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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made April 24, 2017

 Days 3-7Days 8-14Prob. Days 8-14
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Categorical Outlooks
Experimental Probabilistic Outlooks (Information)

Valid Thursday April 27, 2017 to Monday May 08, 2017

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT April 24 2017

Synopsis: 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 Detailed Summary

For Thursday April 27 - Monday May 01: 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 antecedent rains.

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 2010.

Forecaster: CPC Forecaster


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