906
FXUS21 KWNC 091807
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 09 2026

SYNOPSIS: Continued mid-level high pressure over the southern and central
contiguous U.S. (CONUS) supports much above-normal temperatures from the Great
Plains to the Mid-Atlantic early in week-2, though the risk of extreme heat is
diminishing as we move closer to the start of autumn. Another round of enhanced
tropospheric moisture is favored to overspread the western and central CONUS,
potentially bringing heavy precipitation and possible flooding over portions of
from the Four Corners eastward to the Ohio Valley and lower Great Lakes.  A
developing area of mid-level low pressure looks to bring an unseasonably cool
air mass east of the Rockies, possibly resulting in first frost and/or freezes
to the higher latitudes of upper Midwest and into the Northeast later in the
period. Antecedent dryness, combined with persistent above-normal temperatures
supports a Rapid Onset Drought (ROD) risk for portions of the Central and
Southern Plains, Lower and Middle Mississippi, Ohio and Tennessee Valleys.

HAZARDS

Slight risk of much below normal temperatures for parts of the Northern Plains,
Upper Mississippi Valley, and eastern Great Lakes, Thu-Sun, Sep 17-20.

Slight risk of much below normal temperatures for parts of northern New
England, Sat-Tue, Sep 19-22.

Slight risk of heavy precipitation from the Four Corners region eastward to the
Ohio Valley and Great Lakes, Thu-Sat, Sep 17-19.

Flooding possible for portions of the Desert Southwest, Great Basin, and
Rockies.

Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains
and the Middle and Lower Mississippi, Tennessee, and Ohio Valleys.

DETAILED SUMMARY

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

FOR THURSDAY SEPTEMBER 17 - WEDNESDAY SEPTEMBER 23: Towards the end of next
week, mean 500-hPa height forecasts from the ECMWF and GEFS ensembles remain in
fairly good agreement favoring an amplified ridge centered over the Gulf of
Alaska with downstream troughing overspreading much of eastern Canada and the
northeastern CONUS. Underneath the amplified ridging over Gulf of Alaska early
in week-2, ensembles also maintain a positively tilted mean trough extending
from the West Coast. This feature, combined with additional Tropical Cyclone
(TC) activity in the eastern Pacific, may bring another round of enhanced
moisture advection into the western CONUS supportive of heavy precipitation
with continued flood risks over the Four Corners. Over time, both the ECMWF and
GEFS ensembles favor some progression of the anomalous ridging over western
Canada along with a deepening of the mid-level trough over eastern Canada by
the middle of week-2. Concurrent with this deepening trough, much of the
enhanced subtropical ridging is favored to ease and become more suppressed
across the southern tier of the CONUS, allowing for cooler, and more fall-like
temperatures mainly east of the Rockies. Such a cooldown is anticipated to be a
welcomed relief particularly for areas embattled by persistent heat conditions
since August across the Plains and Mississippi Valley, however any cold air
advection forced by a high latitude ridge over western Canada looks to bring
unseasonably chilly temperatures with possible first frosts and/or freezes for
portions of the northern tier of the CONUS during the period.



Consistent with the observed height pattern since last month, continued
enhanced subtropical ridging also remains featured in the ensemble guidance and
looks to reach its peak amplitude late in week-1. This mid-level feature is
expected to contribute to a prolonged period of unseasonably late summer
warmth, though the prospects for heat conditions are lowered due to decreasing
sun angles to limit any extreme daytime temperature maximums. While raw
temperature tools depict elevated chances for double digit temperature
departures extending from the Southern and Central Plains eastward to the
Mid-Atlantic associated with a 595-596dm mean ridge axis in the ensembles,
calibrated Probabilistic Extreme Tools (PETs) indicate low chances for daytime
temperatures exceeding 98th climatological percentile and daily heat index
tools have become less supportive of values exceeding hazard thresholds.
Therefore, no corresponding heat hazards are issued, however, those sensitive
to heat should continue to exercise caution as temperatures are expected to
remain well above normal early in the period before the ridging aloft
deamplifies.



A Rapid Onset Drought (ROD) risk remains designated for parts of the Southern
Plains and Lower Mississippi Valley and adjacent portions of the Central
Plains, Ohio, Tennessee and Middle Mississippi Valleys, and Southeast U.S. This
area has seen 20-40% of normal precipitation and deficits of 1-2 inches over
the past 30 days. Above normal temperatures are forecast for this region over
the next two weeks increasing evapotranspiration rates into the third week of
September.



With troughing situated off the West Coast, with the potential for added
tropical moisture from the eastern Pacific, a slight risk of heavy
precipitation remains posted (Sep 17-19) over the Four Corners and High Plains,
with coverage expanded eastward to include portions of the Mississippi and Ohio
Valleys, and the lower Great Lakes in the updated outlook. This expansion is
based on an uptick in heavy precipitation signals in both the GEFS and ECMWF
PETs, as well as the raw ECMWF ensemble favoring a pronounced increase in
precipitable water anomalies (now nearing 200 percent of normal over the
Midwest early in week-2). The enhanced tropospheric moisture is favored to
round the northern periphery of the subtropical ridge, where both the raw GEFS
and ECMWF show increased chances for 3-day precipitation amounts exceeding 1
inch over portions of the highlighted slight risk area. Additionally, given the
continued indications of potential heavy precipitation, a flooding possible
hazard remains posted for portions of the Four Corners region during the week-2
period. This region is prone to flash flooding and caution should be exercised
near canyons and creek beds, and additional rainfall may hamper recovery
efforts from recent flooding in the Grand Canyon.



Tied to the deepening trough favored over the northeastern CONUS, a slight risk
of much below normal temperatures remains posted over the Upper Midwest for Sep
17-20, with a separate cold risk area designated for northern New England valid
for Sep 19-22 to capture the evolution of the cold air advection. While the
GEFS PET is weaker with the cold potential, the ECMWF PET depicts an expanding
area with at least 20-30% chances of minimum temperatures falling below the
15th percentile shifting eastward with time, and uncalibrated GEFS and ECMWF
tools show increased chances for minimum temperatures falling below 38 deg F
over the Northern Plains, Upper Mississippi Valley and Northeast during the
period. While such temperatures are not markedly unusual for this time of year
(climatological first freezes occur by late September for the highlighted
areas), any frost and/or freezes may be impactful to some vegetation and
unsuspecting populations.



Across the eastern Pacific, the National Hurricane Center (NHC) is currently
monitoring two areas for additional TC development (as of 7am PDT), with 90%
and 40% chances of development during the next week, both located approximately
between 125W and 110W.  While the lower likelihood area may become a factor in
regards to the enhancing moisture advection into the CONUS, probabilistic TC
genesis and activity tools depict the higher likelihood disturbance maintaining
a westerly track steered by a subtropical ridge extending into the Central
Pacific. Although there remains high uncertainty, additional tropical related
precipitation impacting parts of Hawaii cannot be ruled out during the week-2
period.

FORECASTER: Nick Novella

$$