835
FXUS21 KWNC 201804
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 20 2026

SYNOPSIS: A stalled frontal boundary and prolonged easterly flow are expected
to set up across the Florida Peninsula and the Gulf, accompanied by an elevated
risk for periods of heavy rain all week-2. No other hazards are posted under a
deamplifying zonal mid-level pattern across the Contiguous U.S. (CONUS), but a
few situations need to be monitored. Unseasonably hot weather is possible early
in part of the south-central U.S. while enhanced rainfall expected across the
Plains could prove more beneficial than hazardous. Farther west, low pressure
in the Gulf of Alaska has the potential to bring increased precipitation and
winds to parts of southern Alaska at times, and the continuing active tropical
season in the East Pacific increases the chances that at least one disturbance
impacts Hawaii.

HAZARDS

Slight risk of heavy precipitation for the Florida Peninsula and the Gulf Coast
region, Mon-Sun, Sep 28-Oct 4.

DETAILED SUMMARY

FOR WEDNESDAY SEPTEMBER 23 - SUNDAY SEPTEMBER 27:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR MONDAY SEPTEMBER 28 - SUNDAY OCTOBER 04: Dynamical models generally agree
that week-2 starts with a deamplifying mid-level ridge extending from the
central or interior western CONUS northeastward, with the greatest 500-hPa
anomalies over eastern Canada. Meanwhile, a mid-level low pressure along or
near the Atlantic Seaboard will meander near or into the eastern CONUS while
dissipating by the middle of the period. This results in generally zonal flow
across the CONUS with modest positive 500-hPa height anomalies covering most of
the CONUS during the last half of week-2. A flagging ridge axis is expected to
waver around the central to interior western CONUS at this time, extending
northward into somewhat more impressive positive 500-hPa anomalies across much
of Canada. Mid-level heights are relatively lower to the south and east,
establishing a weak de facto mid-level trough near or off the East Coast
despite expected near- to slightly above-normal 500-hPa heights there.
Meanwhile, the European ensemble (ECENS), GEFS, and GEFS-Artificial
Intelligence (GEFSA-AI) means all start week-2 depicting a fairly strong
mid-level trough extending from Alaska into the North Pacific, with negative
500-hPa height anomalies reaching -12 dm well west of British Columbia. The
Canadian ensemble (CMCE) mean has a weaker trough centered farther north, but
this appears to be an outlier, so a stronger solution consistent with model
consensus is favored. With time, this trough remains approximately in place and
weakens, but 500-hPa heights remain at least 6 dm below normal through all of
week-2 over part of the central North Pacific and adjacent Gulf of Alaska.



Marginally anticyclonic mid-level flow is expected to prevail across the
eastern half of the CONUS between the weak ridge axis to the west and a couple
of iterations of mid-level troughing to the east. This induces weak but
persistent surface high pressure across the central and eastern CONUS for most
of week-2, with lower pressures expected to the south and east, resulting in a
weak frontal boundary around the eastern and southern periphery of the surface
high pressure. Initially this feature is draped southward from the mid-level
trough centered near the Atlantic Seaboard, then southwestward across the
Florida Peninsula south of the surface high pressure. With time, surface
pressures are expected to rise in the western Atlantic as the mid-level trough
becomes more dissolute, and the front takes on an east-west orientation south
of surface high pressure, extending from the northwestern Tropical Atlantic
across Florida and the Gulf and potentially into adjacent Texas or Louisiana.
The weak, wavering frontal boundary is expected to serve as a focus for
convective development, with a protracted fetch of easterly winds keeping
moist, tropical air flowing across the region. Models consistently highlight
southern Florida as the area at greatest risk of hazardous rainfall totals, but
most now show considerable amounts along much of the Gulf Coast and
significantly lower amounts inland, although the details of timing and
intensity are uncertain. In addition, the Probabilistic Extremes Tools (PETs)
from the ECENS and GEFS show marginally enhanced chances for 3-day
precipitation to exceed both the 85th climatological percentile and one inch
total across varying parts of the Florida Peninsula and Gulf Coast through at
least the middle of the period. Furthermore, any tropical waves or low pressure
systems that interact with the frontal boundary would further enhance rainfall
over at least part of the region, further supporting a risk of hazardous
precipitation amounts. The boundary is expected to drift around Florida and the
Gulf Coast region all week, so a slight hazard for heavy precipitation is
posted there for the entirety of week-2. Despite most models showing south
Florida as the area expected to receive the most rain, there is too much
uncertainty with this set-up to justify any moderate risk areas. Some areas of
heavy rain could be slow-moving, potentially triggering isolated flash
flooding, although no particular area appears vulnerable to widespread and
persistent flood conditions.



Despite the anticipated deamplification of the mid-level ridge over the central
and south-central CONUS, 500-hPa heights are expected to remain broadly above
normal almost nationwide, allowing unseasonably high temperatures to linger
from the southern Great Plains through the Lower Mississippi Valley early in
the period. Temperatures or heat indices there have a 30 to 50 percent chance
of exceeding the 90th climatological percentile and meeting hazards thresholds
according to internal heat forecast tools based on the ECENS and GEFS, but
declining 500-hPa heights and the time of year both argue against posting any
heat hazards.



Early in week-2, significant rainfall is expected to reach eastward out of the
southwestern CONUS across the Plains and Mississippi Valley. This rain is more
likely to be beneficial than hazardous in most areas as antecedent conditions
have been dry, especially across the southern half of this region. And while
the ECENS PET shows nominally enhanced chances for 3-day rainfall amounts to
exceed the 85th climatological percentile and one inch total, the GEFS and CMCE
PETs show little chance for amounts that high. Overall, there isn`t enough
support for heavy rainfall among the tools to support posting a hazard at this
time, although models overall are slightly wetter today.



A mid-level trough is expected to be centered over the Gulf of Alaska at the
start of week-2 and remain approximately in place through the period while
slowly weakening. This is expected to induce and maintain low surface pressures
south of Alaska, which has the potential to enhance precipitation totals and
winds across parts of southern Alaska at times. Most of the tools, however,
show somewhat lower precipitation totals and less of a chance for hazardous
winds today, so while above-normal precipitation and some gusty winds are
possible, tools are not sufficiently robust to support posting any hazards at
this time.



As the active tropical season in the East Pacific, the increase in tropical
cyclone development enhances the odds that at least one system may influence
conditions across Hawaii. Grounds are saturated in many areas due to periods of
heavy rain over the past few months, and any additional episodes of heavy rain
may renew impacts caused by the wetness, such as flooding and possibly
mudslides. At this time, several areas of potential tropical development are
being monitored in the East Pacific, but no specific system is threatening the
island chain at this time. Refer to products from the National Hurricane Center
for more detailed information, including any tropical cyclone development.

FORECASTER: Rich Tinker

$$