315
AWUS01 KWNH 210704
FFGMPD
IAZ000-KSZ000-NEZ000-SDZ000-211300-

Mesoscale Precipitation Discussion 0294
NWS Weather Prediction Center College Park MD
303 AM EDT Tue May 21 2024

Areas affected...Southern & Eastern NEB...Western IA...

Concerning...Heavy rainfall...Flash flooding likely

Valid 210715Z - 211300Z

SUMMARY...Expected upscale growth of fast moving MCS with
downstream slowly lifting WAA thunderstorms pose track of training
of moderate to heavy rainfall through daybreak. Additional 2-4"
pose scattered incidents of flash flooding.

DISCUSSION...Well defined cyclonic MCV over southwest NEB is
starting to mature into a favorable environment for upscale growth
along ahead of a strong mid-level shortwave lifting out of
Southern CO at this time.  Downstream a well defined surface front
angles a bit WSW to ENE in proximity to the KS/NEB border with a
surface wave near Goodland extending toward the Omaha metro area
where surface pattern gets a bit more complicated.  Deep layer
moisture and higher theta-E air is pooled along/south of the
boundary with upper 60s Tds with solid south-southeasterly
streamlines over-running strengthening northeasterly flow
along/north of the Platte River Valley. CIRA LPW and TPW analysis
also note a wedge of enhanced moisture across Southern and
Southeastern Nebraska into W IA, with 1.3-1.5" values mainly in
the the sfc to 700mb layer...providing ample moisture for solid
flux convergence/efficient rainfall production through the morning
hours.

While the environment is nocturnally stabilizing/capping.   A
narrow ribbon of uncapped MLCAPE also resides along the front with
1500-2000 J/kg. Even if further stabilization occurs in the warm
sector, north of the front; solid MUCAPE is available for stronger
deep isentropic ascent.  VWP shows this is stating to begin in
earnest downstream of the MCV with veering southeasterly/easterly
flow becoming solidly south and increasing from 15-20kts to 40kts
over the last few hours; likely in response to the approaching
DPVA, but also favorable ascent plane though the left exit of an
anticyclonic turning jet streak across KS providing broad
divergence for upscale growth.  Moisture convergence along the
850-700mb shear axis has resulted in some fractured downstream
development already and this is likely to expand.

Given the flux, ample low level moisture convergence should allow
for heavy rainfall production with 1.5-2"/hr rates. Only limiting
factor is likely to be expected forward speed of convection in
proximity to the MCV, but with some repeating/training with the
downstream WAA showers/thunderstorms should allow for scattered
spots of 2-4" to occur resulting in likely scattered incidents of
flash flooding through early morning. This is of greatest concern
in proximity to the Omaha Metro and points downstream where spots
of 2-4" fell earlier tonight.  There is some uncertainty to the
latitude of greatest ascent/training but if this were to occur,
considerable magnitude flooding may occur given the saturated
ground conditions...but there is solid uncertainty to this track
in latitude depending on the strength of the upscale growth
(stronger further north NE NEB/NW IA; weaker a bit flatter into S
NEB/SW IA).

Note: A MPD into IA will be forthcoming for ongoing expansion of
thunderstorms downstream as well.

Gallina

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...DMX...FSD...GID...GLD...LBF...OAX...

ATTN...RFC...MBRFC...NCRFC...NWC...

LAT...LON   42959659 42809582 42209502 41289507 40709567
            40489630 40219740 39879917 39840014 40120064
            40920065 41519998 42099869 42759747