WEATHERAmerica Summer 2007 Season Forecast

[Larry_Cosgrove]

Overview

Trends In Evolution Of 500MB Longwave Pattern
The trend toward a three-storm sequence, with the middle cyclone being the strongest, continues across North America. Typically, the disturbances are noted in a series across the Eurasian continent, migrate to the northern Pacific Ocean, then dig underneath higher-latitude blocking signatures in the PNA and NAO positions. The result has been numerous severe weather events across portions of the lower Great Plains and Old South, unseasonably cold air with very heavy rain and even snow through the Midwest and Northeast, and wild temperature swings through most of the U.S. and southern Canada.

Seasonal and synoptic climatology favors the general storm track relocating farther north with time. As an example, the current suite of numerical models show the series of disturbances (from April 22 through May 3) taking a path from the central Front Range into the St. Lawrence valley (with perhaps some redevelopment scenarios off the Delmarva Peninsula). The next series of impulses (from Turkmenistan into Korea and Taiwan as of April 20) will probably move to the left of that trajectory, and each succeeding group of cyclones should migrate to a mean position just above the Canadian border. This pattern allows for migration of the persistent subtropical high near Cuba to a position over the southeastern states by June, with a maximum northward intrusion of the heat ridge over MO, IL, IN and KY at some point during the middle of the summer season. The impressive subtropical jet stream, which has persisted over the equatorial Pacific Basin, should conjoin with the polar westerlies over BC and OR with a cool, wet summer indicated through the north and western edges of the U.S.


SST Anomalies
The start of a La Nina episode is indicated by many of the climatic models formulated by meteorological offices around the world. CFC modeling points toward a strong -ENSO by middle to late summer; other statistical and dynamic versions suggest anything from a very weak El Nino to a moderate La Nina. Since the bulk of the operational equations point to the latter designation, we can surmise that some summer weather patterns will emerge from past analogues like 1995 and 1998 (warm ENSO periods which transitioned to cool). That would mean extensive heat east of the Rocky Mountains; a fairly active (but not record-breaking) tropical cyclone season; and a rather active "Ring Of Fire" convective array from July through September.


The Tropics
After a long period of westerly flow dominating the equatorial regions, GOES EAST and METEOSAT 2 images show a slow regrouping of deep mean easterlies over north central Brazil and central Africa. Perhaps most importantly (favoring future tropical cyclone formation and growth), mesoscale convective clusters are appearing while monsoonal-type synoptic-scale groupings are disappearing from the Amazon and Congo Basins.


Placement Of Atmospheric Features During The Spring

The spring of 2007 in North America has been marked by a pattern of vigorous polar westerlies, strong storms, and occasional incidences of impressive high-latitude blocking (as was seen in the record-breaking cold spell in early-mid April). But an overlooked feature has been the persistent presence of a flat heat ridge straddling the Greater Antilles and Caribbean Sea. When this subtropical high has percolated, unseasonably warm and dry conditions have enveloped in the Old South. And due to the very existence of the anticyclone, deep moisture fetches have fed storms advancing into the Great Plains and Midwest.

Since the ridging will expand north and west with the coming of summer, it is speculated that:

1) Hot, dry conditions will begin to win out through the Dixie states beginning in May

2) The eventual peak of the heat ridge will be over and to the west of the lower Appalachian mountains, with occasional northward expansion. Fitting the "Great Smokies" variant of the Bermuda High, this system should establish long periods of heat, drought, and air stagnation across the Corn Belt, Mid-South and Mid-Atlantic regions during the upcoming summer months.

3) When all three heat ridges (Sonoran, Bermudan, Azores) interconnect, deep mean easterlies will increase the threat of tropical cyclone landfalls in the Florida Peninsula and along the Gulf Coast (most likely strike: Texas) in late summer and early autumn.

4) Best potential for lower temperatures and heavy precipitation will be along the northern tier of the U.S, and adjacent Canadian provinces.

Other Indicators

While the SST presentation on global oceanic analyses is generally supportive of a developing La Nina episode (and,as noted above, many atmospheric interactions point that way as well) there are signs that a -ENSO signature may not be prominent or long-lasting. Consider that while MEI measures are falling (favoring a colder phase), SOI values are negative and appear to becoming more so. Also, warming is noticeable across the eastern Pacific Ocean, eroding a long-standing negative (Cooler than average) period of the Pacific Decadal Oscillation (PDO) in the Gulf of Alaska. There has never been a case of a positive PDO event with a strong La Nina. From all of this, it seems probable that the El Nino Southern Oscillation will be of a MODERATE La Nina type this summer, with peak intensity occurring in mid to late summer before a weakening trend may set in, as indicated by most of the governmental and academic services dealing with global climate and meteorology. If, indeed, a moderate -ENSO event gets underway, we can expect more widespread heat and dryness across much of the lower 48 states with an above normal (but not record-setting) amount of tropical cyclone formation in the Atlantic Basin. The presence of a La Nina SST anomaly favors easterly flow (and organization of convective vortices) at low latitudes between the Lesser Antilles and western Africa. Below average production of hurricanes in the Mexican/eastern Pacific theater (low latitude westerlies shearing the formative impulses).

