…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
