Dangerous heatwave this weekend
with heat indices over 110° for some Get the Forecast!

*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.

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Precipitation:

Precipitation has been largely few and far in between in term of synoptic events, and at the time this outlook was written on the 19th of July, measurable precipitation has only been found on one-third of those days. The problem is, some of those days involved slow-moving thunderstorms accompanied by heavy rainfall and flash flooding. As is the case in most summers, one or two synoptic events can skew the monthly departure numbers, but doesn’t capture how the month largely played out unless you look at it on a daily basis. As such, most areas affected by flooding rainfall on back to back Thursdays are well above normal in the rainfall department, but this doesn’t tell the broader story. Going forward, we have limited rainfall events as a whole, and aside from Monday’s cold front noted in the chart above, a fairly benign and tranquil precipitation pattern is projected. Precipitation departures from average were maintained after last week’s adjustment in the chart to account for the areas hit hard by flash flooding on a few of those days.

Although there could be an isolated shower or storm over the weekend closest to the NY/PA border, largely most areas are rain free, very hot and humid this weekend. A cold front approaches on Monday and will bring the chance for numerous scattered showers and thunderstorms along and ahead of the frontal boundary, mainly Monday afternoon and evening…with some residual showers/storms overnight until the front completely clears to the south. What follows is lowering humidity and much more pleasant temperatures and conditions on Tuesday and Wednesday. and a welcome break from the weekend heatwave.

Precipitation we maintained for the month of July as a whole to near to slightly above average with the recent synoptic events that caused flooding in some areas, while others largely missed out on that. Aside from a cold front passage Monday/Monday night, the overall pattern outside of this event is largely drier than average for the remainder of July. Early look at August precipitation projections would suggest a near average month (-0.5″ to +0.5″) and we have maintained that since the previous  updates with ECMWF support. As is the case this month, any localized synoptic event can change that, especially on a hyper-local level in summer.

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

July 22nd-July 28th: Slightly below average

July 29th-August 4th: Near to slightly above average

August 5th-August 11th: Near average

August 12th-August 18th: Near average

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

Over the weekend while it is very hot/humid and dry at most locations, the NAM 12km suggests a few areas closest to the PA/NY border could see an isolated shower or thunderstorm; best chance for that is Friday night.

A strong cold front moves through the entire region on Monday and Monday night, accompanied by numerous scattered showers and thunderstorms. This marks the effective end of the heatwave, and cooler temps follow.

Pattern:

The Madden-Julian Oscillation (MJO): The Madden-Julian Oscillation is currently in the circle of death, and is expected to largely remain there for the foreseeable future. Some guidance indicates a brief appearance in a very weak phase 1 and 2 octants before returning there, but this doesn’t largely affect the overall pattern. Tropical forcing projections are expected to be much less influential and more variable for the remainder of July and the beginning of August as a result of this.

Other factors will contribute more to the overall pattern going forward, and most notably this will be tied to the SOI (Southern Oscillation Index) and AAM (Atmospheric Angular Momentum). The SOI continues to maintain negative which is typically found in an El Niño background state, but recent heat can be tied to the drop in global AAM, which is projected to rise toward the end of July and heading into at least the beginning of August. As the AAM rises, this supports some cooler risks (such as next week) and less of the anomalous long-lasting heatwaves…for now. Cooler risks are better defined as largely “near average” temperatures for about a 2 week period, perhaps a few days longer than that, through about the 10th of August. Some temperature variation in that period, but absent of heatwaves most likely.

The core of the ridge over the next 2 weeks or so will retreat to the SW US, and that opens the door to a northwesterly flow aloft and cooler…but still seasonably warm period next week. The flow then becomes more zonal as the ridging breaks down in the SW US through the first week of August, in large part due to a rise in the global AAM. This will keep temperatures closer to seasonal averages with only slight temperature variability, and likely no extended heatwaves. After this point, long range climate models suggest the anomalous heat will return, perhaps in a big way in the 2nd half of August, which may bleed into early September.

The July temperature outlook we continue to project will be an above average temperature month (+4.0°F to +6.0°F) as a whole by month’s end with high confidence. Using a blend of leading analogs from our privately shared Summer Outlook and extended range climate and ensemble guidance, the month of August we project will be another above average temperature month (+3.0°F to +6.0°F) by months end, with heatwaves during the 2nd half the month. Precipitation, as noted above, will also be closer to average in August.

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

The Plains ridging will shift to the SW US this week, which opens the door (at times) for transient cooler periods embedded within a NW flow aloft. “Cooler” periods will be most common at times late July/early August.

The ridge in the SW US breaks down a bit, but the rise in global AAM will likely lead to a more zonal look to much of the US. This likely contributes to a lack of sustainable heatwaves thru at least the first 10 days of August.

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.

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Forecaster: EPAWA Meteorologist Bobby Martrich
Discussion last updated: Friday July 19th, 11:00am