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Being a huge country, a continental climate

The United States offers a wide variety of climates, from frozen tundra to steamy tropical and from dry desert to soggy rain forest, and everywhere in between. In the 50 states, the extreme temperatures have ranged well over 200°F from -80°F (Prospect Creek, Alaska ) to 134°F (Greenland Ranch in Death Valley, CA). Rainfall has varied from virtually nothing in the driest years in the southwestern desert region to 460 inches at Mt. Waialeale on the Island of Kauai in Hawaii.

The wettest regions are those in the Pacific Northwest and the southeastern United States, as well as the coastal and adjoining mountainous areas of Hawaii and Alaska. In the northwestern United States and the Alaska coastal regions, it is the steady barrage of winter season storms that bring the heaviest rains (and mountain snows). In the southeastern United States, strong storms and cold fronts bring rains in winter and then in the warm season it is the daily thundershowers and occasional tropical systems that keep it wet. In Hawaii, it is the moist and steady trade winds that are responsible for the rainfall.

The desert regions and the large expanse of semi-arid climates in the Great Basin get that way because of the shielding effect of the western mountains which remove a much of the Pacific moisture as systems ride the jet stream from west to east across the country. One of the most important factors in the annual cycle is the position and strength of these storms and the polar front that separates warm air from cold. In winter, the storm tracks and the polar front is suppressed well to the south, often reaching the Gulf Coast. Storms move along the polar front bringing snow and ice to their north. Although the cold air can occasionally reach the southern border states, much of the time, the deep south, from southern California and Arizona to Florida, stays relatively mild. This area is a haven for tourists seeking a quick escape from harsh winter conditions to the north.

In summer, the polar front is much weaker and retreats north, often into Canada. Tropical air spreads to the north and brings the heat and humidity and the scattered daytime thundershowers of summer. Low pressure moving along the front can organize these thunderstorms into squall lines. There is often a reversal of tourist destinations in the summer, as many southerners flock to the northern areas to escape the sweltering heat and humidity over the south.

In the transition seasons, the polar front migrates (north in the spring, south in the fall). In the spring, the still strong storms may feed on the reappearing tropical moisture and instability, thereby spawning severe weather. In the late summer and fall, the excess heat of the tropics manifests itself in the form of tropical storms and hurricanes that threaten coastal areas with wind and storm surge and bring flooding rains inland.

The Dead of Winter

The sun is slowly returning to the cold winter hemisphere. The days grow almost a full hour by the end of the month from the solstice minimum. This returning sunshine begins to lift the spirits but not the temperatures. January is the coldest month in all the states. In most areas, the daily average temperatures won't reach its annual minimum until late in the month or early in February.

The jet stream positions, which reflect the extent of the cold air, also approach their southernmost positions. Multiple jets are often obvious. On average, the northern jet stream enters near the Northwest corner of the US across to the Great Lakes and Ohio Valley. A southern jet stream becomes significant and flows on average from Mexico and southern Texas to the mid-Atlantic coast.

A number of storm tracks can ride these jet streams. The Pacific storms can enter anywhere on the west coast though the peak frequency is in the northwest. These storms can be very wet and windy and, not surprisingly, January is the peak month of the rainy season in parts of the west. The Los Angeles area gets near 3 inches of rain most years this month while parts of northwest can get a foot of precipitation.

After the storms enter the West Coast, they usually break up over the mountains and then reform to the east. One of the new storm tracks initiates over Alberta. The 'Alberta lows' usually move quickly southeast towards the Great Lakes bringing light to moderate snow and gusty winds. These storms ultimately reform off the northeast coast and if they linger, threaten coastal areas with snows.

Other storms reform in Colorado or Texas and track northeast towards the Great Lakes and St. Lawrence Valley. When these storms tap Gulf of Mexico moisture they can produce very heavy snows over the Great Plains and upper Midwest. When very cold air masses trail the storm, blizzard conditions can prevail with white-out conditions in snow and blowing snow. Even moderate snow amounts can be piled into huge drifts in the treeless plains during a blizzard.

The southern jet stream can spin up storms in the Gulf of Mexico region or off the south Atlantic coast. Sometimes these southern systems coincide with northern jet stream disturbances. These situations produce the biggest East Coast storm events. That is because the southern systems add tropical moisture and energy to the mix. East Coast storms can create havoc for the major metropolitan areas of the east.

Polar high pressure systems are also at their peak in January. The cold air factory over high latitude continental areas is at peak productivity. If air is allowed to sit for any length of time, the temperature plummets and pressure rises over the snow and ice in the long winter nights. When the jet stream dynamics are favorable, these highs can become very massive. When these monster highs drop into the states, they can at times spread coast to coast, border to border. The typically path for these high pressure centers is south into the northern Plains or more east southeast north of the Great Lakes to maritime Canada.

While the polar highs are at their maximum strength, the sub-tropical highs are at their annual minimum extent. In the Pacific in January, the weakened Pacific high is at its southernmost position stretching from off of Baja to Hawaii. In the Atlantic the Bermuda Azores high is also weaker and furthest south, stretching east-west from the Azores to off the southeast coast. Usually the western end of this high is weak, making room for storms to develop off the southeast coast.

In the heart of winter often comes the welcome January thaw. It is a very real phenomenon. It is likely caused by changes in the high atmosphere triggered by the increased sunshine. Research has shown that late in the month, most often between the 20th and 26th, a warm-up occurs (especially over the Midwest and Northeast). History shows this is most likely during cold winters. In actual fact, in unusually warm winters, a reversal to colder often takes place near the end of the month or early in February.

The extremes in January range from 98F in Laredo, Texas on January 17th in 1954. Just 3 days later in the same year, the coldest January temperature was recorded in Rogers Pass, Montana with -69.7 F.