Drought Outlook-Widespread Expansions Expected for Southern Third of Continental U.S.12/04/2017
The Monthly Drought Outlook (MDO) for December 2017 is primarily based on official precipitation forecasts from both the Weather Prediction Center (WPC, Week-1), and the Climate Prediction Center (Week 2, Week 3/4, and the 30-day precipitation update for December). Dynamical model precipitation guidance from the CFS, GFS, and ECMWF was also used.
The overall picture that emerges is one that is “front loaded”; that is, significant precipitation is predicted to fall during the early stages of the outlook period, in this case Week-1. Beyond this time, a succession of polar air masses and northwesterly flow are expected to dominate the Nation east of the Rockies, which is typically associated with a much drier pattern.
Should enhanced convection associated with the tropics-based Madden-Julian Oscillation (MJO) emerge over the West Pacific early in the period as dynamical model forecasts suggest, it may also play a role in the mid-latitude North American circulation pattern by prolonging the duration of colder, drier weather across the United States, compared to what would normally be expected by extratropical influences alone.
Precipitation patterns typically associated with cold season La Niñas favor drier conditions across the southern tier of states, and wetter conditions across the northern tier of states (the latter being associated with a poleward-displaced jet stream and main storm track).
However, this climate signal is often more robust after the month of December, and the utility of this signal for this MDO is therefore limited.
The factors noted above, as well as climatology based on a long historical record (121 years) favor the maintenance of drought conditions across a significant portion of the contiguous United States during December 2017. Several areas of new drought development are anticipated as well. These potential development regions are indicated over the southern CONUS, and are loosely associated with abnormally dry (D0) regions depicted on the latest U.S. Drought Monitor (valid 28 November).
Though some of the tools used favor precipitation across portions of the South and Southeast during Week-1, it does not look like the forecasted amounts will warrant drought improvement or removal. The normally wet winter season in California is off to a slow start this year, and with the possible exception of the end of December, the next few weeks look relatively dry as well.
With the approach of Hawaii’s core rainy season, most of the islands are expected to see drought improvement and/or removal. One exception may be along and near the leeward slopes of western Maui, which has received well below-normal rainfall during November. The persistent trade wind pattern that has dominated the Hawaiian Islands is forecast to continue in December.
There is no drought in Alaska or Puerto Rico at this time.
Confidence for the Southeast is moderate.
- In the Southeast, precipitation deficits of 1-2 inches were widespread during the past two weeks, with 2-3 inch deficits showing up in much of Alabama, the western Florida Panhandle, and far northern portions of Georgia. Surplus precipitation (1-5 inches) was noted over north-central Florida.
- December climatology favors drier conditions near the coast (2-7% of annual precipitation), with southern Florida reporting the driest percentiles (2.0-3.5%), and near-normal conditions (7.0-9.5%) inland.
- Abnormal dryness (D0) is widespread across the Southeast region, with embedded areas of moderate drought (D1) indicated on the latest U.S. Drought Monitor.
- The precipitation forecasts out to the monthly timescale basically favor a “front-loaded” pattern, with most of the precipitation expected to fall within the first week of December. The most recent runs of the CFS favor a drier pattern across the Southeast, and CPC’s official 30-day precipitation outlook calls for Equal Chances (EC).
- Given these predictors, drought is anticipated to persist in this region, with new drought development possible from the Florida Panhandle northeastward to southern parts of North Carolina.
Confidence for the South is moderate.
- DNPs during the past two weeks across the South generally increased from west to east, ranging from near zero in western areas to 2-3 inch deficits in eastern areas. This is in stark contrast to the 121-year precipitation climatology for December, which depicts the driest conditions in the west, and wetter conditions in the east.
- Dryness and drought categories across the South currently range from D0 (abnormal dryness) to D3 (extreme drought). The extreme drought is concentrated in southwestern Arkansas, near the Arklatex area.
- Though precipitation forecasts out to 30-days suggest the best odds for substantial amounts are over southern and eastern sections of this region, it is thought that the amounts received would not be enough to warrant a one-category improvement.
- Drought development is anticipated across a fairly broad area of the South, extending into southern parts of Kansas.
Confidence for the Midwest is moderate to high.
- Across the Midwestern states, DNPs during the past 14-days were somewhat mixed. The driest conditions (2-3 inch deficits) were noted over southern Missouri, with the wettest conditions (1-3 inch surpluses) indicated over northern Ohio and north-central Indiana.
