Good morning, folks! Luckily for us, the CONUS pattern looks relatively quiet, which is common for this time of year. First we’ll start by looking at the current synoptic picture, then we’ll zoom into some regions and examine model data to see what the future holds, then lastly we’ll take a look at the longer term forecasts as provided by the National Weather Service’s Climate Prediction Center.
Current Synoptic Picture
As of the time of writing, the current 300mb height map indicates the placement of the jet stream in the Northern half of the country. Additionally, a region of relatively stronger winds, known as a jet streak, is present over Northern New England, with the strongest winds in excess of 100 knots currently located over portions of Maine. The 300mb map also shows a dip in the jet stream over the Pacific Northwest, which will correspond to an upper-level low pressure in the Northern Pacific, just of of the frame. The jet stream turns poleward in the Rockies, before gradually becoming more zonal through the Northern Great Plains, hinting at a slight ridging signature for much of the Central and Southern United States. Finally, the jet stream slowly falls equatorward near the Great Lakes, is squeezed between an upper-level low in Northern Quebec (offscreen), and a flattened ridge over the south – causing the locally stronger upper-level winds in the Northeast.
Much is the same in the mid-level, 500mb map, where there is general troughiness in the West, a ridge in the Central United States and Southern United States, and tighter isohypses indicating stronger mid-level winds over the Northeast.
Western United States
Just like when reading a book, the weather (for the most part), moves from left-to-right in the Northern Hemisphere, so let’s start first with the Western United States. As we noted in the section above, the Pacific Northwest is under the influence of an upper-level low that is currently centered a few hundred miles offshore in the Northern Pacific Ocean.
By this afternoon, both the American (pictured above) and the European ensemble models show the upper-level low pressure center moving closer to the Washington coast. Between the two models, the 500mb heights are between 1,300 and 1,400 meters below average for this time of year – this, of course, alludes to a greater risk of below-average temperatures and above-average precipitation.
IMAGE OF EPS 500MB HEIGHT ANOMALIES FOR WED 00Z
GEFS 500MB HEIGHT ANOMALIES FOR WED 00Z
By late Tuesday, however, the global ensemble models show a retreat of this feature further west over the Northern Pacific, where the 500mb height in this region will begin to return to average. The strength of this upper-level low will fluctuate in intensity somewhat throughout the week; but come the weekend, both the American and European ensemble models show the 500mb height anomalies returning back to normal (if not above-average) across the Pacific Northwest and for much of the Northern Pacific Ocean, as well.
So what does this mean for temperatures and precipitation chances across the Northwest? As I said earlier, being under the influence of an upper-level ridge generally leads to cooler-than-average temperatures and above-average precipitation across the region. As the influence of the upper-level low is the greatest earlier this week, both the coolest temperatures and the greatest chance for precipitation be during the Sunday-Tuesday timeframe.
As shown above, both ensemble model solutions display below-average temperatures over the 5-day period from Monday afternoon through Friday, with temperatures being as much as 5 C (about 9 F) below average for some portions of Northern California.
As far as precipitation, models agree that precipitation will be marginally above-average across portions of the Northwest, particularly near the Canadian border and into the Northern Great Plains. Unfortunately for the drought-stricken West, however, much of this rain falls outside of the areas affected by the worst droughts, with the exception of some parts of Washington and Montana.
Central and Southern United States
As a trough paints a cooler, wetter picture in the Northwest, the opposite is true for the Central and Southern United States, where a ridge will be responsible for some above-average temperatures throughout the week. As we saw in the 500mb map at the beginning of the post, most of the United States has 500mb heights above 5,800m – about average for this time of year. If you’re not a fan of persistent above-average temperatures, then you might want to look away…
While it starts out slowly at first, a stronger ridge begins to build into the Southeastern United States by the middle of the week, with parts of the West still feeling the effects of the upper-level low, as well as the Northeast seeing similar effects from a different feature.
By the weekend, though, the ridge becomes well-established in the Central United States, with 500mb heights as high approaching 6,000 meters above mean sea level. In fact, from the middle of the week through the end of the weekend, the majority of the Central and Southern United States will be seeing above-average temperatures due to the anomalous ridging pattern. The only regions missing out on this warm-up are some parts of the West Coast, where the ridge is not as strong, and the Northeast…
Northeastern United States
In the last graphic, we saw that the Northeastern United States was left out of the warm-up affecting much of the country late this week – how come? Earlier, we made note of a strong upper-level low in Northern Canada, responsible for the jet streak over New England. This upper-level low will gradually weaken throughout the week, and instead broaden out into a larger trough stretching across Northern Canada and the Northeastern United States.
As the theme goes, these below-average 500mb heights will usher in some cooler temperatures by the week’s end in the Northeast.
Conclusions
The first weather story this week is the developing upper-level low just off the coast of the Pacific Northwest, which will bring some cooler temperatures to much of the West Coast and some chances for rain for parts of Washington, Idaho, and Montana before the system begins to retreat by the middle of the week.
By this time, a developing ridge in the Southeast will expand westward and strengthen through the end of the week – bringing with it some above-average temperatures. The one exception to this warm-up will likely be the Northeast, where a stubborn upper-level trough will help keep temperatures below normal by the end of the week.
The Climate Prediction Center is showing a similar pattern to continue, as seen in its 6-10 day and 8-14 day temperature outlooks.
