United States rainfall climatology

The characteristics of United States rainfall climatology differ significantly across the United States and its possessions. Late summer and fall extratropical cyclones bring a majority of the precipitation which falls across western, southern, and southeast Alaska annually. During the fall, winter, and spring, Pacific storm systems bring most of Hawaii and the western United States much of their precipitation. Nor'easters moving up the East coast bring cold season precipitation to the Mid-Atlantic and New England states. Lake-effect snows add to precipitation potential downwind of the Great Lakes, as well as Great Salt Lake and the Finger Lakes during the cold season. The snow to liquid ratio across the contiguous United States is 13:1, meaning convert|13|in|mm of snow melts down to convert|1|in|mm of water.Martin A. Baxter, Charles E. Graves, and James T. Moore. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FWAF856.1 A Climatology of Snow-to-Liquid Ratio for the Contiguous United States.] Retrieved on 2008-03-21.] The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics.

During the summer, the Southwest monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the subtropical ridge in the Atlantic ocean bring the promise of afternoon and evening thunderstorms to the southern tier of the country as well as the Great Plains. Equatorward of the subtropical ridge, tropical cyclones enhance precipitation across southern and eastern sections of the country, as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa. Over the top of the ridge, the jet stream brings a summer precipitation maximum to the Great Lakes. Large thunderstorm areas known as mesoscale convective complexes move through the Plains, Midwest, and Great Lakes during the warm season, contributing up to 10% of the annual precipitation to the region.

General

The eastern half of the contiguous United States east of the 98th meridian, the mountains of the Pacific northwest, and the Sierra Nevada range are the wetter portions of the nation, with average rainfall exceeding convert|30|in|mm per year. The drier areas are the Desert Southwest, Great Basin, valleys of northeast Arizona, eastern Utah, central Wyoming, and the Willamette Valley. [NationalAtlas.gov [http://www.nationalatlas.gov/printable/precipitation.html Precipitation of the Individual States and of the Conterminous States.] Retrieved on 2008-03-09.] Increased warming within urban heat islands leads to an increase in rainfall downwind of cities.Illinois State Water Survey. [http://www.sws.uiuc.edu/atmos/statecli/General/Illinois-climate-narrative.pdf Climate of Illinois.] Retrieved on 2008-03-19.] The snow to liquid ratio across the contiguous United States is 13:1, meaning convert|13|in|mm of snow melts down to convert|1|in|mm of water.Martin A. Baxter, Charles E. Graves, and James T. Moore. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FWAF856.1 A Climatology of Snow-to-Liquid Ratio for the Contiguous United States.] Retrieved on 2008-03-21.]

Alaska

The extratropical storm track runs along the Aleutian Island chain, across the Alaska Peninsula, and along the coastal area of the Gulf of Alaska which exposes these parts of the state to a large majority of the storms crossing the North Pacific. Onshore flow into the various mountain chains across the state leads to significant spacial variability in precipitation, with snowfall constituting a bulk of the annual precipitation each year.Western Regional Climate Center. [http://www.wrcc.dri.edu/narratives/ALASKA.htm Climate of Alaska.] Retrieved on 2008-03-19.] This makes the actual liquid equivalent precipitation across the state more uncertain than elsewhere, due to the variety of methods of measuring precipitation originating as snowfall and difficulty of catching snowfall in precipitation gages across the Arctic. [Dennis Lettenmaier. [http://acsys.npolar.no/introduction/impplan/hydrology.php 4. Hydrological Cycle in the Arctic Region.] Retrieved on 2008-03-21.] January through April are the drier months statewide. The interior receives more rainfall during the summer. [Alaska Geospatial Climate Animations. [http://instaar.colorado.edu/QGISL/AGCA/patterns_ppt.html Patterns for Precipitation.] Retrieved on 2008-03-19.] Southern and western Alaska have a maximum during the late summer and fall months. [Alaska Geospatial Climate Animations. [http://instaar.colorado.edu/QGISL/AGCA/AGCA_Precipitation.ppt Alaska Geospatial Climate Animation: Precipitation.] Retrieved on 2008-03-19.]

