Monday, August 22, 2011

The Long Range Zone: August 2011

10:00 AM EDT Monday 8/22/2011 This specialty forecast page is our headquarters of collaborative team analysis of atmospheric, oceanic and current climate patterns in the Northern Hemisphere which may influence U.S. weather in the 1-2 week period ahead. This week's report by Long Range Forecast Coordinator Nic Robeson of North Carolina, prepared on 8/19/2011.

Overview As we head into next week, a strong upper Low is forecast to form above the Great Lakes into the Northeastern U.S. Effects will be felt as far south as the Southeast US. Into late this week all eyes will be on the tropics, as the main forecast models have for sometime now been shown a tropical system (what is now Hurricane Irene) developing and heading toward the Southeast Coast. This is also supported by the European weekly forecast model, which last week showed this system making a bee line for the GA, SC coast then sharply curving north-northeaster along eastern NC and up the eastern seaboard. A cutoff low in the North Atlantic would also influence path of this tropical system, due in part to blocking over Greenland. 

Early September As we head into the first days of September, persistent troughing will hold back any big warm up for the eastern US. Around the 7th or 8th of September, the European model weekly trend shows a classic Southeast U.S. late summer heat ridge setting up. heat ridge may also develop in the western US, bringing places like the Pacific Northwest their first real taste of Summer in many weeks. These ridges will flex their its muscles well into middle of the month, meaning late summer heat and humidity for the Southeast and Mid-Atlantic. 

Mid-SeptemberAfter a possible breakdown of the ridge by mid month, the eastern US is forecast to stay in a southwest flow aloft. At the surface, besides a possible tropical landfall in the eastern U.S., the pattern may stabilize regarding significant outbreaks of severe weather. With a strong ridge forecast by this time, any front attempting to drive south  will likely stall and lead to a more fall-like pattern of showers and occasional thunderstorms in the Mid Atlantic.

Temperature trends Temperatures  will likely be held down by a possible tropical storm along the Southeast coast from GA to NC. Following this event, conditions will heat up again, with a long stretch of little rain and dry conditions. Some forecast models are insistent on highs running in the mid 90s to near 100 in the Deep South, specifically from the Ozark region east to the Carolinas see some late summer heat. Other regions from the Great Lakes to the Northeast should stay mostly cool with temps in the mid 70s to mid 80s in this same period of early to mid September. (Forecaster Nic R. and the Long Range Team ) 

August 7, 2011 Report:
Cooler ahead, for some
A significant change in the weather pattern will bring a much cooler period in the next two weeks, including an increased risk of severe storms in the Central Plains. The storm track, which has been in northern U.S. and southern Canada, will shift south. This shift will create a fall-like period of increased temperature contrasts, with continued excessive heat in the southern plains while the upper plains will receive welcome relief well into mid-August with with high temperatures in the 70's and 80's. The battleground areas in terms of alternating between cool and warm periods will be the central Mississippipi Valley and Mid-Atlantic, as shown in the Climate Prediction Center's 6-10 day outlook (left)

Looking ahead
As we move deeper into the month of August, one forecast model we routinely monitor, the European Centre for Medium Range Weather Forecasting, (affectionally known as "The Euro" ) still shows consistent heat overall for Texas, and the southern plains to mid-month. In addition to a welcome cool-down in the Central and Upper Plains, cooler air will begin to make inroads in the Northeast over the next 6-10 days. The current US Hazards Assessment indicates 

Storm threats & rain hopes
Though many folks with broken air-conditioners will finally get a sample of cooler nights, this will also yield an increased risk of severe weather outbreaks. The primary risk areas toward mid-month may be the Southeast and Mid-South, primarily caused by afternoon or evening convective storms. 

Does the heat return? 
Long Range Forecast Coordinator Nic Robeson warns, "I do want to warn though that highs in the Upper Plains could return into the  upper 80s to  mid 90s as the ridge by end of the month." He cites the rebuilding of an upper level ridge over the central US, which could last into  the first week of September. Precipitation chances are likely to increase in the Southeast in the 10-15 day period ahead from convective storms, while the Great Lakes, Northeast and Upper Plains will receive their fair share from traditional "back door" cold front passage we see this time of year.

