Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Fri Mar 09 2018


Prelim Day 1 QPF Discussion
Valid Mar 10/0000 UTC thru Mar 11/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...


...MS valley into the TN and OH Valleys...

Compact, vigorous shortwave traversing the northern Intermountain
west early this afternoon will track down the east side of the
longwave upper ridge and reach the central Plains by the end of
the day 1 period (00Z Sun). Out ahead and south of this feature
will be a fairly elongated upper jet streak, with peak 250 mb
winds of 110-130kts, which will lead to a quick increase in left
exit region forcing (increased low level southerly inflow/moisture
transport from the western-central Gulf and deepening waa/moist
isentropic upglide). Model QPF spread associated with this feature
remains higher than normal (for a day 1 forecast), owing to
multiple (yet subtle) areas of frontogenetical forcing as the
eastern upper jet streak pushes from the mid MS Valley-TN
Valley-lower OH Valley to off the mid Atlantic coast. WPC opted
for a blend of the 12Z ECMWF, NAM, RGEM, ARW, ARW2, and HREF mean
with the bi-modal distribution -- one axis of 0.50-1.0" amounts
draped w-e across KY (associated along the elevated front),
another across a portion of the lower MS Valley from LA and
southeast AR east-northeast into northern MS and northwest AL
(closer to the surface front). The southern heavier QPF axis will
have more instability to work with, especially closer to the MS
River, where MUCAPES will average between 400-1000 j/kg. Thus
expect more variability from areal average amounts over these
areas, as suggested per the high-res CAMs (isolated amounts
exceeding 1.5").


...Central-Southern CA...

An upper jet streak and associated shortwave energy along the
subtropical jet will approach central-southern CA on Saturday --
and with it an axis of deep moisture as PW values of 1.25" reach
the southern CA coast by toward the end of the period. The gradual
uptick in left exit region upper forcing (favorable deep-layer
waa/moist isentropic lift) within the warm conveyor belt (WCB)
will result in modest rainfall totals across the southern coastal
ranges into the San Gabriel and San Bernarino Mtns.  

Hurley