Juneau averages over convert|50|in|mm|-1|lk=on of precipitation a year, while other areas in southeast Alaska receive over convert|275|in|mm|-1. South central Alaska does not get nearly as much rain as the southeast of Alaska, though it does get more snow. On average, Anchorage receives convert|16|in|mm|0 of precipitation a year, with around convert|75|in|mm|0 of snow. The northern coast of the Gulf of Alaska receives up to convert|150|in|mm of precipitation annually.Western Regional Climate Center. [http://www.wrcc.dri.edu/narratives/ALASKA.htm Climate of Alaska.] Retrieved on 2008-03-19.] Across western sections of the state, the northern side of the Seward Peninsula is a desert with less than convert|10|in|mm|-1 of precipitation annually, while some locations between Dillingham and Bethel average around convert|100|in|mm|-1 of precipitation. Inland, often less than convert|10|in|mm|-1 falls a year, but what precipitation falls during the winter tends to stay throughout the season. [http://www.ocs.orst.edu/pub/maps/Precipitation/Total/States/AK/ak_ppt.gifMean Annual Precipitation in Alaska-Yukon] . Oregon Climate Service at Oregon State University. Retrieved on 2006-10-23.] La Niña events lead to drier than normal conditions, while El Niño events do not have a correlation towards dry or wet conditions. Precipitation increases by 10 to 40 percent when the Pacific decadal oscillation is positive. [John Papineau, Ph.D. [http://pafc.arh.noaa.gov/climvar/climate-paper.html Understanding Alaska's Climate Variation.] Retrieved on 2008-03-19.]

West

From September through May, extratropical cyclones from the Pacific ocean move inland into the region due to a southward migration of the jet stream during the cold season. This shift in the jet stream brings much of the annual precipitation to the region,J. Horel. [http://www.met.utah.edu/jhorel/html/wx/climate/normrain.html Normal Monthly Precipitation, Inches.] Retrieved on 2008-03-19.] and also brings the potential for heavy rain events.Norman W. Junker. [http://www.hpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page1539.htm West Coast Cold Season Heavy Rainfall Events.] Retrieved on 2008-03-01.] The West Coast occasionally experiences ocean-effect showers, usually in the form of rain at lower elevations south of the mouth of the Columbia River. These occur whenever an Arctic air mass from western Canada is drawn westward out over the Pacific Ocean, typically by way of the Fraser Valley, returning shoreward around a center of low pressure. Strong onshore flow is brought into the mountain ranges of the west, focusing significant precipitation into the Rocky Mountains, with rain shadows occurring in the Willamette Valley, [USA Today. [http://www.usatoday.com/weather/tg/wrnshdw/wrnshdw.htm How mountains influence rainfall patterns.] Retrieved on 2008-02-29.] Great Basin, [Arthur C. Benke and Colbert E. Cushing. [http://books.google.com/books?id=faOU1wkiYFIC&pg=RA4-PA658&lpg=RA4-PA658&dq=great+basin+rain+shadow&source=web&ots=-pMqwyyB9H&sig=VP8rAzBhcm7kmLQpLPFtEMY9IWI&hl=en Rivers of North America.] Retrieved on 2008-03-01.] the central valley of California, [William Edward Duellman. [http://books.google.com/books?id=2WScSPkvY0AC&pg=PA44&lpg=PA44&dq=central+valley+california+rain+shadow&source=web&ots=vUstLIg5vk&sig=SeFdfQFKtGGumEpgCSVFpAir2iQ&hl=en Patterns of Distribution of Amphibians: A Global Perspective.] Retrieved on 2008-03-01.] and the lower Colorado River valley. [Arizona-Sonora Desert Museum. [http://www.desertmuseum.org/news/wp-content/themes/default/nusoxa/blog.htm Lower colorado river rain shadow.] Retrieved on 2008-03-01.] In general, rainfall amounts decrease as you move southward along the West coast. [Sierra Nevada Photos. [http://www.sierranevadaphotos.com/geography/sierra_precipitation.asp Sierra Nevada Precipitation.] Retrieved on 2008-02-29.] The biggest recipients of the precipitation are the coastal ranges such as the Olympic Mountains, [Olympic National Park. [http://www.olympic.national-park.com/weather.htm Olympic National Park Weather Page.] Retrieved on 2008-03-01.] the Cascades, [USA Today. [http://www.usatoday.com/weather/news/1999/wsnorcrd.htm Mt. Baker snowfall record sticks.] Retrieved on 2008-02-29.] and the Sierra Nevada range. [Sierra Nevada Photos. [http://www.sierranevadaphotos.com/geography/sierra_precipitation.asp Average Annual Precipitation.] Retrieved on 2008-03-01.] Lesser amounts fall upon the continental divide. Cold season precipitation into this region is the main supply of water to area rivers, such as the Colorado river and Rio Grande, and also acts as the main source of water to people living in this portion of the United States. [Colorado Basin River Forecast Center. [http://www.cbrfc.noaa.gov/wsup/westwide/foreword.html Foreword.] Retrieved on 2008-02-28.] During El Niño events, increased precipitation is expected in California due to a more southerly, zonal, storm track. [John Monteverdi and Jan Null. [http://tornado.sfsu.edu/geosciences/elnino/elnino.html WESTERN REGION TECHNICAL ATTACHMENT NO. 97-37 NOVEMBER 21, 1997: El Niño and California Precipitation.] Retrieved on 2008-02-28.] California also enters a wet pattern when thunderstorm activity within the tropics associated with the Madden-Julian Oscillation nears 150E longitude.Norman W. Junker. [http://www.hpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page1539.htm West Coast Cold Season Heavy Rainfall Events.] Retrieved on 2008-03-01.] During La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track. [Nathan Mantua. [http://www.ccb.ucar.edu/lanina/report/mantua.html La Niña Impacts in the Pacific Northwest.] Retrieved on 2008-02-29.]

