Autumn 2007 Forecast

[Larry_Cosgrove]

Overview

The unification of the Great Smokies and Sonoran heat ridges during August may prove to be short lived. Recent trends in the 500MB longwave pattern would seem to favor a tendency for cooler regimes to dominate Canada and the northern tier states, while warm or hot values control much of the Intermountain Region and Old South.

This prediction for the period September 1 through December 31 favors only slow amplification of the upper level wind and height profiles, despite an ongoing display of +PNA, -NAO, and -AO measures. The reasoning behind the forecast is that while there are positive height anomalies, each display of ridging is at a very high latitude with only occasional connection to ridges near and about the lower 48 states. And formation of ridge complexes in Canada have, for the most part, proved to be short-lived.

There is still considerable potential for tropical cyclone development and displays of high heat and humidity. Ridging in the western states should promote extensive periods of hot, dry weather there. But along and to the right of the Mississippi Valley, extensive heavy rainfall along with very warm and humid conditions will be the rule. The Virginias and Carolinas could experience the highest temperatures of the fall season in the lower 48 states.

Climatic Indices, SST Anomalies And Teleconnections

As the attached SST anomaly chart shows, the La Nina episode continues to hold in a weak-to-moderate configuration through the equatorial Pacific Basin. Interestingly, the negative thermal distortion continues along the equator into the Atlantic Ocean between South America and Africa, albeit very faint. The presence of a -ENSO configuration argues for a somewhat prolonged, more active tropical cyclone season for North America and the surrounding islands.

Note also the radical temperature discontinuity in the waters of the northern Pacific Ocean. Sea surface readings vacillate from warmer over the Sea of Japan to unusually cold east of Sakhalin Island and the Kamchatka Peninsula, then warm below the Aleutian Islands and around the shorelines of Alaska and British Columbia. There persists, however, a negative anomaly "donut" in the central Gulf of Alaska. So a "negative vs. positive, or neither" PDO perspective must be given special consideration in possible teleconnections.

Waters are generally warmer than normal in the western and northern parts of the Atlantic Basin, in a manner which may be supportive of a -NAO alignment. In fact, the widespread positive anomaly across the Arctic regions may be a factor in promoting a broad region of a -AO configuration, but possibly in a lateral sense (more on this effect below). Put together, the summer trend of _NAO, +PNA, and -AO indices favors more cP intrusions from Canada into the U.S., with a trough complex possibly setting up close to the Mississippi Valley and Great Lakes.

One "maybe" wrench in the theory for cooling in the U.S. is the quasi-biennial zonal wind oscillation. Lately, this index has dropped sharply. And while not quite at the historic levels of 2005-06, the presence of a deep negative phase would seem to coincide with greater, stronger presence of westerlies across the Northern Hemisphere. So it would seem prudent to not over-amplify ridge and trough positions as potential for blocking may be impaired, if only slightly, by atmospheric tendencies for semizonal flow. While I am not a big fan of the QBO index (remember that the landmark 2005 hurricane season occurred in an extremely negative, or westerly, phase), the forces surrounding the measure may have had an impact on the milder, more mP-dominated 2005-06 winter.

Ongoing Trends With The 500MB Longwave Pattern

The first two weeks of August have seen a sharply-defined alignment of Alaskan +PNA, Davis Strait -NAO and cross-polar -AO ridging. While the mean anomaly representation shows a merged heat ridge across the Midwest and South, this linkage was temporary and now the subtropical highs have returned to the more classic-looking Sonoran and Great Smokies positions.

Using ongoing trends at the 500MB level, teleconnections favor the formation of a trough in the central portions of the continent. This implies that the negative height anomaly on view in the Northwest Territories and Nunavut AR will be displaced southward somewhat (probably close to Hudson Bay) with the ridge complexes settling back into the Sonoran and Bermudan positions during much of the fall. This outlook would seem to imply cool weather from the Upper Midwest into New England, with the warmest values over the Intermountain Region and along the Eastern Seaboard. Precipitation would be more likely within the weakness and trough, from the Gulf Coast into the Great Lakes and New England.

