Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Mon Mar 19 2018


Prelim Day 1 QPF Discussion
Valid Mar 20/0000 UTC thru Mar 21/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Eastern U.S...

A very dynamic southern stream shortwave will move through the
Tennessee Valley today, inducing a widespread precipitation event
from the Plains to the east coast...and some concentrated severe
weather per the Storm Prediction Center Day 1 Convective Outlook -
associated with a well defined frontal wave and the extensive
back-bent occluded front. Areal average precipitation amounts will
be greater north of the warm front and along the occlusion, where
low level WAA/moisture convergence is maximized. Down along the
front and into the warm sector storms will be moving quickly, with
70-80 knot westerly 500 mb flow...thus QPF will generally be less
in the warm sector. Overall this still looks like more of a severe
than flash flood event...with storms expected to be progressive in
nature. However, still appears like isolated 2" amounts in an hour
or two remain possible...which may approach FFG over the risk
area. Could see some 1-1.5" totals further east into portions of
NC and southern VA with embedded heavier convective
cores...although instability here is weaker and elevated in
nature. Thus not currently thinking rates will be high enough to
pose a flash flood risk here. Further north across the Mid
Atlantic and OH Valley precipitation will be stratiform in nature.

Convection has expanded in coverage over the last few hours over
the FL Panhandle...as suggested by earlier HRRR runs and the 12z
runs of the ARW and ARW2. This activity is forming ahead of a weak
mid level wave and on the nose of an enhanced axis of 850 mb
moisture transport. Storms will remain quick moving, however a
broad area of convective development will allow for some repeat
convection. Some areas of the FL panhandle saw several inches of
rain overnight, and with the potential for localized 1-4"
additional rain today...some flash flood issues could arise in
more susceptible locations.

For QPF WPC utilized a multi model blend, incorporating both the
latest global models and high res CAMs. The one consistent trend
seen amongst all guidance was to push the northern extent of the
precipitation shield over IN/OH further north. This makes sense
given the track of the well defined shortwave currently over MO.
QPF was also shifted a bit north across the northern Mid Atlantic
given the consistent trend seen. From the southern Mid Atlantic
into the southeast changes from continuity were more minor...with
the general idea the same from our previous forecast...just some
of the details changed based on the latest consensus. Over FL,
preferred sticking closer to the ARW/ARW2...which both appeared to
have a better handle of the ongoing convection over the Panhandle.


...CA...

Rain will begin to move into central and southern CA by the end of
the day 1 period...although the heavier rain should hold off until
Tuesday night and last through mid week. Some model differences
were already apparent by 0z Wednesday...with the GFS and high res
CAMs quicker to bring in locally heavy rain onto the coast.
Meanwhile the 0z ECMWF and 12z GEM regional were similar in
depicting a slower and further north axis through day 1. For now
opted to split the difference and go with a consensus in between
the two solutions.

Chenard