Drought Outlook-Some Improvements Expected Across the South12/02/2016
A shift in the upper-air pattern in late November brought welcome precipitation to the lower Mississippi, Tennessee, and Ohio Valleys, portions of the parched Southeast, and most of the Northeast, with additional moderate to heavy precipitation predicted during the next 2 weeks.
Although not every location in the Southeast and Northeast may receive surplus precipitation, it is expected that most of these 2 regions will record enough precipitation to ease drought by at least one category at the end of December as this is a favorable time of the year for soil moisture recharge where the ground is not frozen (currently nearly all areas).
In the central High Plains, a dry climatology, a dry QPF, and only a slight tilt toward above-median precipitation in both ERFs and the updated 1-month precipitation outlook does not favor any improvement. The drought in western South Dakota would be difficult to improve given the dry December climatology and its long-term nature, but the smaller areas of short-term D1 in the eastern Dakotas are more likely for removal with favorable odds for above-median precipitation at all time periods.
Similarly in the Northwest, recent wetness plus a forecast for unsettled and cold weather should aid improvement in the few small remaining drought areas. In California and the Southwest, recent precipitation was welcome but not enough to warrant any improvement.
With conflicting short (QPF) and medium-term (ERFs) forecasts of dryness followed by wet weather (and EC for updated Dec precipitation outlook) during normally one of the wettest months of the year for California, too many uncertainties made it best to leave the area as is (persistence). Temperatures during the first half of December should average well below normal in the West.
Although recent dryness has expanded D0 across portions of Hawaii, improvement is still favored for the small areas of D1 and D2 on leeward sides of Kauai, Maui, and the Big Island by the end of the year. There is currently no drought on Puerto Rico and Alaska (only some D0), and no D1 development is expected by Dec. 31.
Forecast confidence for the Tennessee and lower Ohio Valleys is moderate to high.
- The lower Ohio Valley received under an inch of precipitation (25-50% of normal, 2-3 inch deficits) during the past 30-days while the Tennessee Valley received 0.5-1.5 inches, or about 15-50% of normal precipitation.
- The 30-day shortages, however, were a bit greater in the Tennessee Valley (2-4 inches) as November normals increase to the south of the Ohio Valley. This is consistent with below to much-below normal 7-, 14-, and 28-day average USGS stream flows, very poor topsoil moisture and range/pasture conditions, numerous wild fires, low stock ponds, and a 2-3 USDM drought category decline over the past 4 weeks.
- In addition, temperatures averaging 3-6 degF above normal since Oct. 29 exacerbated the short-term drought.
- In sharp contrast, recent rainfall from a change in the upper-air pattern, and somewhat favorable (wet) December climatology and CPC soil moisture changes, along with all precipitation outlooks out to a month tilted toward above-median totals, indicate relief from short-term drought across the lower Ohio and Tennessee Valleys.
- Improvement confidence is a bit greater here than in the Northeast and Southeast (longer-term drought) due to the short-term length of this drought.
Forecast confidence for the Southeast is moderate to high.
- The ongoing drought, which started in the late Spring, has greatly increased in coverage and intensity during the past 4-weeks due to little or no rains since late October, especially in Alabama, Georgia, the Florida Panhandle, eastern half of Tennessee, and the southern Appalachians.
- And with recent dryness extending westward, the Southeast drought has pushed into the lower Mississippi Valley and southern Great Plains.
- During the past 90-days, minimal rainfall (<2 inches) has fallen on the interior Southeast (central Mississippi northeastward into the western Carolinas), creating deficits of 8-12 inches. Temperatures averaging 4-6 degF above normal during the past 60-days have made conditions worse.
- Numerous large wildfires, very poor agricultural conditions (soil moisture, pastures, winter crops, ponds), and many record low 7-, 14-, and 28-day average USGS stream flows have resulted.
