El Niño has transitioned to ENSO-neutral, and ENSO-neutral conditions are most likely to continue through the Northern Hemisphere winter 2019-20 (50-55% chance). Drought coverage remains very low across the contiguous U.S. (CONUS).
Across the West, as the summer monsoon season winds down in September, the West Coast rainy season will gradually ramp up. Drought currently in place across portions of the Northwest and western parts of New Mexico are favored to persist during the Sep-Oct-Nov (SON) 2019 season, mainly because significant, longer-term precipitation deficits are considered unlikely to be overcome.
Over the Midwest, small areas of moderate drought (D1) were recently introduced to the U.S. Drought Monitor map, but these areas are expected to be removed during the SON season. In fact, heavy rainfall predicted during the next 1-2 weeks may be enough to wipe out budding drought in Iowa and northwestern Illinois.
CPC’s monthly and seasonal precipitation outlooks, valid for September and SON respectively, favor above normal precipitation for North Dakota, supporting drought removal there. Across the southern Great Plains, high temperatures and reduced precipitation have resulted in drought expansion. Drought development is also possible across southern Texas.
In the Southeast, short-term rainfall may be enough to result in the improvement and/or removal of localized pockets of drought currently in place. The exception is southeastern North Carolina and adjacent parts of South Carolina, where 6-8 inch rainfall deficits have mounted over the past 3 months.
In the eastern Interior of Alaska, moderate drought was removed from the U.S. Drought Monitor depiction this week. Autumnal westerlies are climatologically favored to strengthen and shift southward towards the southern coast of Alaska, helping to focus storm tracks and their associated precipitation patterns. This supports at least a one-category improvement in drought conditions across southern Alaska in SON.
Farther south over the Central Pacific, the low-latitude trade winds are expected to strengthen now that the El Niño has ended, and result in increased rainfall across the Hawaiian Islands, especially windward areas. A seasonal increase in mid-latitude storm activity may also bring needed rainfall to the drier leeward slopes.
For Puerto Rico, various dynamical model precipitation forecasts are at odds with the expected uptick in Atlantic hurricane activity for the SON season. With these conflicting indications, and in the absence of strong climate signals for either increased dryness or wetness, it is thought that drought persistence is most likely for the southern portion of Puerto Rico.
Forecast confidence is moderate for the Southeast Region.
- Relatively small-scale areas of moderate and severe drought (D1 and D2, respectively) are indicated on the U.S. Drought Monitor map over the Southeast.
- The SON season is a somewhat drier time of year for Alabama and Georgia, with precipitation close to long-term averages for the Carolinas.
- During the past 60-days, Percent of Normal Precipitation (PNP) ranges between the 50th and 90th percentiles. Soil moisture is near to below normal across this region.
- Between WPC’s Week-1 precipitation prediction of 1.5 to 3 inches of rain (locally greater) over portions of the Southeast, and CPC’s official 6-10 day and 8-14 day precipitation outlooks favoring above normal rainfall, it is thought that most drought areas of the Southeast should be either improved or removed, with the exception of southeastern North Carolina and adjacent parts of South Carolina, where 6-8 inch deficits remain during the past 90-days.
Forecast confidence is moderate in Texas, and moderate to high in Oklahoma.
- Moderate and severe drought (D1 and D2, respectively) expanded in coverage across portions of Texas and Oklahoma this week. This is not surprising, since Departure from Normal Precipitation (DNP) during the past 60-days ranged from 2-4 inches below normal, with localized 4-6 inch deficits in the Lawton/Wichita Falls area.
- Though persistent heat in this region has taken its toll on crops and depleted topsoil moisture, the deeper subsoil moisture profile is still adequate.
- For the 6-10 day and 8-14 day forecast periods, below to near normal precipitation, and above normal temperatures, are predicted. For the monthly and seasonal periods, EC is favored for most of the precipitation outlook.
- During the autumn season, a climatological reduction in both temperature and evapotranspiration make for a much more forgiving environment for crops and soil moisture, even though SON tends to be a somewhat drier time of year. Considering these factors, the current drought areas in the South are forecast to persist, with an area of drought development expected over southern Texas.
- However, an important wild card for Texas in particular is the potential for impacts from tropical cyclone activity in the Gulf of Mexico, especially during September and October.
Forecast confidence is moderate to high in the Midwest Region.
- A number of small-scale areas of moderate drought (D1) were introduced this week to the U.S. Drought Monitor in portions of the Midwest region. Affected states include Iowa, Illinois, and Indiana. This follows an extended period of abundant rains this summer that have kept much of the surrounding Midwest region adequately to overly moist.
- Some dryness (50-90 percent of normal) at 60-days, and a relative minimum in soil moisture, were at least partial contributors towards the recent degradation.
- WPC’s 1-7 day precipitation outlook depicts anywhere from 1.5 to 5 inches of rain over the newly introduced drought areas in Iowa and northwestern Illinois, with less than a half-inch over central Illinois and Indiana. Odds are slightly tilted toward above normal precipitation for all Midwestern areas of drought at the extended-range (days 6-14).
- The monthly and seasonal precipitation outlooks depict slightly enhanced tilts in the odds towards wetter-than-normal conditions only over parts of Iowa, with EC predicted elsewhere.
- Therefore, drought removal is predicted over Iowa and northwestern Illinois in the short-term, and drought removal is anticipated later in the SON season for remaining parts of Illinois and Indiana.
