Quantitative Precipitation Forecast
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FXUS04 KWBC 192218

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

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 20/0000 UTC thru May 23/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Eastern Carolinas...

Will need to continue to monitor portions of eastern SC and NC
through tonight 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 12z HREF shows
moderate to high probabilities of exceeding 3" across portions of
the area, especially into the evening, 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 forecast across Florida as well. PWATs and 850 mb
moisture transport should both be higher tonight into Sun than
previous days, potentially resulting in a wetter overall period.
Activity should initially focus along the west coast sea breeze
boundary, before potentially shifting to the east coast overnight.
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 evening/overnight.

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

A cold front extends from the Southern Plains into the Mid and
Upper MS Valley, with a dryline across western TX. The mid/upper
levels will remain broadly divergent and generally uncapped -- as
the bulk of the vort energy early traverses the central Plains and
eventually the mid MS Valley. Vigorous deep-layer instability
(surface-based CAPES 2000-3000+ j/kg), buoyed by steep mid-level
lapse rates (7.5-8.0 c/km between 700-500 mb) along/ahead of the
front and dry line across north and west TX will spawn new
convection by late afternoon, with the activity expected to grow
upscale later in the evening along an axis of increased 850 mb
moisture transport as the southerly low-level inflow increases to
40-45 kts. Given the degree of directional shear progged, per the
high-res CAM simulated reflectivity expect the segments to be more
progressive/forward propagating than otherwise. Still, there
continues to be a multi-model signal for hefty areal-average
totals across portions of north TX across the Red River into
southwest OK -- maintaining the higher QPF confidence over this
region. The WPCQPF comprised a blend of the previous forecast
(continuity) with the HREF probability-matched mean, NBM, and
WRF-ARW -- which noted a slight timing difference (a little more
progressive) from the previous WPC forecast.

Farther north -- another concentrated area of heavier rainfall is
still anticipated across the lower MO into the mid MS Valley,
based on the additional destabilization later this afternoon
underneath a thinning canopy of last night`s mid/upper level
convective debris, while also ahead of the main vort lobe that
across KS-NE around midday. Areal-average totals up to around one
inch are anticipated given the multi-model consensus; however, per
the high-res CAMs, localized amounts of 3-5+ inches will be
possible within the axis of anomalous PW (deep TROWAL zone). This
especially given the relative weak downwind propagation given the
low-mid layer wind fields.

Days 2/3...

...Mid and Upper Mississippi Valley to the Ohio Valley/Central
Appalachians and east of the mountains in Pennsylvania/Maryland on

A low-mid level wave over eastern Iowa on Sunday evening moves
towards the Great Lakes, with the deformation zone focus over
eastern Iowa/northern Illinois/northern Indiana.

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 Illinois and Indiana late Sunday, supported by upper
divergence maxima. This continues downstream  on Monday and Monday
evening as bands of low level moisture convergence stream across
much of Ohio and then Pennsylvania. 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 West
Virginia and Tennessee.

Manual progs used a blend of the latest available ECMWF/12Z

...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 with less weight given to the UKMET and Canadian global

Marginal risks for excessive rainfall over the southern Florida
peninsula were maintained for both Day 2 and Day 3 excessive
rainfall outlook. The inflow off the gulf advects the high
moisture into the Florida 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

...Great Basin/Northern Rockies...

A long wave trough will slowly amplify this weekend as it crosses
Oregon and California before closing into a low over southern
California Monday. The focus of rainfall develops with upper
divergence maxima just downstream from the trough as it crosses OR
and adjacent northern California and Nevada.

Highest QPF remained over portions of Oregon, northern Nevada
which corresponds 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 farther east into the ranges of Idaho,
western Montana and western/northern Wyoming in tandem with both
the low level front and upper trough.

Good agreement in the 12Z models supported a general model blend
for QPF.

...West Texas/eastern NM...

East of the developing western U.S. trough, southeasterly
confluent low level flow over Eastern NM and across much of Texas
will draw moisture off the Gulf of Mexico inland.  A Marginal Risk
for excessive rainfall was maintained for north of the Big Bend to
the New Mexico border with only minor adjustments.  The airmass
will be characterized by a precipitable water value around 1.5
inches by Monday...which is 2 standard deviations above normal
with high CAPE and low shear which should keep storm motion slow.
There large scale picture does not change much on day 3, which
will lead to another round of showers/storms with a couple of the
models showing locally 2-3 inches of rain.  The excessive rainfall
outlook was maintained at the slight risk category.


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


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