326
FXUS21 KWNC 091856
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 09 2026

SYNOPSIS: Mid-level and corresponding surface high pressure forecast across the
central and eastern contiguous U.S. (CONUS) results in an increased risk of
extreme heat across much of the Midwest, as well as a heavy precipitation risk
across portions of the Central and Southern Plains and Middle and Lower
Mississippi Valley. Surface low pressure over the Northern Rockies and Northern
Plains increases risks for high winds in those regions. Meanwhile, along the
West Coast, high winds remain possible as mid-level low pressure develops.

HAZARDS

Slight risk of extreme heat for portions of the Southern and Central Plains and
the Lower and Middle Mississippi, Tennessee and Ohio Valleys, Sun-Mon, May
17-18.

Slight risk of heavy precipitation for portions of the Central and Southern
Plains and the Middle and Lower Mississippi Valley, Sun-Tue, May 17-19.

Slight risk of high winds for much of coastal California and Oregon, Sun-Tue,
May 17-19.

Slight risk of high winds for portions of the Rockies and the Northern and
Central Plains, Sun-Tue, May 17-19.

DETAILED SUMMARY

FOR TUESDAY MAY 12 - SATURDAY MAY 16:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SUNDAY MAY 17 - SATURDAY MAY 23: A potentially significant pattern change
is favored during week-2. Following a period characterized by persistent
500-hPa ridging over much of the western CONUS early in May, both the ECMWF and
GEFS ensembles feature this anomalous ridge center shifting eastward with
positive mid-level height departures overspreading much of the Plains and
Mississippi Valley by late next week. Such a pattern transition is expected to
allow for the development of more troughing to promote cooler temperatures
upstream across the West.  However, well above-normal springtime temperatures
with possible extreme heat conditions may be felt across the Midwest
accompanying the shifting mid-level ridge center. The increased troughing over
the Interior West also supports the potential for shortwave activity where
tools maintain an increased risk of heavy precipitation across the
south-central CONUS tied to surface low development favored in the lee of the
Rockies early in the period.  Later in week-2, much of the anomalous 500-hPa
ridging looks to continue expanding eastward into the eastern U.S., though a
deepening longwave trough favored over the Canadian Maritimes would look to
suppress any additional heat risks further south across the central and eastern
Gulf Coast and the South Atlantic States.



Late in week-1, WPC surface temperature forecasts for day 7 show a widespread
coverage of anomalous warmth over the Interior West, Great Plains and
Mississippi Valley, with several locations indicating positive temperature
departures in the double-digits. This anomalous warmth is expected to persist
heading into week-2, as the National Blend of Models (NBM) shows an increased
number of locations over the Central Plains and Middle Mississippi Valley with
daytime temperature reaching into the mid 90s (deg F) and nearing record
breaking territory. While temperatures above 90 (deg F) are likely to trigger
heat advisories based on historical heat risk data (above orange and red
levels), the potential for maximum temperatures exceeding this threshold is
much less supported in the raw and calibrated GEFS and ECMWF temperature tools
which both limit this potential further to the west over the High Plains where
heat thresholds during May are higher. The GEFS is also less pronounced with
the heat risk, but both GEFS and ECMWF Probabilistic Extremes Tools (PETs) have
trended warmer since yesterday, indicating 30-50% chances of maximum
temperatures exceeding the 85th climatological percentile over portions of the
Plains, Mississippi, Tennessee and Ohio Valleys. To reconcile differences in
the guidance and to capture this warm trend, a slight risk of extreme heat is
issued from the Southern Plains northeastward into the Ohio Valley, valid for
May 17-18 before a moderation of temperatures is anticipated. The unseasonably
warm spring temperatures may adversely impact unsuspecting populations without
cooling or hydration systems in place, and those who are not yet acclimated to
summer-like temperatures.  Beyond this time, the ECMWF PET maintains elevated
signals for maximum temperatures exceeding the 85th percentile over the
southeastern CONUS. No additional temperature hazards are posted due to the
deeper troughing and cooler temperatures favored in the GEFS over the eastern
U.S. later in week-2, though this potential will continue to be monitored in
upcoming outlooks.



Accompanying the anomalous 500-hPa ridge center, mean surface high pressure
remains favored over the southeastern U.S. and into the western Atlantic, where
the prevailing southerly flow not only supports the anomalous warmth in the
Midwest, but also increases the risk of heavy precipitation due to enhanced
moisture advection from the Gulf.  Mean troughing upstream is expected to
induce one or more surface lows, though there remains a large amount of
ensemble spread in regards to the location and strength of low pressure in the
lee of the Rockies.  Compared to yesterday, there is a stronger precipitation
response focused across the Southern Plains in the raw ECMWF ensemble, which
now depicts 30-50% chances for 3-day amounts exceeding an inch. However, the
raw GEFS is comparably drier, and due to uncertainty in the placement and
timing of synoptic forcing, as well as the calibrated PETs not being quite
supportive of a higher designation of the heavy precipitation risk at this
time, a slight risk remains posted for the south-central CONUS for May 17-20.
Notwithstanding this risk, any enhanced precipitation within the highlighted
region is expected to be beneficial for many areas  in the Plains and
Mississippi Valley currently registered in drought.



The mean troughing over the western CONUS, along with potential for shortwave
activity, supports the continuation of the slight risks of high winds (both
valid for May 17-19) over portions of the West Coast as well as over the
Interior West and High Plains, tied to surface low development.  Both the GEFS
and ECMWF PETs continue to show at least 20% chances for wind speeds exceeding
the 85th percentile from Oregon southward to southern California, and although
the GEFS PET lacks any wind signals downstream in proximity to surface low
development, both the ECMWF PET and uncalibrated wind tools are more supportive
with increased chances for gusts exceeding 34kts (~40mph) over parts of the
Plains. This slight wind risk is also expanded further southward into the
Central Plains in the updated outlook based on more increased support for high
wind gusts in the raw tools.



In Alaska, the typical peak of the river ice breakup season is approaching, and
ice jam flooding can occur this time of year with little or no notice as
conditions on frozen rivers can change quickly. Please check with the Alaska
Pacific River Forecast Center for the latest conditions and advisories

FORECASTER: Nick Novella

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