Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Fri May 18 2018


Prelim Day 1 QPF Discussion
Valid May 19/0000 UTC thru May 20/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...Mid Atlantic and Southeast...

Another wet day with locally excessive rainfall expected across
the area. The mid and upper levels remain broadly diffluent ahead
of the large scale trough, with multiple weak embedded shortwaves
continuing to progress northward in the southerly flow. PWATs
remain in the well above normal to near climatological record
range, supportive of efficient rainfall processes. A nearly
stationary west to east boundary will remain draped across central
VA through much of Friday. This boundary will once again act as a
focus for backbuilding/training convection, with continued 850 mb
moisture transport into the boundary.

Modest instability remains along this front (mixed layer CAPES
500-1000 j/kg), allowing for an uptick in rainfall rate potential
this afternoon/evening -- particularly across southern-central VA.
North of the front, rainfall will be more stratiform in nature,
resulting in generally lower rates. South of the front, convection
will generally be less organized, but still capable of briefly
intense rates. Thus the best focus for heaviest rains and an
increasing flash flood threat will be along/near the front, and
into the northwest NC and southwest VA terrain, where differential
heating across the higher terrain and some upslope component to
the flow may enhance convective coverage and rainfall magnitudes.
Overall a good consensus amongst the 12z HREF members to focus
higher amounts near this front, with high probabilities of
exceeding 3" and low to moderate probabilities (20 to 40%) of
localized 5"+ amounts. Overall appears like a very similar setup
as yesterday. Given this additional rainfall potential is over
increasingly saturated grounds, a more widespread and significant
flash flood threat appears to exist today. Thus have opted to go
with a MODERATE risk of excessive rainfall across portions of
southern and central VA into southern MD.

North of the front, rainfall will generally be more stratiform in
nature, keeping rates lower. However could still see some
localized heavier rates, especially tonight as the whole system
lifts northward. This northward shift should dissipate the
stationary boundary over VA, and thus think by this time we should
see an end to the better backbuilding/training setup over
southern/central VA. However should still see showers and embedded
thunderstorms shifting north across the Mid Atlantic, which may
focus into a narrower north/south axis. Instability should be
decreased by this point, tempering rates, although localized
heavier amounts are possible where any south to north training is
able to occur. Given the lower FFG here, will maintain a Slight
risk across northern VA into portions of MD and DE.

Will also will need to watch portions of eastern NC/SC later
tonight and on Saturday. Looks like a mid/upper level shear axis
(vort lobe) and attendant low-level convergence axis will try to
set up here, with a long fetch of moisture streaming north out of
the Caribbean and over the Gulf Stream (where current and forecast
500-300 mb layer PW values are expected to be around 0.25"). Thus
possible we see south to north streamers of convection through the
day, capable of producing very heavy rainfall rates. The models,
particularly the CAMs, have backed off in terms of the timing (now
mainly after 09Z Sat). FFG is higher here, and not certain we will
see convection organize enough to pose a more widespread flooding
threat. The 12z HREF does indeed shower some higher probabilities
of exceeding 3" and 5" here, though mainly after 12z Sat. Thus see
no strong reason to remove the Slight risk that was in place, and
thus just adjusted it a bit to better fit the newer guidance.


...Central Plains...

Another round of storms will fire across CO/WY late this afternoon
into the evening coinciding with the peak diurnal destabilization
and reasonably steep 700-500 mb lapse rates. Will also be a more
favorable synoptic pattern compared to yesterday, with the broad
mid/upper level trough and embedded shortwaves moving closer. PWAT
values will remain well above normal to near climatological record
values as well. Anticipate a complex evolution to convection
across this area with multiple factors at play. Will have a very
strong gradient in place over the western Plains, with
moderate/high instability forecast. An 850 mb low over southern CO
will strengthen southerly and easterly flow ahead of the
instability gradient feeding into convection. Will also heave a
northern stream cold front progressing south with time across the
area. In general anticipate we will see widespread convective
development, with some chaotic storm motions resulting in areas of
training and repeat convection. Will likely end up seeing all this
convection result in a well defined MCV taking shape as well,
adding even more complexity to the evolution. Thus for QPF, WPC
generally followed a blend of the 12z HREF members. Tough to pin
down exactly where the heaviest amounts will fall, but think at
least some flash flood risk will likely evolve with time into
tonight from northeast CO into southeast WY, western and central
NE, and western KS. Storms further south into OK/TX should
generally be less organized and more progressive being displaced
further form the better deep layer forcing and better 850 mb
moisture convergence. 


...Mid MS Valley into the OH/TN Valley...

Scattered to widespread convection is likely across this region
today underneath the mid level shear axis and embedded shortwave
features. This setup supports slow moving convection, and periodic
cell mergers, and with PWATs running well above normal, locally
excessive rainfall will be possible. WPC generally followed close
to the 12z HREF. As was the case in previous days underneath the
shear axis, amounts will likely be higher than the global models
suggest, and thus not surprising that this blend results in a
pretty significant increase in amounts from our previous forecast.

Hurley