Threats Assessment Discussion

904 
FXUS21 KWNC 241844
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT April 24 2019
SYNOPSIS: Mid-level low pressure is forecast over the Western U.S. in week-2 
favoring periodic impulses of shortwave energy to eject out of the Rockies. 
These disturbances are favored to rotate around the periphery of surface high 
pressure over the East, resulting in intermittent chances of unsettled weather 
throughout the period.
HAZARDS
Moderate risk of heavy precipitation for portions of the Central and Southern 
Plains, and the Middle and Lower Mississippi Valley, Thu-Fri, May 2-May 3.
Slight risk of heavy precipitation for portions of the Central and Southern 
Plains, the Mississippi Valley, the Central and Southern Appalachians, the 
Great Lakes, the Ohio Valley, the Tennessee Valley, the Northeast, and the 
Mid-Atlantic, Thu-Wed, May 2-May 8.
Slight risk of much below normal temperatures for Montana and adjacent areas, 
Thu, May 2.
DETAILED SUMMARY
FOR SATURDAY APRIL 27 - WEDNESDAY MAY 01: 
https://www.wpc.ncep.noaa.gov/threats/threats.php
FOR THURSDAY MAY 02 - WEDNESDAY MAY 08: A noticeable shift in model guidance 
today has resulted in some changes to the hazards outlook compared to 
yesterday. A stronger trough is now favored in the Western CONUS early in 
week-2, along with an eastward displacement of the mid-level ridge and surface 
high pressure in the East. Because of the increased troughing in the West, 
there is an increased risk for cooler temperatures early in week-2, with a 
slight risk for much below normal temperatures (20% chance of minimum 
temperatures in the lowest 15th percentile of the climatological distribution) 
now posted over much of Montana, Northern Wyoming, and the far Western Dakotas 
on 5/2 given the potential of a hard freeze in some areas. This is based on 
guidance from the GEFS reforecast tool. Snow is also possible early in the 
period over this region as shortwave energy may interact with the cold air in 
place.
With the eastward shift in the ridge axis, the precipitation axis is also 
shifted southward and eastward compared to yesterday. A disturbance ejecting 
out of the Rockies early in the period favors a heightened chance for heavy 
precipitation across the Plains early in week-2. The GEFS focuses the heaviest 
precipitation across portions of the Central Plains and adjacent parts of the 
Mississippi Valley, while the ECMWF ensembles favor the Southern Plains to have 
the highest precipitation totals. Synoptically, the overall set-up looks 
conducive for a convective event, and therefore a moderate risk of heavy 
precipitation (40% chance of precipitation in the highest 15th percentile in 
the climatological distribution) is highlighted for parts of the Central and 
Southern Plains along with western parts of the Middle and Lower Mississippi 
Valley. Throughout week-2, this along with other disturbances are favored to 
track around the periphery of surface high pressure over the Southeast and 
Western Atlantic, resulting in intermittent chances of heavy precipitation. 
Therefore, a slight risk of heavy precipitation (20% chance of precipitation in 
the highest 15th percentile in the climatological distribution) is posted for a 
broad area extending from the Central and Southern Plains northeastward through 
parts of the Mississippi Valley, the Ohio Valley, and into the Mid-Atlantic and 
Northeast for the entire period. No areas of hazardous winds are posted, 
although some gusty winds are possible in parts of the High Plains during 
week-2.
Ridging is forecast over Alaska in week-2 with some troughing forecast to 
develop over the Bering Sea toward the middle of week-2. Precipitation threats 
look minimal and therefore, no hazards are posted for the state.
FORECASTER: Thomas Collow 
$$