Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 359 AM EDT Sat May 03 2025 Valid 12Z Sat May 03 2025 - 12Z Mon May 05 2025 ...Upper-level omega block to form across the CONUS, leading to periods of unsettled weather and large temperatures contrasts this weekend... ...Widespread showers and thunderstorms expected ahead of a cold front from the Ohio Valley to the East Coast Saturday with some severe weather and isolated flash flooding possible... The mid- to upper level flow will become increasingly amplified this weekend in an omega block pattern as upper lows develop over the Ohio Valley and West Coast. In the east, the first of the upper-lows will develop Saturday as an upper-level trough passes over the Ohio Valley. An accompanying surface low pressure system/cold front stretching from New England southwest through the Ohio Valley and into the Lower Mississippi Valley/Gulf Coast will provide the focus for widespread showers and thunderstorms across the region this morning and eastward into the Mid-Atlantic and Southeast Saturday afternoon/evening. Sufficient instability along with increasing winds as the upper-low churns overhead may lead to a few clusters of more intense thunderstorms capable of severe weather. The Storm Prediction Center has highlighted portions of southern New England and the Mid-Atlantic southward through the Carolinas, generally along and east of the Appalachians, in a Slight Risk (level 2/5). Large hail and damaging winds will be the main threats, though an isolated tornado or two will be possible. In addition, a moist airmass in place ahead of the front will support moderate to locally heavy rainfall with isolated flash flooding possible. The front will progress slowly eastward overnight Saturday and into the day Sunday, with clearing conditions to the west over the Lower Mississippi/Tennessee Valleys but continued shower and thunderstorm chances along the East Coast and westward through the Ohio Valley under the upper-low. Moderate to locally heavy rain and isolated flash flooding will remain possible, particularly from the Lower Great Lakes eastward through the Mid-Atlantic. To the west, some scattered showers and thunderstorms are expected this weekend in the presence of moist, upslope flow along the southern Rockies as a frontal boundary lingers in the region. Some isolated flash flooding will be possible across portions of eastern New Mexico, particularly for terrain sensitive regions and area burn scars. More widespread precipitation chances will begin to expand southward throughout the West as an upper-level trough dives southward along the West Coast, with a second upper-level low forming and solidifying the blocky pattern across the CONUS. Showers and some thunderstorms in the Pacific Northwest this morning will spread into portions of the northern Rockies, Great Basin, and California through Saturday afternoon and evening. Storms will continue Sunday throughout the northern Rockies and Great Basin, as well as the aforementioned southern Rockies/Four Corners region, as the upper-level low remains in place. Accumulating snow is expected for some of the higher elevations of regional mountain ranges, most notably the Sierra Nevada on Saturday and then portions of the northern Rockies in Montana and the San Juans in Colorado on Sunday. Dry antecedent conditions, gusty winds, and comparatively low relative humidity over southwestern New Mexico, south of the unsettled weather covering most of the West, has prompted a Critical Risk of fire weather conditions (level 2/3) from the Storm Prediction Center for Sunday. The amplifying, blocky pattern will also lead to increasing temperature contrasts from coast-to-coast as some areas of the country will see well below average conditions while others will see well above average conditions. Forecast highs under the eastern upper-low will be well below average, particularly across portions of the Midwest/Ohio Valley, as temperatures only reach into the 50s and 60s. Highs further south into the Tennessee/Lower Mississippi Valleys and southern Plains will be a bit warmer but still below average, only reaching into the 70s for most locations. Highs along the immediate East Coast will remain well above average on Saturday as temperatures rise into the 60s and 70s in New England and the 80s from the Mid-Atlantic south. The approaching cold front and precipitation Sunday will bring temperatures about 10 degrees cooler and much closer to average on Sunday. Forecast highs under the western upper-low will also be well below average, with 50s and 60s expected from the Pacific Northwest south through California Saturday. These cooler temperatures will spread into the Great Basin Sunday, with highs in the 50s and 60s, and into the Southwest, with highs only into the 70s. In contrast, temperatures under a building upper-level ridge between the lows will be well above average as highs soar into the 70s and 80s across much of the northern/central Rockies and into the northern Plains/Upper Great Lakes. Some locations along the northern High Plains may reach the upper 80s on Sunday, upwards of 25-30 degrees above average. Putnam Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php