- In the Nov. 22 US Drought Monitor, D3 was widespread from northeastern Louisiana northeastward into western North Carolina, with D4 entrenched in northern Alabama and Georgia, southeastern Tennessee, and the western Carolinas.
- Fortunately, all bad things must eventually end as a change to the upper-air pattern in late November brought numerous storm systems and ample moisture to the Southeast, first in western and northern sections, and gradually spreading to eastern areas.
- With moderate to heavy rains the past few days, 7-day QPF totals of 2-6 inches, favorable chances of above-median precipitation in the 6-10 and 8-14 day ERFs and the updated December precipitation outlook, low temperatures, evaporation, and water demand, and favorable (wet) December climatology and CPC soil moisture changes, some improvement is bound to occur in most of the Southeast during December.
- With less certainty of wet weather in far southeastern (southern Georgia and northern Florida) and western (eastern Oklahoma) areas, persistence or some development of existing D0 (northern Florida) was made.
Forecast confidence for the lower Mississippi Valley (Delta) and southern Plains is moderate to high in southern and eastern areas, and moderate in western and northern sections.
- During the past 30-, 60-, and 90-days, less than half of normal precipitation had fallen on the lower Mississippi Valley and south-central Great Plains, resulting in 60-day deficits of 4-6 inches, a 1-2 category decline in the 4-week USDM change map, and D2 throughout much of the region.
- Additionally, the short-term drought (D2-D3) in the Delta and south-central Great Plains recently merged with the Southeast drought, forming a rather expansive Southern drought area.
- However, recent rains (1-2 inches) in this region have halted further deterioration. And with favorable (wet) QPF, ERF, and updated 1-month precipitation outlooks, a slightly wet December climatology, positive CPC soil moisture changes observed during December (since 1932), and a shorter period of drought than the Southeast, the odds for improvement during December are quite good, especially in southern and eastern sections.
- The exception is eastern Oklahoma where recent rainfall was less, and all forecasts suggest near or below-normal precipitation, thus persistence was left there.
Forecast confidence for the central and northern Plains is moderate.
- Although 30- and 60-day precipitation was well below normal in parts of eastern Montana, most of the Dakotas, Nebraska, Kansas, and eastern Colorado, a rather dry late fall precipitation climatology created 60-day deficiencies of only 0.5-2 inches. Despite these deficits, USGS stream flows in this region were predominantly near-normal.
- Another factor leading to some D0, D1, and D2 expansion, especially in the central High Plains during the past 4-weeks, was 60-day temperatures averaging 6-9 degF above normal.
- Although recent precipitation (including 5-15 inches of snow blanketing the Dakotas), QPF totals of 0.3-0.5 inches, subnormal temperatures (finally), and tilted odds for above-median precipitation in both ERF periods and the updated 1-month December outlook (mainly northern areas) are favorable, the dry climatology and long-term duration of the drought in western South Dakota is probably not enough for any improvement in December.
- Likewise, a less favorable precipitation forecast for the central and south-central Plains left persistence as the best option.
- The two small short-term D1 areas in the eastern Dakotas, however, may receive enough moisture from Nov. 22-Dec. 31 for improvement.
Forecast confidence for California and the Southwest is moderate.
- During the past 30-days, precipitation was mixed across California, with portions of the coast and the Sierra Nevada above-normal while the interior valleys and the southeast were below normal. At 14-days, however, much of the state was above normal, while at 60-days the northern half was well above normal (thanks to a wet October).
- The Southwest has also seen above-normal precipitation in the short-term (14- and 30-days), but drier conditions at 60-days.
- The recent precipitation and colder weather has provided a good start to this year’s snowpack in the Southwest and Sierra Nevada, although it is still early in the Water Year.
- Not surprisingly, with generally near-normal conditions in most areas, no changes were made to the 4-week DM change map except for some D0 development in southeastern Utah, southwestern Colorado, and northern New Mexico.
