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


Prelim Day 1 QPF Discussion
Valid May 18/1200 UTC thru May 19/1200 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 should
again develop along this front, allowing for an uptick in rainfall
rate potential this afternoon/evening. 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. Also will need to
watch portions of eastern NC/SC through the day. Looks like a low
level convergence axis will try to set up here, with a long fetch
of moisture streaming north out of the Caribbean as well. Thus
possible we see south to north streamers of convection through the
day, capable of producing very heavy rainfall rates.

By later tonight the whole synoptic system begins to shift
northward, which should dissipate the stationary boundary over VA.
Thus, think by this time we should see an end to the better
backbuilding/training setup over this area. 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, thus think rates
will be tempered some.

WPC QPF followed a blend of the 0z HREF members and the 0z
Experimental HRRR. This results in a pretty significant uptick in
amounts over VA near the aforementioned boundary.


...Central Plains...

West to east training of convection will continue across NE into
the morning hours on the nose of strong 850 mb moisture transport.
Anticipate this convection will weaken through the morning as we
see the diurnal weakening of the low level jet. By afternoon
another round of storms will fire across CO/WY. Will be an even
more favorable synoptic pattern today 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 0z 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 0z 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.


...Northern Plains...

Convection over the northern Plains this morning is aiding in the
development of an MCV near the SD/ND border. This feature will
move northeast with time today, interacting with the right
entrance region of the upper jet to the north. Thus will remain
favorable for showers and embedded thunderstorms ahead of this
feature. Instability will be lower with this system resulting in
generally lower rainfall rates. May see some weak instability
develop east of the MCV with time, which could allow for the
development of some heavier convective cores from eastern ND into
northern MN. Although generally think this activity will remain
progressive, keeping rainfall totals from getting too high.


...Great Basin, Northern Rockies, MT...

Embedded shortwaves within the broad troughing will continue to
support showers and embedded thunderstorms over this area. PWATs
will remain above normal, but instability will generally be weak,
inhibiting rainfall rates. A relatively narrow axis of weak
instability is however forecast across ID, although probably not
enough to really get much intense activity going. Thus generally
thinking we will see light to moderate shower activity, with any
heavier rates generally brief in nature.

Chenard