Expected Position Of Sonoran, Bermudan And Azores Heat Ridges

Heat ridges are a VERY big deal when it comes to trading on energy futures and derivatives. And not just because of what these systems represent in their name, heat. Typically, every summer sees an increase in temperatures at the upper levels, and atmospheric heights rise accordingly. Also termed subtropical highs, these ridges are notorious for stabilizing the atmosphere and setting up high surface barometric pressures. If you are located under and slightly to the left of a heat ridge, temperatures will be much above normal and precipitation may be minimal, perhaps nonexistent.

There are three geographic types of heat ridge that affect markets in North America. One is the Sonoran, which tends to form along the U.S.-Mexico border in early May; this anticyclone is named for the nearby Sonoran Desert. If this ridge builds over the western states and shows no migration out into the Great Plains, the West stays hot and dry with a reduced monsoon season (so hydro power suffers accordingly). East of the Rocky Mountains in this scenario, temperatures are closer to normal but with little rainfall. The hottest temperatures of the summer occur when this searing area of high pressure expands and tracks into KS and NE. In such a situation, ethanol-producing corn crops can be devastated.

The most familiar type of heat ridge is the Bermuda High. Typically, the center of this high is situated near the British commonwealth of Bermuda, hence its name. Usually, a classic Bermuda High means warm (not hot), and VERY humid conditions for the Eastern Seaboard, with afternoon and evening showers and thunderstorms each day. If the ridge shifts to the right, the Midwest and East Coast are cooler and in some cases stormy. But when the high retrogresses into the Great Smokies of the Southeast, a long siege of miserably hot, muggy (but not rainy) weather with air pollution issues tends to occur anywhere to the east of the High Plains.

Third on the list is the Azores ridge, often seen in the warm season off of the northwest coast of Africa. This ridge, if it is very strong, will send hot, dry air filled with sand particles and dust from the Sahara Desert into the tropics, thus eliminating hurricanes before they can start. That is part of the reason why the 2006 storm season was a dud. But a flatter, weaker high positioned closer to Portugal and Spain makes for a pattern ideal for tropical cyclone development, allowing thunderstorm growth in tropical waves, as well as westward motion, that can culminate in landfalls in populated areas of the U.S. coastline.

What is the worst case scenario for heat ridge development? When all three anticyclones link into a "super-ridge", and allow for extreme heat in much of the U.S. while at the same time promoting rapid westward motion for developing tropical disturbances. This last happened in the summers of 1995 and 2005, active and deadly seasons across the Western Atlantic Basin.

At this point in time, it appears that the dominant heat ridge for the summer months will be the "Great Smokies" variety of the Bermuda High. We have seen examples of this feature through the past six months, ranging from the northern Caribbean Sea into Cuba, with some percolations into Florida, the Bahamas, and Georgia. Persistence and climatology point toward an eventual position in the vicinity of the lower to middle Appalachian Mountains into the lower Missouri Valley (see graphic) during the peak and later periods of summer.

In almost all cases, a Sonoran heat ridge will appear at some point during the summer. But the footprint of the ridging has been largely missing this spring, which leads me to believe that its presence may be more limited over the next few months. Using the drought criteria as a likely position for the subtropical high, we can probably expect higher 500MB heights to take shape in the Desert Regions with occasional expansion into the northern and central Rocky Mountains.


Last summer there was a powerful, widespread heat ridge over the Mediterranean Sea and Sahara Desert which often bridged southward into the Sahel and equatorial regions of Africa. The anticyclone never really bridged westward and northward into the more typical position near the Azores and Iberian Peninsula, and influx of arid cTw regimes (with high dust and sand levels) eliminated many developing tropical disturbances (the El Nino-inspired low latitude westerlies helped destroy other ITCZ impulses beginning in August). The ridge looks very strong this spring, with less tendency for interruption from higher velocity southwest winds. The projected core of the anticyclonic flow looks to be over the eastern Atlantic Ocean around 30 W Longitude, 40 N Latitude. While the strength of the ridging will advect Saharan Dust and low dewpoints into the tropics, the position favors many tropical waves reaching the various island chains and southeast shorelines of North America (through linkage with the Great Smokies heat ridge). Bottom line: an active, but not extremely so, 2007 hurricane season.


"The Ring Of Fire"

The presence of a "ring" of thunderstorms across North America is symptomatic of widespread heat and drought. A unified, strong heat ridge parked over the middle sections of the lower 48 states will set up a very unique satellite signature. Beneath the core of the heat will be deep mean easterlies, which will then turn north and flow along the Rocky Mountains. At some point, usually close to the border between the U.S. and Canada, the moisture fetch is intercepted by the polar westerlies and turns east through the Great Lakes. Then, upon reaching the right flank of the subtropical high, the higher dewpoint field (with associated thunderstorms) turns southward along (or just east of) the Atlantic coast before linking, once again, with the flow from the lower Atlantic Basin.


Summary

A look at the overall synoptic pattern for atmospheric features and the SST anomalies present around and over North America would seem to favor an abundance of hot weather and increased potential for tropical cyclone landfalls. The presence of abundant warm water through the lower reaches of the Atlantic Basin, an upcoming (probably moderate intensity) La Nina episode and increased ridging from the southeast to the middle part of the U.S. favors disturbances striking the Florida Peninsula and western Gulf Coast. Some of the systems may recurve seaward or toward Newfoundland, depending on the position and strength of the Azores heat ridge.

[rainstorm]

wow, great work mr cosgrove. it appears we are in for extreme heat and drought here.