Lake Effect Snow off Great Salt Lake

The southern and southeastern sides of the Great Salt Lake receive significant lake-effect snow. Since the Great Salt Lake never freezes, the lake-effect can affect the weather along the Wasatch Front year around. [Steenburgh, W. J, 1999. [http://www.met.utah.edu/jimsteen/cirp/lake_diagnostics/ Lake Effect of the Great Salt Lake: Scientific Overview and Forecast Diagnostics.] Retrieved on 2008-03-01.] The lake-effect largely contributes to the convert|55|in|cm to convert|80|in|cm annual snowfall amounts recorded south and east of the lake, [National Weather Service Forecast Office Salt Lake City, Utah. [http://www.wrh.noaa.gov/slc/climate/slcclimate/SLC/table44.php Maximum and Minimum Seasonal Snowfall: 1928-1929 to present.] Retrieved on 2008-03-01.] with average snowfall amounts exceeding convert|600|in|cm in the Wasatch Mountains. [Utah Center for Climate and Weather. [http://www.utahweather.org/UWC/lightning_precipitation/greatest_snow_on_earth.html Utah's Greatest Snow on Earth.] Retrieved on 2008-03-01.] The snow, which is often very light and dry due to the desert climate, is referred to as "The Greatest Snow on Earth" in the mountains. Lake-effect snow contributes to approximately 6-8 snowfalls per year in Salt Lake City, with approximately 10% of the city's precipitation being contributed by the phenomenon. [Deseret Morning News. [http://deseretnews.com/misc/gsl/105002200.htm Lake has great impacts on storm, weather] . Retrieved on 1999-08-05.]

North American Monsoon

The North American Monsoon (NAM) occurs from late June or early July into September, originating over Mexico and spreading into the southwest United States by mid-July. This allows the wet season to start in the Southwest during the summer rather than early fall as seen across the remainder of the West.J. Horel. [http://www.met.utah.edu/jhorel/html/wx/climate/normrain.html Normal Monthly Precipitation, Inches.] Retrieved on 2008-03-19.] Within the United States, it affects Arizona, New Mexico, Nevada, Utah, Colorado, West Texas, and California. The North American monsoon is known to many as the "Summer", "Southwest", "Mexican" or "Arizona" monsoon. [Arizona State University Department of Geography. [http://geography.asu.edu/aztc/monsoon.html Basics of Arizona Monsoon.] Retrieved on 2008-02-29.] [New Mexico Tech. [http://www.ees.nmt.edu/vivoni/hydromet/lectures/Lecture17.pdf Lecture 17: 1. North American Monsoon System.] Retrieved on 2008-02-29.] It is also sometimes called the "Desert Monsoon" as a large part of the affected area is desert.

When precipitable water values near convert|1.32|in|mm, brief but often torrential thunderstorms can occur, especially over mountainous terrain. [Norman W. Junker. [http://www.hpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page1472.htm Maddox Type IV Event.] Retrieved on 2008-02-29.] This activity is occasionally enhanced by the passage of retrograding (westward-moving) upper cyclones moving under the subtropical ridge and the entrainment of the remnants of tropical storms. [Climate Prediction Center. [http://www.cpc.noaa.gov/products/outreach/Report-to-the-Nation-Monsoon_aug04.pdf Reports to the Nation: The North American Monsoon.] Retrieved on 2008-02-29.] [David M. Roth. [http://www.hpc.ncep.noaa.gov/tropical/rain/tcwest.html Tropical Cyclone Rainfall for the West.] Retrieved on 2008-02-29.] Flash flooding is a serious danger during the monsoon season. Dry washes can become raging rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away. Lightning strikes are also a significant danger. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.

