Wednesday Weather Topic in Greater Depth

From: Subject: Wednesday Weather Topic in Greater Depth Date: Thu, 22 Oct 2009 10:28:24 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w05w_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Wednesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Wednesday, 7 = October=20 2009

MONITORING DROUGHT

Many locations across the West remained under severe drought = conditions for=20 much of the summer. Significant monetary losses were felt, with lost = crops and=20 losses from wildfires.

What constitutes a drought? The answer depends upon whom you ask. At = least=20 four types of drought can be defined. To the farmer, an agricultural=20 drought represents an extended interval with a serious soil moisture = deficiency during critical crop growth periods. A hydrologist would = classify a=20 hydrological drought as an extended interval containing = abnormally low=20 stream flow, lake levels and ground water reservoirs. Most = meteorologists would=20 consider a meteorological drought to occur when the accumulated=20 precipitation is well below a prescribed amount that would depend upon = the=20 region or season. A fourth type would be socioeconomic drought, = where the=20 shortage of water affects humans, typically in terms of economic = activities.=20 Economic goods that may be affected may include water, agricultural food = products and hydroelectric power.

Typically, the severity of a drought depends upon the lack of soil = moisture,=20 which is influenced by one or more of the following factors to include a = lack of=20 precipitation, low atmospheric humidity, high air temperature, strong = winds, a=20 lack of clouds and intense sunlight. In addition, the drought severity = depends=20 upon its duration and the size of the affected area.

The start of a drought usually is subtle in that few can tell when a = spell of=20 dry weather really constitutes the incipient phase of a drought. = Similarly, the=20 end of a drought is also difficult to assess, since one rain event does = not=20 necessarily "break a drought". The National Weather Service uses several = indices=20 to assess the severity of a drought. One of the most frequently used = drought=20 indices is the Palmer Drought Severity Index developed by Wayne Palmer = in the=20 1960s. This Palmer Index incorporates temperature and rainfall = information in a=20 formula to determine abnormal dryness or wetness over prolonged time = intervals,=20 such as a month to years. The National Weather Service and U.S. = Department of=20 Agriculture jointly compute the Drought Index weekly for each of 344=20 climatological divisions across the United States. A map=20 of the current Drought Index is available that shows those divisions=20 experiencing drought with negative index values and varying shades of = red, while=20 those regions with excess precipitation have positive values and varying = shades=20 of green.

Many locations across the West remained under severe drought = conditions for=20 much of the summer. Significant monetary losses were felt, with lost = crops and=20 losses from wildfires.

The most recent map (weekly index values ending 3 October 2009) shows = a=20 widespread region of moderate to extreme drought across much of the = nation west=20 of the Rocky Mountains, along with sections of the Midwest bordering = Lake=20 Superior and scattered areas in the Appalachians. On the other hand, = unusually=20 moist to extremely moist conditions prevail across the central and = northern=20 Plains, the lower and middle Mississippi Valley and southern New = England. Near=20 normal soil moisture conditions prevailed over the remainder of the = nation.

In the last several years, the National Drought Mitigation Center, a = group=20 consisting of several governmental agencies along with the University of = Nebraska-Lincoln, has maintained a Drought Monitor = site that=20 provides weekly updates of current drought information and forecasts of = the=20 potential for drought across the nation. Their current summary map of = drought=20 conditions attempts to improve upon the Palmer Drought Severity Index = and=20 synthesize five other indices, together with a certain amount of = subjectivity to=20 arrive at six drought severity categories. They attempt to show the = impacts of=20 the drought upon agriculture and wildfire potential. Their most recent = map (29=20 September 2009) shows extreme to exceptional hydrological drought = extending=20 across southern Texas, while moderate to extreme agricultural and = hydrological=20 drought was found across northern Wisconsin and adjacent sections of = Minnesota=20 and Michigan's Upper Peninsula. An accompanying narrative entitled = "National=20 Drought Summary" also provides a five-day forecast and a 6- to 10-day = outlook=20 for precipitation and temperature across the country. This site also = includes=20 animated Drought Monitor maps for the prior six and twelve weeks. The = Drought=20 Impact Reporter is an interactive tool that permits exploration of = the=20 reported drought impacts across the nation. The goal is to help in risk=20 management that could ultimately help shape drought related policy at = the state=20 and federal levels.

The US=20 Seasonal Drought Outlook released on 1 October 2009 by the Climate=20 Prediction Center and valid through December 2009 indicates that drought = conditions were expected to develop or persist across sections of the = Northwest,=20 primarily in Washington, Oregon and northern California. The regions of = the=20 Southeast, south Texas, the Great Lakes States that have been = experiencing=20 drought should experience definite improvement in drought conditions. = Other=20 drought-stricken areas in California, Nevada and northern Montana east = of the=20 Divide could experience varying degrees of improvement.

Source:

Palmer, W.C., 1988 (12 Jul): The Palmer Drought Index: When and how = it was=20 developed. Weekly Weather and Crop Bulletin, 75 (28), 5.

Return to the Wednesday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2008, The American Meteorological Society. =