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Latest Seasonal Assessment -
Heavy rains toward the end of August and during the first
half of September relieved drought over the central and northern Plains, and the outlook calls
for more improvement in coming months, especially from Oklahoma into South Dakota and
Minnesota. Improvement is expected to be more limited in Michigan and Wisconsin,
although forecast rains during the last half of September will be beneficial. Drought is
forecast to persist in southern Iowa and northwestern Missouri, as precipitation is expected
to be below normal during October-December. Turning toward the West, September rains
have already begun to relieve drought in Washington and Oregon, and additional
improvement is anticipated into December for the Pacific Northwest. Some improvement is
on tap for the Great Basin and central and northern Rockies, although pronounced relief
from the water shortages will depend on the upcoming winter snow season. A tendency
toward above-normal temperatures and below normal precipitation means that drought
should largely persist across the Southwest. Limited drought improvement should continue
in the Hawaiian islands.
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Tools used in the drought outlook included: the official CPC long-lead
precipitation outlook for the next 90 days, the Palmer Drought Index probability projections
for the next 3 months, and various medium and short-range forecasts and models such as the 6-10 day and 8-14 day
forecasts and the 2-week soil moisture forecast. Additional tools included the constructed analogue from soil
moisture forecasts.
As in the previous forecast, with weak signals from the various forecast tools regarding the
3-month outlook for precipitation over most of the country, this forecast focuses on
climatology, the Constructed Analogue Soil moisture tool for December, and the near-term
circulation trends as seen in the latest model runs.
The soil moisture Constructed Analogue tool, CAS, implies a tendency for drought improvement
across the northwest quadrant of the country with below-normal precipitation leading to
persisting or even worsening drought across the Southwest. This would be consistent with a
trough near the Northwest coast. A constructed analogue forecast of 500-hpa height anomalies
based on August SST data does suggest troughing over western Canada with ridging to the west
near the Aleutians. This pattern, including above-normal heights over eastern North America,
also correlates with the negative QBO. This is in contrast to the pattern expected over the next 2
weeks or so, which shows troughing over the central or eastern United States and ridging over
the West. Since the near-term pattern differs considerably from the pattern implied for the long
term by the Constructed Analogue tools, confidence for this forecast is quite low.
CAS shows dryness for October-December extending from the Great Lakes southward through
the Ohio Valley and westward to the southern Plains. With the near-term outlook showing
normal to above-normal rainfall for much of the Midwest, the drought forecast indicates the
ambiguity of the situation by showing limited improvement for the Michigan-Wisconsin area.
Recent trends, short term forecasts, Palmer Drought Index probabilities, and QBO correlations
argue for improvement in much of the Great Plains. The lingering drought shown in southern
Iowa and northwestern Missouri is consistent with the CPC forecast of dryness for Oct-Dec, but
improvement was shown in western Iowa and northeastern Kansas because of ample rain
expected in the near-term. Both short-term and long-range forecasts imply improving conditions
in central Kansas and central Oklahoma.
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www.nws.noaa.gov
