Valid Thursday April 27, 2017 to Monday May 08, 2017
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT April 24 2017Synopsis
: A pattern conducive for miscellaneous
hazards is favored during week-1, with mid-level low pressure forecast across
the central part of the country. Given the transition season timing, both
associated cold- and heat-related hazards are possible on either side of this
feature. Southerly flow out of the Gulf of Mexico is also anticipated during
Week-1, leading to potential hydrological concerns across much of the East. By
Week-2, a more benign setup is anticipated resulting in less hazardous weather
being anticipated for the lower-48 states. Surface low pressure systems may
impact Southern Alaska throughout the forecast period. Hazards
Summary For Thursday April 27 - Monday May 01:
- Severe weather across portions of the Lower Mississippi Valley and the
Southern Plains, Fri, Apr 28.
- Severe weather across portions of the Middle and Lower Mississippi Valley
and the Southern Plains, Sat, Apr 29.
- Severe weather across portions of the Southeast, the Middle and Lower
Mississippi Valley, and the Ohio and Tennessee Valleys, Sun, Apr 30.
- Heavy rain across portions of the Central and Southern Plains, the Middle
and Lower Mississippi Valley, and the Ohio and Tennessee Valleys, Thu-Sat, Apr
- Heavy rain across portions of the Central and Southern Plains, the
Mississippi Valley, the Ohio and Tennessee Valleys, the Great Lakes, and the
Southeast, Sat-Sun, Apr 29-30.
- Heavy snow across portions of the Central Rockies, Thu-Sat, Apr 27-29.
- Heavy snow across portions of the Northern and Central Rockies, Thu-Fri,
- Excessive heat across portions of South Texas, Fri-Sat, Apr 28-29.
- Much above-normal temperatures across portions of the Great Lakes, the
Mid-Atlantic, the Northeast, the Central and Southern Appalachians, the Ohio
and Tennessee Valleys, the Southern Plains, and the Lower and Middle
Mississippi Valley, Thu-Mon, Apr 27-May 1.
- Much below-normal temperatures across portions of the Central and Southern
Plains, Sun, Apr 30.
- Flooding possible across portions of the Central and Southern Appalachians,
the Ohio and Tennessee Valleys, the Mid-Atlantic, and the Southeast.
- Flooding occurring or imminent across portions of the Great Plains, the
Northern Great Basin, and the Upper and Middle Mississippi Valley.
- Flooding likely across portions of the Mid-Atlantic, the Southern
Appalachians, and the Tennessee Valley.
- High winds across portions of the Southwest, Southern Rockies, and Central
and Southern Plains, Thu-Fri, Apr 27-30.
- Slight risk of much below-normal temperatures for portions of the Northern
and Central Plains, the Upper Mississippi Valley, and the Great Lakes, Tue-Thu,
- Slight risk of much above-normal temperatures for areas west of the Rocky
Mountains, Tue-Mon, May 2-8.
- Severe Drought across the Lower Mississippi Valley, the Tennessee Valley,
Hawaii, the Northeast, California, the Southeast, the Southern Appalachians,
the Southern Plains, and the Southwest.
During week-1 a 500-hPa trough is forecast to extend southward from the
Canadian border into Mexico. Over the course of the week this feature is
anticipated to shift from a positive tilt to a negative one, with some
deamplification of the feature late in the period. Given this amplified setup a
wide range of hazards is anticipated with this feature and the associated
shortwaves that are forecast to pass through the trough. The mid-level setup is
also highly conducive for substantial influx of moisture off the Gulf of Mexico
into the eastern U.S., potentially yielding a number of hydrological concerns.
The Storm Prediction Center (SPC) is currently forecasting a 30% chance of
severe weather across much of Oklahoma and western Arkansas on Fri, Apr 28,
with a broader area of 15% probability surrounding this region. With warm,
moist air anticipated to be flowing north out of the Gulf, a dryline may set up
to the west of this area with an associated warm front stretching east through
the Ozarks. Given the southwesterly flow aloft, ample low-level shear is
potentially available for supercell development along with associated
tornadoes, large hail, and severe winds. Eastward shifting severe weather risks
continue to be highlighted by SPC with a 15% probability on Sat-Sun, Apr 29-30,
from Northeast Texas and Central Missouri through Southern Illinois and western
Alabama. On the 29th, this severe weather appears tied to shortwave activity
that may yield discrete storms, while on the 30th storm activity is likely tied
to the passage of a cold front through the region which could yield a more
concentrated line of thunderstorms producing severe weather.
