Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
320 AM EDT Mon May 21 2018


Prelim Day 1 QPF Discussion
Valid May 21/1200 UTC thru May 22/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...Lower MS Valley into the TN/OH Valley and Great Lakes region...

Another day of active convective weather stretching from the Lower
MS Valley northeast into the Great Lakes region. This activity
will develop in a broadly convergent low level pattern, with ample
moisture and instability for locally heavy rains. Across portions
of MS, eastern AR, and western TN/KY flow will generally be weak,
and thus convection should generally remain unorganized and more
pulse in nature. With that said, this will be the axis of greatest
instability (> 2000 j/kg in spots), and thus anticipate some
strong convective cores to develop. As we have seen the past few
days, while most of these storms will be short lived and brief,
some cell mergers along outflows will likely occur...leading to
localized areas of 3"+ rainfall.

Further north across portions of IN/OH, flow and forcing will be
stronger. This area will be closer to the mid level shortwave and
upper jet to the north, and deep layer wind shear increases enough
to suggest at least some convective organization is likely. While
not as high as areas further south, instability should still
become sufficient for convection along and south of the warm front
stretching across the region. Overall think convection will be
progressive in nature, although some brief backbuilding/training
is a possibility along/near the warm front. Overall this setup,
combined with PWATs above average, suggests the potential for some
heavy rainfall totals locally exceeding 3".

WPC QPF generally favored a consensus of the 0z HREF members and
the 0z Experimental HRRR for this region.


...Mid Atlantic...

Another active day expected across portions of WV, into southwest
VA, and northern NC. A backdoor front will likely act as a focus
for convective development this afternoon/evening across this
area. PWATs, while not extreme, will remain solidly above normal,
and thus locally heavy rates are possible. Convection may first
develop across portions of eastern KY/TN into southern WV,
southwest VA, and northwest NC...where differential heating in the
terrain may act as an additional trigger for storms on top of the
boundary. Slightly stronger southerly flow across this region
should result in storms drifting off to the north. Instability is
high enough that will probably have some brief cell mergers along
outflows, and also may see some brief training at cell initiation
on/near the terrain. Thus, while in general storms will both be
moving enough and unorganized enough to prevent widespread heavy
totals, likely to see some localized heavy amounts. And given the
saturated conditions, these isolated heavier totals will result in
at least a localized flash flood risk today.

Further east storms will fire along/near the stationary boundary
near the NC/VA border. Instability is forecast to be even higher
here, and mean flow is westerly, parallel to the boundary. Thus
even though shear is weak and suggestive of pulse convection, a
bit more concerned about heavier rains here. Think convection may
try to organize along the boundary, with some regeneration along
outflows allowing the storms to slowly drift south. Thus seems
like a pretty good chance we will see at least localized 3"+
amounts from near the VA/NC border south into northeast NC. These
areas have not been quite as wet as locations further northwest,
so could likely handle a bit more rain before flash flooding
becomes a concern. Nonetheless, the setup suggests at least a
localized flash flood risk is probable.


...Florida and Southeast...

A weak vort and low is currently moving into the Florida panhandle
this morning. This system will bring locally heavy rains as it
moves northward. The 0z high res models are pretty much unanimous
in depicting some very heavy totals in narrow bands with this
feature. In general, think these are probably overdone given the
overall lack of instability. However, locally heavy rains still
seem probable. As the low/wave drifts northward, it should act as
a focus for diurnally driven convection into portions of southeast
AL and southwest GA. Again, instability will probably be a
limiting factor, although still likely to see some locally heavy
convective cores. Again think the high res models are probably
overdone with amounts, but localized totals exceeding 3" seems
likely, with at least a localized flash flood threat given PWATs
increasing to above the climatological 90th percentile.


...New Mexico into Texas...

Convection will develop this afternoon across western NM along a
tightening low level convergence axis and broadly divergent upper
level flow. Activity should be rather widespread in nature, and
will probably form into one or more linear segments as it
propagates eastward with time. Storms are expected to generally be
progressive in nature, although two periods of slower
movement/training are possible. First near initiation time over
central NM, where storms may linger near the terrain features that
help initiate them. Then later at night an increase in
southeasterly 850 mb moisture transport is expected. As this feeds
into the eastward moving convection, storms may try build
southeast into the low level flow and better instability. This
motion may allow for some brief training to occur. PWATs increase
into the very above normal range across this area, so any repeat
cell or training could result in some flash flood concerns. WPC
QPF leaned towards a consensus so the 0z high res models. Given
the increasing southeasterly flow overnight, do think convection
will tend to maintain itself into west TX, thus leaned towards the
more aggressive models here.


...West...

Troughing and above normal PWATs will lead to another showery day
across portions of the west. Best focus seems to be across the
central Sierras into NV and also across central MT. Instability
may never get high enough over CA/NV to generate rates high enough
for a flash flood concern. Slightly higher instability is possible
over MT, so will need to monitor for localized heavier rates there.

Chenard