Nick Wiltgen and Chrissy Warrilow
Published: Jul 28,2014, 10:59PM,EDT
weather.com
How Cold Will It Get?
A large dip in the jet stream, also known as a trough, will usher cool air south from Canada into much of the central and eastern U.S. behind a cold front early this week. Temperatures will be as much as 20 degrees below average for this time of year in some areas, and most areas will see at least one or two days with temperatures 5 to 10 degrees below average -- a noticeable difference at a time of year when many of us spend the most time outdoors.
(MORE: Coldest July Days in U.S. History)
What is notable about this cooldown is not just the fact that it will take place in mid-summer, but that it results from a pattern that seems to be frequently repeating.
The same jet stream pattern we'll see this week showed up not only in mid-July, but during much of the January-June time frame as well. As a result, much of the country east of the Rockies was much cooler than average for the first half of 2014, while the West was unusually warm.
(MORE: January-June 2014 Coolest in U.S. In 21 Years)
That's a temperature regime we'll see again this week.
Much Cooler
The cool air began to invade the Upper Midwest Sunday, holding highs to the upper 50s and 60s in parts of North Dakota, northern Minnesota, northern Wisconsin and the Upper Peninsula of Michigan.
As the maps below show, the coolest weather will advance farther south over the next few days. The blue shades show areas where high temperatures will be below average. After the maps, we'll touch on some highlights of the upcoming cool spell.
Current Temperatures
Tuesday's Forecast Highs
Wednesday's Forecast Highs
Thursday's Forecast Highs
Midwest Highlights
As is often the case on the coolest July days, cloud cover and rain will help hold temperatures down in some areas.That was the case in many areas of the Midwest Monday, as low cloud cover kept many areas cool. But even in areas where the strong late-July sun broke through, temperatures stayed temperate.
Morning lows will be suitable for a light jacket as far south as parts of Illinois, Indiana and Ohio by Tuesday morning, with a few upper 40s possible in smaller towns and rural areas away from the Great Lakes and the urban heat islands of major cities. Record lows in the 50s and lower 60s are possible Tuesday and Wednesday mornings across a broad area of the Ohio Valley.
The temperate weather will be in no rush to retreat, as Thursday and Friday will continue to feature crisp mornings and mild afternoons across much of the Midwest.
In Indianapolis, the thermometer has not hit 90 degrees yet this summer. Only one year, 2004, has failed to bring a 90-degree day to the city at all. In fact, Indy's five hottest days this year have all reached just 88 degrees. With such a cool July in the books, Indianapolis is on track - if the forecast going forward to the first of August is right - to have its coolest July on record.
(5-Day Forecast: Marquette | Green Bay | Chicago | Detroit | Indianapolis)
Northeast Highlights
Cool air invaded the Northeast beginning Monday, when parts of upstate New York stalled in the 60s while rain fell behind the cold front. Buffalo's lunchtime temperature Monday was 59 degrees under low clouds and sprinkles.The interior Northeast will remain cooler than the coast throughout the week, with widespread highs in the 70s and morning lows in the 50s. However, the refreshing air will make it all the way to the coast, at least for a while, keeping highs from getting much above 80. Lows will be well down into the 60s for the major cities.
Cool as it will be, only a few daily record lows are in jeopardy, mainly in the interior and in places that have a shorter period of weather recordkeeping.
(5-day Forecast: Buffalo | Pittsburgh | Boston | NYC | Philly | D.C.)
Southeast Highlights
Cool mornings are in store as the chilly air continues to push south. Numerous daily record lows are expected to fall by the wayside in this cool snap.Places such as Birmingham and Atlanta will wake up to temperatures in the upper 50s to lower 60s Wednesday and Thursday, which will feel refreshing for early morning joggers. In Montgomery, Alabama, Wednesday morning lows could flirt with the city's all-time July record low of 59 degrees set July 20, 2009.
You may consider putting on a sweater as you step outside Tuesday, Wednesday, and Thursday mornings across Kentucky and Tennessee, since the thermometer will dip into the 50s in many areas.
Though temperatures will rocket back up into the 80s by the afternoon hours, those daytime highs will be below average for much of the region. This is typically the hottest part of the year in the Deep South, with normal highs for most cities running in the 90s.
(5-Day Forecast: Louisville | Huntsville | Nashville | Atlanta | Raleigh)
And if this cooldown isn't enough for you, you won't have to drive far to find even crisper temperatures. The mountains and resort towns of western North Carolina and east Tennessee will see highs in the low to mid 70s, with lows in the upper 40s and 50s.
(5-day Forecast: Asheville | Boone, N.C. | Gatlinburg, Tenn.)
Southern Plains Highlights
Farther west, a gradual cooldown will arrive in Oklahoma and Texas early in the week, but much cooler weather will take hold Wednesday and Thursday as widespread clouds and rain break out across the Southern Plains.This wet weather will hold highs to the 70s in places such as Oklahoma City, where the average high this time of year is 95. Oklahoma City had three consecutive days (July 16-18) with highs below 75 in the last polar plunge, and one more such day between now and Thursday would tie the all-time record of four July days with highs below 75 degrees set way back in 1895.
Before the clouds and rain break out, skies may be clear enough for a few record lows in Oklahoma and Arkansas Tuesday morning.
(5-Day Forecast: Dallas | Oklahoma City | Tulsa | Little Rock)
Early August: More of the Same?
Medium-range forecast models strongly suggest this pattern will continue into the first week of August, keeping temperatures lower than usual across most of the central and eastern U.S.Dr. Michael Ventrice, operations scientist at WSI, says typhoon activity in the Western Pacific may continue to cause disruptions in the jet stream in the northern mid-latitudes (the part of the Northern Hemisphere between the tropics and the Arctic, including most of the United States, Europe, China, Japan and Russia).
In a Monday blog post, Ventrice said powerful typhoons curving northward in the western Pacific tend to export a large amount of heat into the air over the North Pacific, forcing a northward buckle in the jet stream. This, in turn, induces a compensating southward dip in the jet stream over the Lower 48.
"This is a typical response we observe during strong typhoon-mid-latitude interactions, which are then often followed by a downstream rippling effect across the North Pacific in through the U.S. that often results in another polar air mass intrusion over the lower 48," Ventrice wrote. WSI is the professional division of The Weather Company, which also owns The Weather Channel.
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