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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made September 19, 2018

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

Valid Saturday September 22, 2018 to Wednesday October 03, 2018

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 19 2018

Synopsis: Several upper-level low pressure systems are forecast to move over the northern tier of the U.S. during the next two weeks, causing possible heavy rain and some convective weather across parts of the lower 48 states. Much of the central U.S. is likely to be wet through the 3-7 day period, with possible extension into Week-2. Widespread upper-level high pressure in Alaska is likely to be disrupted by a shortwave trough moving over the interior at the beginning of the 3 to 7 day period, while surface low pressure over the Bering Sea, Aleutians, and South Coast of Alaska is expected to cause onshore flow along the Gulf of Alaska for Week-2.

Hazards Detailed Summary

For Saturday September 22 - Wednesday September 26: Surface troughing is anticipated across parts of Texas, as well as a stationary front extending from the Southern Plains northeastward to the Mid-Atlantic throughout the beginning to middle of the 3 to 7 day period. This pattern is expected to support heavy rains across parts of the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Sep 22 to 23. As it shifts further east, this stationary front, in addition to a cold front forming across the Southern Plains and Upper Mississippi Valley is likely to bring heavy rainfall to parts of the Southern Appalachians, the Central Appalachians, the Ohio Valley, and the Tennessee Valley, Sep 24-25. Highlighted areas may receive up to or greater than 1.5 inches of rainfall in 24-hours. A surface low is predicted to form along the northern portion of this cold front and bring heavy rainfall to localized parts of the Upper Mississippi Valley. However, due to significant model differences regarding rainfall amounts and spatial disagreement regarding heaviest rainfall, an additional heavy rain area is not identified at this time.

Flooding in the Mid-Atlantic and Southeast is likely to continue through the end of this weekend, as the region recovers from the impacts of Hurricane Florence and its remnant low. This has led to record flooding for large portions of the Carolinas.

Surface low pressure is predicted to form along the Aleutians by Sep 25, favoring strong onshore flow from the Gulf of Alaska into the South Coast of Alaska. This pattern favors heavy rainfall across coastal portions of the South Coast of Alaska, the Kenai Peninsula, and northern Alaska Panhandle, Sep 25 to 26. Some models are indicating the potential for localized areas receiving up to or greater than 3 inches of rainfall in a 24-hour period. This is a significant pattern change compared to the ongoing anomalously dry conditions in this area.

For Thursday September 27 - Wednesday October 03: An amplified upper-level trough is expected to dominate the eastern two-thirds of the CONUS throughout much of Week-2. This trough may support frontal development across the central Great Plains at the beginning of Week-2. A slight risk of heavy precipitation has been posted over parts of the Lower Mississippi Valley, the Middle Mississippi Valley, and the Southern Plains for the early portion of Week-2, Sep 27 to 29. A moderate risk area for heavy precipitation is highlighted across parts of the Southern Plains and Lower Mississippi Valley, Sep 27 to 29, where the greatest chance for heavy precipitation is forecast. The identified areas indicate where the GEFS and ECMWF Reforecast Tools indicate a 20% or greater chance of 3-day precipitation accumulations reaching the upper 85th percentile, compared to climatology.

The amplified trough is also likely to cause some of the colder air from Canada to spill south over the Great Plains and neighboring regions. A slight risk for much below normal temperatures has been posted for portions of the Central Plains, the Northern Plains, the Northern Rockies, and the Upper Mississippi Valley Sep 27 to Oct 1. First freezes are possible for portions of the northern Plains.

Antecedent amplified ridging over Alaska is anticipated to continue into the beginning and middle of Week-2, weakening towards the end of Week-2, with the possibility of some upper-level low pressure moving in. The GFS and ECMWF probabilistic extremes tool show good agreement regarding an increased likelihood for heavy precipitation along parts of coastal portions of the South Coast of Alaska throughout Week-2, where a slight risk for heavy precipitation has been forecast. Model guidance shows several surface low pressure systems moving through the Gulf of Alaska during Week-2, supporting onshore flow and leading to prolonged periods of heavy rain. For most of the Interior, warm temperatures are forecast throughout Week-2, with the GEFS probabilities tool indicating a 60% a widespread chance of exceeding the 90th climatological percentile toward the end of the week. Although these temperatures are not inherently hazardous, associated coastal flooding and other risks should be monitored.

The U.S. Drought Monitor, valid on September 11, indicates a continued decrease in severe to exceptional drought (D2 to D4) coverage from 18.86 percent last week to 16.7 percent this week. Recent heavy rainfall resulted in improvement across the middle Mississippi Valley along with central and southern Great Plains.

Forecaster: Melissa Ou

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Please consult local NWS Forecast Offices for short range forecasts and region-specific information.