Quantitative Precipitation Forecast
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FXUS04 KWBC 190933

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
532 AM EDT Sat May 19 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 19/1200 UTC thru May 22/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...OH Valley into the Mid Atlantic...

Will be another wet day across much of the east, although the
pattern will finally begin to progress today. The broad mid/upper
level trough will finally begin pushing off to the northeast,
which will push increased moisture into the northeast and push the
Mid Atlantic front north as well. Thus anticipate showers will
overspread the northeast with time, and generally become less
focused over the Mid Atlantic. Given weak to no instability, not
anticipating rainfall rates will be all that high today from the
northern Mid Atlantic into the Northeast, and thus a Marginal risk
should suffice from the eastern Mid Atlantic into southern NY to
account for the locally heavier amounts forecast.

The shortwave that has only slowly been progressing east across
the MS Valley into the OH valley over the last few days will pick
up a bit more speed off to the northeast today. Anticipate another
day of ample instability and moisture ahead of this wave and
trough axis for more scattered to widespread diurnally driven
convection. Main focus will be across the OH Valley into the
western Mid Atlantic. Mean flow is greater today, thus anticipate
storms will be more progressive than we have seen the past few
days. Thus in general would expect rainfall magnitudes to be
lower, although with continued high PWATs, briefly intense rates
are still likely. Normally this setup would probably not be too
much of a flash flood threat, however where recent rainfall is
saturating soils and lowering FFG, this rain may cause additional
flash flood concerns. Biggest threat appears to be across portions
of southeast WV south into northwest NC, where some signal this
convection today could briefly hang up and/or repeat near the
terrain resulting in localized heavier amounts. Given antecedent
rainfall here, and the possibility of intense rates with this
activity, additional flash flooding is possible...and will carry a
Slight risk here.

...Eastern Carolinas...

Will need to continue to monitor portions of eastern SC and NC
through the day for the potential of an axis of heavier rainfall.
An axis of near 2" PWATs is expected to focus across this area for
an extended period of time, with at least weakly convergent flow
in the lower levels. Also appears like one or more weak shortwaves
will push north in the southerly flow, locally enhancing lift and
low level moisture transport. Not expecting convection to be too
organized, and instability is not all that great. However, the
persistent convergence axis, higher PWATs and mean flow does
support the potential for a period of south to north moving repeat
cells capable of producing briefly heavy rates. The 0z HREF shows
moderate to high probabilities of exceeding 3" across portions of
the area, and so think the setup suggests at least a localized
flash flood threat. Will maintain continuity and have a Slight
risk to account for this risk.


Another wet day across Florida as well. PWATs and 850 mb moisture
transport should both be higher today than previous days,
potentially resulting in a wetter overall day. Activity should
initially focus along the west coast sea breeze boundary, before
potentially shifting to the east coast overnight. Most of the 0z
high res guidance is quiet wet over eastern FL with this overnight
activity. The potential is certainly there for these heavier
totals given the increased southeasterly flow, strong convergence
signature and near climatological record PWATs. Appears like the
high res models are hinting at a subtle wave riding north,
interacting with these favorable parameters and producing a period
of south to north training. Tough to say for sure if this will
materialize, but certainly something to watch, as given wet
antecedent conditions, a greater flood threat could exist. With
the above environmental parameters in place, 0z HREF probabilities
of locally exceeding 5" in the moderate to high category, and wet
antecedent conditions, will go with a Slight risk across the
southeast Florida coast. Lower confidence with this risk area, but
the conditional risk appears high enough to warrant it at this
time and will monitor trends through the day.

...Central Plains into the Mid MS Valley...

Another active convective day expected across the area. By this
afternoon the surface setup should feature a cold front extending
from the Southern Plains into the Mid and Upper MS Valley, and a
dryline across western TX. The mid/upper levels will remain
broadly divergent and generally uncapped. Thus by afternoon will
see an uptick in convection along both these boundaries. PWAT
values will remain high enough to support heavy rains. Thus any
flash flood threat will come down to if and where convection is
able to train and or repeat for a period. The general consensus is
that the first area to convect will be along the front across
OK/KS this afternoon. This activity should move northeast along
the front for a period, but then should advect eastward into the
very unstable airmass forecast over MO. The orientation of
propagation vectors relative to the instability gradient does
support the potential for a period of west to east training of
convection across this area. Also, should see storms form further
northeast along the front from southeast NE into southern IA. Some
training along this front is also possible. Overall confidence is
low with exactly where the better convective training threat ends
up...with outflows from this mornings activity potentially playing
a role. Would lean towards the better threat probably being
somewhere over MO, given the placement of the stronger 850 mb
moisture transport, higher instability pool, and the potential for
this mornings convection establishing an effective boundary
further south. However southeast NE into southern IA is certainly
in play as well, likely being closer to the best mid/upper forcing
and having a slow moving cold front to potentially train along.
Thus opted to just go with a broader Slight risk to encompass both
potential areas.