As much as 45% of the annual rainfall across New Mexico occurs during the summer monsoon. [Elizabeth A. Ritchie. [http://ams.confex.com/ams/27Hurricanes/techprogram/paper_108801.htm 27th Conference on Hurricanes and Tropical Meteorology P2.4: The impact of tropical cyclone remnants on the rainfall of the North American southwest region.] Retrieved on 2008-02-29.] Many desert plants are adapted to take advantage of this brief wet season. Because of the monsoons, the Sonoran and Mojave are considered relatively "wet" when ranked among other deserts such as the Sahara. Monsoons play a vital role in managing wildfire threat by providing moisture at higher elevations and feeding desert streams. Heavy monsoon rain can lead to excess winter plant growth, in turn a summer wildfire risk. A lack of monsoon rain can hamper summer seeding, reducing excess winter plant growth but worsening drought.

Great Plains

Downslope winds off the Rocky mountains can aid in forming the dry line. Major drought episodes in the midwestern United States are associated with an amplification of the upper tropospheric subtropical (or monsoon) ridge across the West and Plains, along with a weakening of the western edge of the "Bermuda high". [ClimaTrends. [http://climatrends.com/site/drought/drought-patterns-in-the-midwest-1988-and-2007/ Drought patterns in the Midwest; 1988 and 2007.] Retrieved on 2008-03-01.] During the summer, a southerly low-level jet draws moisture from the Gulf of Mexico. Additional moisture comes from more local sources, especially transpiring vegetation. Maximum precipitation generally occurs in late spring and early summer, with minimum precipitation in winter. During La Niña events, the storm track shifts far enough northward to bring wetter than normal conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers. [Southeast Climate Consortium. [http://www.agclimate.org/Development/apps/agClimate/controller/perl/agClimate.pl/agClimate.pl?function=climforecast/outlook.html&location=local&type SECC Winter Climate Outlook.] Retrieved on 2008-02-29.] [Reuters. [http://www.reuters.com/article/domesticNews/idUSN1619766420070216 La Nina could mean dry summer in Midwest and Plains.] Retrieved on 2008-02-29.]

The convective season for the Plains ranges between May and September. Organized systems of thunderstorms known as mesoscale convective systems develop over the region during this period, with a bulk of the activity occurring between midnight and 6 a.m. local time. The time of maximum precipitation during the day gradually varies from late afternoon near the slopes of the Rockies to early morning near the Ohio River valley, [Ahijevych,D.A., C.A. Davis, R.E. Carbone, and J.D. Tuttle, 2004: Initiation of Precipitation Episodes Relative to Elevated Terrain. J. Atmos. Sci. 61, 2763–2769.] in part reflecting the west-to-east propagation of mesoscale convective systems. Mesoscale convective systems bring 30 to 70 percent of the annual warm season rainfall to the Plains. [William R. Cotton, Susan van den Heever, and Israel Jirak. [http://rams.atmos.colostate.edu/at540/fall03/fall03Pt9.pdf Conceptual Models of Mesoscale Convective Systems: Part 9.] Retrieved on 2008-03-23.] An especially long-lived and well-organized type of mesoscale convective system called a mesoscale convective complex produces on average 8% to 18% of the annual warm season rainfall across the Plains and Midwest. [Walker S. Ashley, Thomas L. Mote, P. Grady Dixon, Sharon L. Trotter, Emily J. Powell, Joshua D. Durkee, and Andrew J. Grundstein. [http://ams.allenpress.com/archive/1520-0493/131/12/pdf/i1520-0493-131-12-3003.pdf Distribution of Mesoscale Convective Complex Rainfall in the United States.] Retrieved on 2008-03-02.] Squall lines account for 30% of the large thunderstorm complexes which move through the region. [Brian A. Klimowski and Mark R. Hjelmfelt. [http://www.crh.noaa.gov/images/unr/soo/sels19.pdf Climatology and Structure of High Wind-Producing Mesoscale Convective Systems Over the Northern High Plains.] Retrieved on 2008-03-01.]

