Quantitative Precipitation Forecast
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FXUS04 KWBC 220700

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue May 22 2018

Prelim Day 1 QPF Discussion
Valid May 22/1200 UTC thru May 23/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...


The wet pattern will continue today across much of the east. A
moderately unstable airmass, moderately anomalous PWATs, and broad
troughing will support scattered to widespread convective
activity. A few forcing mechanisms within the broad troughing will
help focus convection. Will have stronger synoptic forcing and
wind shear across the central and northern Mid Atlantic, being
closer to the mid level shortwave and upper jet to the north. Thus
would anticipate convection will be a bit more organized here,
probably generally in small linear segments. Will also have whats
left of the backdoor front that is lifting north as a warm front,
which should locally enhance convergence. Instability will drop
off north of the front, which should cap rates there, however
along and south of the front will have ample instability to
produce locally heavy rates across portions of northern VA into
MD. Storms should form near the terrain of WV/VA/MD this afternoon
and progress eastward. Generally expect storms to be progressive
in nature, limiting the overall flash flood threat. Although a
localized threat probably still exists given the saturated
conditions over portions of the central Mid Atlantic. Also likely
to have storms form along the actual cold front further west over
the OH Valley, and move eastward into portions of western PA.
These should weaken after sunset as they head into the terrain.

Across the southeast a lingering mid level shortwave will once
again act as a focusing mechanism for storms this afternoon. This
feature is currently over southern AL, and is responsible for the
small scale training shower activity over southeast AL. With
diurnal heating, anticipate a broader area of scattered convection
to form around this wave across portions of AL and GA. As with
past days, not expecting this activity to be organized, and cells
should generally be pulse in nature and thus short lived. However
briefly intense rates are likely, and where cells are able to
merge extending rainfall duration, a localized flash flood risk
exists. Activity should diminish after sunset, with a more
localized band of showers likely near the center of the wave
(similar to this morning).

In between these two areas, the mid/upper level focus for
convection is less evident. However seems likely that we will
again see cells form over the terrain of the western Carolinas,
eastern TN and southwest VA with diurnal differential heating.
These cells will tend to drift eastward off the terrain with time.
Normally this activity would not really pose a flash flood risk,
but given the saturated conditions over portions of this area, a
localized threat likely exists once again. May need to watch
northeast GA into western SC and southwest NC a bit closer this
afternoon. Some chance that we see a bit more enhanced cell merger
threat here, with cells moving northward on the periphery of the
wave over AL/GA possibly merging with cells moving east off the
higher terrain. If this were to occur then may see a bit more
focused flash flood risk develop. Confidence is not high on this
though, and will just need to monitor through the day.

A separate weak shortwave over the Lower MS Valley may also act to
focus some convection over portions of AR/LA/MS today. The QPF
signal amongst the 0z HREF member suggest this activity will be
even more isolated and short lived than the activity further east.
Thus while very localized heavy totals are possible, areal
averaged rainfall should not be all that high.

...Central and Northern Rockies into the Plains...

The approach of the mid/upper level trough will increase
convective chances across the Rockies into the Plains. Upper level
flow will become very divergent over WY by this afternoon, which
combined with increasing instability,and the presence of a low
level boundary, will result in scattered convective development.
Instability should be high enough to fully take advantage of the
anomalous PWATs and produce some locally intense rates. Storms
will generally be progressive in nature off to the north, however
could see multiple rounds of storms given the favorable synoptic
environment. Thus at least a localized flash flood risk probably
exists. Eventually these cells should take a turn to the right and
move into the northern plains, with one or more convective
clusters likely. Low confidence on how this evolves, but locally
heavy rains are possible into SD/ND as convection moves across.
Storms should generally be progressive, although one area to watch
may be southwest SD. Some signs that storms that initiate off the
Black Hills could merge with convection moving northeast out of
WY/NE. Should this happen a more focused flash flood risk could

As we head into the overnight hours appears to be a pretty good
setup for an axis of elevated convection across portions eastern
SD into southwest MN and northwest IA. 850 mb moisture transport
increases and persists through the night, with this elevated
activity likely focusing on the nose of this feature. Storm motion
vectors appear to have a large component parallel to this moisture
convergence axis...suggesting that storms may tend to regenerate
and train for a period from northwest to southeast. Would appear
that both instability and PWATs would be high enough to produce
some pretty heavy amounts should this materialize. Still a good
amount of uncertainty though with both the organization of this
activity and the latitude it sets up at. Generally favored an axis
close to the 0z nam nest and ARW2 for the WPC QPF, although likely
will need to make some adjustments to this as things become
clearer by later today. Either way, something to watch, as
certainly could see some flash flood potential with this setup.



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