US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM
EDT May 10 2021
Synopsis: Regions of mid-level high pressure are
forecast across the northeastern Pacific and Great Lakes during Week-2, with
mid-level low pressure possible across the Intermountain West. An advancing
cold front with accompanying heavy rainfall chances is likely to cross the
Central U.S. during the early portion of the forecast period. Forecast
solutions featuring increased mid-level low pressure across the West would
support some risk of high winds across roughly the southwestern quarter of the
Lower-48 states. Mid-level low pressure is forecast over the Bering Sea which
could bring a wet pattern to the southern coast of Alaska.
Slight risk of heavy rain across portions of the
Central and Southern Plains, and the Middle and Lower Mississippi Valley,
Tue-Wed, May 18-19.
Slight risk of high winds across portions of the Central Great Basin,
Southwest, Rockies, and High Plains, Tue-Mon, May 18-24.
For Tuesday May 18 - Monday May
24: The Weather Prediction Center (WPC) surface analysis for Day 7 shows a
surface low pressure center near the Sandhills of Nebraska with a trailing dry
line and cold front extending southward over the High Plains. As this system
transits eastward the cold front is likely to overcome the dry line and become
the primary focus for heavy rainfall concerns during Week-2. A slight risk of
heavy rain is forecast for much of the Central and Southern Plains and Middle
and Lower Mississippi Valley on May 18th and 19th tied to the advancing
boundary. Discrepancies exist regarding how progressive the system is,
resulting in a slower advance in the GEFS (primarily focused over the Great
Plains) versus the quicker ECMWF (targeting the Mississippi Valley), resulting
in the broad shape covering both days. The GEFS is also quicker to wash out the
precipitation risks beyond the 18th, while the ECMWF lingers precipitation into
the 19th, resulting in the two day forecast period. Flash flooding could be a
concern with this system as currently observed soil moisture levels are
generally above normal across the area highlighted for the hazard, while
another 2-4 inches of rain are possible over the coming week per WPC.
Mid-level low pressure is forecast to varying degrees across the West in
the mean Week-2 circulation field. This feature digs the furthest south in the
ECMWF ensembles, while the Canadian ensembles exhibit similar height departures
with some northwestward displacement. Conversely, today's 0Z GEFS has only
marginally below-normal heights confined to the Northern Rockies. The more
amplified solutions of the ECMWF and Canadian models would support periodic
high wind risks throughout Week-2 extending across the Southwest, Rockies, and
High Plains depending on the timing of shortwave activity rotating through the
longwave axis. A broad slight risk of high winds is posted for these regions
across the entirety of Week-2 where winds of 35+ knots are possible but
difficult to pinpoint in time and space.
Anomalous mid-level troughing is forecast over the Bering Sea during
Week-2, supportive of a wet pattern for areas adjacent to the Gulf of Alaska
and potentially the Bering Sea. Despite this, it's a relatively dry time of
year for the region and forecast precipitation totals appear unimpressive.
Reforecast guidance for temperature across the state is mixed, with the GEFS
generally favoring near-normal temperatures, while the ECMWF tilts strongly
toward above-normal temperatures for the northern two-thirds of the state. If
the ECMWF solution were to verify this could exacerbate river flooding concerns
during break-up season for regions north of the Brooks Range.
Forecaster: Daniel Harnos
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.