Several Cool Fronts in the Pattern; Periods of Heat Precedes These Fronts as Northwest Ridge Weakens

Key Points:

1. A strong front may bring temperatures up in the northern tier between July 6 and 8. This front may have some heavy showers and thunderstorms.

2. Another front will follow late in the 5-day period with waves of showers and thunderstorms along the front from the mid-Mississippi Valley to the Appalachians.

3. An upper trough will move into the West between July 8 and 9, and this could lead to more warming for the East.

4. In the Gulf Coast and Florida, storms may be more spotty, but heavy. Southern Florida lesser chance and hotter temperatures.

5. 11- to 15-day period still looks tricky with two systems in the western Pacific that can impact the pattern in the eastern U.S.

Risks:

1. The late 11-15 day period is risky. There can be a new, deep trough from the Midwest to the East with cooler temperatures or some of the western heat blends in or is pushed out into the northern Plains and Midwest during this time.

Changes:

1. Took up temperatures a little more in the early 11- to 15-day period due the idea that the system looks a little weaker compared to Thursday and Friday. This is discussed in the technical discussion below.

July 5-July 9:

1. Temperatures will run above normal in the Northeast, Ohio Valley and mid-Atlantic over the next day or two ahead of a cool front. However, areas that have been hit hard with rainfall over recent weeks will have lower departures above normal. (lower - Ohio Valley to central/northern mid-Atlantic; higher - upstate New York, northern New England and southern mid-Atlantic)

2. European weeklies will support an early warmup and a late warmup during this period by showing above-normal temperatures from the Appalachians on East, but has had a hard time picking out wet areas that will be held back.

European week one temperatures:

3. Frontal timing between July 6 and 7 for the Midwest and Ohio Valley and July 8 from the Appalachians on east. However, the front may stall between July 8 and 9 with a wave moving along to a boundary producing more showers and thunderstorms from the Ohio Valley into the Appalachians and mid-Atlantic. A second trailing wave can move from the mid-Mississippi Valley through the Tennessee Valley and southern Appalachians between July 9 and 10.

5. There will be a brief, two-day drop-off in temperatures behind this front in the northern Plains July 6-7, Great Lakes July 7-8 where departures can be 4-7 degrees below normal.

July 10-14:

1. With an upper low moving into the West Coast between July 9 and 10, this will keep any major troughs early in this period from digging. This leads to some sort of a warmup again, but we want to play cautious that in the real wet areas, temperatures may have difficulty either reaching normal or going above.

Last 30-day precipitation map from NOAA

2. A weak front dropping south through the Tennessee Valley and mid-Atlantic between July 10 and 11 is much weaker than thought from last week. However, this goes with point one that temperatures may be held back from going too far above normal around the front. However, south of the front, temperatures will rise.

3. An upper high will cause temperatures to rise along the Front Range, central and southern Plains and mid-Mississippi Valley early in this period.

4. A slightly stronger front will arrive between July 13 and 14 in the northern tier with severe weather possible from the Midwest through the Great Lakes, Ohio Valley and perhaps steadier rain for the Northeast. There will be more cooler behind this front, but concentrated more across the northern tier.

5. For the Southeast, the Carolinas and interior Gulf Coast states look active in this period with numerous showers and thunderstorms. Storms will be more spotty along the Gulf coast, and Florida can get the typical round of storms, especially in the afternoon.

July 15-19:

1. This will be a period with two systems, both warm front and cold front combinations from the Plains to the East Coast. The first front between July 15 and 16 with focus of heavier storms in parts of the Ohio Valley and Tennessee Valley. The second system will start storms in the Plains off the front from the previous system, pushing it back as a warm front sometime around July 17. Then a stronger front will arrive between July 18 and 19.

2. In the technical discussion, I will show how the second front can make a bigger impact with cooler temperatures over a bigger area then the first system. The typhoon and six- to 10-day rule may be a big play in this time period leading into the 16- to 20-day period.

Technical Discussion:

Taking a look at the indices, you can see the change in the PNA which will impact the weather not only in the West, but also along the East Coast. The PNA is currently positive as the upper high over the Northwest will slowly shift west off shore as we expected. An upper low will track into California later next week, and this will bring an increase chance in rainfall, especially the higher ground out in the West. But what about the eastern U.S.? The change from a positive PNA to a negative PNA can mean a warmup for the East, especially the Southeast and lower mid-Atlantic. Areas that are so wet right now will have some difficulty.

PNA indices

Also, take a look at the EPO which shows the strong negative side easing back middle of next week. This means that upper troughs will not dig, but rather lift out. This allows the East to warm up as well. Any cooler-than-normal temperatures will be centered around fronts and well moist areas of the country.

As a result, take a look at the 5-day means ending on July 9. You can see the temperatures are above normal east of the Appalachians, a time when both nighttime lows and afternoon highs will be a few degrees above normal.

Looking at the storms in the western Pacific, the forecast models are leaning toward a more westward track of the first system between July 8 and 9, then a sharp re-curvature after the storm has weakened toward Korea then northeast. The pattern surround this system shows a weaker trough moving through eastern Asia and weak upper ridge east of Japan. This may not be your classic six- to 10-day setup but can lead to a trough from the Midwest to the East, but the impacts can be smaller.

Below is the upper pattern around July 10 in the Western Pacific and eastern Asia. Take a look at where the first system is heading at this time. Also, notice how weak the upper high is north of the second system, east of Japan and how weak the upper trough is moving through eastern Asia.

Now look at the six days later at the trough moving through the Great Lakes. This is much farther north than what the models were showing Wednesday and Thursday.

Now let's look at the pattern surrounding the second system which does a re-curve. The upper trough moving through eastern Asia is stronger, and the upper high east of the system is much broader and in a better position.

The GFS ensemble shows a stronger system moving into the Northwest which could mess up the intensity of the trough in the Northeast and Great Lakes. What you are seeing is the first system leaving but nothing else following as the upper heights come up in the northern Plains. This is the low confidence period because what will dictate the pattern, the western Pacific re-curvature or the strong system moving into the Northwest?

However, the new operational leans toward another system with a potential deepening trough for the East holding back the upper high in the northern Plains. I have numbers on the site for days 14 and 15 leaning toward warmer right now, but it may need to be taken back in the Upper Midwest.

Report a Typo