Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
327 AM EDT Wed Mar 28 2018


Prelim Day 1 QPF Discussion
Valid Mar 28/1200 UTC thru Mar 29/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Central and Eastern U.S...

The northern stream jet streak and associated upper vort lobe will
continue to dive se down the upper ridge and into the western
periphery of the southern stream trough. This will result in a
more progressive tough and surface front by Thursday, in-turn
reducing the excessive rainfall threat. Until then however, a well
established, slow moving warm conveyor belt (wbc) is continuing to
fuel the "Atmospheric River" of deep-layer moisture into the
southern Plains northeast across the MS Valley and Ohio Valley, as
noted from the ESRL AR detection tool. The continued subtropical
connection aloft (from south of Baja) along with the strong
low-mid layer moisture transport/flux off the western GOMEX are
maintaining anomalous PW values into these regions -- about
2.5-3.0 standard deviations above normal per the SREF and GEFS. 

Convection should be undergoing a weakening trend later this
morning as it outruns the better forcing/instability, as depicted
by the trends from the high-res CAMs, although some locally heavy
rains should continue from portions of east TX into northern LA,
southern AR, and western TN. By late morning into the afternoon,
we should see convection regenerate across east TX into western
LA, with the short-term rainfall rates bolstered by the uptick in
deep-layer instability (mucapes 1000-2000 j/kg). This convection
will have strong support synoptically as the southwestern U.S.
shortwave finally begins to eject east. Some uncertainty whether
this convection forms along the morning outflow closer to the TX
Gulf Coast or farther northwest along the actual cold front, or
whether both boundaries eventually become active. Given these
uncertainties we preferred to stay close to a consensus of the
high-res ARW/ARW2 (though not quite the same degree of QPF per the
ARW, which noted a fairly broad swath of 5-7+ inch totals from
eastern TX into portions of northern LA). The risk for
training/repetitive convection will increase once again overnight
Wed into Thu morning given the nocturnal uptick in the LLJ, which
as the night progresses, will veer and align closer to the mean
850-300 mb southwesterly flow. WPC noted a fairly broad MODERATE
risk in the new day 1 Excessive Rainfall Outlook (ERO), from the
Upper Tx Coast and eastern TX through northern LA and western MS,
encompassing the heavier rain areas moving in during the day Wed
along with the potential for additional, slow-moving convection
overnight into early Thursday. The moderate risk also incorporates
the areas with the highest probs of receiving >2 inches of rain
per hour/>3 inches within 3 hours.

Hurley/Chenard