Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sat Apr 21 2018


Prelim Day 1 QPF Discussion
Valid Apr 21/1200 UTC thru Apr 22/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Southern Plains into the Southeast...

A closed and not quite stacked cyclone will make its way slowly
and steadily out of the Rockies, through the southern Plains into
the lower Mississippi Valley. Accumulating snow will come to an
end this morning as the system pulls away from the high elevations
in Colorado. Attention turns entirely to the rainfall pattern. At
least some of the D3 to D4 drought areas in the southern High
Plains received some rainfall to finally kick off the spring
season. The parent weather system, itself, carries seasonably
strong wind fields, but is marked by 500 mb height anomalies only
one standard deviation below climo, and instability is somewhat
difficult to find given persistent dry ridging along the Gulf
Coast. Thus, initially moisture is taking the scenic route up
through West Texas. The moist plume will broaden today, but
ongoing rain/storms north of an advancing warm front will curtail
the northward extent of surface-based instability while the
compact nature of the height falls will mean only a narrow zone of
steep lapse rates to the east of the cyclone. Combine all this
with a jet streak cutting straight across central Texas during
peak heating, and the heavy rain potential appears confined to
areas just north of the jet streak, and then more briefly farther
south and east where convection will be more linearly forced in
the low levels.

Given the limiting factors, WPC QPF is more conservative than the
spotty 3 and 4 inch amounts from some of the hi-res models, but
there will be a good soaking rain of areal average 1.00 to 2.25
inches from the ARKLATEX into easertern Oklahoma, with some
potential for excessive rainfall on the local level, especially in
the more unstable areas across east Texas.

Overall, the guidance trend was to contain the heavier rates
closer to the mid level cyclone, given the instability-based
factors discussed above. This resulted in a westward shift of WPC
QPF for Day 1, meaning lesser amounts creeping into eastern AR/LA
and adjacent TN/MS. Still, warm advection should yield increasing
coverage of rain to parts of the Southeast by Saturday night.


...Florida...

Easterly onshore flow deepens today, and the moisture depth /
magnitude increases. Models certainly signal a fair amount of
coverage of convective showers. Some of the hi-res models indicate
spotty heavy rainfall. We were less confident in the intensity
given the looks of the observed and forecast soundings. Upper flow
becomes more supportive and difluent by Saturday night, but mainly
at high levels, perhaps not tapping into the rich low level moist
layer. Convection may instead by governed by diurnal boundary
layer influences and outflows. The mid level lapse rates do not
seem too supportive of organized heavy rain, but we will have to
watch out for small scale organization along Florida's east coast.
WPC QPF leaned toward a global model consensus while using the
WRF-ARW2 for some of the mesoscale details.

Burke