Areas affected…Much of NE…Far Northeast CO
Concerning…Heavy rainfall…Flash flooding possible
Valid 210655Z – 210930Z
SUMMARY…Very heavy showers and thunderstorms will continue to
expand in coverage over the next couple of hours. Areas of flash
flooding will be possible locally due to very slow-moving cell
motions and pockets of training cell activity.
DISCUSSION…The latest radar imagery is showing a rather
progressive MCS and well-defined bow echo advancing across western
NE currently as a shortwave impulse exits the central Rockies and
advances out into the High Plains.
Meanwhile, downstream of this in a west-northwest to
east-southeast fashion, from central to southeast NE, is a broken
axis of very slow-moving showers and thunderstorms that are
producing extremely heavy rainfall rates and are locally training
over the same area. This axis of convection is developing out
ahead of the upstream MCS within a gradually strengthening low to
mid-level warm-air advection regime north of a front draped across
the central Plains. The airmass is very unstable out across a
large area of NE, and in part owing to an EML as validated by the
00Z KLBF RAOB. In the low-levels, there is a substantial amount of
moisture, and surface dewpoints across areas of central and
eastern NE are in the mid to upper 70s.
The 00Z HREF suite of guidance and earlier runs of the HRRR do not
appear to be handling well the degree of organized convection over
western NE, or the activity that is locally training over the same
area downstream. Consequently, they appear to be underdone with
the evolving rainfall potential, and especially since the
convection is continuing to grow upscale with rapid cloud-top
cooling and overshooting tops.
Rainfall rates of 2 to 3 inches/hr will be possible going through
the overnight hours, with storm totals of as much as 5 inches
expected. These heavier amounts will tend to be from central to
southeast NE where the best axis of slow-moving and training cell
activity will likely be focused. Some periodic training of
convective cell activity will also be possible along the southwest
flank of the upstream forward propagating MCS.
Orrison
…Please see www.wpc.ncep.noaa.gov for graphic product…
ATTN…WFO…BOU…CYS…EAX…GID…LBF…OAX…
ATTN…RFC…MBRFC…
LAT…LON 42100083 42009959 41829835 41399678 40989609
40509565 40139591 40019694 40289889 40480084
40670271 40920381 41410383 41960309 42090194