum” option.

Precipitation Discussion:

Precipitation:

The period begins on a drier note for the upcoming weekend. Hurricane Florence will remain inland across the SE US and well SW of the region as it transitions from hurricane to tropical storm, and then a tropical depression. As high pressure exits the NE US early next week, Florence will turn to the NE and its remnants will likely move through our region in the late Monday through Tuesday time frame as a post-tropical low. The good news is, it will not stall over the region for days and will keep moving. But potentially bad news is, it could dump heavy rain over the region that really doesn’t need it. The GFS and ECMWF differ on track and intensity, which is making the difference in rainfall projections. The more aggressive ECMWF suggests a track directly over Pennsylvania and Northern NJ, bringing a swath of 2-4″ with locally higher amounts of rain. The less aggressive GFS suggests a weaker system on a more NW track, moving from the eastern Ohio Valley into southern New York, which brings a modest 0.5″-1.5″ of rain in our coverage area. We will continue to monitor closely and rainfall projections will be updated in the local forecasts and daily forecast videos. At this point, expect the GFS solution at a minimum.

A cold front accompanying the lead trough of cooler air will arrive around next Saturday, and looks to be a quick-hitting chance for scattered showers and thunderstorms. Timing is still in question, and can deviate at this range. Again, follow local forecasts and daily videos next week for updates.

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Breakdown of precipitation departures from normal over the next several weeks:

September 17th-September 23rd: Above average

September 24th-September 30th: Near to slightly above average

October 1st-October 7th: Slightly below average

October 8th-October 14th: Near average

Public/free available maps will be updated when a threat is imminent for significant weather or snowfall on the weather alerts page throughout the Winter:  http://epawaweather.com/weather-alerts/

 

Note:  Any image in the outlook is clickable for larger viewing

More aggressive models track the remnants of Florence over PA and northern NJ in the late Monday through Tuesday time frame, and suggests 2-4″+ of rainfall potential with a more amped post-tropical system.

ecm2

The GFS suggests a less aggressive outlook from remnant rainfall from Florence, as it tracks a weaker post-tropical system well to the NW of our region. Rainfall is more on the order of between 0.5″ and 1.5″ from the remnants. 

gfs1

Pattern Discussion:

Pattern:

The Madden-Julian Oscillation (MJO): The Madden-Julian Oscillation is currently in the circle of death, and should remain there through the end of the 3rd week of September. The lack of tropical forcing will contribute to ridging maintaining in the upcoming week. Placement of this ridge will play a key role in the eventuality of Florence, and where it tracks near or over our region next week. The best tropical forcing is projected to return to the cooler phases of 8/1  octants toward the last week of September and cycles through phases 8/1 into early October. Combined with some El Niño-like atmospheric characteristics, this should lead to some cooler and lower humidity weather locally, especially during the first week of October. This time frame will coincide with fall foliage kicking into gear.

It is important to understand that average highs/lows will be steadily dropping in September, and any indications of slightly above average temperatures at that time are relative. For example, normal average high in Philadelphia on September 22nd is 76°F, so any slightly above average temperatures expected are likely near 80° or lower 80s. 90°F+ days will be hard to come by for the remainder of 2018.

September is projected to be a slightly above average month (+2.0°F to +4.0°F) when the dust settles at the end of the month, with a warmer than average 1st half of the month, and a near to slightly below average 2nd half. This is also supported by climate models and ensembles, with cooler risks shown on the ensemble guidance after September 22nd, but still near relatively close to average or slightly below average at that time. The initial outlook for October using the privately published Autumn Outlook from the EPAWA premium forum suggests a slightly above average month (+1.0°F to +2.0°F) with near average precipitation departures, and a slightly cooler than average start to the month.

Note:  Any image in the outlook is clickable for larger viewing

GEFS ensembles show generally slightly above average temperatures continuing through at least September 22nd aside from one or two days, which will continue until the next trough arrives around the 22nd/23rd.

gefs2

As we move past the 22nd, the ensemble guidance, both GEFS and the Euro/EPS suggest some cooler risks enter the picture. Most likely a near to slightly below average temperature stretch from late September to early October.

gefs3

Next 3 weeks of US temperature departures

Images below are clickable for better viewing

Days 1-5 – confidence VERY HIGH

1-5

Days 6-10 – confidence HIGH

6-10

Days 11-15 – confidence MODERATELY HIGH

11-15

Days 16-20 – confidence MODERATE

16-20

Current U.S. Soil Moisture Anomaly

Below is a look at the latest Soil Moisture Anomaly for the Contiguous United States. This is used by NOAA/CPC for the purpose of drought monitoring and drought outlooks.

This image will automatically update daily.

My Pocket Meteorologist

The My Pocket Meteorologist text alert program is a year-round program, and seasonal options for Spring/Summer will be available soon. Click on the image below for more information.

mpm3

Forecaster: EPAWA Meteorologist Bobby Martrich
Discussion last updated:  Friday September 14th, 11:15am