Trends In Tropical Weather

The potential exists for an active tropical cyclone season continuing well into the autumn months over the Atlantic Basin. Just as the La Nina episode is associated with shearing, westerly wind profiles in the Pacific Ocean (thereby limiting chances for warm-core cyclogenesis), deep mean easterlies have dominated the lower latitudes between the Horn of Africa and central Mexico. Water temperatures are not quite favorable for tropical development close to the Cape Verde Islands, but are warm enough from the Sargasso Sea into the Caribbean Sea, Gulf of Mexico, and Gulf Stream. Add the potential impact of a Great Smokies heat ridge trending eastward toward the Bermuda position with time, and the cyclonic environment will be suitable for impulses to strike the Greater Antilles as well as the eastern shorelines of Central and North America.

About ten additional named storms are possible during the next 101 days. Cape Verde-generated systems will probably account for 4 or 5 of these disturbances before the "window" closes around the first week of October. Another 2 or 3 tropical cyclones may form in the western portions of the Atlantic Basin from convective groups, diffuse frontal structures, or descending TUTT signatures. Also, there is the usual scenario of about two subtropical storms forming from late October into November, often far at sea and of no threat to the U.S.

Precipitation Forecast

On the idea that the presence of a strong upper level ridge implies a lack of rainfall, then we can surmise that the Intermountain Region will continue under the ongoing drought danger that has been present for the past couple of years. But the eastern states are a more complex situation, as the slow (and probably erratic) eastward migration of the Great Smokies heat ridge may be punctuated by a tropical feature or slowed frontal structure. Still, on the idea that a ridge will be present more often than not in the Southeast, I opted to keep a an area of near or below normal rainfall over middle Appalachia and the East Coast between Jacksonville FL and New York City NY.

Under the weakness and to the right of the mean 500MB trough (see attachments), rainfall will be prevalent and sometimes heavy. This includes a stripe through the Mississippi Valley into the Great Lakes, St. Lawrence Valley and New England. Westerly flow off the Pacific Ocean could account for better rainfall chances from coastal CA into BC.

Temperature Forecast

Since blocking has been noted, but only at very high latitudes and with an unstable presence, some amplification of the jet stream configuration is likely. How the polar westerlies react with the slowly diminishing heat ridges, however, is open to question. Does the flow stay semizonal, essentially meaning near normal readings over the northern half of the U.S. and warmth below 35 N Latitude? Or, with the aforementioned +PNA, -NAO, -AO alignment act to drive colder air into the eastern two-thirds of the nation. While the -QBO argument is a risky one, the broad base of west winds aloft across the Pacific Ocean argue against severe buckling of the upper flow. So a modified ridge/trough/ridge scenario, actually close to recent trends, seems reasonable enough.

As the westerlies increase in velocity and carry stronger shortwaves, there is a chance of interaction with a tropical feature from the Atlantic Basin during October. Such an event could, like Opal in 1995, initiate more amplification to troughs in the central and eastern states. In such a case, a colder turn may occur later in the season with a deeper mean trough at 500MB and better chances for cold advection from central Canada. And with it, a reduction in the strength of the heat ridge lingering over the Southeast.

Summary
There are two key issues confronting forecasters for the period September through December: La Nina and the ongoing presence of a +PNA, -NAO, and -AO configuration. Since the higher latitude height anomalies are arranged in a manner which will allow for a semizonal flow, the argument presented in this discussion is that amplification, while present, will be somewhat muted. Slow erosion of the Great Smokies/Bermuda High is expected which could enable a tropical system to recurve into the eastern third of the nation at some point this fall. Cooler than average readings are likely from the Midwest into parts of New England, as well as in the Pacific Northwest (mP intrusions) and Gulf Coast (clouds from diffuse frontal structures and tropical disturbances).