Autumnal equinox is set to arrive
this Wednesday 9/22 at 3:20pm…

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 snow, ice, or rain events, not publicly. To join this discussion and hear the updated thoughts from our team, visit The My Pocket Meteorologist Page by clicking HERE and select the “Premium Weather Forum” option.

Precipitation:

OVERVIEW: The month of September began on a very wet note with the remnants of the former Hurricane Ida interacting with an existing frontal boundary and dumping copious amounts of flooding rainfall across the entire region. This along with training thunderstorms exactly a week will end up skewing the monthly mean precipitation departures as a whole to finish above average, however it doesn’t tell the story from this point forward. Through the end of the month from the time this forecast was written, we expect near average precipitation, a trend which is likely to continue into at least the first half of October. As with any long range precipitation departure anomaly forecast, tropical systems and/or remnants that are currently indeterminable can skew monthly projections, as was the case with the September 1st heavy rainfall. At present time, there are no forecast tropical systems or remnants thereof in the next 7-10 days. Typically September takes a considerable step back in precipitation compared to the summer months as we begin seasonal transition.

Shorter term, we’re expecting very limited precipitation chances, and both are cold fronts within the next week. The first crosses the region Saturday night, and a stray/isolated shower or perhaps thunderstorm will affect a few locales on Saturday afternoon or early evening, but this activity will be very moisture limited, and likely a quick shower that doesn’t ruin outdoor activities for long. After several days of high pressure dominance, the next chance for precipitation will be ahead of a more organized cold front in the Wednesday-Thursday time frame. Showers and periods of rain are expected to accompany the front, which may move in as early as Wednesday PM and end overnight as the faster GFS suggests, or could wait until Wednesday evening to arrive and last into Thursday morning as the slower ECMWF suggests. Timing will be narrowed down in the coming days and updated on this website in the daily forecasts and daily forecast videos once the timing is better known.

The month of September we project will finish with above average (+1.0″ to +3.0″) precipitation departures for the month as a whole, with high confidence. The above average outlook is due to Ida influences earlier in the month, and training thunderstorms a week later that will end up skewing the monthly mean departures, but the remainder of the month is likely near average precipitation. Early look at the month of October suggests a near average precipitation (-0.5″ to +0.5″) anomaly for the month as a whole, using long range climate guidance and global observational trends. It does appear that the month of October will begin on the drier side of average, but may pick up slightly in the 2nd half of the month. The caveat to the September and October precipitation projections will be any tropical systems or remnants thereof that have the ability to skew projections higher in that instance. Projections for monthly precipitation departures considered this possibility, but assume no tropical remnants in order to paint the bigger picture of monthly precipitation.

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

September 18th – September 24th:  Near average

September 25th – October 1st: Near average

October 2nd – October 8th: Near to slightly below average

October 9th – October 15th: Near average

October 16th – October 23rd: Near to slightly above average

October 23rd – October 30th: Near average

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

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

A weak cold front will cross the region Saturday night, and in a few locations there will be a stray/isolated shower or perhaps a t-storm preceding it Saturday afternoon or early evening.

A more organized cold front on the lead of a trough will cross the region by Thursday morning, preceded by scattered showers and periods of rain ahead of the front as early as Wednesday afternoon.

Pattern:

OVERVIEW: Warmer temperatures in the nearer term will transition to slightly below average temperatures in the wake of a midweek cold front and a trough that settles in behind it. Temperatures then close out the month of September favoring the slightly cooler side of average. This will allow September monthly mean temperatures to also fall on the slightly cooler side of average by month’s end most likely. A very temperate outlook for the month of October as a whole with temperatures very close to seasonal average as average highs/lows continue to drop off in seasonal transition.

The Madden-Julian Oscillation (MJO) is currently straddling phase 3 (Indian Ocean) and phase 4 (Maritime Continent) and is expected to remain in near phase 3 over the next week. At the end of September and perhaps extending into the first half of October we may see a migration toward the Circle of Death (COD) as most guidance continues to suggest, but a migration into the Maritime Continent is possible. MJO waves are monitored for possible favorable periods of tropical cyclone development in the Atlantic. Climatologically speaking, August and September are typically the most active months for Atlantic tropical cyclone development, and there are two notable areas of interest currently with an elongated area of low pressure off of the North Carolina coast that may reach tropical depression or storm status as it moves NE and away from our region over the weekend. The latter is another broad area of low pressure moving across the south-central Atlantic moving toward the Lesser Antilles, which could attain tropical depression status itself over the weekend. Regardless, none of the three pose a credible threat to our area at the moment, which will be monitored going forward.

Average highs and lows are continuing a slow and gradual drop-off week to week. Normal high temperatures are currently in the 75-79°F range from north to south across our coverage region, which is about 2°F lower than last week at this time. Average highs and lows will continue to gradually decline over the next few weeks, and by about 2° per week in September, which accelerates to a 3° per week drop in October. Any above or below average stretches listed in the outlook will be relative to seasonal norms at that time, and it is important to note that cooler/warmer periods will be relative to the time of year we are talking about. For example, slightly below average temperatures listed late next week will generally feature highs in the upper 60s to lower 70s. Use the average highs/lows chart below that provides a point of reference for what average is for each corresponding date, with the examples used for the Philadelphia region.

The month of September we project will finish with near to slightly below average temperatures (+0.0°F to -2.0°F) when the dust settles September 30th with high confidence. Looking at the month of October as a whole suggests a near to slightly above average month (+0.0°F to +2.0°F) using long term climate models and seasonal/global observation trends.

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

Warmer than average temperatures are expected to continue for the next several days, with highs in the 80s to begin the period. Above average continues until the arrival of a midweek cold front.

The midweek cold front will be on the lead of the strongest trough we’ve seen thus far in September, and behind we’ll see a taste of Autumn almost on cue with the arrival of Fall September 22nd.

Outlook table last updated: Friday September 17th, 11:05 AM. Next scheduled update: Friday September 24th.

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:

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.

Images below are clickable for better viewing

Days 1-5 – confidence VERY HIGH

Days 6-10 – confidence HIGH

Days 11-15 – confidence MODERATELY HIGH

Days 16-20 – confidence MODERATE

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.

Forecaster: EPAWA Meteorologist Bobby Martrich
Discussion last updated: Friday September 17th, 11:05 AM