Latest Seasonal Assessment -
Powerful Pacific storms striking the West during the early part of the forecast period, especially during the last
10 days of October, will bring substantial rain and snow to parts of the region. The greatest drought benefits are
expected over the western portions of the western drought region, with the October 19-20 storm dropping 1 to 3 feet
of snow over the high Sierra just before the start of this outlook period, and another major storm forecast during
the following week. Unfortunately, the water supply situation in drought-affected areas for most of the interior
West is not expected to change significantly through early winter, and the overall drought outlook calls for only
limited improvement in these areas. The ongoing multi-year drought has left major reservoirs near their lowest
levels in decades, and it will take more than a few storms to recover. Also, the recent stormy circulation pattern
will likely not persist through the forecast period, with a drier and milder regime expected for much of the
November-January period. Looking beyond this outlook period, various seasonal forecast models, as well as
historical analogues with weak to moderate El Niño events, suggest improving odds for significant rain and snow
toward late winter and spring across the Southwest, including the Colorado River Basin, and that could bode well
for mountain snowpack this spring.
The outlook also calls for limited improvement in the High Plains drought region, with the odds for improvement
less in northeastern Wyoming and southeastern Montana, an area that is experiencing extreme to exceptional
drought. Farther east, storms over the short term should ease lingering drought in the Wisconsin-Michigan area. The
odds favor a return to drier weather for the Great Lakes region, as well as southward into the Ohio Valley, later
in the outlook period, so the entire region will be monitored closely.
El Niño tends to favor dryness in Hawaii, and the seasonal precipitation outlook for November-January indicates
a tilt of the odds toward dryness in this region. Although some rain is expected for the short-term, the longer-term
outlook implies a risk for expanding drought by the end of January. The U.S. Drought Monitor already depicts drought
over eastern and northern parts of the Big Island, with some abnormally dry conditions in Maui.
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Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for November-January, the
drought termination and amelioration probabilities for January, various medium and short-range forecasts
and models such as the 6-10 day and 8-14 day forecasts, and the soil moisture tools based on the
GFS model and the Constructed Analogues for the season. For this outlook, a number of analogues based on
historical instances of weak to moderate El Niño events were consulted, as well as updated runs of the
Coupled Forecast System (CFS).
Once again, the circulation pattern at the start of the forecast period features a deep upper level trough near
the West Coast. The mean trough position is expected to persist near the West Coast at least into late October. This
should lead to more storms dumping rain and snow on the western drought area. The storm lashing California on
October 19-20, just before the start of the outlook period, was expected to bring 1-3 feet of snow to the high
Sierra and heavy rains to lower elevations. Models indicated another storm should strike the Pacific states
around October 26-28, with a deep upper level trough or cutoff low positioned near California. Snow and rain
would again spread eastward well beyond the Pacific states. Although a highly amplified pattern is likely to
continue into November, there are some indications that the mean trough will migrate eastward, with ridging
developing over the West, resulting in a transition to a positive PNA pattern during early November. This would
lead to a milder and drier pattern for the West. Seasonal models, such as the CFS, as well as weak El Niño
analogues, suggest a tendency toward positive PNA (ridge west, trough east) patterns during the winter, and
experimental RSM and GSM weekly forecasts depict this pattern by January, as well as periodically during
November-December. Drought improvement is shown for the western fringes of the drought region largely because
of the short-term storm pattern, and because the drought is less firmly entrenched in California, where low
reservoirs are less of an issue than farther east. Only limited improvement is indicated for most of the western
and High Plains drought region because of the difficulty in relieving water shortages this early in the snow
season. Also, the official November-January precipitation outlook calls for equal chances wet or dry. The latest
CFS run, updated on October 20, shows an area of below-normal precipitation during November-January for the Upper
Colorado River Basin extending northwestward to Washington and Oregon. The expected benefits from this El Niño, should
they occur, would tend to take place more toward late winter and spring, based on historical analogues and the
CFS model, and this would mainly affect the Southwest and Colorado River Basin. The area of persisting drought in
Montana and Wyoming is based on climatology and the expectation of relatively limited benefit from the near-term storms.
The near-term outlook for the lingering drought areas in the Great Lakes region is promising, with a number of
low pressure systems expected to bring rain during the first two weeks of the outlook period. The longer-term
outlook is problematical, with both the CFS model runs and El Niño analogues showing a tendency for dryness
during November-January from the Great Lakes southward into the lower Mississippi Valley. The drought outlook
is focusing more on the shorter-term outlook in this area, where the confidence is higher, and the fact that
recent rains have already made the drought area in Michigan very marginal. If drought does persist or
return, indications are that this is more likely in Wisconsin, where the drought is more firmly established and
analogues and models both tilt toward November-January dryness.
In Hawaii, dry weather since September has already resulted in drought development in parts of the Big Island
and dryness in parts of Maui. Further drought development is indicated because of a dry November-January
precipitation outlook due to the El Niño. Confidence is low for the drought development outlook given a near-term
forecast for rain and historical analogues showing dryness tending to kick in late in the outlook period rather
than early. In addition, the complex climates on the islands make long-range forecasts especially difficult, so
a broad-brush approach was used for this drought outlook.
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