Valid Wednesday September 29, 2021 to Tuesday October 05, 2021
US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM
EDT September 21 2021
Synopsis: A strong area of mid-level high
pressure is expected to prevail over the central U.S. where anomalously warm
temperatures are predicted to end September. Conversely, below normal
temperatures are favored for Mainland Alaska due to the presence of mid-level
low pressure. Increasing onshore flow is expected to bring a wetter pattern to
the Pacific Northwest next week. A tropical wave in the Atlantic is likely to
become a tropical cyclone later this week and its future track should be
Slight risk of much above
normal temperatures for the Great Plains and Ozarks, Wed-Thu, Sep 29-30.
Slight risk of much below normal temperatures for parts of Mainland Alaska,
Wed-Thu, Sep 29-30.
For Wednesday September 29 -
Tuesday October 05: Excellent model agreement and continuity continues
regarding a high amplitude ridge centered over the middle to higher latitudes
of central North America during late September. Ensemble means for the week-2
period depict the largest 500-hPa height departures of more than +120 meters
across the northern Great Plains and Upper Mississippi Valley. Although maximum
temperatures and heat index values are forecast to remain below heat advisory
criteria, given the time of year, a slight risk of much above normal
temperatures is maintained for the Great Plains since this anomalous warmth may
lead to intensifying and developing drought. Although the GEFS reforecast tool
supports a moderate risk, probabilities for maximum temperatures to reach the
85th percentile of the climatological distribution are much lower in the ECMWF
reforecast tool. Due to these large differences in the reforecast tools, only a
slight risk of much above normal temperatures is posted. Maximum temperatures
are expected to range from the low to mid 80s across the northern high Plains
to the low and mid 90s across the southern Great Plains. The rapidly drying
fuels may also result in elevated wildfire danger, especially across the
southern Great Plains during the next two weeks.
An amplified 500-hPa trough favors below normal temperatures throughout
much of Mainland Alaska. Due to its better skill recently, the GEFS reforecast
was preferred in outlining the slight risk of much below normal temperatures
for parts of Mainland Alaska early in week-2. The outlined area includes where
the GEFS reforecast tool indicates that minimum temperatures have at least a 20
percent chance of falling into the lowest 15th percentile of the climatological
distribution and below 20 degrees F. The ECMWF reforecast tool remains the
coldest model solution and shows a 20 to 40 percent chance of subzero lows
across the North Slope of Alaska.
Below-normal precipitation is favored for much of the CONUS during week-2
as the full-latitude ridge at 500-hPa persists. However, the deterministic GFS
and ECMWF model solutions are depicting a closed 500-hPa low developing over
the east-central U.S. by the middle part of next week. If this occurs, there is
the potential for heavy rainfall to the east of the closed 500-hPa low across
the central Appalachians, Mid-Atlantic, or Northeast.
A tropical wave currently located a few hundred miles southwest of the Cabo
Verde Islands is likely to become a tropical cyclone (TC) over the Main
Development Region (MDR) of the Atlantic this week. Although large spread
exists among the GFS ensemble members on the future track of this potential TC,
the number of more southern tracks among the individual members has increased
in recent model runs. Also, the deterministic 0Z Canadian and 0Z ECMWF models
depict a track near or over the U.S. Virgin Islands and Puerto Rico on Sep 29
and 30. Please refer to the National Hurricane Center for the latest updates
and forecasts along with the Climate Prediction Center’s Global Tropics Hazards
Outlook which will be updated on Friday, September 24.
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.