Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
308 PM EDT Sun May 13 2018


Prelim Day 1 QPF Discussion
Valid May 14/0000 UTC thru May 15/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...Midwest to Mid-Atlantic...

Instability was developing near a somewhat wavy frontal zone
across the Ohio Valley and Mid Atlantic. The synoptic background
remained loosely favorable for diurnal convective development,
with flat cyclonic flow and slight 12 hour 500-mb height falls,
despite this being quickly followed by warm advection and height
rises later tonight. Thunderstorms initiating in the high terrain
of eastern OH into WV/PA and northern VA may become locally
organized and evolve eastward, though persistent cloud cover near
the coast will likely yield a weakening trend to anything that
tries to move toward eastern Maryland up through New Jersey.
Meanwhile, farther south in Virginia there may be insufficient
forcing to produce much more than isolated storms.

Back west a remnant convective cluster may find new life as it
exits eastern Iowa and eventually taps into the growing surface
based instability over the region. This cluster/system could hold
together through evening before weakening.

The pattern is then primed for another nocturnal event to develop
over the Midwest - Iowa into IL/WI. The 12Z model suite came into
very good agreement as to this, with many of the individual models
developing a progressive MCS / bow echo by late tonight, and one
that could survive through the day tomorrow - a very summer-like
event if it occurs. There is a lot of model evidence for this, and
the consensus QPF increased somewhat along the path of this
expected MCS. The current pattern has consistently verified on the
north side of guidance given warm low to mid level temperatures
and marginal low level moisture, so we leaned north a bit,
especially from Lake Michigan eastward where we favored the
WRF-NMMB and NAM CONUS Nest.

The QPF crosses some sensitive areas with wet antecedent
conditions and high river stages, and we did introduce a Slight
Risk of excessive rainfall for parts of IA/IL/WI.


...Florida...

Again a difficult forecast here. An injection of tropical moisture
around the periphery of a broadly developing baroclinic trough
yields the potential for heavy rain. The models are depicting very
heavy downpours anywhere that lift occurs, given the tropical
thermodynamic profiles. Today, however, any substantial
instability is located offshore, and rainfall is rooted in the
synoptic lift within the middle and upper layers of the
atmosphere. Widespread moderate rain and embedded heavier cores
are expected, but we reserved the very heavy amounts for the more
unstable near-surface air which advects up into the Keys and along
the east coast of the peninsula beginning tonight and continuing
tomorrow. Certainly the consensus trend was toward spilling more
of the heavy rainfall inland along the east coast on Monday, and
we followed this trend given the eventual arrival of stronger
surface to 850-mb inflow beginning tonight.

WPC QPF was based largely on WPC continuity and the NAM CONUS Nest.


...Southern Plains...

This will be a very difficult scenario in which to pin down
specific thunderstorm evolution with much confidence. The region
near and east of the dryline has been strongly capped the past few
days owing to both warm mid level temperatures and marginal
moisture depth. Both parameters have improved only a smidge,
judging by the SAT - SUN soundings. This evening moisture will
back up to the caprock beneath weakly difluent upper flow and very
subtle height falls ahead of the western U.S. trough. A few
thunderstorm clusters are possible or even likely, but we have
less confidence in the scope of this activity. The global models
are much more conservative than the hi-res models in this case,
and the hi-res may be over-playing the instability and
under-playing the inhibition that will likely squash convection
during the late evening. On the other hand. If enough cold pool
production can take place, then a heftier QPF swath could result,
such as near the Red River in some of the hi-res guidance. We
played this conservatively in the WPC QPF, using to some degree
the GFS and WRF-NMMB. There would seem to be a better chance of
scattered coverage of storms on Monday as westerly flow improves a
bit and the dryline becomes more SW to NE oriented, increasing
convergence over especially NW OK and adjacent KS/TX.


...Western U.S. and Front Range...

The western U.S. low will continue to spin down, while occasional
convergence and instability will support showers, particularly
over eastern Nevada near the low center. Southwest flow atop
post-frontal upslope will lend toward scattered low-topped
convection from northeast Colorado into adjacent WY/NE both today
and tomorrow. We used the NAM CONUS Nest and WRF-NMMB for some of
the smaller scale details.

Burke