(Long Range Forecast Coordinator Nic Robeson, High Point, NC; Lead Advisor Mr. Foot and the Long Range Team) 

8:45 AM EDT Sunday 7/17/2011 

The headline:  The central and eastern U.S. face a prolonged period, starting by late this coming week, of highs reaching or exceeding 100 degrees with extremely hazardous  heat indices near 110 for millions of people from the Midwest to the Mid-Atlantic. 

The Week Ahead as of 7/16/2011
By Long Range Forecasters Nic Robeson, High Point NC and Mintong Nan of Montgomery County, MD.This report is an overview of the pattern analysis based on the weekly mean projections for North America by the European Centre for Medium Range Weather Forecasting (ECMWF or Euro) and the U.S. Global Forecast System. Image to left is the NOAA Climate Prediction Center's 6-10 day temperature outlook for the period July 21-25, for comparison. This map shows fully two-thirds of the U.S. at a high probability for an excessive heat wave.  

The Synopsis (Forecaster Mintong Nan) Excessive heat appears to be the big story as we head into mid to late July. I've been looking at the long range on the Euro and U.S. based Global Forecast System (GFS): Both models indicate a strong ridge building over the Central US initially and working its way east. With such a strong ridge building it would be hard to get much in the way of relief over the eastern two-thirds of the country. Computer models suggest a weak front might snake its way through the Northeast and Mid-Atlantic come Monday or Tuesday of next week. This trough over the east doesn't appear strong enough to weaken the ridge enough or displace it. With upper level "heights" or a measure of how thick the air has become, trending toward 594 Decameters, this suggests a prolonged period, starting once again by late this coming week, of highs reaching near 100 degrees throughout the Midwest to the Mid-Atlantic. 

Teleconnections Meanwhile, the west coast of the US is relatively cool with a trough digging in. There doesn't appear to be much change in the pattern near the end of the month as the ridge still remains firmly in control of the eastern US pattern. The NAO trending positive from mid month to near the end of the month, the PNA trending negative, and the AO trending positive would support the idea of a prolonged heat wave for much of the central and eastern US. 

Will this end soon?(Forecaster Nic R.) The heat will not be ending anytime soon for a good portion of the country, though there may be breaks for some, while others continue experiencing a "hellish"  summer. Starting with week one (July 14-21), the upper ridge as forecasted by most of the operational models and Euro weekly projection, may ride up into the upper mid west/Great Lakes, then make a move into the Ohio Valley. Areas of the east, from the Carolinas northward to the I-95 corridor, will likely see 100 degree readings. Elsewhere, widespread mid-90s are possible by the end of next week from Chicago to the Great Lakes and Ohio Valley. The worse of this upcoming heat wave is forecasted to back out over the Mid-West and Plains again while the east may see a break. 

I want to note that while the deep south COULD escape the worst of the heat (by afternoon convection), the Mid Atlantic and East could bake yet again by end of July.  A weakness forecasted to develop in the central US ridge and Atlantic ridge, this could "open the Gulf of Mexico" for business in terms of increased tropical development after the 23rd into the first few days of August. 

Where's the break? The only breaks in the heat foreseen during this time would be afternoon convective thunderstorms.  With an upper level short-wave flow aloft and possible tropical moisture being pulled into the Southeast,   this area may have an increased probability of much needed rain. Evidence of this outcome developing already lies with the Azores High/Atlantic High, progged at nearly 1030 mb over the North Atlantic extending into the Tropical Atlantic). This setup will provide for an increased flow of surface moisture from the Gulf of Mexico into the Southeast. 

About the tropics Our Tropical Team will be watching the Southwest Atlantic and Gulf for signs that an upper ridge, now over the North Central US as well as the Atlantic containing a strong surface high, will lead to a busy time in the tropics once August begins. 

About the Long Range Zone
Statements posted on this page are not presented as forecasts but projections based on review of projected data. Once a week, a member of the Long Range Team will map out indications of upcoming weather patterns, including potential for future winter storm development, severe weather outbreaks, or extent and duration of a heat wave or a cold wave.

Specific forecasts on winter storm possibilities come forward once the system is considered "inside 5 days" from arrival in a certain area. Those details would be posted in the Winter Stormcast Zone.

PROJECTIONS Models & Analyses | HPC QPF | US Hazards

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