US Weather Commentary

Cool final week of September is the finishing touch on mild Summer for Eastern and Northern Cities.

For each U.S. region listed below, the first bullet-point will focus on medium-range comments (updated each discussion), while the second bullet-point will focus on Seasonal comments (updated as research warrants).

Northeast:

• The seasonable pleasant to cool early-Autumn pattern continues straight into early October, with only brief and muted warm-ups on the horizon. This is in stark contrast to September 2005, where in New York City the first 23 days all broke 80°F! The temperature pattern is being locked-in by a definitively negative phased NAO, which has been a recurring theme over the past 2 months. Very saturated ground from one of the wettest Summers in recent history should actually start to elevate overnight dew-points and prevent overnight temperatures from getting too anomalously cool. As we are still several weeks away from Winter patterns, a flip to a positive-NAO and/or a negative-PNA would easily bring about an Indian Summer episode or warm-up in October, and should be anticipated.
• Frequent troughs and routine frontal passages have ensured that locations such as Detroit, Pittsburgh, Buffalo, Burlington, Hartford and New York, have indeed had some of the largest reductions in cumulative Cooling Degree Days from Summer 2005 to Summer 2006 [June-September].

Southeast:

• The southern portions of a NAO-induced Trough will continue keep temperatures near-seasonable levels across most of the Southeast into October. Lingering Summer drought inland from the Atlantic Ocean will help cool the Gulf Coast States into the 40's at Night, but also boost some late-Summer days into the mid-80's, topping off what has been a blistering Summer for many Gulf Coast States.
• With (warmer) ridging extending eastward into the South at times, critical days (i.e. days > 90°F, 95°F, 100°F) are more numerous than in 2005, and with Hurricane Season upon us, substantial stresses on both the Agricultural and Energy Industries are likely.

North-Central:

• North-Central cities have started to experience the first consistent run of overnight temperatures falling into the 40's, supported by drought-induced radiational cooling for many regions. But it is important to view this as a crisp conclusion to the Summer Season, and not the "onset of Winter". This point will be further reinforced when temperatures surge back into the 80's this weekend as far North as the Dakotas, kicking off October 2006 on an anomalously warm note. This will reinforce the very warm Great Lakes water temperatures (above 60°F for all except Superior), which sets the stage for Lake-Effect clouds, rain showers and eventually snow.
• The potential for warm days and cool nights often seen in "Indian Summer"-like bouts are still likely in most of the North-Central states through October. Agrarian and Hydrological stresses which reached levels not witnessed since the infamous Summer of 1988, have begun to alleviate.

South-Central:

• September has been appreciably more temperate than August, but lingering drought (away from the Gulf) have been helping to yield some late-Summer hot days (Dallas hit 99°F on Friday), topping off what has been a record-setting Summer for many South-Central locales. These regions become much more vulnerable to cold frontal passages as we approach and go through October, but afternoon readings nearing 90°F remain possible for weeks to come.
• Being near the epicenter of our forecasted "Omega" or "Horseshoe" Summer pattern, these aforementioned States will be most consistently under the influence of ridging, subsidence and compressional warming, almost regardless of how the Hemispheric patterns oscillate this Summer.

Northwest:

• Temperatures have already rebounded to more Seasonable levels as the trough weakened and slid eastward. Surface conditions have dramatically dried out from the Northern Rockies to the Pacific shoreline over the past 2 months, so with ridging shifting West at times, the potential for more anomalous warm periods remains probable for the next few weeks (as does corresponding wildfire threats). Any abatement or reversal in the current positive PNA will quickly cool off the Northwest in October however, as was evidenced most of the past 2 weeks.
• The protection of moist surface conditions which much of the Northwest enjoyed for much of 2006 has now completely eroded and will remain a big issue with regards to warm Autumnal days and night-time radiational cooling as the Summer concludes.

Southwest:

• Following a period of cooler, drier weather, Ridging has already pushed temperatures back to very warm levels. Until the ridging slowly slides eastward by the end of the week locations like Sacramento and Fresno will break 90°F and Palm Springs and Phoenix will see more 100's. The wildest swings will be featured across the Rockies where Denver has already hit the freezing mark earlier this month and should break 80°F in early October. Unfortunately wildfire risk remains critical across most of Nevada and California.
• Westward oscillations of upper-air ridging along with bouts of interior drought & monsoonal moisture versus coastal maritime influences, should make for quite a heterogeneous and chaotic Summer for the Southwest quadrant of the Nation.