- December is a dry time of year climatologically, with most of the Midwest typically receiving only 2-7% of its annual precipitation, with the driest percentiles favored over Minnesota, Iowa, and western Wisconsin. Percentiles near and along the Ohio River (including all of Kentucky) are a somewhat more generous 7.0-9.5%.
- Dryness and drought categories from the U.S. Drought Monitor map currently range from D0 to D2, focused mostly over Missouri and southeastern Iowa. However, much of the Midwest region is free of dryness and drought.
- With modest precipitation amounts predicted across the drought areas during Week-1, little precipitation anticipated during the Week-2 and Week 3-4 periods, and the official 30-day outlook favoring below-normal precipitation, drought persistence appears to be the best bet.
Confidence for the High Plains is moderate to high.
- During the past two weeks, DNPs were close to normal across most of the High Plains region, with the exception of eastern Kansas, where 0.5-2.0 inch deficits exist.
- As is the case with the Midwest, December is one of the drier months climatologically for the High Plains region. Only 1-7% of the annual precipitation is typically received in December, with the lowest percentiles (1-2%) depicted in southwestern and south-central Nebraska, and adjacent northwestern Kansas.
- The latest U.S. Drought Monitor indicates the most serious drought conditions (extreme drought, D3) are indicated over western South Dakota and northeastern Montana.
- Little precipitation is forecast at all timescales out to a month in advance. Therefore, drought persistence is considered most likely.
Confidence for the southern half of the West is moderate to high.
- Across the southern half of the West during the past two weeks, PNPs ranged from near zero in southern areas to 150-300% in northern areas, peaking around 400% in the Sierra Nevada mountains.
- For most of this region (the southern half of the West), December is typically a wetter month, especially in much of California, where about a quarter (20-26%) of the annual precipitation is received. Incidentally, the normally wet winter season in California is off to a slow start this year, and with the possible exception of the end of December, the next few weeks look relatively dry as well.
- The latest USDM map indicates widespread D0 and D1 conditions across Arizona, Utah, western Colorado, western New Mexico, parts of southeastern Nevada, and over a significant portion of southern California. A small area of severe drought (D2) is indicated by the U.S. Drought Monitor over extreme southern Arizona.
- With most tools favoring drier conditions, it is thought that drought persistence is the best bet for December. New drought development is possible across portions of Arizona, and neighboring parts of Nevada, California, and New Mexico.
- During a typical La Niña winter, there is enhanced dryness across the Southwest, as the jet stream is normally displaced well to the north across the Pacific Northwest and northern Rockies. However, this dry signal generally becomes more robust after the month of December (this particular monthly forecast period).
Confidence for the northern half of the West is high.
- During the past 14-days over approximately the northern half of the West, Percent of Normal Precipitation (PNP) values (which are generally more useful in this region than DNPs) ranged from about 25-50% (interior Oregon) to 200% elsewhere, with localized maxima of 200-300% in some of the higher mountain peaks.
- December is normally a wet month across Washington, Oregon, Idaho, and far western portions of both Montana and Wyoming, but is normally a dry month for most of the remaining parts of Montana and Wyoming. Eastern portions of these last two states typically receive only about 2.0-3.5% of their annual precipitation during the month of December.
- Nearly all of this region is free of dryness/drought, and this is expected to continue through December.
Confidence for the Northeast is high.
- During the past two weeks, precipitation departures from normal (DNPs) across the Northeast ranged primarily from near-normal to 1-2 inches below-normal. Regional climatology (over a 121-year period from 1895-2015) indicates that anywhere from 7.0-9.5% of the annual precipitation is typically received during the month of December.
- At present, there is no drought over the Northeast, but there are several areas of abnormal dryness (D0). With WPC predicting 1-2 inches of precipitation within the first week of December, it is unlikely that any drought will develop.
Confidence for Hawaii is moderate.
- Persistent trade winds have brought beneficial rains to windward slopes of the Hawaiian Islands, and favorable conditions (which support at least some improvement in the drought) are generally anticipated in December. One exception includes the leeward slopes in western Maui, which has received well below-normal rainfall during the month of November.
- Assuming the trades persist in December (and most rain falling over the eastern slopes of Maui), western Maui can expect drought to persist.
- There is no drought in Alaska or Puerto Rico at this time.
Source: Climate Prediction Center