Gulf coast and Southeast

Cold season

The subtropical jet stream brings in upper level moisture from the Pacific ocean during the cold season. Ahead of storm systems, significant moisture becomes drawn in from the Gulf of Mexico, which increases moisture within the atmospheric column and leads to precipitation ahead of extratropical cyclones. During the El Niño portion of ENSO, increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, polar jet stream. [Climate Prediction Center. [http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/ensorain.shtml El Niño (ENSO) Related Rainfall Patterns Over the Tropical Pacific.] Retrieved on 2008-02-28.] Rare ocean effect snows are possible along the coast of eastern Florida. [National Weather Service Forecast Office Melbourne, Florida. [http://www.srh.noaa.gov/mlb/surveys/012403/flurries.html Cold Temperatures and Snow Flurries in East-Central Florida January 24, 2003.] Retrieved on 2008-03-01.] In the area around Memphis, Tennessee and across the state of Mississippi, there are two rainfall maxima in the winter and spring. [David M. Gaffin and Joseph C. Lowery. [http://www.srh.noaa.gov/meg/images/pcpfig5.gifMonthly Precipitation for Memphis CWA.] Retrieved on 2008-03-19.] [Southern Regional Climate Center. [http://www.srcc.lsu.edu/southernClimate/atlas/msdescription Climate Synopsis For Mississippi.] Retrieved on 2008-03-19.] Across Georgia and South Carolina, the first of the annual precipitation maxima occurs in late winter, during February or March.Netstate. [http://www.netstate.com/states/geography/ga_geography.htm The Geography of Georgia.] Retrieved on 2008-03-19.] South Carolina State Climatology Office. [http://www.dnr.sc.gov/climate/sco/ClimateData/cli_sc_climate.php South Carolina Climate.] Retrieved on 2008-03-18.] Alabama has an annual rainfall maximum in winter or spring and a dry summer. [Jason C. Elliot. [http://www.srh.noaa.gov/hun/research/Elliott_PrecipClimo.pdf A Rainfall Climatology of the WSO Huntsville Hydrologic Service Area.] Retrieved on 2008-03-19.]

Warm season

During the summer, the subtropical ridge in the Atlantic ocean strengthens, bringing in increasingly humid air from the warm Atlantic, Caribbean, and Gulf of Mexico. Once precipitable water values exceed convert|1.25|in|mm, afternoon and evening thunderstorms break out at the western periphery of the subtropical ridge across the Southeast on a daily basis. Summer is the time of the second rainfall maximum during the year across Georgia, and the time of the main rainfall maximum in Florida.Netstate. [http://www.netstate.com/states/geography/ga_geography.htm The Geography of Georgia.] Retrieved on 2008-03-19.] Southeast Regional Climate Center. [http://www.sercc.com/climateinfo/historical/historical_fl.html Historical Climate Summaries for Florida.] Retrieved on 2007-06-02.] Evidence shows that increased air pollution is causing rainfall to reach a maximum in intensity during the middle of the week. [ScienceDaily. [http://www.sciencedaily.com/releases/2008/02/080201215416.htm Air Pollution May Be Causing More Rainy Summer Days In The Southeast US.] Retrieved on 2008-02-29.] During the late summer and fall, tropical cyclones move into the region from the Atlantic and Gulf of Mexico, supplying portions of the area with one-quarter of their annual rainfall, on average. Fall is the time of the rainfall minimum across Louisiana. [Southern Regional Climate Center. [http://www.srcc.lsu.edu/southernClimate/atlas/ladescription Climate Synopsis for Louisiana.] Retrieved on 2008-03-19.] Sometimes, Gulf moisture sneaks up the Front Range of Rockies as far north as the northern High Plains, bringing higher dewpoint air into states such as Wyoming and Montana.