With the aforementioned flow forecast out of the Gulf to help fuel the
severe weather, widespread heavy rain (exceeding 1 inch in 24 hours) is
anticipated. On Thu-Fri, Apr 27-28, heavy rainfall is forecast associated with
a shortwave descending from the Pacific Northwest into the Central Plains while
developing surface low pressure in the lee of the Southern Rockies is forecast
to be lifting north into Kansas. A second period of heavy rain is forecast
Sat-Sun, Apr 29-30, extending from the Western Gulf of Mexico northward toward
Ontario. Widespread rains of 3" or greater could occur during the weekend for
much of the Mississippi Valley. This four day period of wet conditions could
yield possible flooding, while also potentially exacerbating flooding from
With anomalous southerly flow forecast ahead of the mid-level trough
forecast for the central portion of the country, much above-normal temperatures
(anomalies of at least +12 degrees F or high temperatures in the 90s for first
time of the year) are possible for much of the southern and eastern CONUS for
Thu-Mon, Apr 27-May 1. Temperature anomalies of +16 to +20 degrees F are
generally favored in the Mid-Atlantic, where some areas could reach 90 degrees
F for the first time this year. Across the South, high temperatures are likely
to soar into the mid-90s, despite temperature anomalies generally closer to +12
degrees F. The hazardous area highlighted on the map may have its western
extent shrink over the course of the week, as a cold front pushes eastward.
Across far South Texas excessive heat is possible on Fri-Sat, Apr 28-29, where
maximum heat index values above 105 degrees F are currently forecast.
Behind the 500-hPa trough axis, cold-related hazards are possible. Heavy
snow (6" or greater accumulation in 24 hours) is possible Thu-Fri, Apr 27-28,
for an area surrounding the Tetons associated with a shortwave dropping towards
the Central U.S. from the Pacific Northwest. Further south, heavy snows are
also possible in the Front Range Thu-Sat, Apr 27-29, as moisture from the North
Pacific may stream southward into the area and lee cyclogenesis occurs to the
southeast of the region. Much below-normal temperatures (minimum temperature
anomalies of at least -12 degrees F) are possible on Sun, Apr 30, that could
accompany a hard freeze with widespread low temperatures in the low- to mid-20s.
High winds (exceeding 30 kt) are forecast to shift from the Southwest into
the Southern and Central Plains over Thu-Sun, Apr 27-30. Initial winds are
forecast to be associated with the base of the trough positioning, while the
wind threat may expand due to isallobaric effects with lee cyclogenesis
occuring over the Southern Plains by the weekend. Marginally elevated fire
weather conditions are also possible across the Southwest, Southern Rockies,
and West Texas on Thu-Fri, Apr 27-28, where low humidity and the presence of
dry fuels is forecast to accompany the breezy conditions.
Relatively quiet weather is forecast for Alaska during Week-1. Periodic
surface low pressure systems are possible in the Bering Sea and Gulf of Alaska,
but forecast minimum pressures appear unremarkable for this time of year, so no
associated wind, wave, or precipitation-related hazards are forecast. Ensemble
guidance supports only modest temperature anomalies during Week-1. For Tuesday May
02 - Monday May 08:
During Week-2 the 500-hPa pattern is forecast to
become less amplified. Spaghetti diagrams indicate substantial uncertainty
across most of the country with varied ridge/trough solutions across both
coasts. In general a slight progression from Week-1 and a lower amplitude
forecast is favored during Week-2, with anomalous ridging over the West and
anomalous troughing over the Upper Mississippi Valley. Associated with this
setup, a slight risk of much above-normal temperatures is possible west of the
Rockies throughout Week-2. A slight risk of much below-normal temperatures is
possible May 2-4 for the Northern and Central Plains, Upper Mississippi Valley,
and Western Great Lakes. Here at least both GEFS and ECMWF ensemble reforecasts
indicate elevated probabilities of below-normal temperatures during Week-2,
while the GEFS probabilistic extremes tool supports a greater than 20% chance
of daily minimum temperatures below the 15th climatological percentile and
being below freezing.
According to the U.S. Drought Monitor released on April 20th (using data
through 8 AM EDT, April 18th), coverage of severe, or greater intensity,
drought throughout the CONUS remains the same as last week (1.44 percent). This
remains the lowest coverage of D2-D4 drought over the CONUS since Aug
Forecaster: CPC Forecaster
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.