- The climatology for December is a wet one for most of California, and somewhat wet for Arizona and Nevada, while the CPC soil moisture history during December depicts a strong positive (wet) change in California, and smaller changes in areas to the east.
- In the short-term (QPF and 6-10 day ERF), odds favor little or no precipitation but much colder conditions. The 8-14 day ERF is a bit wetter in California, but still favors sub-median rainfall and temperatures in the Southwest.
- The updated 1-month precipitation outlook is EC across southern sections, and slight odds for above-median precipitation in northern locations.
- Based upon the multi-year drought in central and southern California, a less severe and shorter drought in the Southwest, and no obvious forecasts that lean toward improvement or deterioration during December, persistence is the forecast for this region.
Forecast confidence for the northern Rockies and Intermountain West is moderate.
- AHPS and ACIS precipitation anomalies during the past 30-days indicated variable conditions (above and below normal precipitation) for the northern Rockies, and subnormal precipitation for the northern and central Intermountain West.
- Some slight improvements were made in the 4-week USDM change map (northwestern Wyoming, central Oregon, northwestern Nevada), but these occurred early in the period (late October) and reflected the unusually wet October across the northwestern quarter of the lower 48 States.
- During the past 14-days, much drier conditions have prevailed. But with a wet December climatology, a normal gain in the historic CPC soil moisture during December, 60-day surpluses, and wet and cold forecasts for the QPF, ERFs, and updated 1-month outlook, improvement is expected for the small drought area in western Montana and in northeastern Oregon.
- Southeastern Oregon missed the surplus October precipitation, so it is less primed for improvement, thus persistence was left there.
- Similarly in the Great Basin (Nevada and Utah), the past 60-days were near or below normal, and with lower odds for above-median precipitation, these areas were left unchanged.
Forecast confidence for the Northeast is moderate.
- Varying amounts of precipitation fell on the Northeast during the past 30-days, with near-normal totals in parts of the eastern Great Lakes region, northern Maine, and eastern New York where some slight USDM improvements occurred. Much less precipitation across coastal New England and the mid-Atlantic generated 30-day deficits of 2-4 inches as the drought worsened in these regions.
- Current USGS 7-, 14-, and 28-day average stream flows were at near- or record low levels for the period ending Nov. 29 from Massachusetts southward into northern Virginia.
- During late November, however, a change in the upper-air pattern brought light to moderate precipitation to much of the Northeast during the past few days, and both the short and medium-range precipitation outlooks from WPC and CPC (7-days out to 1-month) favor continuing the wet pattern currently ongoing across the entire Northeast well into December.
- Although it appears as though this expected wet pattern should provide improvement to the entire region, the northern portions of the region, especially coastal New England, are in long-term drought (hydrologic impacts) and may require more precipitation for relief than areas to the south where the drought is shorter in duration.
- But the fall and early winter are ideal times of the year for soil moisture recharge (before the ground freezes) with low temperatures, little or no evaporation and evapotranspiration, lower water demand, more infiltrating rains (as compared to summertime thunderstorm deluges that tend to runoff), and wet snow (melts slowly into soils).
- In addition, the CPC soil moisture climatology (since 1932) has a low frequency (10-25%) of negative soil moisture changes during December, with an overall slight 1-month median increase (5-10%) in the Northeast during December. Thus, this is the time for improvement in most areas if decent precipitation occurs, and it appears as though it will.
Forecast confidence for Hawaii is moderate.
- There is currently no drought in Alaska and Puerto Rico, and none expected by Dec. 31.
- Abnormal dryness (D0) in Hawaii expanded slightly in coverage during the past 1-2 weeks, though stream flows are currently near normal. Improvement is anticipated throughout the Hawaiian Islands as wetter weather is forecast during early December, and the odds favor above-median rainfall for December.
- With December looking wet, some improvement is expected to the small D1-D2 areas on the leeward sides of Kauai, Maui, and the Big Island.