12:20 PM | *A nation divided…cooler in the East…hotter in the West*

A split nation in the 6-10 day period in terms of temperatures with cooler-than-normal conditions likely in much of the eastern half and hotter-than-normal in much of the western half. Map courtesy NOAA

Overview

The overall weather pattern is shaping up to feature cooler weather in the eastern US where temperatures will be well below-normal at times in coming days and very hot weather in the western US where numerous record highs will be challenged.  A large upper-level trough of low pressure will extend from eastern Canada to the Gulf of Mexico by the middle of next week at the same time high pressure ridging will intensify over the Southwest US.  The transition to the cooler weather in the eastern US has come about with plenty of rainfall and the buildup of heat in the western US has come with an increase in wildfire activity that often ramps up during this time of year.

Cooler-than-normal conditions will be the rule in the eastern US in coming days at the same time very hot weather dominates the western US. Map courtesy NOAA, tropicaltidbits.com

In the East…

There has been plenty of rainfall in the eastern US during the past few days and another wave of low pressure will likely bring more significant rain on Saturday night and early Sunday to at least parts of the Mid-Atlantic region; primarily, south of the PA/MD border.  At the same time, strong high pressure is building into southeastern Canada and this system is already generating some cooler air for the Northeast US as a low-level northeasterly flow of air has formed.  The combination of the high to the north and the low pressure system which will be ride along a stalled out frontal boundary zone will likely keep temperatures confined to the 70’s on Sunday in the DC-to-Philly-to-NYC corridor - well below-normal for this time of year (normal high now is 87° at DCA, 86° at PHL, and 83° in NYC).  The below-normal temperatures will likely continue for much of next week in the Mid-Atlantic region and throughout much of the eastern US as the upper-level trough intensifies overhead. 

An intense area of high pressure aloft will be centered over the Southwest US by the middle of next week at the same time an upper-level trough intensifies in the eastern US. Map courtesy NOAA, tropicaltidbits.com

In addition to the temperatures, this upper-level pattern featuring a trough in the eastern US will have an impact on the Atlantic Basin tropical scene.  Specifically, as a result of the upper-level trough that will develop near the eastern seaboard, the chances for Tropical Storm “Josephine” to impact the US east coast will be greatly diminished.  First, southwesterly winds on the front side of the west-to-east moving trough will tend to have a weakening effect on “Josephine” (through vertical wind shear) in a few days and second, those same southwesterly winds will help to steer the system away from the US east coast and towards Bermuda and then the open Atlantic.

Soil moisture is abnormally high in the Mid-Atlantic region due to recent heavy rains and abnormally dry in much of the western US. Map courtesy NOAA/CPC

In the West…

As the eastern US cools down, the western US is already heating up dramatically as high pressure ridging intensifies aloft over the Desert Southwest.  The heat wave that has begun will last well into next week with temperatures later this weekend as high as 115° in Phoenix, AZ and 125° in Death Valley, CA.  In addition to the intense high pressure aloft, one other contributing factor to the heat wave in the Desert Southwest is a lack of rainfall in recent weeks.  Typically, there are monsoonal-type rains in states like Arizona during the month of August, but they have been lacking this year likely due to the typical drying effect of La Nina conditions (i.e., colder-than-normal sea surface temperatures) in the tropical Pacific Ocean. 

The abnormally low soil moisture in much of the western US is not only adding to the heat, it is no doubt aiding in the development of wildfires in recent days.  In fact, there have been four new large fires reported in the past couple of days with all four in the western US (one each in California, Arizona, Nevada and Oregon).  Fortunately, the number of fires and acres burned are below-normal so far this year when compared to last year and also below-normal when compared to the 10-year average of 2010-2019 (source: National Interagency Fire Center).

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

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