Quantitative Precipitation Forecast
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FXUS04 KWBC 200720

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Sun May 20 2018

Prelim Day 1 QPF Discussion
Valid May 20/1200 UTC thru May 21/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Mid MS Valley...

Low confidence on convective evolution today across this region.
Will, however have opportunities for multiple rounds of convection
through the day. Already seeing some activity this morning over MO
forming in a broadly convergent low level regime in the right
entrance region of an upper jet streak passing by to the north.
Depending on cloud cover and recovery, additional storms will be
possible this afternoon into tonight ahead of the shortwave energy
moving east across the Plains. Also may see some interaction this
afternoon/tonight with the front draped across the area, possibly
helping focus convection north with time into portions of IA/IL
and east into portions of IN/OH. No strong signal that an
organized flash flood risk will develop across this region.
However, with PWATs above average, the potential for multiple
rounds of storms, and a stationary front that could act as a focus
for some brief training, at least an isolated flash flood threat
probably exists. Again confidence is low on the details and thus
for QPF generally took a consensus of the 0z high res guidance.

...Lower MS Valley into the TN Valley and Southeast...

Anticipate we will see widespread diurnally driven convection
across this region today. Moderate to high instability will
develop, and will generally be uncapped given the broad troughing
remaining over the area. PWATs, while not extreme, will continue
to run above normal. Generally not anticipating much organization
to convection, however weak mean flow supports some slow moving
cells, and likely to see some cell merging along outflows. Thus,
while any individual cell may be short lived, some chaotic cell
motions and cold pool generations should allow for localized heavy
rainfall through the period. Tough to really pin down areas of
focus at this point, although one area of more focused activity
should be across portions of AL/GA along a weak cold front as it
drifts southward. Also possibly some remnant outflows from the
Plains activity act as a focus across portions of
AR/KY/TN/MS...although nearly impossible to pin down these details
at this point. Overall, high confidence in scattered to widespread
pulse convection across this area, but low confidence on the
details and areas of greatest focus. Given the uncertainty WPC
stayed pretty close to the 0z HREF for QPF across this region.

...Mid Atlantic into Northeast...

A weak cold front will push across the Mid Atlantic and Northeast
today. Showers and thunderstorms are expected to develop along
this front. The greatest coverage should be across the Northeast
closer to the better forcing. Further south activity should be
more isolated. The exception may be across the western Carolinas
and southwest VA, where differential heating in the terrain may
act as an additional trigger for storms. Most cells with the front
will be progressive in nature. However areas of the Mid Atlantic
that have saturated soils could see a localized flash flood risk.
The best chance of this is probably across the terrain of the
western Carolinas, where activity may be more intense and could
briefly hang near the terrain increasing the duration. PWAT values
will not be as high previous days, but still high enough for
briefly heavy rates.


Two separate convective complexes are expected across Texas today.
One is ongoing this morning across north central TX. Anticipate
this complex will slowly shift southeastward with time this
morning. As we go through the morning, 850 mb moisture transport
does weaken some ahead of the line. However, the airmass ahead of
the line should become increasingly unstable. Thus would appear
that 850 mb moisture transport will remain strong enough given the
very unstable airmass in place, to allow the complex to generally
persist through the day. All of the normally better performing 0z
high res models do indeed maintain the complex, adding confidence
to this idea. In general expect the complex as a whole will become
more progressive through the day. However, a good consensus that
cells should form ahead of the complex in the unstable airmass,
then move off to the northeast with the mean flow, and merge with
the southeastward moving MCS. This evolution, combined with PWATs
forecast 1.5"+, would suggest that some flash flood risk will
likely persist with the MCS through the day. WPC QPF for this MCS
followed closest to a blend of the 0z ARW/ARW2 and Experimental
HRRR (weighted towards the HRRR and radar trends to start the

The other area of focus will be near the Rio Grande this afternoon
into tonight. Convection should initiate early to mid afternoon
off the higher terrain of northern Mexico and near the dryline.
These storms should organize as they move towards the TX border,
where they will encounter an axis of stronger southeasterly 850 mb
moisture transport. This interaction may result in storms
backbuilding for a period into the flow, before progressing more
off to the southeast along the Rio Grande. This evolution would
suggest a flash flood threat where the cells backbuild and make
the turn more southeasterly. WPC QPF for this area again stayed
closest to the ARW, ARW2 and Experimental HRRR.


Troughing will move into the northwest today, with multiple
embedded shortwaves present. Steepening lapse rates underneath the
trough will result in an uptick in shower and thunderstorm
activity today. PWAT values are forecast around the 90th
percentile, suggesting locally heavy rates are possible. Main
uncertainties regarding a flash flood threat will be whether we
get unstable enough to generate strong enough convection to get
the more intense rates, and cells should generally be moving at a
decent clip off to the north. With that said, the widespread
nature of showers, anomalous PWATs, and at least marginal
instability, suggests that at least a localized flash flood risk
may develop across more susceptible areas.


Will be another wet day across Florida, with moisture remaining
high. As usual, low confidence with the details today. The
potential will certainly be there for heavy rates and some
flooding concerns, just a question as to if/where we see enough
instability and localized convergence to generate more intense
convection. Some signs a weak mid level wave may be approaching
the state this morning, and southeasterly 850mb flow will remains
strong enough to support the persistence of organized convection
shall it develop. Thus will just need to continue to monitor
trends through the day.



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