...Southern Plains...

Higher confidence in what evolves over TX/OK...with widespread
development appearing likely along/east of the dryline. Would
appear like there is a window of potential training near the
intersection of the cold front and dryline from north central TX
into southern OK. This is depicted by a good amount of the 0z high
res guidance, and given the orientation of the fronts at play and
increasing 850 mb moisture transport, it seems plausible.
Eventually would anticipate any complex to become more progressive
off to the south into the instability pool. Will issue a Slight
risk to account for this flash flood potential. Further south
along the dryline it is less favorable for training. However could
see some backbuilding into the dryline for a period, which could
produce localized flash flood issues, and thus will go with a
Marginal risk there.


Days 2/3...

...Mid and Upper MS Valley to the OH Valley/Central Appalachians
and east of the mountains in PA/MD on Mon...

A low-mid level wave over IA on Sunday moves east northeast
towards the Great Lakes, with the deformation zone focus over
eastern IA/northern IL/northern IN. The Marginal risk for
excessive rainfall continues with slow moving cells possible near
the elongated circulation. More progressive showers/storms develop
in an axis of the low level jet with well defined 700 mb
convergence near the jet maxima crossing central IL and IN on Sun,
supported by upper divergence maxima. This continues downstream
on Monday as bands of low level moisture convergence stream across
much of Ohio and then PA.
The mid level westerly flow provides an upslope component of flow
so moisture is lifted by the mountains with showery conditions
expected in the terrain of WV.

Manual progs used a blend of the 12-00z ECMWF/00z NAM/00z UKMET
with the least weight on the GFS, which depicted a weaker wave
across the Great Lakes.

...FL/eastern Gulf coast/southeast...

Deep moisture within confluent flow with corresponding lift
provided by the passage on an inverted 850 mb trough will support
bands of showers over FL on day 2 and 3.  The NAM hinted a closed
low developing which led to strong low level wind fields and
resultant convergence and lift, and thus showed heavier totals
than other models.  Manual progs blended continuity with the
gfs/ecmwf/ukmet/Canadian global models.  Towards the end of day 2
into day 3 the nam/gfs/ukmet closed a closed 850 mb low with the
gfs on the eastern edge of the suite of solutions.  Consequently,
the gfs was given less weight than the ukmet/nam/ecmwf.  Marginal
risks for excessive rainfall over the southern FL peninsula were
maintained for both days 2 and 3. The inflow off the gulf advects
the high moisture into the FL panhandle across into southern
Alabama and Georgia, so combined with diurnal instability and sea
breeze initiated lift, the higher amounts are shown in these areas
for Mon.

...Great Basin/Northern Rockies...

A long wave trough will slowly amplify this weekend as it crosses
OR and CA before closing into a low over southern CA Monday. The
focus of rainfall develops with upper divergence maxima just
downstream from the trough as it crosses OR and adjacent northern
CA and NV.  The higher amounts over eastern OR Sun correspond to
low-mid level frontal convergence plus lift aided by terrain in
the area, like the Blue Mountains.  On Monday the focus shifts a
little further east into the ranges of ID and western MT in tandem
with both the low level front and upper trough. The maxima in NV
continues as the closed low and low level front are slow to move
across the state.  Good agreement in the 00Z models supported a
general model blend for QPF.

...West Texas/eastern NM...

East of the developing western US trough is confluent flow over
Eastern NM and Texas with southeast flow bringing gulf moisture
into NM. A Marginal Risk for excessive rainfall was raised for
north of the Big Bed to the NM border for 1.5 inch PW Sunday night
which is 2 standard deviations above normal with high CAPE and low
shear which should keep storm motion slow.
The continuation on day 3 promises to bring another round of
showers/storms with a couple of the models showing locally 2-3
inches of rain.  The risk was raised to slight on day 3 given both
trends towards continued showery conditions plus potential for
locally heavy amounts.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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