…A continued omega block pattern over the Lower 48 will lead to
prolonged periods of unsettled weather and cooler temperatures to start
the week…
…Increasing flash flood threat as well as severe weather expected for
the southern Plains/Lower Mississippi Valley Monday into Tuesday…
…Locally heavy rainfall with flash flooding possible for the Greater NYC
area/southern New England Monday…
A blocky pattern remains in place over the Lower 48 to start the week with
a slow moving upper-low over the Ohio Valley, another slow moving
upper-low over the Southwest, and an upper-level ridge over portions of
the northern Plains/Upper Midwest in between. The upper-low over the
Southwest, which has brought unsettled weather to much of the West more
broadly this weekend, will continue to bring precipitation to the
Southwest/Four Corners region, eastern Great Basin, and Rockies today.
Showers and thunderstorms with moderate to locally heavy rainfall are
expected, with the greatest chance for heavy rainfall and some isolated
flash flooding over portions of the Desert Southwest. Higher mountain
elevations will see some heavy snowfall accumulations of 8-12″+,
especially for regional ranges of southwestern Montana and northwest
Wyoming Monday, and Colorado/north-central New Mexico Monday into Tuesday.
Precipitation chances will come down significantly for the Southwest/Four
Corners, Great Basin, and northern Rockies Tuesday as the upper-low shifts
eastward.
A more focused heavy rainfall event will develop to the east along
portions of the central/southern High Plains, southern Plains, and into
the Lower Mississippi Valley over the next couple of days. An influx of
moist Gulf air will flow northwestward ahead of a low pressure/frontal
system emerging onto the Plains coincident with the eastward progression
of the upper low. The combination of this anomalously moist air,
upper-level dynamic support, and upslope flow with increasing proximity to
the Rockies will bring widespread showers and thunderstorms producing
heavy downpours and the potential for several inches of rainfall. This
will begin across portions of the southern High Plains on Monday, with a
Slight Risk of Excessive Rainfall (level 2/4) in effect for the threat of
scattered instances of flash flooding. In addition, cooler temperatures
aloft and strengthening winds with the approach of the upper-low will lead
to sufficient instability and deep-layer shear for some more intense,
supercell thunderstorms. The Storm Prediction Center has included an
Enhanced Risk (level 3/5) over southwest Texas with a Slight Risk (level
2/5) more broadly over the southern High Plains for the threat of several
instances of very large hail along with damaging winds and a few
tornadoes. Then, on Tuesday, the threat will shift north and eastward and
expand in coverage along the arcing frontal system. A Moderate Risk of
Excessive Rainfall is in effect across portions of the ArkLaTex and Lower
Mississippi Valley where upper-level support from the low and continued
storm development along a warm frontal boundary extending eastward through
the region will bring repeated rounds of slow moving storms capable of
heavy downpours later Tuesday and into Tuesday night which may lead to
scattered to numerous instances of flash flooding. A broader Slight Risk
extends northwestward across the southern Plains and into the central High
Plains where increasing upslope flow along the Rockies will support
widespread storms with additional scattered instances of flash flooding.
Severe storms will also be possible along and south of the warm front from
central Texas east through western Louisiana, with a Slight Risk of severe
weather in place for very large hail, damaging winds, and a few tornadoes.
Showers and thunderstorms will also continue Monday across much of the
northeastern U.S. under the eastern upper-low, with storms most likely
from the Lower Great Lakes east through the Mid-Atlantic and into southern
New England. A more focused corridor of very moist southerly Atlantic flow
along a coastal front through the greater New York City area and into
southern New England may lead to some locally heavier downpours and
scattered flash flooding, with a Slight Risk of Excessive Rainfall in
effect. Storm chances will linger into the day Tuesday, though will become
increasingly more scattered with less rainfall expected, before storms
begin to come to an end late Tuesday night and into Wednesday morning.
Elsewhere, some scattered thunderstorms are forecast over the Florida
Peninsula ahead of a cold front Monday. The rest of the country outside of
the influence of the upper-lows will see dry conditions the next couple of
days.
The omega block pattern will continue to lead to focused areas of both
well below normal and well above normal temperatures to start the work
week. Well below average highs are forecast for much of the Ohio and
Tennessee Valleys under the influence of the eastern upper-low, with 50s
and 60s expected. Conditions should moderate and warm a few degrees on
Tuesday. Highs will be closer to average along the East Coast ahead of the
upper low, with 50s and 60s in New England, 60s and 70s in the
Mid-Atlantic, and 70s and 80s from the Carolinas south into Florida.
Conditions will also be well below average across much of the Interior
West and into the central/southern Plains under the influence of the
western upper-low. Forecast highs Monday and Tuesday range from the 50s
and 60s for the Rockies/Great Basin and much of the central/southern
Plains to the 60s and 70s for the Desert Southwest and more southern areas
of Texas. In contrast, well above average conditions continue under the
upper-ridge over the northern Plains/Upper Midwest, with highs into the
70s and 80s. The West Coast will also see increasingly above average
conditions as an upper-ridge builds in the wake of the upper-low, with
highs in the mid-60s to mid-80s.
Putnam
Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php