Forecast confidence is moderate to high in northern North Dakota, and low to moderate in southwestern Kansas.
- For the High Plains region, there are two drought areas of concern. The first is over northern North Dakota and the second is over southwestern Kansas. The former area has missed out on most of the heavy rainfall and high soil moisture values that have dominated the north-central CONUS during the past several months.
- CPC’s extended- range (6-10 day and 8-14 day) precipitation outlooks, and the new outlooks for September and SON, all support elevated odds of above normal precipitation. Therefore, a one-category improvement is forecast over North Dakota.
- The second drought area of concern is southwestern Kansas. During the past 60-days, surplus rainfall (PNP values ranging from 110 to 300 percent) fell to the west, north, east, and southeast of this dry area. This week’s U.S. Drought Monitor introduced moderate drought (D1) to southwestern Kansas.
- Precipitation climatology for SON favors somewhat drier conditions in southwestern Kansas. NLDAS soil moisture profiles for the root zone (top 1 meter of soil) indicate near normal conditions.
- The extended-range precipitation outlooks (days 6-14) favor below normal precipitation, while the September outlook depicts enhanced odds of above normal.
- The SON precipitation outlook indicates EC for southwestern Kansas. Given the fluctuating signals out through September, and the prediction of EC for the SON season as a whole, it was thought that drought persistence was the most likely outcome.
Forecast confidence is considered moderate for the Northwest, and low to moderate for western New Mexico.
- In the West, there are two areas of concern related to drought.
- The first area encompasses northwestern Oregon, much of Washington, far northern Idaho, and extreme northwestern parts of Montana. There is moderate to severe drought (characterized as D1 and D2, respectively, on the U.S. Drought Monitor) currently in this area.
- Water-Year-To-Date (WYTD, since Oct 1, 2018) precipitation deficits range from 12-20 inches over much of this region, and this shortfall is not likely to be made up any time soon.
- One factor in its favor is that the climatological rainy season for the West Coast states begins during the first half of the SON season. However, the large deficits noted above, and the lack of a wet tilt in the odds in the monthly and seasonal precipitation outlooks both favor persistence as the most likely scenario across this region.
- The second area of concern related to drought is western New Mexico, which is currently experiencing moderate drought (D1). Though CPC’s SON outlook for precipitation tilts slightly towards above normal in western New Mexico, there are other factors to consider.
- The Southwest monsoon (which has been fairly weak so far) typically winds down in September, and this area is already contending with substantial longer-term deficits that have yet to be overcome. Therefore, it is thought that drought persistence is the best bet for this area during SON.
- There is a significant wildcard to consider for this area, especially during September and October; recurving eastern Pacific tropical cyclones. These systems typically initiate Gulf moisture surges, which can result in heavy rainfall across portions of the American Southwest. Unfortunately, these events cannot be predicted this far in advance.
- Elsewhere, California remains drought-free, despite their dry season. This is due to sufficient water supplies, such as mountain snow pack, and adequate reservoir levels.
Forecast confidence is moderate to high for the Northeast Region.
- The Northeast region has remained drought-free since early November 2018. Year-to-Date (YTD) precipitation is still normal to wet, and CPC’s monthly and seasonal precipitation outlooks do not favor significant deviations from climatological odds. Therefore, no drought development is forecast.
Forecast confidence is moderate to high for Alaska.
- During autumn (SON) the upper-level westerlies typically strengthen and shift southward, helping to focus the mean storm track and associated precipitation across the southern coast of Alaska.
- CPC’s monthly and seasonal precipitation outlooks favor above normal precipitation over this region. Unfortunately, since the (primarily) hydrologic drought over the southeastern Panhandle is longer-term, it will take a prolonged period of wetness to reduce the deficits.
- Therefore a general one-category improvement is predicted across the Alaska drought areas. The area of moderate drought (D1) over the eastern Interior has been removed from the U.S. Drought Monitor.
Forecast confidence is moderate to high for Hawaii.
- With the transition from a weak El Niño to ENSO-neutral now complete, the trade winds are expected to ramp up to normal strength again during SON, with increased rainfall more likely along windward (east-northeast facing) slopes of the Hawaiian Islands.
- As autumn progresses, mid-latitude systems are likely to have an increasing seasonal influence; this would improve the chances for even leeward areas to get rainfall.
- Regional sea-surface temperatures are at or slightly above 27 degrees C (an important threshold for the initiation of atmospheric convection), and the climatological Central Pacific hurricane season is far from over.
- These factors, in addition to CPC’s monthly and seasonal precipitation outlooks, favor a general one-category improvement in drought conditions across the volcanic archipelago in SON.
Forecast confidence is low for Puerto Rico.
- During the past month in southeastern Puerto Rico there has been some deterioration in drought conditions, with little or no change in drought status over the southwestern portion of the island.
- Various dynamical models predict near to below normal precipitation for the autumn season. This is in contrast to the idea that with El Niño now defunct, upper-level wind shear should gradually diminish, allowing for a more favorable convective environment in the tropical Atlantic.
- Hurricane season normally extends through SON, and there is always the chance of a substantial rain producer affecting Puerto Rico during the autumn.
- Given the conflicting indications and high uncertainty regarding which areas of the Atlantic basin may be impacted by tropical systems this far out, it was decided that drought persistence may be the best bet for Puerto Rico.