Great Lakes

Cold season

Extratropical cyclones can bring moderate to heavy snowfall during the cold season. On the backside of these systems, particularly those moving through the eastern United States, lake effect snowfall is possible. Low level cold in the winter sweeping in from Canada combine with relatively warmer, unfrozen lakes to produce dramatic lake-effect snow on the eastern and southern shores of the Great Lakes.Thomas W. Schmidlin. [https://kb.osu.edu/dspace/bitstream/1811/23329/1/V089N4_101.pdf Climatic Summary of Snowfall and Snow Depth in the Ohio Snowbelt at Chardron.] Retrieved on 2008-03-01.] Lake-effect precipitation produces a significant difference between the snowfall around the Great Lakes, sometimes within small distances. Lake effect snowfall accounts for 30 to 60 percent of the annual snowfall near the coasts of the Great Lakes.Randall J. Schaetzl. [http://www.earthscape.org/t2/scr01/scr01hb.html Climate: Impacts of the Great Lakes.] Retrieved on 2008-03-19.] Lake Erie has the distinction of being the only great lake capable of completely freezing over during the winter due to its relative shallowness. [Great Lakes Information Network. [http://www.great-lakes.net/teach/geog/intro/intro_6.html 6 | Introduction to the Great Lakes.] Retrieved on 2008-03-01.] Once frozen, the resulting ice cover alleviates lake-effect snow downwind of the lake. The influence of the Great Lakes allows the region to lie within a Humid Continental Climate regime. [Troy M. Kimmel, Jr. [http://www.utexas.edu/depts/grg/kimmel/GRG301K/grg301kkoppen.html GRG301K - WEATHER AND CLIMATE KOPPEN CLIMATE CLASSIFICATION FLOW CHART.] Retrieved on 2008-03-19.]

Warm season

Late spring through summer is the wettest time of the year for the region. [Weather Channel. [http://www.weather.com/weather/wxclimatology/monthly/graph/USIL0492?from=36hr_bottomnav_undeclared Average Weather for Great Lakes, IL.] Retrieved on 2008-03-19.] [Ohio Department of Natural Resources. [http://www.dnr.state.oh.us/water/pubs/fs_div/fctsht11/tabid/4094/Default.aspx Precipitation in Ohio.] Retrieved on 2008-03-19.] Weather systems in the westerlies that cause precipitation move along jet stream, which migrates north into the region by summer. This also increases the likelihood for severe weather to develop due to stronger upper-level divergence in its vicinity. [ [http://www.msstate.edu/dept/GeoSciences/CT/TIG/WEBSITES/LOCAL/Spring2002/Michael_Marsicek/weather.htm Upper Michigan Weather.] Retrieved on 2008-03-19.] Mesoscale convective complexes move into the region from the Plains from May through August, with June the peak month for Michigan. These systems contribute about 2% of the annual precipitation for the region.Walker S. Ashley, Thomas L. Mote, P. Grady Dixon, Sharon L. Trotter, Emily J. Powell, Joshua D. Durkee, and Andrew J. Grundstein. [http://ams.allenpress.com/archive/1520-0493/131/12/pdf/i1520-0493-131-12-3003.pdf Distribution of Mesoscale Convective Complex Rainfall in the United States.] Retrieved on 2008-03-02.] Also, remnants of tropical cyclones occasionally move northward into the region, though their overall contribution to precipitation across the region is minimal. [David M. Roth. [http://www.hpc.ncep.noaa.gov/tropical/rain/tcmidwest.html Tropical Cyclones in the Midwest.] Retrieved on 2008-03-02.] From the spring through the summer, areas near the shores of the relatively cooler Great Lakes develop sea breezes, which lowers rainfall amounts near the immediate coastline.Randall J. Schaetzl. [http://www.earthscape.org/t2/scr01/scr01hb.html Climate: Impacts of the Great Lakes.] Retrieved on 2008-03-19.]

Northeast

.]

Cold season

Coastal extratropical cyclones, known as nor'easters, bring a bulk of the wintry precipitation to the region during the cold season as they track parallel to the coastline, forming along the natural temperature gradient of the Gulf stream before moving up the coastline. [George J. Maglaras, Jeff S. Waldstreicher, Paul J. Kocin, Anthony F. Gigi, and Robert A. Marine. [http://ams.allenpress.com/archive/1520-0434/10/1/pdf/i1520-0434-10-1-5.pdf Winter Weather Forecasting throughout the Eastern United States. Part 1: An Overview.] Retrieved on 2008-03-01.] The Appalachian Mountains largely shield New York City and Philadelphia from picking up any lake-effect snow, though ocean-effect snows are possible near Cape Cod. [Frank P. Colby, Jr. [http://storm.uml.edu/~colby/mesobands.pdf Mesoscale Snow Bands in an Ocean-Effect Snowstorm.] Retrieved on 2008-03-01.] The Finger Lakes of New York are long enough for lake-effect precipitation. Lake-effect snow from the Finger Lakes occurs in upstate New York until those lakes freeze over. [Neil F. Laird. [http://fli.hws.edu/powerpoint/FLI-Dedication-Presentation-2004.ppt Understanding Lake-Effect Snow Storms in the Finger Lakes & Great Lakes Regions.] Retrieved on 2008-03-01.] Bay-effect snows fall downwind of Delaware Bay, Chesapeake Bay, and Massachusetts Bay when the basic criteria are met. Ocean effect snows are possible downwind of the Gulf Stream across the Southeast. [COMET. [http://www.comet.ucar.edu/class/smfaculty/byrd/sld003.htm Lake Effect-type Phenomena in Other Regions.] Retrieved on 2006-11-05.]

