The strong El Niño that is currently in place is considered a major factor in the Seasonal Drought Outlook for December-January-February 2015-16. The southern jetstream and associated storm track that are characteristic of El Niño winters are expected to bring some improvement of drought conditions to California, though given the severity and longevity of the drought in that region, it’s unlikely to completely alleviate precipitation deficits and replenish reservoirs in just one season.
For the Southwest, central and southern Plains, Lower Mississippi Valley, Florida and Georgia, improvement or removal of drought is anticipated. Drought areas in Indiana and Lower Michigan may be removed within the next two weeks, as dynamical models are forecasting an extended period of heavy precipitation.
For New England and the New York Tri-State region, proximity to the climatological storm track should be sufficient to justify drought removal. For the northern tier of states, prospects for substantial drought improvement or relief are less optimistic.
With the expected southward shift of the storm track during El Niño winters, drought is more likely to persist from the Pacific Northwest and central Great Basin to the Upper Great Lakes region. Drought development is favored for central and eastern Montana, and nearby portions of Wyoming and Idaho.
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Drought development is also favored for the Hawaiian Islands, which is something that is commonly observed during El Niño winters. In Puerto Rico, the dry season is beginning. This is at odds with historical El Niño precipitation patterns, which favor above-median rainfall during the cold season. It is thought that the wetness associated with El Niño during the upcoming winter may be enough to justify improvement and/or removal of drought across the island.
Forecast confidence for California is moderate.
- The wet season in California begins in November, and climatological precipitation increases substantially during December and January, so drought reduction would be more likely to occur during these months.
- El Niños have historically enhanced the wet season across California through an active Pacific jet, with the highest climatological anomalies across central and southern California beginning in DJF. This is reflected in the most recent CPC monthly and seasonal outlooks.
- Uncertainty about whether the precipitation will be sufficient to begin eroding the largely extreme to exceptional multi-year drought, and whether temperatures will be supportive of significant snowpack building across the Sierras is high.
- The CPC seasonal outlook indicates enhanced chances for above-normal temperatures, which is not ideal for snowpack development, but the impacts of warmer temperatures can be partially offset by increased total precipitation amounts due to El Niño.
- Based on these considerations, the best chances for drought reduction are in the regions favored for enhanced precipitation, which are central and southern California.
- Historically, strong El Niño’s have been associated with enhanced convection over the far eastern Pacific, influencing all of California. This is why the anticipated Improvement area has been expanded northward to include most of the state.
Forecast confidence for the Desert Southwest and Great Basin is moderate to high.
- The Advanced Hydrologic Prediction Service (AHPS) shows that during the past 30-days, much of New Mexico, Arizona, Nevada, Colorado, and southern Utah have received from 100 to 300 percent of normal precipitation (PNP). In contrast, northern Utah has been missing out on precipitation, with most PNP values ranging between 25 and 75 percent.
- As of November 18th, SNOTEL Snow Water Content (SWC) is greater than 200 percent of average in the Mogollon Rim area of Arizona, and in the Sierra Nevada Range straddling California and Nevada. In contrast, northern Utah reports SWC values between 50 and 75 percent of average.
- As for the upcoming winter outlook, El Niño favors a southern jet across the Southwest, which is expected to lead to either improvement or removal of existing drought.
Forecast confidence for the northern Plains and upper Mississippi Valley is moderate.
- During the last 30-days, eastern sections of both Montana and North Dakota, and northwestern Minnesota, reported a precipitation surplus of 0.5-2 inches, with the remainder of the northern Plains receiving near-normal precipitation.
- Little if any precipitation is predicted in the next 7-days by WPC. CPC’s Week 2 precipitation outlook, as well as the North American Ensemble Forecast System (NAEFS) both predict above-median precipitation for the area.
- For the December and DJF periods, CPC anticipates enhanced chances of below-median precipitation amounts.
- Overall, the drought areas in northwestern Minnesota and east-central North Dakota are forecast to persist.