Warm season

During the summer and early fall, mesoscale convective systems can move into the area from Canada and the Great Lakes. Tropical cyclones and their remains occasionally move into the region from the south and southwest. [David R. Vallee. [http://www.mass.gov/czm/coastlines/2002/c12.htm A CENTENNIAL REVIEW OF MAJOR LAND-FALLING TROPICAL CYCLONES IN SOUTHERN NEW ENGLAND.] Retrieved on 2008-03-01.] Recently, the region has experienced a couple heavy rainfall events that exceeded the 50-year return period, during October 1996 and October 1998, which suggest an increase in heavy rainfall along the coast.Barry Keim. [http://www.necci.sr.unh.edu/Nera-products/NERA-Climate-2pager-6.99.PDF Current Climate of the New England Region: New England Regional Assessment.] Retrieved on 2008-03-19.]

Pacific islands

Hawaii

Snow, although not usually associated with tropics, falls at the higher elevations of Mauna Kea, which elevations reaching convert|13796|ft|m elevation, as well as Mauna Loa on the Big Island in some winter months. Snow only rarely falls on Maui's Haleakala. Mount Waiʻaleʻale ("Waiokinaaleokinaale"), on the island of Kauai, is notable for its extreme rainfall, as it has the second highest average annual rainfall on Earth, with convert|460|in|mm. [Diana Leone. [http://starbulletin.com/2002/05/27/news/story3.html Rain supreme.] Retrieved on 2008-03-19.] Storm systems affect the state with heavy rains between October and March. Showers are common across the island chain, but thunderstorms are relatively rare. Local climates vary considerably on each island due to their topography, divisible into windward ("Kookinaolau") and leeward ("Kona") regions based upon location relative to the higher mountains. The Kona coast is the only area in Hawaii with a summer precipitation maximum. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover. [Western Regional Climate Center. [http://www.wrcc.dri.edu/narratives/HAWAII.htm Climate of Hawaii.] Retrieved on 2008-03-19.] In the late winter and spring during El Niño events, drier than average conditions can be expected in Hawaii. [Pao-Shin Chu. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0442(1995)008%3C1697%3AHRAAEN%3E2.0.CO%3B2 Hawaii Rainfall Anomalies and El Niño.] Retrieved on 2008-03-19.]

Northern Marianas

The islands have a tropical marine climate moderated by seasonal northeast trade winds. There is a dry season which stretches from December to June, and a rainy season from July to November. [Central Intelligence Agency. [https://www.cia.gov/library/publications/the-world-factbook/geos/cq.html The World Factbook -- Northern Mariana Islands.] Retrieved on 2008-03-19.] Saipan's average annual precipitation is convert|82.36|in|mm, with 67 percent falling during the rainy season. [BBC. [http://www.bbc.co.uk/weather/world/city_guides/results.shtml?tt=TT004880 Average Conditions: Saipan, Northern Mariana Islands.] Retrieved on 2008-03-19.] Typhoons frequent the island chain, which can lead to excessive rainfall. [BBC. [http://www.bbc.co.uk/weather/world/country_guides/results.shtml?tt=TT004880 Weather Centre - World Weather - Country Guides - Northern Mariana Islands.] Retrieved on 2008-03-19.]

Guam

Guam's climate is moderated by east to northeast trade winds through the year. The average annual rainfall for the island is convert|86|in|mm. [United States Navy. [http://www.guam.navy.mil/climate.htm Naval Forces Marianas: Guam Climate.] Retrieved on 2008-03-19.] There is a distinct dry season from January to June, and a rainy season from July to December. [Central Intelligence Agency. [https://www.cia.gov/library/publications/the-world-factbook/geos/gq.html The World Factbook -- Guam.] Retrieved on 2008-03-19.] Typhoons frequent the island, which can lead to excessive rainfall. During El Niño years, dry season precipitation averages below normal. However, the threat of a tropical cyclone is over triple what is normal during El Niño years, so extreme shorter duration rainfall events are possible. [Pacific ENSO Applications Climate Center. [http://www.soest.hawaii.edu/MET/Enso/peu/2006_4th/guam_cnmi.htm Pacific ENSO Update: 4th Quarter, 2006. Vol. 12 No. 4.] Retrieved on 2008-03-19.]