- In central and eastern Montana, where CPC’s seasonal precipitation outlook depicts higher odds for below-median precipitation, an area of drought development was added.
Forecast confidence for the central and southern Plains and the Lower Mississippi Valley is moderate to high.
- Precipitation anticipated within the next two weeks is expected to be heavy across the southern Plains, and the central and southern portions of the Mississippi Valley, in advance of a mean upper-level trough.
- The total precipitation forecasts from the latest two runs of the GFS model (6z and 12z) predict 3-5 inches of rainfall across a sizable portion of this region. Kansas and Nebraska may miss out on the anticipated swath of heavy precipitation in the next two weeks.
- However, with CPC’s one-month and three-month precipitation outlooks calling for increased odds for above-median precipitation across much of the central and southern Plains, and the Lower Mississippi Valley, Kansas and Nebraska are still expected to experience drought removal, even if these two states miss out on the heavy precipitation predicted early in this Outlook period.
Forecast confidence for the Great Lakes region is moderate.
- Across the Great Lakes region, the North American Land Data Assimilation System (NLDAS)shows current total column soil moisture deficits between 1-4 inches in southern Lower Michigan, and part of Upper Michigan.
- WPC’s shorter-term precipitation forecast for Week 1, and CPC’s extended-range precipitation forecasts for Week 2 anticipate significant precipitation for Michigan.
- CPC’s monthly and seasonal precipitation outlooks, however, call for enhanced odds of below-median precipitation, which is consistent with ENSO composites for these periods.
- The CFS, NMME, and IMME, predict varying amounts of relative dryness across the Great Lakes region.
- It is reasoned that enough precipitation could fall and settle into the ground across Lower Michigan (and Indiana) before the ground freezes, but this appears less likely across Upper Michigan.
- Therefore, drought removal is indicated for Lower Michigan, while drought persistence is favored in Upper Michigan.
Forecast confidence for the East is high.
- Remaining drought areas in the central Florida panhandle, southwestern Georgia, and the Florida Everglades are expected to be removed during the upcoming winter season, as these areas are typically influenced by the southern jetstream and cyclonic activity usually associated with an El Niño.
- In the Northeast, it is very difficult to get persistent, well-established drought areas due to the high climatic variability across this portion of the country.
- These considerations, in addition to the official CPC precipitation outlooks for December and DJF which favor above-median rainfall, warrant the removal of drought in these areas.
Forecast confidence for the Northwest is moderate.
- Recent onshore flow events in the Pacific Northwest resulted in significant precipitation surpluses across western Washington state.
- For example, AHPS depicts surpluses of 4-8 inches across much of this area, warranting the recent removal of moderate drought (D1).
- As the traditional dry signal associated with cold season El Niño’s is generally back-loaded (i.e., occurs later in the winter) across the Northwest, no drought development area has been indicated on the map for western Washington.
- For the remainder of the Northwest, including the northern Rockies, drought persistence is favored during DJF.
Forecast confidence for Hawaii is moderate to high.
- El Niño’s are typically associated with a suppressed rainy season across Hawaii. This is especially true of a strong El Niño, as the associated enhanced convection is usually displaced from the central Pacific to the eastern Pacific.
- In recent weeks, Hawaii has been drought-free, but that is expected to change. CPC’s seasonal rainfall outlook for DJF 2015-16 depicts elevated odds of below-median rainfall for Hawaii.
- Based on these factors, drought development is considered likely across all of the archipelago.
Forecast confidence for Puerto Rico is moderate.
- The climatological dry season begins during November in Puerto Rico. During El Niño winters, however, there is a preference for wetness across not only Puerto Rico, but also the Virgin Islands and northeastern Caribbean in general.
- The main uncertainty is whether or not wetter-than-median conditions is actually enough to improve drought conditions by a full category.
- It was decided to go with a 1-category, across-the-board improvement to the depiction, so that areas currently in severe drought (D2) are expected to improve to moderate drought (D1), and areas currently in moderate drought (D1) are expected to be removed.