American Samoa

American Samoa's climate regime is dominated by southeast trade winds. The island dependency is wet, with annual rainfall averaging near convert|120|in|mm at the airport, with amounts closer to convert|200|in|mm in other areas. [National Park of American Samoa. [http://www.nps.gov/npsa/ American Samoa.] Retrieved on 2008-03-19.] There is a distinct rainy season when tropical cyclones occasionally visit between November and April. The dry season lasts from May to October. [Central Intelligence Agency. [https://www.cia.gov/library/publications/the-world-factbook/geos/aq.html The World Factbook -- American Samoa.] Retrieved on 2008-03-19.] During El Niño events, precipitation averages about 10 percent above normal, while La Niña events lead to precipitation amounts which average close to 10 percent below normal. [Pacific ENSO Applications Climate Center. [http://www.soest.hawaii.edu/MET/Enso/enso/rain/asamoa.html RAINFALL VARIATIONS DURING ENSO.] Retrieved on 2008-03-19.]

Atlantic islands

Puerto Rico

There is a pronounced rainy season from April to November across the commonwealth, encompassing the annual hurricane season. Due to the commonwealth's topography, rainfall varies greatly across the island. Pico del Este averages convert|171.09|in|mm of rainfall yearly while Magueyes Island averages only convert|29.32|in|mm a year. [Southern Region Climate Center. [http://www.sercc.com/cgi-bin/sercc/cliMAIN.pl?pr6992 Pico del Este Climatology.] Retrieved on 2008-01-23.] [Southern Region Climate Center. [http://www.sercc.com/cgi-bin/sercc/cliMAIN.pl?pr5693 Magueyes Island Climatology.] Retrieved on 2008-01-23.] Despite known changes in tropical cyclone activity due to changes in the El Niño/Southern Oscillation (ENSO), [Dr. David Hodell and Ray G. Thomas. [http://ess.geology.ufl.edu/usra_esse/ENSO_Impacts.html Impacts of ENSO.] Retrieved on 2008-03-19.] there is no relationship between rainfall in Puerto Rico and the ENSO cycle. However, when values of the North Atlantic Oscillation are high during the winter, precipitation is lower than average for Puerto Rico. [Björn A. Malmgren, Amos Winter, and Deliang Chen. [http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0442(1998)011%3C2713%3AENOSOA%3E2.0.CO%3B2 El Niño–Southern Oscillation and North Atlantic Oscillation Control of Climate in Puerto Rico.] Retrieved on 2008-03-19.] There have not been any documented cases of snow falling within Puerto Rico, though occasionally it is brought in from elsewhere as a publicity stunt. [Associated Press. [http://www.puertorico-herald.org/issues/vol4n50/CanSnow-en.html Canadian Snow Sails To Puerto Rico.] Retrieved on 2008-01-23.]

United States Virgin Islands

The climate of the United States Virgin Islands has sustained easterly trade winds through the year. There is a rainy season which lasts from September to November, when hurricanes are more prone to visit the island chain. [Central Intelligence Agency. [https://www.cia.gov/library/publications/the-world-factbook/geos/vq.html The World Factbook -- Virgin Islands.] Retrieved on 2008-03-19.] The average rainfall through the island chain ranges from convert|51.55|in|mm at Annaly to convert|37.79|in|mm at East Hill. [Southeast Regional Climate Center and The University of North Carolina. [http://www.sercc.com/climateinfo/historical/historical_pr.html Historical Climate Summaries for Puerto Rico and the U.S. Virgin Islands.] Retrieved on 2008-03-19.]

ee also

*Climate of the United States
*Floods in the United States
*List of wettest tropical cyclones in the United States

References

External links

* [http://www.hpc.ncep.noaa.gov/mklein/pptgif/cpc/cpc12_p24i.gifLast 24 hours of rainfall over the lower 48 - National Weather Service rainfall network]
* [http://cocorahs.org/Maps/ViewMap.aspx?state=usa Last 24 hours of rainfall over the lower 48 - CoCoRAHS rainfall mesonet]
* [http://www.hpc.ncep.noaa.gov/qpf/qpf2.shtml Rainfall forecasts for the lower 48]